Vari-mu compressor question - Side chain - diyAudio
Go Back   Home > Forums > Live Sound > Instruments and Amps

Instruments and Amps Everything that makes music, Especially including instrument amps.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 12th August 2011, 07:18 PM   #1
diyAudio Member
 
Brouilly's Avatar
 
Join Date: Mar 2009
Location: London
Default Vari-mu compressor question - Side chain

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
__________________
Broke solderer with delusions of grandeur
  Reply With Quote
Old 13th August 2011, 03:40 AM   #2
diyAudio Member
 
Join Date: Apr 2010
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.
  Reply With Quote
Old 13th August 2011, 07:53 AM   #3
diyAudio Member
 
Brouilly's Avatar
 
Join Date: Mar 2009
Location: London
That certainly clears that up, thanks Printer2.
__________________
Broke solderer with delusions of grandeur
  Reply With Quote
Old 14th August 2011, 04:05 PM   #4
diyAudio Member
 
Brouilly's Avatar
 
Join Date: Mar 2009
Location: London
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?
__________________
Broke solderer with delusions of grandeur
  Reply With Quote
Old 14th August 2011, 09:25 PM   #5
diyAudio Member
 
Brouilly's Avatar
 
Join Date: Mar 2009
Location: London
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
__________________
Broke solderer with delusions of grandeur
  Reply With Quote
Old 14th August 2011, 09:40 PM   #6
Tajzmaj is offline Tajzmaj  Slovenia
diyAudio Member
 
Join Date: Mar 2011
Location: Ljubljana
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
  Reply With Quote
Old 14th August 2011, 09:52 PM   #7
diyAudio Member
 
Brouilly's Avatar
 
Join Date: Mar 2009
Location: London
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
__________________
Broke solderer with delusions of grandeur
  Reply With Quote
Old 14th August 2011, 11:59 PM   #8
diyAudio Member
 
Michael Koster's Avatar
 
Join Date: Jan 2008
Location: Eureka, CA
"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
  Reply With Quote
Old 15th August 2011, 08:06 PM   #9
diyAudio Member
 
Brouilly's Avatar
 
Join Date: Mar 2009
Location: London
Definitely clearer, thanks Michael.

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

Cheers
__________________
Broke solderer with delusions of grandeur
  Reply With Quote
Old 12th November 2012, 10:45 AM   #10
diyAudio Member
 
Brouilly's Avatar
 
Join Date: Mar 2009
Location: London
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
__________________
Broke solderer with delusions of grandeur
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
A New approach of Volume Control: AAVA (Accuphase Analog Vari-gain Amplifier) tiefbassuebertr Analog Line Level 14 4th July 2011 07:26 AM
Newbie audio chain question dasfreak Analog Line Level 1 19th September 2010 10:12 AM
DIY build sound better than a good commercial design in a side-by-side comparison? smellygas Multi-Way 66 25th November 2009 08:26 PM
Any recommendations for tube for vari-mu compressor? leadbelly Tubes / Valves 5 8th April 2009 08:23 PM


New To Site? Need Help?

All times are GMT. The time now is 05:55 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2