Electronics > Beginners
Building a sound mixer within my guitar amp with line IN & mic input
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dazz:

--- Quote from: dazz on June 07, 2019, 04:20:37 pm ---but don't know how referencing R14 to Vref instead of ground affects the math, if it does.

--- End quote ---

Could the answer be a Thevenin equivalent, where both resistors (22K) in the divider would be in parallel (11K equivalent), and that would in turn be in series with R14 (6K8)? That would increase the value of R14 to 17K8 and ruin the CMRR.
In that case, the addition of the opamp as a buffer for the divider would fix the problem, right? because the output impedance would be very high, and R14 would no longer be in series with a 11K equivalent resistance from the Vref circuit. I believe that's what Yansi meant when he said it should be fine as long as the output impedance of the Vref circuit was high it should be fine. Does that sound about right?
dazz:
Wait a minute, I can totally simulate common mode noise and test all this in LTSpice with another voltage source... I think
Zero999:
Here's a quick simulation of what I was talking about, with the constant current source. In the end I decided to use one current source, rather than two and the emitter degradation resistors AC bypassed with the variable resistor.

This isn't the way I'd choose to do it. Unless there was an issue with clipping, I'd make the gain of the input stage high and add a potentiometer after the op-amp to attenuate it, if needs be.

dazz:
Thanks Hero!, looks great. I think I see why only one current source is needed. That's not a lot more complex than the original design. Brilliant

In other news, I finished the prototype today (on perf-board). Haven't tested it yet, but all the voltages seem right. Compared to the simulation they're all bang on.
dazz:
Welp, what do you know, it works! Still not sure how well though, but it's definitely amplifying and doing so differentially as it's supposed to.
So far I've only tested it with a function generator app from my tablet. When I feed it the same signal into both inputs, the sound goes away as it should.
Well, it almost goes away. I tried the following rudimentary test to approximate the CMRR: 20V dc supply, 1KHz input in one channel, maximum gain, I measured 17.7V RMS at the output of the power amp. Then I switched on the other channel so that the function generator inputs the same signal in both channels (common mode). Measured only 0.3V RMS. So if I'm not mistaken that would be 20(log(17.7/0.3) = 35dB of CMRR, which seems a piss por result. Considering how precarious the test is, I should probably ignore the 35dB figure, or take it with a pinch of salt.

But then I redid the same test, this time referencing the opamp to GND through C13 instead of referencing the opamp to vref... same exact result, which leads me to believe that it probably makes no difference and I can safely leave the opamp referenced to Vref.

Also tried both the TL082 and LM358 (NC5532's are on their way) and it didn't seem to make much of a difference in terms of noise
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