Adding gain control to Mic Preamplifier

[nguitar12]

Can someone please tell me how to add a gain control into this schematic? Not just a volume control. Many thanks.

 

[JonSnell Electronic]

Place a pot between the +ve of the 220u Cap (across R6) and the junction of R5 and R6. That will allow you to reduce the gain by introducing negative feedback.

 

[nguitar12]

Place a pot between the +ve of the 220u Cap (across R6) and the junction of R5 and R6. That will allow you to reduce the gain by introducing negative feedback.

Thanks for you help.
Can you tell me if this is what you mean?

An externally hosted image should be here but it was not working when we last tested it.

I have a few extra question:

1: What is the value of the pot added?
2.I found 220uf electrolytic capacitor is a bit big in size for my build. Can I replace with smaller value one? If so what is the smallest value that can be used?
3. I am going to use the following mic capsule:
http://industrial.panasonic.com/www-data/pdf/ABA5000/ABA5000CE22.pdf
How can I decide the best value of resistor in used?

I am so sorry for so many question. I am just a beginner on electronic.

 

[sgrossklass]

1. Depends on the minimum gain that you want to achieve. Av is about 1 + R6 / (R5 || Rpot), so with a 1k pot you'd get a minimum of 3 (about 10 dB). I'd prefer a log type for better gain control, or a "log'd" linear. Make sure to short the "upper" end to wiper.

2. The C at the pot, you mean? It should be sized such that the RC time constant formed by it and a typical value of Rpot is greater than about 1 / (2 pi * 20 Hz).

3. R1? I would size it for a short-circuit current of about 1 mA, i.e. R1 ~= Vs / 1 mA.

Make sure that the supply voltage is squeaky clean, particularly to the left of Q2. I would rather downsize the 220µ supply buffer cap to 100µ and implement another RC filter with 470..1k (between Q1 and Q2) and 100µ instead. Put the circuit into a shielded box if possible (connect box to ground in one spot). A 9V block battery tends to be a handy supply for things like these - I wouldn't expect more than a handful of mA of current draw.

 

[nguitar12]

1. Depends on the minimum gain that you want to achieve. Av is about 1 + R6 / (R5 || Rpot), so with a 1k pot you'd get a minimum of 3 (about 10 dB). I'd prefer a log type for better gain control, or a "log'd" linear. Make sure to short the "upper" end to wiper.

2. The C at the pot, you mean? It should be sized such that the RC time constant formed by it and a typical value of Rpot is greater than about 1 / (2 pi * 20 Hz).

3. R1? I would size it for a short-circuit current of about 1 mA, i.e. R1 ~= Vs / 1 mA.

Make sure that the supply voltage is squeaky clean, particularly to the left of Q2. I would rather downsize the 220µ supply buffer cap to 100µ and implement another RC filter with 470..1k (between Q1 and Q2) and 100µ instead. Put the circuit into a shielded box if possible (connect box to ground in one spot). A 9V block battery tends to be a handy supply for things like these - I wouldn't expect more than a handful of mA of current draw.

Truly thanks for your detail info. I will try to build this circuit right now.

 

[frank1]

Why not just make R6 a 1k pot with the 220uF from slider to ground?