Help: Audio Preamp with a DC Passthrough

[Good Brown Gravy]

Hi Guys,

I am working on a preamp that has a bit of unconventional functionality. Below is the part of the preamp that I have got working to date.



An audio signal comes into the input on the left of the schematic, passes through a couple of DC protection caps, and passes into the SSM2019 preamp. Resistor Rg controls the gain and for now I have pot a potentiometer in its place. The signal is amplified and then outputs to my audio amp that outputs to my headphones.

The unconventional part is what I propose below.



My audio amp, shown to the right on the schematic, has the ability to push a +15V to -15V bias back down the input line. This voltage is used to power small microphones like this. I want to be able to pass this voltage back around the preamp to a microphone that is waiting down the line. However, when I arrange the jumpers as shown and push dc bias down the line, the microphone seems to come on but any audio is really, really, really, low. The only way I know it's working a little bit is because when I touch the mic, I hear a little response but no voice like when the jumpers are removed and the mic is powered externally.

The only other thing I'm doing is using a TC7660 for the negative rail of my preamp.

I've been so pleased with how the preamp works by itself and would really like to add this last piece of functionality. Thank you for any help you guys might be able to provide. Have a great day!

Jason

 

[BradtheRad]

Your link shows electret mic's of the two-wire type. You have plain wires carrying power. I believe the power supply drowns out the audio.

There needs to be some resistance in those long wires carrying power. Every schematic I've seen puts it in the wire carrying V+, anyway.

Example:

 

[chuckey]

I am confused you have a pre-amp with a short from input to output and you complain about low gain. the loop around the amplifier I would break and insert two resistors in series and decouple the centre point on both loops. Try 1K and 1000MF as a start. if the resistors are too large there won't be any DC at the far end for the mic.
Frank

 

[Good Brown Gravy]

BradtheRad - Thanks for the help. The audio amp I am using has the ability to apply dc voltage to the microphone alone without the need of a resistance in line with the mic. I've pushed a mic on and off with only the audio amp on many occasions. Thanks again for the response though.

chuckey - Thanks for the idea. I tried out your approach and did get more audio out of the preamp. The only problem was that even when I found the best values of resistance, the amplification wasn't very much. However it was much better than zero, which was what I had before. As I increased the resistance, the audio got better, but of course the resistors limited the amount of current that the microphone could pull. If I decreased the resistance, there was more power available for the mic but then the input would start to short to the output again. I'd have to say that your approach was the best so far.

My next steps to try:

1) I was going to try to send each feedback loop through one side of the same transformer and see if I can null out the AC
2) Try to use a FET to pass the DC and eliminate the AC current -> this was suggested by someone at work
3) Try to find a big inductor online to kill the AC signal feeding back through to the input

If you guys have anymore suggestions, I'd love to hear them.

Thanks again for your help. I'm learning a ton from this.

 

[chuckey]

As the opamp's output impedance can be quite low, the resistor in the loop nearest the output can be low (100-> 1K). The resistor in the loop nearest the input, shunts the input impedance so this should be high, but this drops the DC. So this resistor should be the inductor. Whose reactance is high compared to the input impedance of the opamp. These inductors should be your anti-phasing transformer. You will also need this arrangement between the DC feed and the AC signal at the input of the main amp-feeding the pre-amp.
Frank