PassDIY Aleph-X 100w amplifier construction notes, parts, & mosfet matching

This project was made possible through the generous assistance of Nelson Pass, Grey Rollins, Hifizen, Carpenter, William, and the rest of the community at DIYAudio.com. The notes on this page are a combination of my own experiences and those reported in the thread listed below. This amplifier is a DIY effort to duplicate the exceptional ~$12,000 XA commercial amplifier offered by Pass Labs. The XA series amplifier combines the best features and performance characteristics of the legendary Aleph series and the highly acclaimed X series amplifiers (see the passlabs.com web site for more information on these). A number of people at DIYAudio contributed to the circuit design, the design and production of printed circuit boards, as well as construction advice. This particular design is not so much a "project" that comes with a schematic, parts list, and step-by-step building instructions but rather is an open design that can be relatively easily be scaled to meet any set of output requirements dictated by your particular set of speakers and listening preferences. For example, the a40 amp that I recently completed was available from the PassDIY site as a downloadable PDF file containing everything that you need to know to build one 40wpc stereo amplifier. No such single document exists for this amplifier. There are several versions of it in existance that range from 40w to 100w versions of the amp, each with different power supply rail voltages, different bias points, different numbers of output mosfets, etc. As each of these designs works, there is no "right" way to build the amp except the way that best suits your needs. The schematic above runs on 15 volt rails, 4.5A bias, and will deilver approximately 38 watts into an 8 ohm load. The following references are good (and lengthy!) reading about this amplifier and its evolution:

The Aleph-X is a high current, pure Class A (low distortion, simple design, lots of heat) amplifier that uses a Mosfet output stage. The circuit is best described (by Nelson) as balanced single-ended Class A, consisting of two balanced Aleph amplifiers sharing a single differential input pair of transistors. This innovative design allows distortion from both halves of the output to cancel at the speaker.

The particular version that I am building delivers approximately 100wpc into an 8ohm load and about 150wpc into a 4ohm load (thanks to William on DIYAudio for sharing his hard work - I'm just a copycat...). Each channel uses 12 output devices arranged in 4 banks of 3 matched devices each and dissipates approximately 300 watts all of the time. Total draw from the wall is estimated at approximately 325 watts for the completed mono amp. Each channel has over 200,000uF of capacitance, runs on +/-22v power supply rails, and is biased at about 7.5A, or 1.25A per output transistor. At 1.25A bias, each of the transistors in the output stage will dissipate approximately 28 watts. Total heatsinking per mono amp needs to be a minimum of about 0.06c/w to limit thermal rise to somewhere near 25c above ambient.

Until I finish mine, I'll have to rely upon Nelson's impressions on the commercial version of this amp: The sound of this amplifier is a quantum leap over the parent’s. They retain the sweet warmth and lushness of the Aleph series without the fluffy colorations. The dynamic contrast is even better than the X series. The bass has as much control over the speaker as the original X amps, but is a bit more neutral and carries more subtle nuance. The midrange is a little deeper and the soundstage wider than the Alephs. Are these amps better than Alephs and X’s in every respect? Yes, except for the higher power/current ratings of the X amps. Perhaps more eloquently, Nelson says that it sounds "like chocholate and peanut butter."

There are at least two versions of this amp (low- and high-power) that have been built and tested by a number of people. The parts list for each can be found by following the appropriate link below: Choose the one that best suits your power needs.

Grey's Original Version that runs on 15v rails at 4.5A bias, uses 4 fets per channel, produces about 40w into both 8 and 4 ohm speakers (at 50% AC Current Gain), dissipates 34w per mosfet, or 135w per channel. [Each Q is biased by 0.5v^2/0.22ohms= (v^2/r, ohm's law). This brings the total bias current per channel to 4.55A, total dissipation would be 15v x 2 sides of the circuit x 4.55A = 136.5W.] If you bump the AC Current Gain to 66%, it will boost the power output for 4 ohm speakers to about 72w without increasing the heat dissipation. Heat sinking for this amp needs to be 0.12c/w or more per mono. The transformer needed for this level of power output is a minimum of 300VA with dual 13v secondaries per channel.

High Powered Version that runs on 22v rails at about 7.2A bias, uses 12 fets per channel, produces about 100w into 8 ohm speakers, 150w into 4 ohm speakers (at 58% AC Current Gain), dissipates 26w per mosfet, or 320w per channel. Changing AC Current Gain to 66% will increase power output to near 180w into a 4 ohm load. Heat sinking for this amp needs to be 0.06c/w or more per mono. The transformer needed for this level of power output is a minimum of 750VA with dual 18v secondaries per channel. If you build the high-powered version (or any version requiring you to use multiple output transistors in parallel to share the bias load), you will need to duplicate everything that is enclosed in the turquois boxes in the schematic above. For example, using 12 mosfets per channel means having three of each output transistor in the schematic (Q1, Q2, Q10, Q11) as well as three of each gate resistor (R7, R9, R36, R38) and three of each source resistor (R5, R6, R40, R41). Plan accordingly...

If you desire something different in terms of power output than what is listed above, you are venturing off into strict Do It Yourself territory. Without a fairly good understanding of how the circuit works and a fair amount of test equipment, you won't get very far. Its much easier just to pick one of the linked versions and copy it (as I am doing).

Grey provides a good summary of the design: "The Aleph-X circuit is capable of driving an arbitrarily low impedance as long as the bias is set high enough. As I've noted once or twice, the voltage sets the wattage capability and the bias tells you how low an impedance you can work into. Obviously, there are tradeoffs. Power dissipation gets ugly, fast. MOSFET reliability decreases. Cumulative Gate capacitance begins to be a burden, although different people will give you different answers as to when this happens. I tried to make the Aleph-X scalable. How large or small you build it is up to you, but please do it intelligently. "

Grey explains: Taking it from the front end:

Input Differential--Q5, Q7, R23, R25
Current Source--D1, R17, R20, Q6, R24, R26, V2
Protection & anti-ground loop--D2-5, R21
Input Network--R18, R19, R28, R29
Feedback--R16, C2, R30, C4
Current limiting/fault protection--R10, R13, Q4, Q9, R35, R39
Output--Q2, R6, R9, R38, R41, Q11
Aleph current source--R2, R3, Q1, R5, R7, R8, V1, R11, R12, C1, R14, R15, Q3, C3, C5, R31, R32, Q8, V3, R33, R34, C6, R36, R37, Q10, R40, R42, R43
Output grounding/DC offset control--R1, R4, R44, R45

Yes, you can quibble a bit about some of the groupings, like whether R6 and R41 ought to be included in the limiting/protection unit for instance, but it'll do for a start. If you want to, circle each group on the schematic with a pencil. Some units, like the Current source, are completely, absolutely up for grabs. Wanna put in a different current source? Knock yourself out. Pull out the parts I listed above and insert the current source of your choice. Your one and only concern is that your new current source deliver something on the order of 20mA and be a bit variable. Unless you want to start changing other things...

The Aleph current source is really the only unusual thing, and the patent is actually pretty readable. The X part is nothing more than the way the feedback runs across from one side to the other. That patent is a little more obscure to read, thouogh. The Outputs are nothing special. That's a normal way to hook up a gain device--grounded Source. Wanna put in bipolars? Okay. Tubes? Okay. Just be sure that you make some adjustments so that they bias up properly and you'll be fine. Having a differential is pretty much necessary, it's inherent in the X scheme. Besides, it's a nice, elegant way to do the feedback, the phase splitting (if needed), and get some gain going, all in one compact package. The Current limiting/fault protection unit can be removed entirely if you want. The Protection and anti-ground loop stuff, too. Just be careful with static at the front end if you take it out. Incidentally, you can do voltage feedback to the Aleph current source instead of current feedback and it'll still be Aleph. But that's another story.

