# [SOLVED]Mic amp, very simple design. Help with gain

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#### zerodegreec

##### Junior Member level 3
I have a simple circuit that enables me to use a Dynamic Mic on a system designed for a Electret mic. It uses 3 parts.

I am limited for the # of wires in and out of my pre-amp. The amp I am connecting to uses 3 wires (shares the neg of the Mic and speaker). my input has 4 wires (2 for each the mic and Speaker)

My problem is that I would like to have a little bit of feedback from the mic to the speaker. Right now I get none at all. Any ideas of how I could figure this out?

BTW the application is for a communication setup inside a headset used in a very loud environment. When I talk I would like to hear my own voice (not very much). right now its just silence and its kind of weird talking without hearing yourself even a little bit.

Thanks for any help you can throw my way.

#### kak111

##### Advanced Member level 4
I can not see mic amp power supply
Bias resistor R1 has very small value

Circuit dont work as amplifier ................

#### zerodegreec

##### Junior Member level 3
Actually it does. I have used it many times.

It takes the very small voltage intended for an electret mic and it does amplify the signal of my dynamic mic.

The person on the receiving end of my mic say the volume is excellent. I tried using just a patch cable without any electronics and the person on the other end said they could barely hear me.

I would love to see another option if someone has one. (Always learning)

Thanks

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#### erikl

##### Super Moderator
Staff member
Circuit dont work as amplifier ................
Of course it works. S. above!

I can not see mic amp power supply
It arrives via the yellow wire (Mic +). The load resistor (a few kΩ) resides inside the amplifier. This is standard for electret mic. amplifiers.

Bias resistor R1 has very small value
It's just a feedback resistor to establish the base bias voltage. Admittedly it seems rather low. Perhaps 47 or 470 kΩ ?

I would like to have a little bit of feedback from the mic to the speaker.

@0°C: I think you need a bit of feed-forward from your speech (microphone) amplifier to your headset amplifier!

#### zerodegreec

##### Junior Member level 3
Thanks for the reply erikl

I guess I do need feed-forward

I am pretty sure I used a 470KΩ resistor. Sorry for the error.

I had help with this circuit so I am not 100% familiar with what changing some of the components would do to affect the output or maybe solve my feed-forward problem. What would happen if I put a resistor from the mic output (yellow) to the Speaker + (Green)?

#### erikl

##### Super Moderator
Staff member
What would happen if I put a resistor from the mic output (yellow) to the Speaker + (Green)?
A terrible noise. This means feedback from the speaker to the mic, which would create the same terrible sound as if you would hold the mic rather close to the speaker.

You need a connection (feed-forward) from your speaker amplifier to your headset (amplifier).

#### zerodegreec

##### Junior Member level 3
Ok, can you give me an example of how I could do this? or would this have to be something integrated into the amplifier I am connected to?

Could I make something with the existing wiring to do the job?

#### kak111

##### Advanced Member level 4
Yes i know the circuit, but normally electret mic input channel has
dc-bias resistor 4.7k...47k , volts 5...12V

Sorry my answer is a bit late

I am pretty sure I used a 470KΩ resistor. Sorry for the error.

KAK

#### zerodegreec

##### Junior Member level 3
Yes i know the circuit, but normally electret mic input channel has
dc-bias resistor 4.7k...47k , volts 5...12V

Sorry my answer is a bit late
KAK

Sorry I don't understand the symbols being used. Could you clarify?

#### erikl

##### Super Moderator
Staff member
If I understand your system correctly, I believe you have a separate 2-way communication system:
1. From your mic + amplifier system (shown above by you) to a speaker, which you can't hear.
2. Some external input (directly, or from a second remote mic.) via a headset amplifier (or directly) to your headset.
Pls. correct me, if I'm wrong!

In this case, you'd need a connection between the output of your mic. amplifier = input of your speaker, and the input of either your headset amplifier or -- directly -- of your headset loudspeakers.

Without schematics of both (2-way) amplifiers inputs/outputs wiring I can't help you further, sorry.
I'm quite sure, however, that it can be realized with a simple addition of a passive device (resistor and/or capacitor) to the existing wiring.

#### zerodegreec

##### Junior Member level 3
If I understand your system correctly, I believe you have a separate 2-way communication system:
Yes. the other side of the amp from my drawing is the same. We each have a headset with mic. (this is actually a com system for a Rally car. We are trying to sort out a headset issue. The other side of the amp, my driver uses a headset with a Electret mic)

1. From your mic + amplifier system (shown above by you) to a speaker, which you can't hear. 2. Some external input (directly, or from a second remote mic.) via a headset amplifier (or directly) to your headset.
Pls. correct me, if I'm wrong!
This is correct

In this case, you'd need a connection between the output of your mic. amplifier = input of your speaker, and the input of either your headset amplifier or -- directly -- of your headset loudspeakers.

