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Variable gain amplifier, DC inverting amplifier problems?

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tdkehoe

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I'm trying to make a circuit that inversely changes the volume in headphones as you talk louder, i.e., as you talk louder the volume in the headphones gets quieter. Is a variable gain amplifier the best solution to this problem?

This circuit doesn't work (see attached drawing). I'm using a +4-power supply. U1 is the microphone pre-amp, it works fine. D1, D2, C3, and R4 are a rectifier, it works fine. (BTW, I invented this type of rectifier years ago but have never seen it in books, etc.) The signal at this point is DC and corresponds to the volume of the user's voice.

U2 is intended to invert the rectified signal but it doesn't work. I can't figure out how to invert a DC signal. The problem may be that U2 is set up single-supply. Maybe with dual power supplies U2 might work. Putting a capacitor between D1 and R5 doesn't work because it kills the DC signal. I tried using a 74xx04 inverter, but it's digital so it flips from +4V to ground, nothing in-between. Maybe a PNP transistor could invert a DC signal? I've found NPN DC amplifier circuits but I can't find any PNP DC inverting amplifier circuits.

U3 is a variable gain amplifier design I found on a website. It takes a second audio source in the inverting input, and the output volume is controlled by the voltage on the non-inverting input. I tested it and it seemed to work, but I need to do more testing. I tried swapping the inputs to make it an inverting amplifier but that didn't work.

- Any suggestions how to invert a DC analog signal?

- Any suggestions for a better variable gain amplifier (VGA)? I tried a LMH6505 but it requires split power supplies, minimum 7 volts (or maybe 14 volts, the datasheet was confusing). I also tried a AD8331 but couldn't get it to work (even with the pre-assembled evaluation board). The LMH6505 and AD8331 are designed for ultrasound machines, their performance (bandwidth) is way beyond what I need, so I'd rather find a simple solution using a general purpose op-amp.
 

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audioguru

Well-Known Member
Most Helpful Member
Look at a good Audio Compressor/limiter circuit in Google. They usually use a jFet as a variable resistance to attenuate the signal.
 
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