Official M2 schematic

Hi all,
After the successful diy assembly of another PASS amplifier (the Aleph 30) and after enjoying it a LOT, I'm very motivated and planning to build the M2.
As the M2 has less components I'm thinking on using a 160x100 mm perforated pcb and making point to point soldering with copper wire (it's not the first time I do this, so I feel confident. In fact my TDA1541A nos DAC was made this way).
In any case I would like to share my first sketch of the pcb layout (it's hand-made, sorry for that) just to ensure it's correctness and maybe get some advice.
The overall dimensions of components should be substantially correct. The lines red highlighted with crayon shall be interpreted as galvanic bridges and the view is from above. Two upper corners are to be cut to place the mosfets.
I have a question regarding C2 (10uF). I have at home some MKP big caps and two of them can also be seen in the photo, so I would like to use one of them (the blue or the grey type). The blue is a big 10uF MKP high voltage X2 type and the grey is a Vishay 20uF (700V model 1848). I guess that the MKP shall be the best option in this position (signal path) and I also guess that 20 uF can work as well (or even marginally better regarding the low frequency cut-off) as the 10uF. I'm right?
(I would prefer the 20uF as it's the smaller of the two). View attachment 1241514 .
Many thanks
wow, exactly what I was waiting for.
Thanks for sharing
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well, the initial layout changed a bit while building. The final sketch I used to guide myself is attached for convenience.
I added some on-board filtering and later a led (and led series resistor) not indicated in the sketch.
If this can be useful to anybody I'm happy for that, but I haven't tested the board, so cannot say it's 100% right.
If everything is fine red shall be bridges under the board and black shall be the bridges in the upper side. I used 0.8mm copper nude wire (to be tinned before soldering to components), and some thin standard bridges.


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M2 Left channel gets between 2 listening session (3 days difference) low?...left channel is very low value..

after check:
i found out that my auto former PC600/15k is not really working any more. i try to re-solder the thin copper cables (before i put away the isolation) but i had no success. This has 4 windings 2x primary + 2x secondary which are connected like in the original schematic.

as i read here the XS4400 is a very good updated but i cant ordered directly at edcor cost about 42,56USD.
at the European seller (Don-audio) 92 euro excl ship !!!!...holly..**it

so i want to try a cheaper version and found the Hammond 109N (10k with CT :600 with CT). so i have to swap primary and secondary windings. i can leave with factor 1:4 instead of factor 1:5. (600/15k)

measurements with scope and RL at the 10k load resistor( lower resistors are not working because of bad frequency response):
2Vrms input i got about 18,8Vpp so i got 3,53 factor and about 10.96dB..nearly from 30Hz to 30kHz without its fine for me.

so i think about similar to M2X to build daughter pcb:

i build a kind of daughter board with a small pcb but it doesnt work if i implement this into the place of the original edcor.
How should i implement this into the M2 amp pcb? could i use the CT? it is "just" a CT and not like the edcor PC600/15k 2x prim and 2x sec windings.

i have no clue ...i tried different things but i always got 1:1 ...same signal input as output?



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as you can see it is working fine.
test with 100mVrms input and check the input Voltage and output voltage with scope and RL 10K.
at 1Vrms , 2Vrms and 3Vrms i just check the lower and upper frequencies to check the fg response.


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i did the update..
it seems to work_ but over 200mV input i got clipping....hhhmmm...why?

here is the result:


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+/- 22,5V rail, 1,8A bias
yellow input
blue output inverted

2,8Vrms into 4R gives about 25,5Watt.
because different gm of my Mosfets i got unsymmetrical clipping at max..
PCH is weaker then the N CH...but ...

fg is now 36kHz
old xformer had 55kHz


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Last edited:
Official Court Jester
Joined 2003
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if xformer is connected as autoformer, then there is no phase inversion ( in general, there is some phase change at extremes, but way far from inversion)

same as no phase inversion in input buffer (preceding autoformer) and no phase inversion in output buffer/OS (following autoformer)

it is irrelevant , in this context, that you sort of backward connected xformer, regarding nominal primary and secondary
Joined 2008
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I posted a question on a different thread, but the OP thought it was better asked here instead. So here it is, rephrased...

I'm planning to try EUVL's amp that uses the M2 output stage. ( I have a PSU I built for a different M2 build a couple of years ago that I could use, but I'm wondering about doing something different.

One of my weaknesses is e**y; in particular I just can't resist nice transformers when I see them at a low price. I'm like a magpie - see something pretty, gotta have it... I recently picked up a pair of Avel Lindberg 160VA transformers (old stock made in the UK) with 25V-0-25V secondaries. At less than $9 each and reasonable shipping I couldn't help myself.

So they're a little small but much higher voltage than the usual transformers you'd choose if you were building Pass-style monoblocks. Ignoring the input stage and potential cascoding or whatever, suppose I were to build M2 monoblocks with these transformers. After the rectifiers I'd have +35V - 0 - 35V before smoothing, and the usual rule of thumb I've seen on this site for class A amps is to use a transformer with 2.5 times the power demand of the amp, so I should have about 70W available. So it seems to me that the highest bias I could use would be around 1A. Say 800mA to leave a little headroom. If I understand things correctly then the amp will leave class A when the output current is twice the bias, so 1.6A, but that's an AC number, so RMS power is 1.6^2 x 8 / 2 = 10.2 W into a 8R load. (Does that sound right?)

10W should be enough for my usual listening needs, but hypothetically suppose I did use more. Obviously the rails would be much higher than needed for class A operation (around 32V, say, after filtering and maybe a cap multiplier or whatever). But what would happen in class AB? It seems to me that the transformer can't provide enough power for the amp to clip, since the amp could swing 64V p-p, which would mean an AC current of 8A, and RMS power of 248W, which is way too high. There's going to be heat loss from the continuous bias current; 32VDC rails at 800mA means 64 x 0.8 = 51.2 W to be dissipated when no music playing, and I guess close to that when there is music playing. So there's about 110W available from the transformer; is this the limit for output power in
class AB? Seems awfully high, so I suspect I've misunderstood something. As usual.

Anyway I will find something to do with the transfomers eventually, I'm sure. But playing with the M2 in class AB sounds like fun.
Joined 2011
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Analyze the M2 circuit schematic (found in post #1 of this thread) again. You'll discover that "the power demand of the M2 amp" is far greater than 28 watts per channel. Since M2 operates in class A, the power it draws from the power supply is a LOT more than the power it delivers to the loudspeaker load.

You can do a "sanity check" on your calculations by visiting the First Watt website and downloading the owner's manual for the M2. In the specifications table you'll find a line-item that tells you the power an M2 draws from the AC mains. Hint: it's a lot more than 28 watts per channel.

*you calculated 28 watts * 2.5X safety factor = 70 watts for the power transformer. But I suggest that 28 watts is far too low.

If this helps: I used a 400 volt-ampere toroidal transformer to build my (stereo) M2x. That transformer had 2 x 18VAC secondaries at 11 amperes each. I actually planned to use a 300 VA transformer, 2 x 18VAC at 8.3 amperes each . . . . . but those were out of stock on the day I bought. So I paid a little more money and got the 400VA because it was available immediately.
Last edited:
Joined 2008
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Hi Mark,

Yes, I know the M2 draws more than 28W per channel. A normally biased M2 dissipates something like 80 - 90W in the heatsinks, I believe. And draw from the AC mains is higher still, of course.

But that isn't my question. I'm wondering about how to estimate output power in class AB under the circumstances I described. (And whether my calculation of class A power is correct for the circumstances I described.)

Or did I misunderstand what you're saying?