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Old 17th August 2007, 04:14 PM   #1
mhtplsh is offline mhtplsh  India
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Default agc preamp distortion

I found this circuit on the net. Accordingly i assembled it with 2N5019 fet. I changed the opamp to njr4558 also for better sound. But the tv sound is getting distorted after using this preamp. I am using as the input from satellite receiver & output to tv audio input.
Waiting for suggestions & solution.
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Old 17th August 2007, 04:25 PM   #2
hermanv is offline hermanv  United States
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Old 22nd August 2007, 08:43 PM   #3
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The J-Fet used as a variable resistor, needs to be linearized, or distortion occurs above about 25mv input. Even then, it's best to keep the signal below 100mv. you can do this with an input attenuator.

I think it's Vishay that have an ap note about using J-Fets in this manner, but simply, you want to create feedback between the drain and the gate of the fet.

With this circuit, you would need to take a cap, say at least 0.1uf from the drain, with a 100k r in series with the cap, and connect the other end of the r to the gate. You will then have to put a 100k r from this junction, to the junction of the cap and resistor at the gate of the fet and the collector of the transistor, removing the direct link between them.

As it is, it is a very simple circuit, and not really meant for high quality repoduction. With these changes, you should be able to eliminate audible distortion.

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Old 22nd August 2007, 08:46 PM   #4
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Also, put a 1k resistor between the base of the transistor and the junction of c3/p1 and see if that lowers distortion as well.

jD
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Old 23rd August 2007, 06:22 PM   #5
hermanv is offline hermanv  United States
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A number of manufacturers now make digital potentiometers or level controls. While few, if any, qualify as the last word in audiophile signal purity, most will be better than simple FET circuits.

For example look at the LM1972 data sheet from National Semiconductor .003% distortion 78 dB of gain rage and a signal to noise of better than 100dB. This was just the first one I found. Other people make them such as Aanalog Devices, it might be worth a search.
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Old 23rd August 2007, 09:15 PM   #6
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Quote:
Originally posted by hermanv
A number of manufacturers now make digital potentiometers or level controls. While few, if any, qualify as the last word in audiophile signal purity, most will be better than simple FET circuits.

For example look at the LM1972 data sheet from National Semiconductor .003% distortion 78 dB of gain rage and a signal to noise of better than 100dB. This was just the first one I found. Other people make them such as Aanalog Devices, it might be worth a search.

I think the original poster is using the circuit as an agc preamplifier. The LM1972 is essentially just a volume control, and would not work (well) as an agc preamp.

Of course there are better ways of doing this than with a fet, but certainly not as cheap, and not within the scope of this thread.

For alternatives, see www.thatcorp.com
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Old 24th August 2007, 05:58 AM   #7
mhtplsh is offline mhtplsh  India
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How about this circuit?
http://sound.westhost.com/project67.htm
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Old 24th August 2007, 05:43 PM   #8
hermanv is offline hermanv  United States
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Quote:
Originally posted by electrode10101

I think the original poster is using the circuit as an agc preamplifier. The LM1972 is essentially just a volume control, and would not work (well) as an agc preamp.

Of course there are better ways of doing this than with a fet, but certainly not as cheap, and not within the scope of this thread.

For alternatives, see www.thatcorp.com
An AGC circuit has two basic requirements, a method of controlling gain and a method of detecting the output level. While ICs that do both exist, all the ones I know of are srictly Lo-Fi. By buying a quality solid state "volume" control and designing your own rectifier, a reasonable AGC circuit can be developed with a few Op-Amps or use discretes if the last word in quality is the goal.

Having designed a few commercially, there is a real issue of attack and decay speed to consider. AGC circuits for voice or music turn out to be tougher than they apear because our ears quickly pick up the "un-natural" resulting sound if the gain down and up speeds are not done very carefully. It is also necessary to limit the maximum gain or the output just become hiss and noise when no input is present.

I don't quite know what the scope of the thread is, the original poster asked for ideas or a working circuit.
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Old 24th August 2007, 06:16 PM   #9
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Default Re: agc preamp distortion

Quote:
Originally posted by mhtplsh
I found this circuit on the net. Accordingly i assembled it with 2N5019 fet. I changed the opamp to njr4558 also for better sound. But the tv sound is getting distorted after using this preamp. I am using as the input from satellite receiver & output to tv audio input.
Waiting for suggestions & solution.
If you want to use the original circuit with the fet variable resistance, there is a trick that EVERYONE uses in this case. Put a resistive divider (2 x 100k should work) from the input to ground and connect the midpoint to the gate. That cuts the distortion by an order of magnitude. And, make sure your input level stays below 100mV (preferably below 10mV) and put some gain after it.

Jan Didden
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Old 24th August 2007, 08:19 PM   #10
mhtplsh is offline mhtplsh  India
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hermanv
Quote:
the original poster asked for ideas or a working circuit.
If u can give working circuit, what more can i ask for?
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