Vari-mu compressor question - Side chain

[CSlee]

Looking for some clarity in quite a specific topic. Im sure there is someone here who has delved into vari-mu compressors....?

So as I understand it, a vari-mu compressor works very basically as follows...

The signal of the final stage is decoupled through a cap and then rectified to a DC voltage. This is then sent to the input valve's grid, where it will make the grid more or less negative to change the mu of the valve, and therefore compressing the signal.

And there will be circuitry for threshold, attack etc...

My question regards a side chain bass cut.

I was playing around with a vari-mu compressor the other day and it had a bass cut switch. Where you can change the decoupling cap value of the side chain, which acts as a high pass filter. This filter cuts bass frequencies from passing to the rectifier, so audibly you dont compress these bass frequencies and compress the higher frequencies.

This was quite a sublte effect, but noticeable. I was told it was to make the mix less muddy etc etc...

So, I can not understand why changing the value of this cap would give this effect. If the signal is being rectified to DC, and sent to the grid of the input valve how can it know what frequencies to compress?

DC isnt frequency relative right?

Hope someone can clear this up.

Cheers

Charlie

[Printer2]

If you have a lot of bass energy the rectification and subsequent DC level will be greater so you would get greater compression. Filtering it out would cause a lower DC voltage and therefor less compression. The compressor is not frequency specific, it just adds less compression which may make it sound less muddy to some.

[CSlee]

That certainly clears that up, thanks Printer2.

[CSlee]

Actually no, this doesnt clear up my confusion.

This suggests that the high pass filter on the signal when recitifed only produces a different DC voltage, so the input valve compresses less.

So all frequencies are still being compressed, just at a different rate. So it is effectively just another type of threshold control.

If you were compressing a track with a wide frequencie spectrum, this side chain bass cut would specifically allow bass frequencies to not be compressed would it?

[CSlee]

Appalling English sorry, my question is..

Is this 'side chain bass cut' actually allowing bass frequencies (set by the coupling cap)to not be compressed? And if so, how is it doing it?

How does a change in grid voltage at the input valve compress certain frequencies?

Cheers

[Tajzmaj]

Side chain has it's input Z which determinate cut off frequency of hi pass filter.......because it is relatively high there is small value of coupling cap.....
It is similar to tweeter and it's coupling cap.
Regards, Taj

[CSlee]

Sorry I may have not been clear in my question.

By side chain I mean the circuitry that decouples, and rectifies the signal, to then give a DC voltage at the input valve. Different from a side chain input where an external signal can be used to trigger compression.

Cheers

[Michael Koster]

"How does a change in grid voltage at the input valve compress certain frequencies?"

It doesn't. Can't.

The gain element, in this case a remote cutoff vacuum tube, is not frequency selective. It compresses all frequencies equally, depending on the DC or impulse signal from the sidechain.

Filtering the sidechain determines which frequencies the compressor will act on if present in the signal, but the entire signal will be compressed as a result.

Think about a de-esser which compresses the signal when there is excess energy in the sibilance band from 5 KHz to 7 KHz. It compresses all frequencies equally but since the "ssss" is all that's likely to be present in a vocal signal at that time, it works well to remove only the "ssss".

The bass filter prevents the compressor from responding to bass energy, which I believe is more useful in the context of a mix bus compressor, when one wants to avoid the "ducking" effect where the whole mix is reduced in volume when loud bass or bass drum notes are played.

I hope this is clearer.

Cheers,

Michael

[CSlee]

Definitely clearer, thanks Michael.

I just found it very confusing from reading blurb from manifacturers to actually understand the science.

Cheers

[CSlee]

To save starting a new thread I thought I would post my question here.

I have built a vari-mu compressor, very similar to the Altec 436C. http://www.dvq.com/hifi/images/436c.pdf

Everything is working well, however I am having a stability issue when using it at quick release settings. The circuit goes unstable at low frequenices with quick release settings - release being the discharge time of C4 I believe. Instability is cured if I use a bigger cap here, but then I have a very slow release so that is not the solution I can use.

I was hoping someone could give any suggestions of has any experience in improving stability. I have read about curing instability when using gloal feedback and 3 LF time constants, and have followed those guidelines but this is something a bit different... as usual a little bit stumped.

Thanks

Charlie