Hiss filtering OTA based compressor - which frequencies? Concept diagram inside.

[nelson]

Hi folks,


I am working on a multi band compressor based around the SSM2164 quad VCA that smallbear kindly got in for me from cool audio.

I already have the compressor basically designed and it is waiting for me to breadboard and fine tune it. However, I have a spare OTA that I would like to dedicate to de emphasising the hiss inherent to sustained notes in compressors. As it is multi band this will be even more apparent if I don't include this feature.

Below is the basic concept for the de hissing. I would like suggestions for frequencies of the filters and any other suggestions you kind knowledgable people may have.

I would also like to have a discussion about hum debugging - it would also be possible for me to apply the same de emphasis on low frequency noise.

Here is the diagram.

[caress]

it might help to record your playing and use a software visualizer of some sort to find the frequencies you need to cut.  i've done this in the past with some fuzz pedals and from what i've seen, it's usually somewhere between 2k-6k.  i know, pretty broad range...  it might be a little higher and in a compressor which is just boosting and not fuzzing... maybe 8-10k?  if you have access to a graphic eq, you might want to just experiment with cutting some different frequencies and finding out what works best.

[nelson]

it might help to record your playing and use a software visualizer of some sort to find the frequencies you need to cut.  i've done this in the past with some fuzz pedals and from what i've seen, it's usually somewhere between 2k-6k.  i know, pretty broad range...  it might be a little higher and in a compressor which is just boosting and not fuzzing... maybe 8-10k?  if you have access to a graphic eq, you might want to just experiment with cutting some different frequencies and finding out what works best.

I think I will start with 5K and above, then increase until the hiss that is produced by the noisiest signal chain I can put together gets too loud with sustained notes.

[StephenGiles]

What about a high pass filter to generate a hiss only signal from the compressor output, invert it and then mix back with the compressor output - most of the hiss should cancel out. I used that to reduce FM hiss years ago and it was very successfull - it was even published in ETI as the Hiss Reducer!!

[nelson]

What about a high pass filter to generate a hiss only signal from the compressor output, invert it and then mix back with the compressor output - most of the hiss should cancel out. I used that to reduce FM hiss years ago and it was very successfull - it was even published in ETI as the Hiss Reducer!!

Good idea. What I'm after is a gradual reduction of treble content as the note decays. I could keep the comp envelope in phase, place the inverter after the OTA and have a gradual cancelling of the hiss as the note decays.

[StephenGiles]

The ETI Playmate used a 13600 as a compander, that circuit must be around somewhere on the net. I might even still have the issue (I use the word in it's rightful non problematic sense - ie edition )/

Also didn't the Roctave use the envelope from the 570 compressor to drive the expander?

[StephenGiles]

I found a rough scan of the ETI Playmate on my PC

http://www.4shared.com/file/53888401/416bef85/playmt.html

[Processaurus]

Random idea, rather than compressing the highs, and then expanding them, why not skip both, and just not compress the highs as much as the mids and lows?

[nelson]

Random idea, rather than compressing the highs, and then expanding them, why not skip both, and just not compress the highs as much as the mids and lows?

Screw it, I will add another mid band instead.