Level selctive attenuator

[tomerbr]

Hi,

I have a project where I feed line level amplifier from a high gain amplifier.
The main problem that I have if with the gain difference so I introduced an attenuator which solved my problem when the high gain amplifier was at his peak volume but now I have a problem with the low level signals when the high gain amplifier volume is very low.

I would like to build an attenuator that will start to work only from a certain input level, is it possible?
I have searched the net and could not find how to do it although I am sure there is a way.

I saw for example this device https://www.ti.com/lit/ds/symlink/tpa6132a2.pdf but the problem is that the max input levels are 2.2V (HPVDD + 0.3), the output level of high power amplifiers can reach 15-20V (even more) so I guess this is not an option.

Any help is greatly appreciated.
Tomer

 

[BradtheRad]

It sounds as though you're looking for automatic gain control (AGC). You can find many circuits that do this, although you'll need to decide where to install it, and which amplifier it should go before or after.

One simple and popular method is (or was) a photosensor illuminated by an led or bulb. To work directly with AC signals, consider a light dependent resistor. Often this is just an inexpensive CdS cell.

 

[tomerbr]

Thank you Bead,

The AGC/DRC should be placed before the line (or mic) input.
Can you point me to an example of a circuit please?

 

[BradtheRad]

AGC is the sort of function that a search can find several circuits that do it, but none are that easy to remember. They are not simple nor easy to work with. Expect to use one or two op amps. Expect to adjust a lot of component values, in order to get the response you desire.

Also consider whether you want to apply compression to your signal. This is a more sophisticated audio tool than AGC.

before the line (or mic) input.

Microphone output is normally weak (5-50mV). It is customary to apply voltage gain through a pre-amp to raise it to line level (1 volt). Then it is not as prone to pick up noise. Then it can go through the same circuitry as other line level signals.

Perhaps you need a simple potentiometer-type volume control?

 

[tomerbr]

Thanks Brad,

Mostly I prefer to work with IC but building an AGC from scratch is also an option for me.
I do not need a pre-amp since the input to the mic port is not an actual microphone but a signal that is originating from a high power amplifier - that is actually the source of my problem, to reduce the levels no to increase them.
I already build a circuit that works and added an attenuator, the only issue with the circuit is when I attenuate low signals coming from the amplifier (when volume is very low).

Do you (or anyone) have any suggestions how to overcome it?

 

[FvM]

An AGC, dynamic compressor, limiter circuit or whatever you name it implements a specific level transfer (threshold, compression factor) and time dependent behavior (attack and release time). Both must specified to design a suitable circuit.

There nothing special about using a fixed attenuator in front of a dynamic processor. A standard situation when connecting instrument amplifiers per DI-box to a mixing console.