R10/R13 and R39/R35 form voltage dividers that set the 'on' point for Q4 and Q9. Think of them as setting .65V for the Vbe. If the voltage drop across R6 or R41 gets too high, the voltage across R13 and R35 will approach .65V and the NPN will switch on, limiting the output MOSFET it is attached to. You can set this wherever you like or remove it entirely if it bothers you. To disable the current limiters, remove (or never install) R10, R13, Q4, R39, R35, and Q9. The circuit will hum along quite happily assuming that you don't have a fault condition at the outputs.
The sum of the resistance of V1/R11 and V3/R33 will alter the bias somewhat. In principle, you can remove them entirely (infinite resistance) and the Vbe of Q3/Q8 will take over entirely. Putting in resistance lowers that voltage, hence lowering the current in the MOSFET. By all means, experiment with different values there. This is going to be one of those issues that is influenced by the temperature of the MOSFETs, which in turn will be set by the bias current, rail voltage, and heatsinking.

I had the very devil of a time figuring out why my Alephs weren't biasing up properly. Eventually, I figured out that it was because they were stone cold (they're water cooled) compared to Nelson's production models which ran at supernova temperatures. Once I got that riddle solved, I was much happier. Everyone's thermal environment will be different so experimentation will be the order of the day. The only way around this that I can see would be to specify heatsinks for specific variations on the circuit, but since DIY people are notorious for taking matters into their own hands, that solution wouldn't work well in the real world. This is not a good beginner project...but then again, there are those budding DIYers who will scoff and try it anyway. Baptism by fire, you might say. A learning experience. The value of the resistors from the output to the current source will be interrelated with the value of the resistors from the outputs to ground. The lower one is, the higher the other (there are limits to this, obviously--you don't want a dead short in either location).

How much heat sinking do you need for this (or any other Class A) amplifier? Here is my rule of thumb that seems to work very well. In general, you want to limit the thermal rise of the heat sink to 25c above ambient (thus providing a final temperature of about 45-50c). You typically want to be able to comfortably rest you hand on the heat sink of the operating amplifier for 5 to 10 seconds. Anything shorter than this and you are probably running too hot, which threatens the longevity of your output transistors, and possibly also the life of your speakers (once your output transistors fail). Nelson provides a good description of different thermal operating points:

So how do you figure all of this out BEFORE you build your amp and discover that it is too hot (because your heat sinks are too small)? Glad you asked! Here is what I recommend. Thermal ratings of heat sinks are specified in units of c/w, or thermal rise (c) divided by the wattage that they need to dissipate (w). This measure assumes a reasonable ambient air temperature (typically 20-25c), that the heat sinks are black anodized, have free air curiculation (not using a fan and given plenty of breathing room-usually 8 inches on all sides), and that the fins of the heat sink are oriented vertically. Be aware, though, that different vendors will have different assumptions when measuring and quoting the thermal dissipation of their heat sinks - sometimes you cannot reliably trust the figures. The bottom line is that you need BIG and HEAVY heat sinks - and LOTS of them. To get an idea of what you need, follow these steps:

So here are a few examples: The a40 amp (see link at the top of the page) is specified to dissipates 200w total for two channels (its a stereo chassis). Dividing 25c by 200w yields 0.125c/w. Next, derate this by 25% by dividing your c/w rating by 1.25. This yields a more realistic figure of 0.10 c/w. Then round it down (resulting in a larger heat sink specification). Thus, the final heat sink rating should be about 0.09c/w in order to dissipate 200w and keeping thermal rise limited to 25c. How close does this come to reality? My completed a40 amp actually draws 190 watts from the wall socket (slightly less than specified), has a total heat sinking capacity of 0.084c/w (slightly more than calculated), and has final thermal rise is 24c (almost exactly to target specs). If you do anything screwy with your heat sinks like use ones that do not have black anodize, spray paint them, mount them horizontally, block the air flow above or below them, or place them in a closed cabinet (all of which are bad ideas, by the way), you need to increase your de-rating fudge factor to something more like 1.50 or even 1.75 depending on how bad things are.

As another example,the Pass Labs Aleph2 monoblock delivers 100wpc into an 8 ohm load and dissipates 300 watts of heat all of the time. Nelson Pass states that the Aleph2 requires heatsinking of 0.06c/w per monoblock. Using the theoretical calculation dissipating 300 watts of power while limiting thermal rise to 25c above ambient (25c/300w) should require 0.083 c/w worth of heatsinking. Derating the theoretical result of 0.083c/w by 25% (0.083/1.25) provides us with a more realistic figure of 0.0656c/w and rounding down to the nearest 100th yields exactly the target value of 0.06c/w indicated by Nelson.

In conclusion: Perform your calculation for the power that you need to dissipate, determine the appropriate sized heatsink, then derate its dissipation by 25% of the calculated dissipation rating and round this answer down. This is the actual size of a heatsink that you need - unfortunately, you alway need more heatsinking that you think! Now, while it is next to impossible to find a single heat sink that is rated at 0.06c/w for a chassis that needs to disppipate 300w, it is much easier to find 4 heat sinks each rated at 0.25c/w and make sure that your output transistors are evenly distributed across your sinks. When using multiple sinks, just add their c/w ratings to one another to obtain the value that you need.

You can arrange your power supply in a number of ways. Some people just use the standard transformer-bridge-caps approach (which I did for my a40 amp), while others recommend CRC or even CLC for the higher powered configurations in order to reduce power supply ripple due to the higher current draw from the caps. Below are two power supply configurations, the first (left) is a CRCRC from Hugo of DIYAudio (including a photo), and the second (right) is a CLC from Kristijan. The first power supply provides 30v rails, while the second one will provide 20v rails. After reading through the power supply information below, you might want to download a copy of Duncan Amps Power Supply Designer software. Its a very nice piece of software that will let you simulate the size of your transformer, secondary voltages, various configurations such as CLC or CRC, and bias load placed on the power supply.

With Class A amps, the calculation of how large a transformer to purchase is pretty straight forward: the VA rating of your transformer should be a minimum of 7.5x the output power. For example, if your Aleph-X amp will provide 100wpc into 8 ohms, you need a 750VA transformer. Many people opt for a 1000VA transformer in this case to provide an increased margin of flexibility for increasing the bias or trying different configurations. The a40 amp that I built provides 40w into 8 ohms and is a stereo chassis fed by a single transfromer. Multiplying the output power (40w) times 2 channels and then multiplying by the 7.5 factor results in a minimum rating of 600VA. This is exactly the transformer that I used. While I could have gone for a higher VA rating, the one I have now works just fine without any problems. In this spirit, I would recommend a 750VA or 1000VA transformer in the Aleph-X power supply rather than a 600VA transformer that it specified in the power supply schematic (on the right).

OK, so now we know how large the transformer needs to be (its VA rating), the next thing to figure out is the required voltage to be supplied on the secondaries of the transformer. The (theoretical) calculation for specifying the proper secondary voltage for your transformer depends on the type of power supply you want to create. The most typical configuration is a capacitor input power supply (two are shown above). For a cap-input power supply, take the rail voltage that you need and divide it by the square root of 2 (1.414). Thus, if you need 32v rails, you should get a transformer with 22v secondaries (32/1.414 = 22.6, in theory). In reality, you lose about 0.7v due to rectification and sometimes a little more at the caps. For my a40 amp, 24v secondaries provide about 32.5v after rectification and at full load. Thus, dividing the needed rail voltage by 1.35 is more accurate in this case. If you are planning a CRC or a CLC filter (see below for details), you will probably lose another little bit due to the extra resistance, so plan accordingly. Each of the power supplies shown above is for a single channel. If you are looking to build a stereo chassis, plan on doubling these. For a choke input power supply, take the rail voltage that you need and divide it by 0.9. Thus if you need 32v rails, you should get a transformer with 36v or 37v secondaries. You will still lose voltage over the rectifier and caps, plus a little additional voltage with the resistance of your inductor. When choosing the inductor that you will use, keep an eye on the resistance (measured in DCR) of the coil as higher resistance will cause the inductor to heat up considerably. Consider using 12g or 14g inductors.

