Simple FET Compressor

[gyraf]

Tamas,

Remember that these are FET-devices, so they won't behave well at signal levels higher than ~30mV AC. This is why you always see signal being heavily attenuated before the attenuator FET in compressors - followed by a powerfull make-up gain stage. This is also why FET compressors are relatively more noisy than LDR-based electro-optical circuits.

The LDR-based designs dosen't care at all about ac voltage level, so they design for much higher ac on the cell - and lower makeup gain.

Jakob E.

 

[jdiamantis]

"Remember that these are FET-devices, so they won't behave well at signal levels higher than ~30mV AC. This is why you always see signal being heavily attenuated before the attenuator FET in compressors - followed by a powerfull make-up gain stage. This is also why FET compressors are relatively more noisy than LDR-based electro-optical circuits."

If you linearize the FET with feedback from the drain, you can increase the input handling to about 0.5v peak. This value varies with Vgs off, but is a good rough estimate. Also, you'll want a circuit where the FET is biased close to pinch-off, but remaining on, as this will also minimize distortion. Different FETs require different op points here.

 

[Flatpicker]

[quote author="jdiamantis"]...This is why you always see signal being heavily attenuated before the attenuator FET in compressors - followed by a powerfull make-up gain stage. This is also why FET compressors are relatively more noisy than LDR-based electro-optical circuits."...[/quote]Anyone ever tried to make one using a MOSFET? No signal attenuation and maybe less noisy?

As far as the original circuit in the first post, the FET?s controlling the gain-setting resister. Is noise a problem there?

 

[tk@halmi]

[quote author="gyraf"]Tamas,

Remember that these are FET-devices, so they won't behave well at signal levels higher than ~30mV AC. This is why you always see signal being heavily attenuated before the attenuator FET in compressors - followed by a powerfull make-up gain stage. This is also why FET compressors are relatively more noisy than LDR-based electro-optical circuits.

The LDR-based designs dosen't care at all about ac voltage level, so they design for much higher ac on the cell - and lower makeup gain.

Jakob E.[/quote]

Jakob,

I completely forgot about the FET's Vgs range limitation. I was bedazzled by the Vds spec. I will cook up something with them yet. Perhaps I could use them as a variable resistor in an amp feedback loop.

Thanks,
Tamas

 

[martinnez]

8) hi friends!!
  just to add a comment, i remember one fet circuit from sound westhost  ESP,
called simple fast fet compressor or something,didn't try it , but looked ok.
  also you can try one called orange squeeser, but tha is more guitar oriented.
anyway i think both are worth a try, good luck!!
    martinnez