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Troubleshoot Simple NTE123AP Amp

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Junior Member level 3
May 4, 2011
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I have an amp for use with Dynamic mics being used on a system designed for Electret mics. (I had posted a question on here a few weeks ago regarding the circuit and side-tone issues.)

Here is the circuit.
**broken link removed**

I had a guy using this circuit last weekend and he said that after about 2 hours of use, the volume on speaker B became to low and static was being heard.

I have a fear and a concern.

Fear, that somehow the dynamic mic was damaged. What part of the circuit would be an area of concern. What do I need to test for in order to determine if the dynamic mic could be damaged by this circuit.

Concern is that the circuit has failed somehow. Obviously its easy to test the Cap and res but how do you test an analog amp?

I know I will need more info for you to help me, I will get what I can.

It is unlikely that the dynamic mic has been damaged. I would disconnect every thing and start of with one loudspeaker and one mic. Once that works , add one item at a time until it stops working. A two hour delay suggests a thermal effect, i.e. some thing was overheating and failed. It is bad practise to common the grounds of mics and loudspeakers, as you have done on the right hand side of your diagram - thats why the amplifiers manufacturers gave you a separate mic- terminal.

Thanks for the reply Frank,

The design in regards to sharing the ground was not by choice. :) The system is an intercom for rally cars. The amp uses a 3 wire system (I edited the original drawing). My helmet is a 4 wire setup with a dynamic mic. We tried to wire the intercom in without a pre-amp but we had low volume levels since a dynamic mic has less "gain". Plus from what I understand, if you keep a dynamic mic on a circuit designed for a electret mic, it can damage it.

Interesting thought about the temperature issue. how much heat could be generated by a circuit like this. Typically a Electret mic circuit would draw a very low amount. What part of my circuit would cause a higher amperage and thus heat?
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