Good sources for a transformer are Plitron (choose from available models) and Victoria Magnetics (custom wound to your needs).

Due to the high current draw of this amp, an extreme amount of capacitance is called for. The typical "bigger is better" clearly applies here, but some care is needed (which is typical for Class A amps due to the higher-than-typical current draw when compared to Class AB and B amps). Hugo's power supply (left) features nearly 150,000uF per rail at 30v; Kristijan's (right) has 100,000uF per rail at 21v; my power supply will have approximately 250,000uF per rail at 21v. Do be sure that your power supply caps are rated about 20% higher than your planned rail voltage so that the caps can comfortably handle the voltage and to provide a safety margin. 25v caps are just fine for an amp with 21v - 22v rails. Also, when you get into capacitance this large, it is often useful to have an inrush limiter (typically a thermistor) on the primary (line side) of the transformer to reduce the stress on your power switch and rectifiers as you power up the amp (it will also keep the lights in your house from dimming quite so much). One recommended thermistor is a CL-60 (Mouser #527-CL60) that is rated at 10ohms (at no load) and 5A maximum steady state current. Under load, the resistance drops to about 0.18 ohms. Also, since we are talking about high currents in the power supply, the best power supply switch is a double pole switch that has 4 or 6 terminals. Essentially, this is two switches that operate in parallel, so the current across any single set of contacts is shared, thus reduing the stress and potential for arcing. To further reduce the potential for arcing at the swtich, you can install a 0.1pF 600v ceramic disk capacitor across each pole of the switch.

A Word of Warning: This much capaticance is HUGE! It can be LETHAL! Before working on an amp that has been powered up, make sure that your caps are discharged! The best way to do this is to install discharge capacitors across the terminals. Alternatively, you can connect a low-ohm high power resistor across the terminals with aligator clips once the amp is powered down. DO NOT use your screwdriver to short the terminals! This will destroy your screwdriver (arcing literally melts metal away), possibly your caps, any maybe you in the process. Please be careful!

If you really want to design a great power supply (one that minimizes ripple [cyclic highs and lows in the rail voltage] and provides cleaner power to your amp) the designs of choice are CLC (capacitor, inductor, capacitor) or CRC (capacitor, resistor, capacitor) power supplies. So which do you choose, CLC or CRC? Nelson had some comments on the DIYAudio forum about this topic. His comments indicate that either a CRC or CLC will do the job. However, he tends to prefer CRC networks because 1) they are cheaper 2) they have no resonance like a CLC does 3) no mechanical noise like a CLC and 4) no magnetic field to be picked up as you might with a CLC. Additionally a CRC takes up less room in your chassis. On the other hand, CLCs are slightly more efficient (less voltage loss on the rails) and reduce ripple further than a typical CRC can. I'll probably be using a CRC in my set of amps... Inductor values should be in the neighborhood of 1.5mH to 2.2mH (don't fret over whether you should use 1.4mH or 1.8mH - each will have largely the same effect) whereas resistor values near 0.15ohm will work well. In both cases, be sure to watch the power handling capability of your inductor or resistor. The inductor should be able to handle 5-10A, whereas the resistor will need to be an aluminum housed type that is bolted to the chassis for heatsinking and able to handle 25-50w. These amps draw large amounts of current.

There are a number of options here. Some have used individual STPS 80 H100 TV Schottky's, while others have just used a standard 600V 35A bridge. In either case, be sure that your rectifiers can handle the current surge at turn on (I would think a 10A steady-state diode is plenty, check the specs of the device you are considering) and are mounted directly to the chassis (with an electrical insulator, of course) or on their own heat sink to help them stay cool. For reference, a typical IRF bridge rectifier rated 600v 35A can handle temperatures up to about 130c while carrying a load of about 5 amps. In practice, mounting your rectifier to the bottom or back of the chassis will keep its temperature somewhere near 40c.

Since this current gain stage in this amplifier design is essentially two Aleph circuits operating in parallel, it is important to make sure that certain components are carefully matched to one another. Of critical importance is matching the input differential pair, followed by the output mosfets. Many people will stop here and then use 1% tolerance resistors for the rest of the circuit. While ending your matching efforts here is probably sufficient, one of the points of DIY is doing things because you can and because you can make something better. Following that thought, I also matched the resistors across the halves of the circuit, especially the mosfet source resistors to help insure proper current sharing. Finally, and perhaps not imporant at all, I also matched the capacitors across the two halves of each amplifer. One useful technique for organizing the matched sets of resistors was to use a piece of foam (left over from insulating the concrete block walls in my basement before I finished it) with a strip of masking tape on it. I could write the part number and its measurement on the tape and then poke the resistor lead into the foam to hold it in place. This way, I was able to store all of the matched resistors for each amp in a single piece of foam as shown below. Since the lead is long enough, there is no danger of the parts falling out should the whole thing tumble to the floor as mine did several times.

A single channel of an Aleph-X amplifier is essentially two (traditinal) Aleph amplifier circuits running in parallel: one amplifier for the + signal and a second amplifier for the ground signal. Thus, the Aleph-X circuit requires careful matching of components across the two sides (each amplifier) of the completed circuit in order to perform at its best. Also, in order to avoid problems with DC offset at the speaker output (having your amp put out a constant +5v to your speakers), it is necessary to match several sets of components. I matched the following sets of items, highest priority and most citical matches are indicated first:

**Resistors to Match to One Another**
R1/4 and R44/45	R10 and R39	R18 and R28 0.1%
R2/3 and R42/43	R11 and R33	R19 and R29 0.1%
R5 and R40 (all) 0.1%	R12 and R34	R22 and R27
R6 and R41 (all) 0.1%	R13 and R35	R23 and R25 0.1%
R7 and R36 (all)	R14 and R31	R46 and R47
R8 and R37	R15 and R32
R9 and R38 (all)	R16 and R30 0.1%

Matching mosfets seems to cause a great deal of distress among amplifier builders. It really is not terribly difficult to accomplish, but it does require access to equipment that many people do not have lying around. Also, purchasing the necessary equipment just to build a few amps is cost prohibitive. You need to have access to a regulated bench power supply and a precision volt meter, preferably one that reads to the mV level such as a 4 1/2 digit or greater DMM (the 6.5 digit multimeter that I used retails for $1200 - yikes! Glad I had access to the EE lab on campus. Alternatively, make friends with someone who is an electrical engineer...). Your typical $10-$30 DMM is not well suited for this task as its precision may not be great enough. Nelson posted a set of articles about matching and testing mosfets on the PassDIY web site that are helpful reading. Essentially, you want to set up the circuit below for making your measurements. The figures in the diagram have been optimized to test the IRFP240's and IRF9610's at the actual voltage (22v) and currents (10mV for 9610s and 1.125A for the 240s) that they will see in the completed amp. Note the changes in the test jig: the 9610 is a P-channel mosfet, whereas the 240 is an N-channel mosfet.

