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Old 12th November 2012, 10:55 AM   #11
DF96 is offline DF96  England
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If there is an overlap between the lowpass filter action of the AGC chain and the highpass filter action of the main signal path then you have an unstable loop. If you want a quick response then you have to exclude low bass from affecting the compression.
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Old 12th November 2012, 11:40 AM   #12
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There must be an overlap, at fast attack times it is very unstable. So I should address that before moving on.

It does perform very well apart from that, but it would be a much better comp if it was a bit quicker. I assume there is a way to do this.... But then a vari-mu will never be as responsive as a optical I guess.
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Old 12th November 2012, 11:52 AM   #13
DF96 is offline DF96  England
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Vari-mu valves operate quite well up to 1GHz so that is not the issue. The issue is loop design and passive component values. You can't have a quick response on bass frequencies, except perhaps by using a signal delay so that causality ceases to be a constraint.

Fast servo loops are difficult to get right.
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Old 12th November 2012, 04:57 PM   #14
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It is not uncommon for the combination of fast attack/release times and low bass notes to cause instability. The reason is simply that the with fast attack/release times, the gain control voltage will actually follow the waveform of low bass notes and causes them to be flattened. It is equally common to include a high pass filter in the side chain to prevent this happening.

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Old 13th November 2012, 07:51 AM   #15
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Good to hear from you Ian.

I guess it depends on how much the instability is a problem, and if actually it is just part and parcel of the vari-mu design and sound. I will aim for a good balance between stability and fast attack/release - I thought it was worth asking as there might be another approach to get quick settings and stable bass.

I have a high pass filter at 50,75,120,250,300 which I found to be the most useful frequencies and the unit has no stability problems with that in. I am also finding that the release is a little bit sluggish in mid settings, so I imagine some experimentation with the discharge time of that cap will give better results.

Thanks, learning lots.

Charlie
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Old 13th November 2012, 10:04 AM   #16
DF96 is offline DF96  England
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You can get the best response by ensuring that the loop design is excellent, with a dominant pole and other poles much higher in frequency. The temptation is always to add extra smoothing so there is no audio on the control voltage. This adds phase shift and so destabilises the loop.

You could try reading up on 'hang AGC', as used in some communications receivers. Receiver AGC and audio compressors are doing almost the same thing, so techniques used in one can sometimes be applied to the other.
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Old 13th November 2012, 10:12 AM   #17
disco is offline disco  Netherlands
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Last year I build this compressor and experienced similar distortions on faster settings (smaller value for the .22uF).

Do I get this right:
P1 ?= amplification level
CW ?= quantity of compression
C4 ?= both attack and release times
M1 ?= indicates AC level
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Old 13th November 2012, 10:37 AM   #18
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P1 is gain
P2 release
P3 is threshold
R12 is attack, but I have a different arrangement here.
M1 is gain reduction shown as current through input valve.

Thanks DF96, I will look into hang AGC. Better get the calculator out and make sure the poles are where they should be.
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Old 13th November 2012, 11:25 AM   #19
disco is offline disco  Netherlands
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Of course, CW= clock wise

What iron did you use Charlie?
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Old 13th November 2012, 11:41 AM   #20
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That's an interesting thread, thanks for posting.

I have some 9:1 Sowter transformers I am using with 5965. But have just ordered some LL1689 from Per, really looking forward to hearing the difference between the two!
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