circuit questions for Pass Thagard A75


2022-08-06 7:13 am
hi fellows, have been reading the original A75 publish and got a few questions wondering if i can find consultation here.

my electrical engineering study experience went no further than high school level.
DIY audio-wise, have successfully assembled and run AMB β22 singled-ended and balanced in these past 3 years.
started reading and assembling an A75 these 2 months.
can sense my understanding for audio amplifier components and circuits growing, but still won't say i am knowledgeable a all.

  1. output stage circuit
    so i see in the amplifier wiring figure depicted in Part 2, it has speaker+ output connecting line coming from 'right in front of' the output stage circuit, rather than having OUT going through the output stage and then connect to speaker.
    i understand that the output stage is common drain mode in parallel, and it takes signal input from DRV+ and DRV-, right? so what would OUT be comparing to DRV? because in the complete amplifier circuit in Part 1, the OUT goes in to and through the whole output stage, and comes out as input to speaker terminal.
  2. gerber drawing of power supply board
    is there an error with the section of where transformer secondary output goes into regulated circuit? R3 and R4 should definitely be between C4, C5 and AC, right?
    same thing with transformer secondary + and - marking in the wiring figure. (but probably most DIYer can make out these typo so no big deal?)
    after reading the article in these 2 months, i am 'guessing' there are minor errors with the gerber drawing but circuit diagram of amplifier and power supply are correct.

Would appreciate guidance on both A75 as well as suggestion on reading/studying direction to solve these questions.
1. OUT (speaker+) The common connection point of all those 1 ohm source resistors is the same electrically. There is no "before" or "after".

DRV+ / DRV- is the gate drive for the power MOSFETs (always pin 1 on those guys). This is a "push-pull" output stage so we've got N channel and P channel output devices. Current flow between Drain and Source is controlled by how much we "open the faucet". The faucet handle is a DC voltage we put on the gate. We need ~4 volts DC from G-S to get the current flowing. The "follower" output stage only supplies current gain, not voltage. We modulate these gates with what he is calling "DRV", which is our DC bias voltage, combined with the voltage amplified AC signal riding on it. Your favorite Barry White tune I'm assuming..

2. Those 10 ohm resistors are being used to dampen the current pulses, and slightly drop the voltage. The resistor and cap are in series so the order shouldn't matter. I'm don't believe that's an error. When you do the wiring you'll want 30V from the transformer secondary to (-) of C4, and 0V to (+) of C5 as shown on the schematic. Confusing at first glance but these are voltage doubler circuits that are being employed here.
Hi seann,
great to see you building an A75, its an excellent amp, mine running for 14 years now.

About Gerbers i don´t know, but schematics and wiring are correct, i designed my own PCBs from them. Understanding the circuit was sometimes hard as drawings are quite low resolution and some connection points in the schematics can hardly be recognised.
Schematic attached is of my front-end (sinlge-ended input, DC-coupled without input caps) with voltage-doublers and 50V-regulators, maybe it is helpful.
Connectors named LS... are test-points on PCB i added for measuring.

If i was to build an A75 again, i would additionally try to match all corresponding FETs for similar transconductance, and do matching at their estimated operating temperature as described by Michael Rothacher:

Also thanks to BDP and Alain Dupont for additional info. Good to know this, though i never had problems in these regards.


  • A75-V2 FrontendCompleteSch.pdf
    55.3 KB · Views: 28


2022-08-06 7:13 am
thanks for the schematics! will take a look.

indeed, as I am not a professional nor from EE background, what I did was corresponding the original schematics to the gerbers, and used principles of the electrical components and common sense of electronics to figure out if a cross point is actually non-intersecting or intersecting.
after this effort I realized the original schematics was drawn brilliantly, for the amplifier schematic, except for at R81, all the '+' crosses are non-electrically-intersecting.

I planed to build A75 by just lay out the components on universal boards according to original layout, as I am almost figuring all connections out (and at least a good portion of how the circuit works).


2022-08-06 7:13 am
I saw this not somewhere on the web earlier, and I corresponded the gerber with the schematic.
I think the note is partially correct, as it didn't point out which R24 in the gerber drawing was mis-labeled. And I don't think R26 was mis-labeled, R26 wasn't labeled on PCB.


2022-08-06 7:13 am
thanks for the info about push-pull, will look further into it.
before I posted my question I realized it must be 'Gate taking input signal, and Drain taking DC power input and signal was amplified', but didn't realize the connecting points of N bank and P bank are electrically the same.

regarding power supply, I watched some nicely made videos explaining voltage doubler circuit, and realized the earlier video learning I did were with bridge rectifier regulating AC to DC, and my previous experience assembling power supply had same DC output voltage to transformer secondary input AC voltage.
so there was voltage regulation but probably no voltage amplification. I guess that is where the differences are.

what still bugs me is that, why one 30V and one 0V secondary from the transformer would go together into Ground? Is there a keyword to search for breakdown of dual rail power supply with voltage doubler? or I can only try to analyze how the circuit would work by myself?
voltage manipulation circuits fundamentals are quite easy to find, but just adding one more side and dual rail power supply analysis seems like something limited to audio DIY field.
Here's some notes on the original article.
These notes are in reference to the original Audio Amateur published One/93


  • AA0ne93.jpg
    446.2 KB · Views: 16