Matching IRF9610s (Q5 and Q7): My design calls for 22v rails and the circuit biases the pair of them at 20mA. Thus, each one should be tested at 10mV. For matching your mosfets using this technique, you can start by either marking each device with a piece of tape or sticker with a number on it, or just use the number that is imprinted on the casing itself. Incitentally, if you have a series of mosfets with sequential numbers imprinted on them, they were made at the same time and from the same silicon wafer. Having 10 or more from the same lot greatly increases your chances of finding well-matching pairs. Make a piece of paper that has a place to record the device number and its voltage measurement at several points in time - I measured mine at specific time intervals of 1, 2, 3, 4, and 5 minutes to see how they behave over time. Next, build yourself a test jig like the one pictured above. Use half of a dip8 socket (available from Radio Shack for about $0.70) and a small piece of perf board (also from RS). In order to make the test jig more stable during use, I screwed it down to a small block of wood and then labelled the connection points so I wouldn't forget which was which... Paralleling a 2k2 with a 10k resistor will achieve 1k8 as specified in the circuit and normal 1/4 or 1/2 watt resistors are fine since we are only dealing with miliamps of current. The value of the test jig is that you don't have to disconnect/reconnect all of your probe wires each time. I simply socketed each device, then connected the positive power supply clip. When the test period was over, I disconnected the positive supply clip and then removed the transistor from the socket.

Before you start measuring, lay all of the pieces out on the table in close proximity to one another and let them sit for 20-30 minutes so they are all at the same starting temperature. Find a comfy chair - you'll be here a while because you need to measure all of your devices at the same time. If you measure half of them now and half of them on another day, you won't be able to compare the results because the environement will be different. I found that it took about 1 hour for every 10 devices I tested. You also have to be very careful how you handle these devices while you measure them as they are extremely sensitive to their temperature environment. Use pliers instead of your fingers to place them into your test measuring jig. Even brief contact (1-2 seconds) with your fingers will heat up the part prior to testing and will adversely impact their final measurements (hotter devices result in a lower voltage reading). Also, simply walking past them or breathing on them while testing them will affect your results as well!! Be sure any room airconditioners or heaters are turned off, all fans are off, windows and doors are closed, etc. You even have to be careful where you place them in relation to your regulated bench supply as many of these have built in fans that create a breeze. If you don't belive me, just try gently blowing on one once the voltage reading has stabalized - the voltage reading will change quickly as the device changes temperature! I placed my test jig dowin into a box that would help isolate the devices from vagrant breezes while I measured them. Using your pliers, gently insert the device into the socket and connect your power supply. Use a stopwatch that clearly indicates seconds to keep careful track of time.

At first, the voltage reading will decrease fairly rapidly as the device begins to heat up, but after about 4-5 mins the vgs is getting stable and the voltage drop is getting very slow - the forth decimal place was changing by one digit with each 5 to 8 seconds that passed after 4 minutes. To measure these, I used a 6.5 digit multimeter with a regulated bench power supply at the voltage and current settings indicated above. This particular meter provided 4 decimal place resolution (0.0001V) and within my set of 40 devices (that included 3 groups of sequentially numbered fets) yielded nine sets of 3 closely matched devices. In three cases, I found pairs of devices whose voltage change tracked identially across the entire 5 minute test period at the 0.0001v level (they also had sequential numbers stamped into them)! Looks like I've found my input differentials for my front left, center, and right amps... Nelson indicates that he matches these devices to within 10mV for the Aleph series of amps, but those who have built the DIY version of the Aleph-X indicate that the amp is extremely sensitive to the matching of these devices to minimize DC offset problems (see below) and recommend matching them to within 1mV or better over a period of up to 30 minutes! My pairs are matched to the 0.1mV level after 5 minutes.

Rematching these same devices a few days later revealed different final measurements (I suppose due to different ambient temperatures), but the original groupings were still within 0.001v of one another.

Matching IRF240's (Q1, Q2, Q10, and Q11): Measuring these devices is essentially the same as indicated above for the 9610's, but you cannot test these for as long of a period of time without attaching them to a heat sink as they really heat up quickly! All of the same precautions about temperature variation for the 9610's apply here as well. Also, if you are testing the 240's anywhere near 0.5A or higher, regular 1/2w resistors do not have sufficient power dissipation ratings, it is recommended that you use resistors rated at 30w or more. Since the pin spacing on the 240's is different from the 9610's, they won't fit into the same DIP-8 socket that you made for the 9610's. The 240's do, however, fit into every other hole in an 18- or 20-pin socket, but they are a little tighter... I purchased several sets of IRF240's from Matthew Olson of DIYAudio that he matched in groups within 10mV using 15v and 0.5A power supply for 30 seconds. Since I have access to good test equipment, I re-tested them at 22v and 0.75A for 45 seconds. From what I have read on the subject of matching, it seems that many people prefer to match the devices at the actual voltage and current levels that they will see in the completed amp. Unfortunately, the regulated power supply I am using can deliver a maximum of 1.0A at 25v, so I cannot test them at 1.125 as I had intended to.

As expected, when measured at different voltage and current levels for longer periods of time, my final measurements differed from Matthew's by as much as 0.15v, but his original groupings held up very well! In contrast to measuring the 9610's, the voltage reading for the 240's changes very quickly as they heat up, making precision voltage readings difficult to perform at 0.001v levels. I had measured the 9610's with the meter set to 4 decimal places, but this level of accuracy is not feasible for the 240's, so I made measurements with the meter set for 3 decimal places. Even at this setting, the last digit changed rapidly, making it challenging to record precise readings at specific time intervals (15s, 30s, and 45s) for each mosfet.

After 45 seconds at 22v and 0.75A, the transistors got quite hot and could still be handled without burning my fingers, but I don't think I'd test them at much higher current or for any longer without using some form of heatsinking... Also, I used three 10w power resistors in parallel in order to achieve 25 ohms (100 + 100 + 50 ohm resistors). Together, these resistors should be able to handle about 30w of power, but they got quite hot to the touch and after an hour of testing, are capable of burning your fingers! When you arrange your test jig, make sure your power resistors have some breathing room to cool off. While testing a higher voltage and current levels for longer periods of time allowed me to be more selective, but did not fundamentally alter the groupings that Matthew had made. Overall, it looks like testing at 15v and 0.5A is just fine for this application. In the end, I was able to match six sets of 6 devices at a time to within 0.003v to 0.005v of one another (that's within 0.03% accuracy for both the 9610s and the 240s!). Nelson recommends matching at a level of 0.1 to 0.01Vgs, so it looks like my groupings will be just fine!

Matching at Different Bias Points: If you need to measure your IRF240's (or 9610's) at a different bias point, just use the formula Current = (Voltage - 4 ) / R, where Voltage is your actual rail voltage (and the setting for your regulated power supply), the "-4" accounts for the approximate voltage dropped by the mosfet in the circuit, and R is the value of the resistor. Adjust the output of your test equipment and the resistor in your test jig accordingly and measure at whatever bias point you need.

IRFP044 vs IRFP240 vs IRFP244 vs IRFP250 : A number of amps have been built using each of these transisitors. Nelson has characterized the differences among the three latter transistors as follows: The 250's have greater current capacity, but twice the capacitance of the 240's and 244's. They will give a better bottom end, and will sound a bit different, for better or worse, depending on your situation and taste. There is very little practical difference between the 240's and 244's. Some have reported that the IRFP044 sounds a little "darker" than the 240, though this is not really surprising as the 044 has greater capacitance than the 240 series. Essentially, greater capacitance provides slightly better bass, but at the cost of slightly reduced treble. The practical difference, though, is their current capacity. The maximum current and operating temperature ratings have lead many to use the IRFP240/244 for rail voltages of 20v or higher, while using the IRFP044 for 15-20v rails. The lower voltage versions of the amp tend to feature higher current, and the 044's can handle significantly more heat.

