AMB Laboratories DIY Audio

[amb]

If your build is anything like mine, you can look at the photos to get some ideas. A lot of the transformer wiring are hidden under the bottom sub-plate on which the transformers are mounted.

[oneplustwo]

Since my sigma22 trafo has dual outputs (same as yours from sumR i think) can I split them and take two wires to one sigma and the other two to the other? (Red blue to one and yellow gray to the other)? Where would the ground come from in that case? I thought I saw only three wires going to your sigma so not sure how you did it.

[Pinnocchio]

This is the original AMB schematic using the SumR transfo



Hope this helps
Do

[oneplustwo]

Ah, got it. Star ground it is. And it looks like I can indeed split the secondaries. Cool, thanks Do!

[oneplustwo]

One channel setup! Woot!



Sigma secondary wiring isn't what I expected though. I was only getting 11Vdc at the output until I connected all four wires. Did I use the wrong pairs?

[Pinnocchio]

You should have

Primary (115V)
Black* / Brown* together
White / Orange together

Secondaries
Red* = 35V
Yellow/Blue tied together. This will form the 0V (center tap that will connect to star ground)
Gray = 35V

This is with the SumR transfo that I have. Fuse it and check your voltages before connecting to the o22

Ciao!
Do

[oneplustwo]

Ok. Guess i'll have to split the red and gray wires to go to each sigma them, ya? Yellow and blue can go to star ground and then to each sigma.

Ok. Second channel already stuck. R7 I get 2.9V. R9 is 4, r8 is 2.2 and r10 is 3.2. And the pot is maxed out.

If it helps, I tried to match r11/r13 and could only get them even at 90mv.
Thoughts?

[Pinnocchio]

I had this issue as I remember... Play with one of the other pots slowly and it should start getting some traction... I would personally unplug the power then measure each trimpot of the working amp and adjust the pots so they read just about the same resistance on the second amp. Fire up and do the final adjustments.

BTW, leave the amp running for at least 15 min since with the heat, the settings will drift a little and you'll need to adjust again.

Do

[oneplustwo]

I had this issue as I remember... Play with one of the other pots slowly and it should start getting some traction... I would personally unplug the power then measure each trimpot of the working amp and adjust the pots so they read just about the same resistance on the second amp. Fire up and do the final adjustments.

BTW, leave the amp running for at least 15 min since with the heat, the settings will drift a little and you'll need to adjust again.

Do

Where are you measuring trim pot resistance?

[oneplustwo]

Update: I get through all steps ok now up to offset. With 40mv on both sides, I get like 400mv offset. When I dial that down, the quiescent current goes down with it and I can't raise it up again. I tried the process twice with the same result.