Electrical – Amplifier volume potentiometer causes voltage offset

amplifierarduinopotentiometer

Using answer about microphone I have built a simple amplifier.

schematic

simulate this circuit – Schematic created using CircuitLab

I changed some of the parts due to my needs and resources.

  • R1: is now potentiometer and allows me to set the "0" of the sound to 2.5V
  • R3: should control amplification (but it doesn't seem to), so again a potentiometer
  • C1: unless the original answer meant electrolytic capacitor, there's no way to get 22uF capacitor. So I used just two 22nF caps.
  • C2: I have 10uF capacitor (salvaged from old sound card ironically), so I used that

So I have set up R1 to get 2.5 volts on silence. The image displays 80Hz sine wave rather than silence, I generated it here (works on Android).

image description

Now if I move the R3.2, instead of change to gain, I just get voltage offset, and an insignificant one. Picture describes full range (min – max – min) of that particular potentiometer:

image description

Note also that increasing resistance increases voltage on output.

I could use my 50k potentiometer instead but that would give me no precision.

So what is wrong with my circuit? How to control the amplification?

Note: I displayed the analog input using this processing script and this arduino program, both mine but I thought I might share.

Best Answer

Try this...

  • Remove R1 the way it is currently wired and all wires leading to it.
  • Connect a ~1k resistor from Vcc to the transistor collector.
  • Connect R1's end leads from the transistor collector to ground, respectively. A larger value for R1 might be a good idea, such as 10k, but it isn't critical.
  • Connect a 1µF ceramic cap from R1's wiper to the arduino.

Edit:

  • Also connect a 10k from Vcc to the arduino pin, and a 10k from that to ground. This will cause the output to center around 2.5v, which seems to be what you are after.

This way, the position of R1 forms a "tap" between full signal, and no signal. Then adjust R3's value to give the cleanest output with the loudest microphone signal expected. There is a technical term for adjusting these resistor values - biasing. Also this. Have you calculated what the values should be?

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