Source resistors in this circuit are R5, R6, R40, and R41 and typically have values of less than 1 ohm - the ones I'm using are 0.333 ohms each (achieved by using three 1 ohm resistors in parallel). Most readily available DMMs are really not well suited to accurately measuring low value resistors. The best way to accomplish this is by using a regulated bench power supply to pass a small voltage and current through the resistors and then measuring the voltage drop across the resistors. Since voltage drop is directly related to its resistance (through Ohm's law), matching the resistors according to voltage drop is essentially the same as matching them by resistance - but is much more precise. What is most important here is that they match in value since they will be placed in parallel and directly control the bias level for the output mosfets.

To match mine, I used a regulated bench power supply set to 0.1V. With this setup, each set of three paralleled resistors drew approximately 0.232A. Just use the aligator clips from your power supply and connect them across the bundle of resistors. Then, connect the leads from your volt meter across the resistors too. Depending upon your equipment, you may need to wait several seconds for the voltage reading to stop fluctuating before taking the reading. Be sure to "wiggle" the aligator clips back and forth on the resistors leads a little to make sure they are making good contact before taking your reading. On the meter I was using, the forth decimal place was in constant fluctuation even after a few seconds, so I rounded and only used three decimal accuracy in determining the resistance values of each resistor bundle (3x1ohm). To determine the precise resistance, just divide the voltage by the current. This method easily provides resistance accuracy to 3 decimal places and allowed matching of the resistor bundles to 0.3% accuracy. I now have several sets of resistors that measure 0.333 ohm and several sets that measure 0.332 ohm.

Here is the place where your standard DMM will come in handy. Matching resistors with values that exceed about 10 ohms is a task that your typical DMM is well suited for. Measure them one by one, lay them down where they won't be disturbed and make a list of the readings. When you're done, just go through your list and pick the resistors that match. It may be helpful to re-measure to make sure you picked the right ones. After confirming their measurements, just poke them into your foam block, and write their value on the tape underneath the resistor. With my three digit LCR meter, I was routinely able to match all of my resistors to tolerances of 0.1% to 0.01%. Most of them matched in the range of 0.05% to 0.01%.. Having all of the matched resistors readily available in the foam blocks helped enormously while I was stuffing the boards. I could pull the resistors I needed and double check their values against the parts list and schematic before I put them in the circuit.

In addition to matching the components as described above, I also took one extra step before soldering anything in place: I used a small piece of 0000 steel wool to polish the leads. Now, you may think its not worth the bother, but have a look at the left most picture below. The resistor on bottom has had its lead cleaned while the resistor on top has not. The difference does not show up in the picture as much as it does holding it in your hand, but the difference can be both seen and felt as you work with the part. I'm not sure it will make a realistic difference, but intuition seems to indicate that a cleaner lead will result in a better physical and electrical solder joint. The center photo below is the Panasonic 220uF FC series cap bypassed by a Wima 10nF FKP2 polypropylene film capacitor. The piggybacked assembly was then inserted into the circuit board. Finally, the right-most picture below is the 100K ohm multiturn potentiometer (V1 and V3). These are great little pots, they vary continuously from about 1 ohm to 100K ohms. Turned fully clockwise while looking at the printing on the front sets it to about 1 ohm, while each full 360 degree turn counter clockwise increases the resistance by 4K ohms. For the 200 ohm pot (V2), each 360 turn changes the resistance by 10 ohms. While they allow for fairly high-precision adjustment, several people have indicated that multi-turn pots may be less robust than single turn pots as they are more susceptible to changing their value over time or when bumped or dropped.

The pictures below are of the actual PCB itself. After spending several evenings measuring and matching my parts, it took me about 3-4 hours to stuff each board. Looking at the completed board, you'll also notice that not all of the spaces are filled. If you build the stock ~38w amp, all of your components will fit neatly onto the board and there will be no empty spaces. However, because I'm looking for higher power output and using paralleled output devices, they will be mounted directly to the heatsink. All of the source and gate resistors will be attached to the mosfets in a point-to-point fashion, and then tied back to the PCB. The same is also true for the output resistors that attach to the source of the mosfets.

Installing Jumpers: This particular circuit board has a great deal of flexibility built into it in order to accommodate different needs and philospohies of building. I am simply copying someone else's parts layout, so I installed the following jumpers: J1a, Q12a (jumper the outer pins together), R48, and R49.

Grey's Original Version If you have opted to build Grey's original version of this amp (15v rails, 4.5A bias, ~40w output), then the only external connections that need to be made to the circuit board include the power supply rails, signal input, and speaker output. For this design, the circuit board is designed to hold all of the necessary components (except the power supply). No other external connections are necessary.

High Powered Version If you have opted for increased power output, you will need to make several external connections. There are a number of "EXT" pads on the circuit board. These will be described shortly...

The input differential pair seems to benefit from added heat stability as it will help to limit DC offset drift while the amp is running (see notes above on its temperature sensitivity). To add stability what you want is mass (for thermal inertia) rather than surface area (for thermal radiation). Hence, a heat sink will actually increase your sensitivity to air currents because it'll act like a sail, catching air currents. What may help is to screw the two front end MOSFETs to a block of aluminum or copper to raise the thermal mass. Just a rectangular block, not a heat sink. Another (and perhaps better) option is to place these devices side-by-side on the main heatsink so that they track with one another and reach steady operating temperature more quickly, thus improving stability. Just remember, Q5 and Q7 must remain electrically isolated from one another! Use a silpad between them, mount them with plastic shoulders, or use plastic screws...

Some notes from Peter Daniel about adjusting the trimpots: If you didn't make the enclosure yet and the amp is not in a permanent chassiss, I wouldn't rush with replacing trimpots with fixed resistors. The chassis will change the thermal heat distibution inside the amp and I can almost guarantee that you will have to readjust everything again once the chassis is complete. This is also one reason that the amp wasn't very stable with DC offsets. When I lift the cover of my AlephX, DC can run 30mV one way or another in a first minute. That's why also, in my A75 amp I made holes in a chassis to adjust DC and bias without having to remove the top panel.

One thing I have learned from the prototype is that I am going to mount the board horizontally and on top to have ready access to the ccs trimpot. However, V2 is really the only thing that needs tweaking in situ to trim the absolute DC when everything is at the steady state. V1, 3 and the current gain stay the same. - Grataku

V1: Used to adjust bias level of "left side" of amp, directly biases Q3, which then affects current gain through Q1. See "Setting Bias" and "Adjusting Relative DC Offset" below.

V2: Used to adjust absolute DC Offset. Adjust only after setting V1 and V3. See below.

V3: Used to adjust bias level of "right side" of amp, directly biases Q8, which then affects current gain through Q10. See "Setting Bias" and "Adjusting Relative DC Offset" below.

V1 and V3 are used to set and/or fine tune the bias current. Set V1 and V3 before adjusting V2. In keeping with the traditional Aleph values, we're looking for a nominal 0.5V across the Source resistors (R5, R6, R40, and R41) of the output MOSFETs. Regardless of what you set it to, make sure both sides of the circuit match. The pots give you a fair amount of latitude in setting the bias. As a general rule, more bias current is better, but remember to keep an eye on the power being dissipated in your output devices; don't want to cook them. Begin with both V1 and V3 set to 0 ohms (full clockwise when looking at them from the front). This results in the lowest possible bias setting for your amp (approximately 4A for the 100w design). Then, slowly increase (turn counterclockwise) the bias by increasing the value of both pots. Keep turning them up until you get approximately 0.5V across each of R5, R6, R40, and R41.

