Electronics > Beginners
Building a sound mixer within my guitar amp with line IN & mic input
Zero999:
C13 won't make any difference to the CMRR. The reason why I recommend having it is because it allows the value resistors in the op-amp's feedback loop to be changed, without affecting the DC operating point of the circuit.
The CMMR of a simply differential pair will never be great and will vary depending on the gain, in this case. I still think you should use a three op-amp instrumentation amplifier, with a dual low noise op-amp, such as the NJM2122 for the inputs and a cheap op-amp on the output. It will offer an excellent CMMR, without any messing around.
U1 and U2 are the low noise op-amps. U3 and U4 are the cheap ones.
The 8V can be generated from +V (9V to 24V) using a low drop-out regulator.
Yansi:
You need to keep in mind, the gain needs to be variable, and you will get the same poor performance out of the circuit at low gain setting, where all the CMRR will be handled by the U3.
dazz:
--- Quote from: Zero999 on June 09, 2019, 10:03:47 am ---C13 won't make any difference to the CMRR. The reason why I recommend having it is because it allows the value resistors in the op-amp's feedback loop to be changed, without affecting the DC operating point of the circuit.
--- End quote ---
Oh, I see what you mean now. As the resistors in the opamp are increased to get more gain, the bias voltage in the inputs decreases, which could potentially lead to early clipping. I think I could get around that by shifting the Vref voltage upwards as you suggested. But I still don't know if I need more gain. It seems to have plenty, I will know when I can test the preamp with a proper mic.
--- Quote from: Zero999 on June 09, 2019, 10:03:47 am ---The CMMR of a simply differential pair will never be great and will vary depending on the gain, in this case. I still think you should use a three op-amp instrumentation amplifier, with a dual low noise op-amp, such as the NJM2122 for the inputs and a cheap op-amp on the output. It will offer an excellent CMMR, without any messing around.
U1 and U2 are the low noise op-amps. U3 and U4 are the cheap ones.
The 8V can be generated from +V (9V to 24V) using a low drop-out regulator.
--- End quote ---
--- Quote from: Yansi on June 09, 2019, 04:42:43 pm ---You need to keep in mind, the gain needs to be variable, and you will get the same poor performance out of the circuit at low gain setting, where all the CMRR will be handled by the U3.
--- End quote ---
I'll probably build that circuit too and see if I can compare their performance with the (very) limited equipment I have here.
Meanwhile I put my prototype in the amp's box, which has a heavily filtered dc supply, and it's now almost dead quiet at maximum gain with a 2m lead connected at the input.
Zero999:
I don't see why the gain needs to be adjustable. As long as it doesn't clip, it can be attenuated by the next stage.
I still think an instrumentation amplifier will have a better CMMR than a simple differential pair, especially if 0.1% tolerance resistors are used.
Yansi:
you don't see, so tell us why yet all manufacturers of mixing desks make the input preamp gain variable, including those digital ones?
Mic preamps are not all about CMRR only. There are other important factors, such as noise, PSRR, linearity (distortion THD, IMD, TIM), headroom, etc.
However that is nothing new, that circuit design is a mixture of compromises.
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