Without schematics of both (2-way) amplifiers inputs/outputs wiring I can't help you further, sorry.
I'm quite sure, however, that it can be realized with a simple addition of a passive device (resistor and/or capacitor) to the existing wiring.

I don't have the schematics of the amp itself. (without some exploratory surgery)

Here is the general layout for clarification.

#### kak111

##### Advanced Member level 4
I am limited for the # of wires in and out of my pre-amp. The amp I am connecting to uses 3 wires (shares the neg of the Mic and speaker)

So this is your circuit no:1.......

if we look the common wire ( neg of the Mic and speaker) between points A and B.

We find out , that wire resistance and speaker current makes positive feedback
for your mic amp circuit between A and B

ie. this causes oscillating effect to your circuit

If your mic amp is at B-side of ground wire, effect is stronger

To avoid that there should be separate ground wires for mic and speaker

KAK

#### zerodegreec

##### Junior Member level 3
I am actually connecting it to the "B" side in your diagram.

If I simply moved my Emitter to the "A" side would that fix my problem?

I know that its best if they each had there own Neg line, but this type of amp is not wired like that (other manufactures do use a 4 wire system)

#### Syncopator

##### Full Member level 6
What you are trying to add (your own voice to your earpiece) is called "sidetone". Telephones use it all the time - haven't you noticed?

You need to explore the amplifier circuit, and feed an attenuated sample of the microphone signal to the corresponding earpiece's amplifier. And, presumably, you will need to do the same thing for the other headset.

It will need a bit of trial and error to get the level just right.

#### zerodegreec

##### Junior Member level 3
Here is how it is currently wired (physically)

Oh and here is the finished product

#### kak111

##### Advanced Member level 4
Now when i saw picture of this amp.
it barely dont help because wire A to B is 1cm long

I suppose that from connector to headset mic and speaker has 4 wires

That case its possible that this feedback is acoustical via mechanical
construction.

You can test it by replacing headset speaker(s) with same size resistor ( same ohms ) ( check with ohm meter )
And test by listening is there anymore oscillations in other headset.

If interference is gone.
Try first change speaker phase (interchange speaker side wires + and - )
If it dont help try insulate acoustic waves and vibrations from speaker
to mic

One point more.
Can it be inductive or capasitive feedback in wire between connector and headset ??

KAK

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#### zerodegreec

##### Junior Member level 3
Now when i saw picture of this amp.
it barely dont help because wire A to B is 1cm long

I suppose that from connector to headset mic and speaker has 4 wires

That case its possible that this feedback is acoustical via mechanical
construction.

You can test it by replacing headset speaker(s) with same size resistor ( same ohms ) ( check with ohm meter )
And test by listening is there anymore oscillations in other headset.

If interference is gone.
Try first change speaker phase (interchange speaker side wires + and - )
If it dont help try insulate acoustic waves and vibrations from speaker
to mic

KAK

I wonder if we have a mis-communication.

I dont have a feedback problem in my headset. I would like to get "Sidetone" as Syncopator mentioned.

#### kak111

##### Advanced Member level 4
Once more
You want to get sidetone
This is the principle

Values R1,2,3 depends on speaker impedance Z

So your voice goes from mic A to speaker B
this circuit takes a part of that back to your speaker (A)
Level of sidetone can control with potentiometer R2
R1 set max. level
R3 protects amplifier for overloading

KAK

( I come back tomorrow evening )

#### zerodegreec

##### Junior Member level 3
Ok I get it.

Now is there a way to get the side tone without taping into the speaker B line? Somehow use my mic output (either before or after my pre-amp) through what ever black magic is needed and into my own speaker?

#### BradtheRad

##### Super Moderator
Staff member
Hi - jumping in with my thoughts here.

Your headphone speaker is probably between 10 and 50 ohms. To get it to carry some of your mic signal you need to get power to drive it.

Where is DC+ power? The only source coming out of the amp is at your mic+ line. It's probably in series with a few K resistance (designed to operate an electret mic. as post #4 states.).

So it doesn't appear easy to get enough power to drive a low impedance headphone speaker. You would have to fit a battery into your plug somewhere. Then you could make a separate amplifier circuit from a transistor.

The solution is not easy.

But if you can open the amplifier housing, then you should have easy access to speaker terminals. I think that kak's arrangement (post #18) is likely to work for you.

Or another solution may be what Syncopator said in post #14. Create a slight amount of sidetone (crosstalk) between the two mic signals. This can be done by a capacitor and resistor in series, connected to the two mic terminals at the incoming jacks.

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