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Old 25th May 2014, 02:50 PM   #1
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Default Adding gain control to Mic Preamplifier

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Can someone please tell me how to add a gain control into this schematic? Not just a volume control. Many thanks.
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Old 25th May 2014, 03:27 PM   #2
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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.
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Old 26th May 2014, 02:57 AM   #3
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Quote:
Originally Posted by JonSnell Electronic View Post
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?

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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-...BA5000CE22.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.
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Old 27th May 2014, 09:08 AM   #4
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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 220 supply buffer cap to 100 and implement another RC filter with 470..1k (between Q1 and Q2) and 100 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.
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Old 28th May 2014, 09:04 AM   #5
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Quote:
Originally Posted by sgrossklass View Post
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 220 supply buffer cap to 100 and implement another RC filter with 470..1k (between Q1 and Q2) and 100 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.
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Old 1st June 2014, 12:11 PM   #6
frank1 is offline frank1  United Kingdom
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Why not just make R6 a 1k pot with the 220uF from slider to ground?
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