# Mic Pre amp and head amp sharing one power supply problems

#### Circuitfreak101

Joined Apr 13, 2016
15
Hi

If I've got the following 2 circuits in the same box would it be best to bias both op amps non inverting inputs from the same resistors rather building both complete circuits and only sharing + volts and ground ?

Here's the mic pre :

http://circuitswiring.com/circuit-diagrams/0/2011/10/electret-microphone-pre-amp.jpg

Heres the headphone amp :

http://www.redcircuits.com/Page119.htm

The reason I'm asking is with the circuits only sharing + volts and ground I'm getting motorboating in the cans when the electret mic is connected to the pre amp via a capacitor, which I need to do to keep DC out of it's volume pot which isn't shown in the schematic but mic hot is to one end of pot, ground to the other and the centre wiper goes to input of amp in normal way

#### wayneh

Joined Sep 9, 2010
17,152
Are you talking about using R6 and R7 to also serve as R2 and R3, or vice versa?

I was going to tell you that's a bad idea but after looking more closely, it seems like it should be fine. Sort of. By using a resistor voltage divider as a voltage reference, any change in the supply voltage causes the reference to move. So a pulse on the output causes a voltage dip, and this gets sent to the input.

You might have other wiring issues that cause the headphone amp circuit to interfere with the mic circuit. For instance the mic amp should have its own wires to the power supply, so that any voltage drop on the wires to the headphone amp don't disrupt the mic amp.

A regulated power supply might also help.

#### #12

Joined Nov 30, 2010
18,222
I'm thinking, add a 2.2k before R1 in the mic amp and then add a capacitor to ground. 47 uf should do the job, so use 100 uf.

#### ian field

Joined Oct 27, 2012
6,539
Hi

If I've got the following 2 circuits in the same box would it be best to bias both op amps non inverting inputs from the same resistors rather building both complete circuits and only sharing + volts and ground ?

Here's the mic pre :

http://circuitswiring.com/circuit-diagrams/0/2011/10/electret-microphone-pre-amp.jpg

Heres the headphone amp :

http://www.redcircuits.com/Page119.htm

The reason I'm asking is with the circuits only sharing + volts and ground I'm getting motorboating in the cans when the electret mic is connected to the pre amp via a capacitor, which I need to do to keep DC out of it's volume pot which isn't shown in the schematic but mic hot is to one end of pot, ground to the other and the centre wiper goes to input of amp in normal way

Someone on this forum has a link to a copy of my Elektor article for a TL431 electret mic booster.

The entire circuit can be reconfigured to put the whole thing on the end of a twisted pair.

Basically; you just take the cathode load resistor from the head end amp and put it in the base unit.

Its capable of wildly excessive gain, but you can tailor that to what you want with the nfb network around the TL431.

#### Circuitfreak101

Joined Apr 13, 2016
15
Thanks for the reply's and suggestions. As it stands with C1 of the mic pre amp left out the whole thing works as intended with the electret mic being biased by the DC voltage on pin 2 of the op amp. But this also puts DC into the volume pot I have installed at the beginning of the circuit causing scratching noise when the pots turned. If I use C1 and obtain bias for the mic via a resistor like normal the circuit starts to oscillate in a kind of motorboat fashion

I'm unsure why blocking the DC with a cap should cause any problem as it wouldn't normally but will try the 2.2K before R1 with a 100u to ground.

@wayneh Yes I have both circuits using their own wires to the power supply which presently is a 9v battery but the circuit will be powered by a regulated supply, probably 12V in and then a 7809 with decoupling caps on the board should do it maybe ?

#### Circuitfreak101

Joined Apr 13, 2016
15
Thanks for the reply's and suggestions. As it stands with C1 of the mic pre amp left out the whole thing works as intended with the electret mic being biased by the DC voltage on pin 2 of the op amp. But this also puts DC into the volume pot I have installed at the beginning of the circuit causing scratching noise when the pots turned. If I use C1 and obtain bias for the mic via a resistor like normal the circuit starts to oscillate in a kind of motorboat fashion

I'm unsure why blocking the DC with a cap should cause any problem as it wouldn't normally but will try the 2.2K before R1 with a 100u to ground.

@wayneh Yes I have both circuits using their own wires to the power supply which presently is a 9v battery but the circuit will be powered by a regulated supply, probably 12V in and then a 7809 with decoupling caps on the board should do it maybe ?

#### ian field

Joined Oct 27, 2012
6,539
I'm thinking, add a 2.2k before R1 in the mic amp and then add a capacitor to ground. 47 uf should do the job, so use 100 uf.
That could probably be described better...................

The bias supply for the electret capsule needs very good supply decoupling.

With 9V supply, the total feed resistance can be as much as 10k. I'd go for adding another 4k7 in series with the one already there, and add a decoupling electrolytic from the junction between them to GND.