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Building a sound mixer within my guitar amp with line IN & mic input

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Yansi:

--- Quote from: Yansi on June 06, 2019, 11:47:07 am ---The minimum gain is basically set by the gain of the differential amplifier (opamp) part of the circuit.

--- End quote ---

I just forgot to add this:

The resistor between the emitters of the PNPs define the differential gain of that stage. The single-ended gain of the transistor is still mostly the same (very low) due to the Rc/Re ratio being quite low.

If you make the resistor between emitters of the transistors large (ie, none is present, gain pot set to maximum resistance or disconnected), then what you get is just a two separate common-emitter amplifier stages with just low gain (defined by approx. Rc/Re), hence why the minimum gain is mostly defined by the opamp following the transistor amp stage.

By decreasing the gain pot resistance, you increase the differential gain of that stage. The single-ended gain of the circuit is still low (Rc/Re), hence why the stage has a good CMRR at very high gains.

dazz:
Oops, that was a typo. I meant 6k8. I'll rise that to 8k2 or 10K. I get minimums of 10dB and 12dB respectively with those.
I see what you mean re: the gain pot. By lowering it's value it seems to perform more evenly across it's range. I don't have any C5k pots though

Thanks for the explanation of the common emitter gain stage, very helpful! So we get a high CMRR because the gain is low, and then we can "trick" the circuit to increase the gain by means of the resistor across the emitters. So you mentioned that's called emitter degeneration... off to google!

One thing I'd like to ask you if I may about those bandwidth considerations you mentioned, do those apply to the mixer or the mic preamp? IOW, Do I really need that wide of a bandwidth in my mic preamp? I see most dynamic mics have a bandwidth of 50Hz-15kHz or whereabouts.

Yansi:
Yes, emitter degeneration. But also look for differential amplifiers* with two discrete transistors, that is where it gets interesting.

*discrete ones are also often called a  "long tailed pair".

You may also find this interesting & practical explanation from w2aew:


He does not explain the emitter degeneration though (zero ohms in between emitters).
A lot of it can be found on the web and again in interesting books from Douglas Self (Audio Power Amplifier Design) etc.
 

Audioguru:
A Shure SM-58 vocals mic has its frequency response rise above 2kHz for "presence" then a sharp dropoff above 12kHz. Most people who have been deafened by sounds too loud and who are used to hearing a muffled AM radio think it sounds live. You do not want to cut high frequencies any more. If your first-order lowpass filter has a -3dB (half the power) cutoff at 20kHz then it is flat up to only 4kHz.

When you increase the voltage gain of a transistor then you also increase its distortion. Negative feedback can be as simple an unbypassed emitter resistor reduces the gain and also reduces the distortion.

dazz:
Crystal clear. I think this should do. Added the EMI protection components too, so I think I'm finally ready for the build

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