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Old 10th February 2011, 01:06 PM   #11
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Thanks, I'd limited my search at mouser to 3/8" square. Every extra turn helps.
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Old 12th February 2011, 01:13 AM   #12
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Could you tell me what your main DC power supply AC ripple is running? Mine is about 95mV rms on all capacitors. The bias and capacitance is right out of the article. Seems over the years the hum has increased and I thought one of you could give me a reading for comparison.

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Old 12th February 2011, 01:43 AM   #13
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I don't remember but that seems plenty low enough on the outputs.

Check your grounding connections for corrosion.
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Old 12th February 2011, 04:20 AM   #14
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There is about 3mV rms on the outputs with the inputs shorted and its audible when you get your ear up to the woofer. I will probe around the grounds and see if I can find some improvement.

By the way. I have never had much problem dialing in the offset and bias and temperature changes with respect to output offset track well, which is one of the benefits of dual differentials.
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Old 12th February 2011, 04:25 AM   #15
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It would be nice to see some better pictures of the original A75 from the article. The pictures in the magazine weren't the best quality and I never did figure out where the front end board was located. Maybe Mr Pass is watching?
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Old 12th February 2011, 12:05 PM   #16
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Is the hum 60 Hz or 120? If you don't have a scope, to me 60 Hz sounds like a nice hum, 120 a bit buzzy. Also, 60 Hz is what you get when you touch the open input.

Did you try to duplicate the chassis in the article? I suspect that inside there is a shielding shelf above the transformer and PSU on which the front end board resides centered between the heat sinks. All in all, get as much space as you can between the mains circuitry and the front end board. Do you have decoupling caps added at the power connections to the front end board?
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Old 12th February 2011, 09:55 PM   #17
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I tried moving the grounds to other places on the buss ground bar with no success. Bypassing the front end power supply made no difference. Its the same whether in balanced or unbalanced mode.
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Old 12th February 2011, 10:02 PM   #18
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Picture of my A75

Pictures of your diy Pass amplifier
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Old 19th February 2011, 01:22 AM   #19
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Ok...I'm almost there but am still struggling with the DC Offset. With .9V across both R7 and R8 i get -1v offset and 3.5V to ground from +DRV and -DRV. To get the offset down to less than 50mv I have to take the bias down across R8 to almost 0v. Nelson stated it wasn't necessary to match the TO220's but only make sure the VGS is between 4-4.5V. The best I could do was 4.5V on the N's and 3.5V on the P's. the inputs are matched to with 30mv. Is it possible this VGS difference between the P/N is why I'm struggling with the offset? And, if so how would I go about compensating for this difference? I'v spent half the day trying to figure this out. Any suggestions would be really appreciated. I would also mention that when adjusting P1/2, adjusting one pot also changes the V across the R7/8 on the other side? Is that normal?
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Old 19th February 2011, 01:58 AM   #20
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Yes, the two are related. Look at the schematic, you'll see that the current in R7 ends up in R8. Yep, it's touchy as all get out because what you are doing is balancing a current source and a current sink.

Ignore the voltage across the DRV terminals for now. Just set it at minimum.

You might want to turn the back down the pots and try bringing the the R7/R8 voltages up together keeping the offset nulled. It can be a pain, but the only way I could get it reasonably close was to try from zero a couple times.

Once you get your offset nulled at full R7/R8 voltage, then reconnect the output stage and bring up the bias slowly.
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