Relative DC offset is measured from the positive speaker output to the negative speaker output on the same channel. The pots (V1 and V3) on the outputs will allow you a bit of control over the relative offset. If you used matched devices across the amp [that is the Aleph constant current source devices (Q1 and Q10) match on each side and the gain devices match on each side (Q2 and Q11)] then relative offset should be pretty low. Set the pots such that you have the same voltage drop across the Source resistors on the output MOSFETs comparing one side to the other. This fine tuning is performed with small adjustments once the initial bias level has been set and the amp is fully warmed up.

When you're done adjusting the relative DC offset, its time to check the absolute offset (see below), as the two interact. Thirty minutes to an hour should be enough to get the amp nice and warm. The output current adjustments can help with small relative DC adjustments at the output. - Grey

One particular thing I have noticed is that all adjustments affect each other. V2 takes care of absolute DC as well as differential DC be it of a smaller magnitude. Adjusting V1/V3 also has influence on DC in some way. Increase or decrease the bias by as little as 0.5A and a new adjustment has to be made to the offset. I tend to believe there’s a sweet spot somewhere. - NetList on 04-08-2005

Adjusting Absolute DC Offset with V2 and the 'McMillan Resistors' (R46 and R47) :

V2 sets the absolute DC offset (speaker positive output relative to power supply ground) after the bias has been set with V1 and V3. Absolute DC offset is measured by attaching a test lead to one output, the other to ground. Set the front end current source adjustment (V2) so that the absolute DC offset is as close to zero as possible. Let the amp warm up for at least 30 minutes first. An hour is better. Readjust. It'll drift a little. As long as you're close, you're okay. The speaker does not see the absolute DC offset. The only reason you fiddle with it at all is that it will cut into your potential output voltage swing to either positive or negative, causing premature clipping. It will drift a bit, particularly with heat variations. Don't worry about it. - GRollins on 03-15-2004

There are numerous ways to stabilize the absolute DC offset, but the easiest is to load each output to ground with a resistor (R1/R4 and R44/R45). In any case, the issue is usually reduced maximum output when the absolute DC is more than a volt off. 200 mV is better than you generally need. - NelsonPass on 04-06-2005

Changing R46/47 (the McMillan Resistors) from 4k7 to 3k9 to 3k3 (perhaps even 2k7 or 2k2) decreases the initial level of DC Offset - William on 04-08-2005

From William on 02/01/2007:

The McMillan resistors (R46 R47) are connected between the outputs (+ and -) and the sources of the input diff pair. Their value ranges from 2k2 to about 10k and they form feedback loop. Suppose the output at the speaker terminal absolute output becomes positive. Now the current through the diff pair will become higher and the voltage over the drain resistors (R23 / R25) becomes higher. Now the output fets will open up a bit (Rds gets lower) and the dc voltage at the output becomes lower. It works the same way the other way round. I tried different R´s starting from 2k7 and ending up with 10k. The higher the value is the better the sound quality will be and the higher the absolute dc startup value.

For the McMillan resistors you should use the highest value you can get away with for a decent absolute offset behaviour. I noticed big improvements up to about 10-12k with the standard input and up to 20-22k with the Jfet input. I didn´t install a pot but tried different values, measured and listened. As for absolute DC Offset the startup value normally doubles when the Mcmillan resistors are doubled. So 4V dc offset with 5k will change to 8V dc offset with 10k resistors.
This is the cold startup value for a 0V value when warm.

Another alternative for lowering absolute DC offset is to reduce R24 from 470 to 320 ohms.

When using the Aleph-X with an unbalanced source (RCA inputs instead of XLR), you should connect the negative input to ground. This provides 6dB of gain for the amp.

Question: I can adjust the bias nicely and can also adjust the relative DC offset to zero. BUT as soon as I connect the negative XLR input to ground (For RCA input) I get a relative DC input of 4V! Absolute DC offset is 6V with XLR and 8V!!! negative XLR input grouded (for RCA input). What did I wrong?

Answer: I had the same problems myself using single ended inputs.Whatever I tried the dc offset at the output would be always very high and always drifting- very difficult to adjust.With balanced input there was never a real problem after adjustment and warm up things were quite stable despite initial high dc.I don´t know how Pass labs manage this in their amps.
When you connect one input to ground the absolute dc adjustment is feeding back to the relative adjustment and is almost impossible to adjust as it will slowly drift away.It is as frustrating as keeping water in your cupped hands.It will never stay.I think Nelson suggested lowering the output resistors to ground. - from Protos on 10/30/2007

Do I Need to Bother With This? If you are simply copying a known design (as I am), you probably won't need to bother with AC current gain settings or how to make them. Where this is important is if you are building your own custom version of this amp.

Some Background on Current Gain: As used here, 50% AC current gain means that at maximum output half the current comes from the Aleph Current source's AC current gain, half is from the static bias. Grey's original design is biased to approx 4.5A. 4.5A = 3A + 1.5A. The 3A is DC bias current and 1.5A (50% of 3A) is maximum peak AC current gained by the active current source. 67% gain would mean that at max output, 67% of the current comes from the AC gain, so a bias of 1 amp could produce an ouput current of 3 amps (1 amp bias, 2 from AC gain)

As shown in the Aleph 30 service manual, the Ratios of R114 and R115 and the parallel combinations of R120,121 and R124-7 set the current gain. Since the output resistors add up to half the value of the emitter resistors in parallel, the AC gain would be 50% if R114=R115. Reducing R114 increases the AC gain. Since the Aleph 30 has R114 at 750R and R115 at 1K, the gain is greater than 50% - Bob Ellis

Nelson Responds to Bob's Post: This calculation ignores the inverse transconductance of the Mosfets and limited gain of the npn circuit, so it is a first order approximation. From experience, the actual gain will be less than this calculation, which is why I always prefer to measure the current ratios through the resistors.

This should not be adjusted until both the relative and absolute DC offset has been adjusted to as close to 0v as possible and the amp is fully warmed up.

As far as I believe setting the ACS is very straight forward. The output resistors (the lot in parallel which are also with the normal aleph's) should put out twice the current than the negative resistors. I will explain my case:

I have 8 times 0.56 as output resistors and 4 negative mosfets (as also 4 current source mosfets which totals to 16 mosfets per channel) . So when you measure the AC RMS voltage drop across the output resistors and across one negative 0.56 resistor you can calculate the amount of current running through these stages. I use a 1Khz sin wave with a output of 9V AC RMS as a base to measure the working of the ACS.

