Electronics > Beginners
AC Coupling and OP Amp Stages
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Benta:

--- Quote from: eev_carl on November 20, 2018, 05:30:12 pm ---Is there a rule-of-thumb about when one is needed?

--- End quote ---

Not a rule-of-thumb, but a rule: whenever the DC levels change, you need a coupling cap. And this is exactly what happens at the points in your link.
Example: the opamp in the volume stage has an output cap. Why? Because the volume pot is ground referenced. If it was referenced to 4.5 V it wouldn't be necessary.
And so on.
floobydust:
It's not an issue over coupling capacitors. Your circuit is wrong, the single 500k resistor base-bias resistor to Vcc (not 1/2Vcc).

I dislike emitter followers in audio circuits as I find (at high levels) they don't have symmetrical input/output impedances. Your EF can source many mA but sink almost nothing with the 10k pulldown. The op-amp stage has a low 1k input impedance. The result is some rectification of the signal and a DC average that moves around, and it does sound not so great with the high distortion.
eev_carl:

--- Quote from: floobydust on November 20, 2018, 06:47:23 pm ---Your circuit is wrong
--- End quote ---

Thanks.  Here's the circuit with the correct connection to Vcc/2.

Benta:
Why do you insist on the emitter follower? Start from scratch again and stop leaning on the "Tube Screamer" schematic. Either copy it 100% or do your own thing. A mix won't work.

Read my reply #5 again.

eev_carl:
I'm at scratch.  I've made and tested each stage individually, but still am missing something when biasing two or more stages.  I plan on using a 4 op amp chip and was getting similar results when I made the unity gain amp from an op amp (rather than the NPN).  I can post an op amp-only LT Spice example later. 

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