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Building a sound mixer within my guitar amp with line IN & mic input
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dazz:
I'm building a sound mixer in my DIY guitar amp, so that I can play the guitar and sing over a backing track. So far I have successfully implemented the mix of the guitar preamp output and the line in, which I use to play backing tracks from my phone. I went with a simple summing amplifier using an opamp as shown in the first pic.

So now I want to add a balanced microphone input, and I'm simulating a couple of designs in LTSpice to get an idea of what might work and how.

Initially I thought I would implement the circuit shown in attachment #2. As you can see I have an inverting opamp to shift the phase of one of the balanced mic inputs, then I sum that to the other mic input. I believe that's how balanced signals are supposed to be handled to cancel out common mode noise, right?

So here's my first question. If you noticed, I used a non inverting opamp configuration to add both mic inputs, by mistake, because according to the articles I googled, it should be an inverting one (like the one I used for guitar + line in). The simulation seems to work fine, and seems to me I would also get the added bonus of high input impedance / low output impedance out of this non-inverting opamp config. Is there a reason to pick one over the other?

Then I thought this design doesn't make too much sense. Instead of adding the guitar with the two line in channels, and then that to the mic, it would be more sensible to add both line in channels separately, both mic inputs separately, and finally sum the resulting line in, mic & guitar signals in a final summing stage. Problem with that is that the simulation doesn't work properly. I think I have issues with impedance because increasing the guitar level doesn't result in an increase in total opuput level. But I think I'd better leave that for later for now, to keep things simple and one question at a time.
Audioguru:
1) A lousy old LM358 (and its sister the LM324) are never used for audio because they produce crossover distortion because they are low power. But for a guitar maybe you want the awful buzzing.
2) Your mic balancing circuit cancels the mic signal. Look in Google for Balanced Microphone Preamp Circuit to see how a single audio opamp (not LM358) is used.
I recommend an OPA134 single, OPA2134 dual or OPA4134 quad audio opamps.

dazz:

--- Quote from: Audioguru on June 01, 2019, 09:48:35 pm ---1) A lousy old LM358 (and its sister the LM324) are never used for audio because they produce crossover distortion because they are low power. But for a guitar maybe you want the awful buzzing.
2) Your mic balancing circuit cancels the mic signal. Look in Google for Balanced Microphone Preamp Circuit to see how a single audio opamp (not LM358) is used.
I recommend an OPA134 single, OPA2134 dual or OPA4134 quad audio opamps.

--- End quote ---

1) Thanks for that. Honestly, I haven't noticed any distortion from the LM358 I'm using now, but I admit I don't have a very good ear. I'll definitely get a bunch those other opamp you recommend.
2) I found one using a differential opamp. I was using an inverter for one input and then a summing opamp, which seems really stupid when I could do it in a single stage with that differential configuration. Doh! That should work since the balanced inputs are out of phase, if I got right
Zero999:
Here's a video demonstrating the LM358's crossover distortion. It can be fixed with a pull-up/down resistor.
Audioguru:
The crossover distortion of an LM324 and LM358 is reduced when its gain is low because then it has a high amount of negative feedback that cancels distortion. Your line level inputs have a gain of 1.
But your mic preamp has a gain of about 200 so its crossover distortion will be bad. Audio opamps do not need to be "fixed" with an added resistor.
Since the LM324 and LM358 are not designed for audio then they produce a lot of hissss noise all the time.
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