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Building a sound mixer within my guitar amp with line IN & mic input
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Yansi:
Is it an input for guitar or microphone?  This circuit will do anything but suck at being microphone amplifier. For standard microphone inputs, you need low impedance (~2kohm) and low noise amplifier.  Neither TL0xx nor NE553x will cut the mustard here, with the buffer in front of it not by any chance. And going balanced inputs helps also by a lot. The easiest way to design a balanced MIC input is to just extend the differential opamp amplifier with a differential pair of suitable PNP transistors. Gain is then regulated by emitter degeneration in that differential pair.

If you are designing inputs specifically for guitar (instrument input), then for a good sound higher the impedance the better. 1M or more is good enough. Then a FET opamp with large input impedance is the way to go.

Do not forget good EMI and ESD protection on all inputs!

dazz:

--- Quote from: Yansi on June 02, 2019, 05:38:16 pm ---Is it an input for guitar or microphone?  This circuit will do anything but suck at being microphone amplifier. For standard microphone inputs, you need low impedance (~2kohm) and low noise amplifier.  Neither TL0xx nor NE553x will cut the mustard here, with the buffer in front of it not by any chance. And going balanced inputs helps also by a lot. The easiest way to design a balanced MIC input is to just extend the differential opamp amplifier with a differential pair of suitable PNP transistors. Gain is then regulated by emitter degeneration in that differential pair.

If you are designing inputs specifically for guitar (instrument input), then for a good sound higher the impedance the better. 1M or more is good enough. Then a FET opamp with large input impedance is the way to go.

Do not forget good EMI and ESD protection on all inputs!

--- End quote ---

It's a balanced mic preamp, the preamp for the guitar is already in there. I had no idea that low input impedance was required for this application.
Zero999:

--- Quote from: dazz on June 02, 2019, 07:20:00 pm ---
--- Quote from: Yansi on June 02, 2019, 05:38:16 pm ---Is it an input for guitar or microphone?  This circuit will do anything but suck at being microphone amplifier. For standard microphone inputs, you need low impedance (~2kohm) and low noise amplifier.  Neither TL0xx nor NE553x will cut the mustard here, with the buffer in front of it not by any chance. And going balanced inputs helps also by a lot. The easiest way to design a balanced MIC input is to just extend the differential opamp amplifier with a differential pair of suitable PNP transistors. Gain is then regulated by emitter degeneration in that differential pair.

If you are designing inputs specifically for guitar (instrument input), then for a good sound higher the impedance the better. 1M or more is good enough. Then a FET opamp with large input impedance is the way to go.

Do not forget good EMI and ESD protection on all inputs!

--- End quote ---

It's a balanced mic preamp, the preamp for the guitar is already in there. I had no idea that low input impedance was required for this application.

--- End quote ---
What's your budget?

How about an instrumentation amplifier? The INA163 and  AD8429 look good but are pricey.
http://www.ti.com/lit/ds/symlink/ina163.pdf
https://docs-emea.rs-online.com/webdocs/10fa/0900766b810fa3b4.pdf

As far as low noise op-amps are concerned, the best one I could find in terms of price vs low noise is the NJM2122, but I've not actually used it before.
https://www.njr.com/semicon/PDF/NJM2122_E.pdf
dazz:

--- Quote from: Zero999 on June 02, 2019, 08:01:03 pm ---
--- Quote from: dazz on June 02, 2019, 07:20:00 pm ---
--- Quote from: Yansi on June 02, 2019, 05:38:16 pm ---Is it an input for guitar or microphone?  This circuit will do anything but suck at being microphone amplifier. For standard microphone inputs, you need low impedance (~2kohm) and low noise amplifier.  Neither TL0xx nor NE553x will cut the mustard here, with the buffer in front of it not by any chance. And going balanced inputs helps also by a lot. The easiest way to design a balanced MIC input is to just extend the differential opamp amplifier with a differential pair of suitable PNP transistors. Gain is then regulated by emitter degeneration in that differential pair.

If you are designing inputs specifically for guitar (instrument input), then for a good sound higher the impedance the better. 1M or more is good enough. Then a FET opamp with large input impedance is the way to go.

Do not forget good EMI and ESD protection on all inputs!

--- End quote ---

It's a balanced mic preamp, the preamp for the guitar is already in there. I had no idea that low input impedance was required for this application.

--- End quote ---
What's your budget?

How about an instrumentation amplifier? The INA163 and  AD8429 look good but are pricey.
http://www.ti.com/lit/ds/symlink/ina163.pdf
https://docs-emea.rs-online.com/webdocs/10fa/0900766b810fa3b4.pdf

As far as low noise op-amps are concerned, the best one I could find in terms of price vs low noise is the NJM2122, but I've not actually used it before.
https://www.njr.com/semicon/PDF/NJM2122_E.pdf

--- End quote ---

Those INA163 go for 12€ on ebay, for a one-off like this that's perfectly fine, thanks for the suggestion, I'll check it out to see what it does. I was planning on doing this on vero board though, but I can do a pcb too if needed be.

Meanwhile I removed the buffer from the simulation and reduced the resistor values to 6k8 to lower the input impedance to some 6K
Yansi:
Jeez. Why 12GBP part for something that can be solved with a couple of discretes?

Your differential mic amp is useless. Instead of simulating stuff, where it tells you nothing more that the simulation is worth, try breadboard it.

First, the amplifier has different input impedances in each input. Quite a fail, regarding CMRR.  There is no easy fix for this, unless using a proper instrumentation amplifier configuration (three opamps).

Second, whats up with R9? Wtf is there doing?

And lastly, you're missing input protection (EMI) completely. Without it, microphone amplifier can't stand much chance.

Instead of trying to reinvent the weel, why not lookin for the easily available literature I have suggested? Or to also find some schematic service packs for good old analog mixing consoles? (Allen&heath, Behringer, ... a lot of schematics available). Most of them used off-the-shelf parts costing peanuts, yet delivering performance.

But, to be honest, even I have tried to reinvent mic preamps in the past, but I have done it with a soldering iron, so have hands-on experience with what works and how good. Just sayin, trying to save yerr some time  :P

Here you go.  Open the damn book I have suggested and read! :P

Ya'll be surprised how well this works.  THis circuit still needs some tweaking (at least adding 470p-1000p caps from inputs to ground and to add small resistors in series with input pins, about 5ohms for damping. C3 should be increased to 470-1000uF, 6.3V rating sufficient.).

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