I'm trying to build a single supply balanced preamp for an electret mic. To further complicate matters, I'd like to impedance balance the mic output to take full advantage of the preamp's noise rejection capabilities. Here's what I've got so far:
There are a few problems with the circuit:
- gain is not as expected. Removing the 2.2K resistor from pins 1 and 3 on the left of the diagram increases the gain substantially, but the cable is no longer impedance balanced.
- there is a lot of hum, which is corrected by replacing the 470K resistor at pin 3 on the opamp with a wire to the voltage divider. However, I'm assuming that the opamp is no longer functioning as differential in this case.
I've run out of ideas - can anyone suggest anything that might remedy these problems?
Thanks for the replies so far everyone. I've made a slight change to my original design that seems to have fixed the problem:
The only difference is that I moved the 470K resistor from pin 3 on the opamp to ground (as it would be in a split supply design) and connected pin 3 directly to the voltage divider. This has given me the desired result of the correct gain with the hum reduction. Is this the proper way to set up a single supply balanced differential opamp?