I hv the following circuit assembled. I hv used 2N 5019 fet as per the instruction of the designer. I tried with replacement of LM 358 with njr4558 ic to reduce distortion.
I hv tested the circuit by connecting satellite receiver audio out as input & tv audio in connected to the output of the circuit mentioned.
The sound is distorted after connecting the unit.
waiting for suggestions of the fellow members.

Attached Images

agc amp.jpg (89.2 KB, 28 views)

 

Is your 2n5019 has linear resistance to Vds characteristics.
It seem that 2n3904 create some distortion.
You need one more amplifier (same input at pin 3) to isolate the output from 2N3904.
4580 is better option for low noise low distortion wideband but higher power needed than LM 358.

 

The FET is a very non-linear voltage controlled resistor. Its linearity can be made much more symmetrical if half the drain signal is fed to the gate as in my modification to your circuit.

The FET will cause additional distortion if the signal at its drain exceeds about 100mv peak. Add a fixed attenuator to the input of the circuit so that the signal is not as high.

Attached Images

AGC circuit.PNG (49.9 KB, 26 views)

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Uncle $crooge

 

Thanks audioguru,
I will try this circuit on Monday & will give u feedback.

This is the description of the project from the author:
Using the circuit presented here, you can construct a very inexpensive AGC amplifier with the following features: a dynamic range greater than 50 dB; negligible distortion to the output waveform; fast attack and slow decay; an adjustable output level from 0 to 1.2 V p-p; operation from a single 5-V supply; less than 1-mA current drain; and low cost (uses one half of a dual 8-pin op-amp package at less than $2.50 in parts). Better yet, if you need a second channel, the remaining half of the op amp can be used for that circuit.
Referring to the diagram, Q2 (a Pchannel JFET), coupled with R2 and the equivalent resistance of R3 and R4, form a voltage divider to the input signal source. With input levels below 40 mV p-p, the input is evenly divided between R2 (120k) and R3 ¦ ¦ R4 (120k). The output amplitude of U1A isn’t large enough to turn on Q2, which acts as a positive peak detector. The gate of the JFET is pulled to +5 V, pinching its channel off and creating a very high resistance from drain to source. This essentially removes it from the circuit.
At input levels above 40 mV p-p, Q1 is turned on at the positive peaks of the output of U1A, lowering the JFETs gate to source voltage. The channel resistance decreases and attenuates the input signal to maintain the output of U1A at approximately 1.2 V p-p.
The circuit, as shown, was tested with a sine-wave input ranging from 300 Hz to 30 kHz at 40 mV to 20 V p-p, a 54-dB range. It maintained the output level at 1.2 V p-p, ±0.5 dB, with no visible distortion when comparing it with the input waveform. With a 40 mV to 20 V p-p input signal, the amplitude of the signal across the JFET (VDS) measured less than 20 mV p-p.
Other JFETs with VGS(OFF) of 5V or under, such as the 2N5019 or 2N5116, should work equally well in this circuit, although they haven’t been tried. To use JFETs with higher VGS(OFF), such as the 2N3993 (it was tried and worked equally well), increase the supply voltage to 12 V.

 

Another serious failing of the circuit is that it only reacts to positive peaks, a precision full wave rectifier would improve it as well.

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PIC programmer software, and PIC Tutorials at:
http://www.winpicprog.co.uk

 

Thanks Nigel Goodwin,
Let me know where to put precision full wave rectifier in the circuit?
Is it to put after volume control & before 2N 3904?

 

In the base lead of the 2N3904, but you would need to add a good few more components as well - easier to find a circuit that does it already? - this one is really pretty poor, and like Audioguru already mentioned it's VERY bad to leave out the extra components round the FET.

__________________
PIC programmer software, and PIC Tutorials at:
http://www.winpicprog.co.uk

 

thanks Nigel Goodwin,
i will appreciate if i can some good circuit.

 

Here is a good Peak Limiter circuit that cuts the level down to its setting.
It has a full-wave rectifier and its attack time is very fast.
http://www.sound.westhost.com/project67.htm

Attached Images

Fast audio peak limiter.gif (6.8 KB, 14 views)

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Uncle $crooge

 

Here's a mod of the original circuit which should respond to positive and negative peaks. I have not tested it, so it's worth what you pay for it.
Audioguru's circuit is probably better (I'm no audio guru).

Attached Images

AGC circuit with FW.PNG (58.0 KB, 11 views)

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Ron (aka Rube)

 

Thanks audioguru & roff.
I really appreciate the way u r guiding me.
Just one question.
Is the circuit will just work as peak limiter for high audio output or will also boost the weak channel audio signal.
From the satellite receiver few channel audio output is very weak. So it needs to be boosted & high channel output to be reduced.

 

I doubt if anybody can hear the difference between a half-wave compressor and a full-wave one.

The modification I made to the first posted circuit reduces the distortion of the FET but causes the attack time of the FET to be much too long.
The "better" circuit I posted is not mine, it is from Rod Elliot. It has parts added to speed up the attack time of the FET.

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Uncle $crooge

 

The circuit isn't really a 'booster' it's an attenuator of loud signals, but this is really just a difference in description.

However, it's not really intended for satellite channels, you perhaps need to find why some are quiet?.

__________________
PIC programmer software, and PIC Tutorials at:
http://www.winpicprog.co.uk

 

It appears to me to have a maximum gain of 71.2 (37dB) for outputs less than 1.2V p-p.

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Ron (aka Rube)

 

If the gain is high or if the input signal level is high on the limiter then it limits the output to its max output level, the same as a compressor. Since the gain is high then low level signals are amplified, the same as a compressor.

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Uncle $crooge