This Week on the Breadboard: The Dark Esbat

The MID switch affects the gain mainly at the mid frequencies, however the left position produces boost over a very broad freq range.

I intentionally reduced the bandwidth of the 3rd stage because there is very little going on above 10KHz. Output transformers and speakers all roll-off in the 5-10KHz range. The feedback cap (C9 in the schematic above) can serve two purposes.
1. It reduces the bandwidth so we don't amplify frequencies we can't hear.
2. It helps maintain stability by ensuring that the opamp has sufficient phase margin.

In the Dark Esbat, R19 is low enough that C15 is not required for stability.
 
Chuck D. Bones said:
We can take up the headphones amp in a convo or separate thread. Althought it works well, I would not build another one to that schematic. I've found what I think is a better stereo amp chip. I'll post something once I actually breadboard it.
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Did you ever post the better amp? I searched but didn't see anything.
 
MattG said:
@Chuck D. Bones - is C7 actually necessary? I believe both sides of that cap are centered at +Vb, and there's no DC path to 0v/GND, right?
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If opamps were perfect, we might not need C7. Opamps have offset voltage, which is a DC error at the input. It's usually a few mV, but it gets multiplied by the DC gain of the stage. With C7 present, the DC gain of 2nd stage (U1B) is 1. If we jumper C7, then the DC gain depends on the DRIVE setting. If DRIVE is dimed, then the DC gain is 22.4. A 3mV offset at the input results in a 64mV offset at the output. Not a huge amount of error, but it will unbalance the clipping diodes a little bit. In this circuit, we probably won't hear the difference. The other thing C7 can do is limit the bass response. In this circuit, C7 cuts off the bass below 7Hz. In other words, all of the bass gets thru in the Esbat.
 
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