I get for instance with a 1Khz 9V AC RMS:

0.080V across the 8 times parallel output resistors and 0.078V across one of the negative output resistor. This means:

0.080 / (0.56 / 8) = 1.14A
0.078 / (0.56 / 4) = 0.557A

So I have about (1 - (0.557 / 1.14)) * 100 = 52% The CS puts out 1.14A and the negative side puts out 0.557A so the CS is responsible for about 52% of the output. So the CS can output a bit more than double the output of current. The higher gain the more the CS can multiply the current. -Edwin Dorre

In order to get symmetrical clipping as it should be, the absolute dc (output to gnd) should be allowed to settle down to its steady state value, and if not 0, it should be set to 0 with the diff. ccs pot before proceeding to adjust the ac gain. - grataku

When setting the amp up for more than 50% ac-current-gain the upper half (current source) will shutt off during negative current peaks causing the amp to get in some sort of AB mode, however, it's worth noting that if you bias the amplifier at the expected peak output current, you won't get this effect, and you can set the current gain arbitrarily. - Nelson Pass

Question: Your ran a test for R12 and R34. What values did you choose?
Answer: I used 2k potmeters to start off, I set the initialially at 1k. After applying a 1khz sinus wave of about 9V RMS into a 8 ohm load, I adjusted these potmeters to get an even gain of 50% of the active current source . After adjusting I measured the 2k potmeters and they were about 780 ohm ! So it is best to go this route as the value is very dependend of amount of mosfet's used and the value of the bias resistors. - Edwin

Question: Are R12 and R34 the resistors to set current gain? The lower the resistor the more current gain? If the negative clips before the positive do you need more or less current gain?
Answer: Indeed R12/34 are the resistors to set current gain.. A lower R12/34 increases the percentage of the current source. This is the explanation by Mr. Pass himself: http://www.passdiy.com/projects/zenv2-4.htm . Clipping should be symmetrical if the ac gain is set to be the same on both sides. If clipping is asymmetrical the current gain is probably off on one side.

To increase power output (into the same load), you'll need to increase the number of devices, the supply voltage, the bias current, the closed loop voltage gain and the trigger point of the protection circuit (if you use it).

More simply, Nelson suggests going up to 66% AC Current Gain for low impedance speakers. For example, you could set bias to 3.5A , 66% ac current gain and 26w pd with only 4 fets giving 53W into 4ohm loads.

If you want to change gain, you've got several options. You can:
--Change the feedback loop (R16 & R30)
--Change the ratio of the resistors in the inputs (R18/R19 & R20/R29)
--Change the loading of the front end differential (R23 & R25)

Note that all of these effect other things. The feedback ratio, for instance, will change the gain, but will also change the distortion characteristics. If you get really radical and throw in a cascode, you're going to have to do some serious thinking about how that's going to influence the DC offset on the load resistors...which in turn biases the output MOSFETs.
The design looks complicated, but isn't really. Just take a blank piece of paper and cover half of the schematic. Suddenly, you're back to something very much like the original Mini-A from whence it came. (I have a master plan, you see...everything dovetails together...) If you've got sufficient parts on hand, you can build a Mini-A and test any ideas on it before trying to fit them into the Aleph-X. Remember, I tried to make the Mini-A a 'cheap' project. There will be some things that don't translate exactly--in particular things that relate to output DC offset, but many things will and the Mini-A is a great test bed for possibilities.

The Beta AX lives...
Using Grey's original CCS.
From the beta schematic I changed the following:
R1/R4 and R44/R45 total 62-ohm instead of 31-ohm
C2 & C4 are 5pF instead of 10pF
R19 & R29 are 10K

The Results:
PS Voltage 14.3VDC w/ 220,000uF per rail
Avg DC offset is running 1.4mV
Avg Abs DC offset is 47mV / 55mV
R23 4.24V
R25 4.31V
R5 .499V
R40 .501V
R6 .504V

I will try to be as specific as I can be, and I will be referring to Grey schematic for nomenclature:
First of all I setup a proper rig to test match the mosfet, and matched both the output and differential mosfets. Contrary to Peter findings I got a pair of 9610 with Vgs within 2mV over a 1 minute monitoring period and that was enough to get a corresponding voltage drop on r23 and r25 when I soldered them in place back to back with a silpad in the middle. The Output mosfet could not be matched to better than 10 mV on my sample of 20 I could only find 4 between 3.85-3.86 V Vgs. This translates in about 40mV DC offset. That IMHO is the result of different transconductance. Maybe this large effect would average out if several mosfets would be paralleled resulting in a much lower relative DC offset.

TO FIRST ORDER things that DON'T MATTER AT ALL:

-Value of out to gnd resistance.
I am assuming that some type of value is needed to control the DC offset between + and - out this to work HOWEVER, short of eliminating that completely, playing with values between 20 and 100 ohms made no difference at all in the absolute or relative DC offset. THEREFORE, this resistor might as well be 100 ohm to save some power. It is POSSIBLE that while music is playing the value of the DC will be kept to a more constant value with a lower resistance but as I said this is all to first order.

Things that matter ALOT:

VR1 and VR3 have a large impact, since they set the bias current through the output transistors. You can measure this across R6 / R41. I was a little surprised to find that very large variances in this current had little to no effect on the DC bias at the outputs. In any case, I'm running 2.5A bias per side for a total of 5A bias per channel, and I haven't had any difficulty with the standard component values. You may want to increase the value of R10 and R39 if you are running high bias currents.

-Values of R24 to R26 and V2.
R24, 26 and V2 decide how much current is going through the output differential.
R23 and R25 control the threshold at which Q2 and Q11 will start conducting.

-Values of R15 and R32. The voltage drop across these resistor control the conduction of the Q1 and Q10 mosfets. Not as effective as r24, 26 and V2 but somewhat effective to control absolute DC value at the outputs. The reality all these resistances need to be adjusted simultaneously I have a 2k trimpot in right now which is turned all the way up to 2k.

I used a strange CS a hybrid of HH and the standard that used an LM329 instead on the 9.1 zener and a 5k to –15V for r17. The current R24 and 26 are 205 ohms and V2 is 200 ohm. I initially used 301 ohm for r23 and 25 but reverted back to 390 ohms. It should make no difference at all. At one point I was listening to the sound of the input differential biased to 15mA just for kicks. On my test crappy speaker it sounded ok.
I went back to the 100K and 10pf loop (r16 c2) from the 200k and 5 (more XFB?).

Here are some voltages:
PS voltage +/- 13.5 VDC on my test setup. As soon as the VM guy replies to my email I will order the real transformer.
+/- DC ~50-70 mV when playing music with balanced in and 6.6 uF input caps on both hot and cold.
The absolute DC hangs around 0 and +/-35 mV.
Source R drop top 490 mV, bottom 460mV
Vgs top 4.2 V, bottom 4.07V
Differential load drop (r23,25) 4.510 and 4.512V.

At this point it would be nice to figure out how to mess with R9,10, and 13 to increase the bias on Q2 and Q11 thereby evening out the source resistor drops. Since adjusting the CS resistor affects both the lower and upper output mosfets AND r15 and r32 take care of the top bias only a third way of adjusting the bias of only the bottom mosfets could prove somewhat useful. Maybe at that point the resistor to ground would just be there for show.

At this point the only comment about the sound is that it was really audibly distorting when the absolute DC offset was 8V and now it sounds a heck of a lot better. Very promising actually.

Yes the rectifier need it's own heatsink. So far I don't hear any turn on-off noises or hums. I have a high efficiency test speaker that I want to try before I really say.

Problem: Amp seems to be oscillating - getting a motorboat sound on the speakers

Solution: 1) I used 3n3 for C9 and C10 (4n7 also worked) to get rid of oscillations. 10pF for C2 and C4 (5 would probably be enough). C7/8 are still not used and the amp works fine - William. or 2) Mmotorboating occurs when bias pots are turned all the way down. I suggest playing with V1, V2 and see what happens - Hugo.

R1/R4 getting hot means that too much absolute DC is present at the outputs. (That is from output to ground and should preferably be less than 1V). Assuming your amp is technically OK, you should be able to adjust that DC with VR2. - NetList 08-20-2004

Make sure your current source and diff pair works. Check the voltage across R23/R25. Something between 4V and 5V is OK - NetList 08-20-2004

Voltage over R2, R3, R42,R43 drops only when current is flowing throu the speaker and that means this is only AC, so in normal quiet state there is no DC drop at those resistors. Tell me the voltages over: R5, R6, R40 , R41; they schoul be equal arround 400..600 mV depending on biasing. But they should be as close together as possible, assuming the Q1, Q2, Q10, and Q11 were matched properly and VR1 & VR3 were setup properly. - piotrzurawski on 08-21-2004

If absolute dc offset gets too high (above Nelson's 66% ?) my A-X starts to hum - dieringe on 09-22-2004

Problem: Heat sink stays cold after 5, 10 minutes; R23 and R25 measure only about 3.3V : Solution: VR2 somehow end up at the highest 200 ohms. After I found this out and turn it all the way down, I got my Q7 up to 4+V and the heat sink starts working now....

