This is another simple FET-based compressor. It is designed for "line level" signals, but can easily be preceded by another op-amp stage to bring the guitar signal up to 500 mV or so. If you do that, you have to follow the circuit with a volume control. A 10k log (audio) pot will do the trick.
This circuit was originally used to give final level control at the output of a pedal board (rather than for sustain), as the pedal board's output to the PA (we didn't ever use backline at the time) went through a radio link. The radio gear sounded truly horrible if it was overmodulated, so this circuit prevented that kind of distortion (I prefer the more controllable, pedal-generated kind!).
Here you go...
Enjoy.
"Why is it humming?" "Because it doesn't know the words!"
The mod is quite simple: just change the gate 10 kOhm resistor with the 470 kOhm and add one more 470 kOhm one from FET drain directly to the gate (National Semiconductor recommendation).
As for me, while playing with the similar circuit I've seen the great effect even on the scope.
DDD wrote:The mod is quite simple: just change the gate 10 kOhm resistor with the 470 kOhm and add one more 470 kOhm one from FET drain directly to the gate (National Semiconductor recommendation).
As for me, while playing with the similar circuit I've seen the great effect even on the scope.
I tried it out earlier this evening - the distortion was reduced. It went down from ~0.6% to ~0.2%. This was measured by sending 600 mV of 800 Hz in, and nulling the output with the original. You're just left with the distortion products, and you can measure their relative levels (and look at them on your 'scope, too). The residual signal was reduced from about 3.5 mV to 1.2 mV. If you're concerned about that degree of fidelity, you shouldn't be playing with stompboxes!
"Why is it humming?" "Because it doesn't know the words!"
Gerry de la Sel wrote:
Remarkably similar to this compressor:
And your point is?
The simple FET compressor came from equipment that I designed and built built in the early 70s. It's simple, it works, and was copied in a number of commercial products in the years since. The only downside is that the range of control is more limited than other compression methods, but with low noise op-amps, it works quite quietly. You'll also find that the simple sidechain circuit confers a useful (though unexpected at first glance) characteristic - it tends to limit rather than compress at higher input levels.
"Why is it humming?" "Because it doesn't know the words!"
DDD wrote:The mod is quite simple: just change the gate 10 kOhm resistor with the 470 kOhm and add one more 470 kOhm one from FET drain directly to the gate (National Semiconductor recommendation).
As for me, while playing with the similar circuit I've seen the great effect even on the scope.
I tried it out earlier this evening - the distortion was reduced. It went down from ~0.6% to ~0.2%. This was measured by sending 600 mV of 800 Hz in, and nulling the output with the original. You're just left with the distortion products, and you can measure their relative levels (and look at them on your 'scope, too). The residual signal was reduced from about 3.5 mV to 1.2 mV. If you're concerned about that degree of fidelity, you shouldn't be playing with stompboxes!
I reckon that, if you're using this compressor circuit with a distortion pedal after it, then the fidelity is probably going to be a moot point, so I'd agree with you....
Genius is not all about 99% perspiration, and 1% inspiration - sometimes the solution is staring you right in the face.-Frequencycentral.
seems boring as bat shit this one the side chain would have a horrid response. It would be like sticking your soldering iron into a glue pot and sniffing it. Hmmm maybe that would be better
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