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Mic preamp dynamic gain

inklen

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So, as some of you may know (and thank you guys!) I built - finally - a mic preamp and it works like a charm. It has a gain ~60dB. But now what I'd like to do is to have this gain "dynamic". I mean, when there's low level SPL let the preamp work at its maximum but when the input signal is "loud" the preamp would better lower its "amplification ability" to keep the output at the proper level (and not clipped).

I'm trying to read books and I only finished ones for beginners where opamps are just touched. And I'm curious how to solve this. One idea (probably weird) is to have 2 mics and 2 pramps on the same board/circuit and have different gain ratios (different feedback resistors). In this case the sum of two signals could be somehow mixed. But I'm not sure.

I'm sure this has been done before. I'd appreciate any articles and circuits just to have an idea how it should be done.
Thank you!
 

KlausST

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Hi,

you did an internet search?
What documents did you read?

Then you already should have find out that this also is called "audio compressor" .. or similar.
A search on those phrases may give new results.

There are many solutions, OTA, integrated analog compressors, digital solutions, digital pot solutions, analog multiplier...

Klaus
 

inklen

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Klaus, now I understand what to search for! Let me see...

I don't want to go digital.
--- Updated ---

You have to know what you don't know ;)
 

Audioguru

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Most audio compressors react to a loud sound slowly allowing a blast of distorted loud sounds before reducing the level.
When compressing the level of loud sounds and weak sounds occur then there is a delay before the gain ramps up.
But my digital hearing aids compress loud sounds perfectly without these problems.
 

danadakk

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BradtheRad

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Automatic gain control / Automatic volume control.

It's not necessarily easy to do electronically.

1.
One strategy is to illuminate a photosensor (example, CdS cell), with a bulb/led powered by the incoming signal. Then arrange a feedback circuit so the changing resistance of the photocell automatically holds signal amplitude to a certain level.

2.
A jfet conducts more or less of a signal depending on bias voltage. The link below is a project telling how to make a variable resistor using a jfet. By arranging a feedback circuit the signal amplitude can be adjusted automatically.

 

betwixt

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All the above is good advice but beware of two ways to do this which are not quite the same:

1. Compression - this is best done in real time by having an amplifier that does not have a linear input to output relationship. For example, at low levels the gain is high then as the level increases, the amount of gain is reduced. It causes the higher levels to be amplified less while retaining the gain at lower levels. It restricts the output successfully but distortion gradually increases at louder levels, whether this is important or not is your choice. Each cycle of waveform is individually adjusted.

2. AGC - this is the method where an attempt is made to hold the output level constant, at least beyond a certain threshold. A proportion of the output level is used to control the gain of the amplifier. As the output increases the gain is reduced in the amplifier to keep it within acceptable levels. This is the most commonly used method but it has some serious drawbacks. It adjusts according to the envelope of the signal rather than individual cycles so there is an inevitable time delay. It tends to cause 'breathing', an effect where the background noise level rises slowly when little sound is present because the gain adapts to maximum. In extreme cases, especially in music, it can cause a quiet 'dead' time for example after a loud beat of a drum.

Brian.
 

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