Question: What is the generall opinion on the compensation caps/ are there already values known to be used with the boards? I could start without them and ad when neccessary. How can I test this, what input signal and what load should I use? - William. Answer: I usually start without them, but you'll need a scope to watch the circuit. As a rule, an ordinary resistive load is adequate to provoke oscillation. If you are looking more extensively, try hanging a .047 to 1 uF across the output terminals. If you really want to see how bulletproof it is, drive an unterminated length of Litz or Mogami or similar low inductance/high capacitance cables. - NelsonPass on 12-14-2004

Problem: (This is from a discussion of the A-30 circuit) I fussed a little and it played for a couple seconds after a power cycle then went quiet. Then I noticed that it smoked a little RC circuit I placed on the GND connector of the PS boards

Solution: (Nelson's reply) When the RC to ground at the output is cooked, it always means high power ultrasonics, almost always system oscillation.
I say system because an amplifier's internal oscillation is almost always at low power levels, but if you bleed the output of an amp back to the input, you often see full power up around 100 KHz or so. Most often this is due to bad input grounding on the cables. It could also be the MOX putting out power ultrasonics.

Problem: Powering up I can adjust the bias just fine and have 0.44v over the 0.22ohm source resistors, but then the problem I'm having is that I cannot fix the absolute DC offset with the 3rd pot, both sides measure ~ -15.5V for the entire travel of the pot. I've replaced the differential input and current source fets thinking it is probably them, which didn't help. All parts were hand matched very well

Solution 1: -15.5 volt means the output fets are completely open. This means that the current through the current source is too high.
You can measure it by looking at the voltage over the drain resistors from the input fets. Try to raise the value of the resistor parallel to the 3th pot and turn the pot to max resistance. This should do the job.

Solution 2: I got the same, and I found the Zener Diode of the input current source was installed in wrong direction.

Question: Has anybody tried to increase the input impedance for aleph x? I'd like to change it from 20kohm balanced to 40-47kohm balanced. I have built the 100W amps with 220kohm resitors instead 100kohm to have better input sensitivity. Lets say it is kristijan schematic, only 100kohm changed to 220kohm. In what direction should I go when replacing 10kohm input resistors with 20-23kohm?

Solution: Input impedance is roughly sum of two series resistors (+ and - side); you can up them to, say, 27 K -but you must scale up 100K resistors for same factor (2,7x) . Also you must scale up (2,7x ) 10K resistors from inputs to gnd. After that you need to scale down (2,7x) 10pF caps across (now) 270K resistors. I found using a high input impedence caused a phase variation between the channels of my aleph-mini amp. It was only a few degrees at 20kHZ, but could be seen very clearly at 100kHZ. Additionally, increasing the input impedance will decrease the bandwidth due to the Gate capacitance of the MOSFETs, though Nelson indicates, "Not by much, as it turns out."

So this takes us to an approximate (the cost of smaller items has been rounded up) total cost of $525 (USD). Using this list, it will be necessary to order parts from about 3-4 vendors, so figure another $30-50 for overall shipping, bringing the grand total somewhere near $575. Building the lower powered version (in a stereo chassis) is substantially cheaper for several reasons: 1) you can get by with a single transformer for both channels (I may even run both channels off of the same cap bank...), 2) depending upon your output power, the same heatsinks for a single high powered amp will be sufficient for two channels at lower power, 3) fewer output fets, less matching required, 4) source resistors are readily available in 0.22ohm ratings and fewer of them are needed. Overall, I suspect that I can build a lower powered stereo chassis for somewhere near $400 to $450.

Yes, many of these were learned to hard way, so the reason for this section is to try to prevent you from going through the same frustrations that I did! In no particular order, here are a few things I've learned by working on this project off and on for the past few years.

1) Measure each and every part prior to including it in your circuit. Mistakes happen at many different levels. In one case, I was sent resistors that measured 220k ohms when I ordered 220 ohm resistors. Also, in many cases, it is useful to match parts across a stereo amp or match components in the output stage with one another. Another benefit of doing this is that it will decrease the likelihood of making a dumb mistake like soldering the wrong part value into your circuit.

2) If building more than one channel at a time, install all of the same parts at the same time. Fir instance, if you are building 3 monoblocks (like me), install the R1 resistors on each of the three boards before you move on to installing R2 on each of the boards. This helps to reduce mistakes becuase you have just measured each resistor again before installing it (to verify that you are installing the correct part), and you can visually compare each of the three circuit boards after installing each part to make sure that you have installed the part into the correct position.

3) Once the amp(s) are working, make changes to only one channel at a time when working on a multi-channel amplifier! Often times, a change that you intend for the better results in the introduction of a problem. This is especially true when moving wires inside your chassis in order to reduce or eliminate hum. Other changes, if improperly applied, have the potential to let the smoke escape from vital components of your amp - its far better to toast one channel rather all of them at once! As a parallel to this rule, power up only one channel at a time after making a change...

4) Be especially careful when soldering to avoid cold solder joints. A cold solder joint occurs when the materials that you are soldering together have not yet reached the melting point of the solder, thus you have a hot blob of solder on a cold piece of metal. The result is a solder joint that is neither electrically nor mechanically solid. A cold solder joint can cause a great deal of trouble because when visually inspected, it looks good. When the component itself is measured with a meter, it measures good. But, the connection with the rest of the circuit is intermittent at best. I've been soldering for years and have never had a problem with this before, but it caused some frustration in fixing a problem with this amp. A cold solder joint on one of my resistors resulted in running the full rail voltage to the speaker outputs which would have immediately fried any speaker! Ironically, I suspect the cold solder joint resulted from the use of a soldering heatsink that I had clipped to the leg of the resistor to prevent the resistor itself from overheating while I was soldering it. This problem is easily solved by carefully and methodically re-melting each of your solder joints with the tip of your soldering iron and then letting them cool again.

Electrical Grounding 1, 2

Building Power Supplies 1, 2, and 3

Power Supply Decoupling

Heat sink selection & transistor mounting

Links to other power amps

ePanorama.net

Surplus Parts Vendors:

MPJA, C&H Sales, All Electronics, MECI, Surplus Sales of Nebraska, Skycraft, MCM Electronics, B.G. Micro, Fair Radio Sales, Excess Solutions, Electronic Goldmine, Alltronics, Jameco, HSC Electronic Supply, Brigar Electronics, Apex Jr., Cascade Surplus, Excess Solutions

General Retail Vendors:

Digikey, Mouser, Newark, Allied,

Boutique Parts Vendors (exotic resistors, capacitors, etc.):

Michael Percy Audio, Borbely Audio, Angela Instruments, Audio Note, The Parts Connection, Welborne Labs, Williams Hart Electronics, Sonic Craft,

Blimey Hot	10 seconds hands on	45c
Crikey Hot	5 seconds hands on	50c
Bloody Hot	2 seconds hands on	55c
X*?@!		60c

Device	Max Voltage	Max Current	Max Temp
IRFP044	60v	57A	175c
IRFP240	200v	20A	150c
IRFP244	250v	15A	150c