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

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dazz:
And I believe this is how I should buffer the Vref source with another opamp

Zero999:
Just a few comments, as I don't have time at the moment to thoroughly read everything and perform simulations.

C13 should have been there to start off with. There's no reason why it shouldn't be there with a dual, rather than a single PSU and vice versa.

Why not use the LM358? Just saying it's crap for audio, without saying why isn't very helpful. The main three issues with the LM358 are: noise, crossover distortion and insufficient slew rate. These can be problems for an audio amplifier, but they're not always as bad as many people make out.

The LM358 has 40nV/√Hz of input voltage noise, so is no good for amplifying small, low level signals, such as those from a microphone. However isn't going to make much difference on a 400mV signal.

Crossover distortion has been covered before and can be cleared up by adding a pull-down resistor, as mentioned earlier. The LM358 may introduce other forms of distortion and I admit, I haven't done any tests or seen any experiments to quantify it. I suspect it will be more of a problem, at higher gains.

The LM358 only has a slew rate of 0.5V/μs or 500 000V/s. Try to get it to amplify a high frequency, high amplitude sinewave and it'll turn it into a triangle wave.

SR = 2xΠxfxVp

A 20kHz, 5Vp signal, will require an op-amp with a slew rate of at least 628 319V/s or 0.628319V/μs. In reality, you won't get an audio signal like that. Even a 10Vp signal which does have some content at 20kHz, won't have 5V of 20kHz in there, so it's a non-issue. Another thing is slew rate distortion doesn't sound nasty, like crossover distortion does, given the same figures of total harmonic distortion. Crossover distortion sounds very bad, while slew rate limiting distortion is very subtle, because it happens anyway when listening to sound, for a long distance, which attenuates the higher frequency content.
https://www.electronics-notes.com/articles/analogue_circuits/operational-amplifier-op-amp/slew-rate.php

Try putting stickers over the part numbers of op-amps, swapping them in the circuit. Look at the waveform on the oscilloscope and listen to it. Is there and audible difference?

Finally, as I said before, using constant current sources, rather than emitter resistors, in the differential pair will solve the problem varying power supply voltage, as well as boosting the gain and improving the common mode rejection ratio. I'll do a simulation if/when I get time.

magic:

--- Quote from: Zero999 on June 06, 2019, 11:02:30 pm ---Why not use the LM358?

--- End quote ---
Because 4558/4559/4560 are better if you really insist on cheap crap.

Zero999:

--- Quote from: magic on June 05, 2019, 10:31:45 pm ---
--- Quote from: Zero999 on June 04, 2019, 01:36:25 pm ---I'm currently designing an intercom system, using the NE5534 as a dynamic microphone pre-amplifier, simply because there's no point in using anything better.

--- End quote ---
Consider NJM2068, it's a dual, dirt cheap and very low noise - around 3nV/√Hz or so IIRC.

--- End quote ---
Yes, that was considered. It's only a little less noisy than the NE5534, which was chosen because it's more widely available and we already have a stock of them.


--- Quote from: magic on June 07, 2019, 10:19:10 am ---
--- Quote from: Zero999 on June 06, 2019, 11:02:30 pm ---Why not use the LM358?

--- End quote ---
Because 4558/4559/4560 are better if you really insist on cheap crap.

--- End quote ---
No, those op-amps won't work close enough to the negative rail, for this application. At least the LM358 will work. The problem is the volt drops across the collector resistors, at low supply voltages, is too low for most jellybean op-amps to work.

dazz:
The good thing is that the NC5532, TL082 and the LM358 are all compatible pin to pin. I'm socketing the opamp so I'll try them all.

As for C13, I'm confused. Didn't we agree I could omit it and reference the opamp to Vref through a buffered resistor divider?
I'm in the process of building the prototype and I will test both configurations but it would be nice to understand how to determine when CMRR is maximized. I think that's when the resistors ratios in the differential opamp are the same (R18 = R14 in my simulation, since both input resistors are missing and determined by the pnp outputs, which are the same), but don't know how referencing R14 to Vref instead of ground affects the math, if it does.

I'm also going to simulate the constant current source in place of the emitter resistors. Apparently that's a basic building block that I need to figure out too. It has enough advantages and is simple enough to justify it going in the final pcb version, I think.

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