How overcome AGC on MIC input on domestic recorder?

I record my phone calls at home using a line adapter. The adapter output goes into the MIC input of a small recorder (it does not have a line input).

The recorder has automatic gain control on the mic input and I get a lot of hiss & noise when no one is talking. When someone does talk the recording is a bit distorted.

How can I improve this without buying a different recorder. If I put an attenuator between the adapter and the recorder then wouldn't the mic AGC kick in more frequently?

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AGC reduces gain, so an attenuator will reduce the number of times it kicks in. Also, the attenuator will reduce the line noise (but not any internal to the recorder) when the gain is full on.

This is not an electrical engineering problem.

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David Woolley

AGCs normally are present on line inputs too.

Noise "breathing" effects are a well known problem with recorders of this type and they are a PITA

An attenuator will reduce the risk of overloading the mic input but make other matters worse as the mic gain will be wound up higher and these circuits are designed down to a price not for any "low noise" attributes. They often pick up noise from the motor circuitry as well and, as you say, the AGC will still be winding the qain up and down.

You would be better buying a quality recorder, however, there is a scheme which might work.

Build yourself a small two channel mixing circuit. Into one channel you feed your telephone pickup, into the other the output of a high frequency oscillator - start with a frequency you "just can't hear". The oscillator provides a constant level which will keep the AGC happy - you will have to experiment with it's level to get the best recording. Since the frequency is above hearing you won't hear it on playback - nor is it likely to get through the limited bandwidth of the tape system anyway.

If there is any sort of bandwidth limiting before the AGC monitor point, or it causes interference with the bias oscillator, you will have to drop the frequency lower and use a notch filter on playback.

The other mixer channel can be used to control your telephone signal level so that it does not overload the mic input.

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Yes. The attenuator in this case would be a potentiometer. Try a 10K ohm skeleton preset "pot". Wire the source to the top (hot), and bottom (ground) terminals. Wire the lead to the mic jack to the centre and bottom terminals.


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