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Old 13th November 2012, 11:53 AM   #21
disco is offline disco  Netherlands
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Yep, just found it looking for the iron involved. Seems more people are in to this (Dave, that's not your potato we're looking at?). It might be interesting to study the Collins 26J and reported refinements called 'EMI mods'.
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Old 28th November 2012, 12:25 PM   #22
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Stability was pretty good, even at very fast attack and release settings. The next stage in the build was to add Salas regs - as they worked so well in my #26 pre I thought they can only benefit here.

Unfortunately stability is much worse with the boards in! I thought if anything they would improve stability, but i am getting lots of oscillation at quick attack/release settings now. Now, I do not know why this would be the case - is there a technical explaination for this? Or maybe I have changed something else without realising.

I think the best step is to rebuild the old RC filter supply to see if I get back to where I started. If I do, I guess there is a problem with using the board, or I have implimented them wrong.

The circuit is fine and sounding great with the Salas boards and with no compression...

Thanks for any insight, as usual.
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Old 29th November 2012, 01:32 AM   #23
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Old thread, so I do not know if someone will see it. I am posting because answers are pretty much off here.

The instability you talk about is surely the famous thump effect. In varimu compressors there will always be some degree of thump. Designers struggle to minimize it. You simply need to match the tubes in the vari stage as perfect as possible in the entire compression curve. Even this is hardly enought, so you use trimmers on the plate and possibly cathode resistorsI in the vari stage and maybe the output stage also, for even better results. Then trim it for lower thump. If it doen't work, you need another pair of tubes. Paralleling tubes makes it easier, as in the 670. Then use at least 1k as the lower attack time resistor, as it should be low enogh for any transient limiting, and adjust all coupling caps and audio transformers in the unit for 20-30Hz low frequency cut. Oh, interstage transformers helps, as these have near infinit CMRR, and if they are geometrically balanced, even better! There is also some more near-voodoo secrets In the cathode vari mu stage, but are not really necessary.

All that together makes the perfect clean vari mu comp.

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Old 29th November 2012, 01:34 AM   #24
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This old thread belongs over in Instruments & Amps so I'll move it over there.
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Old 29th November 2012, 06:28 AM   #25
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Quote:
Originally Posted by rafafredd View Post
Old thread, so I do not know if someone will see it. I am posting because answers are pretty much off here.

The instability you talk about is surely the famous thump effect. In varimu compressors there will always be some degree of thump. Designers struggle to minimize it. You simply need to match the tubes in the vari stage as perfect as possible in the entire compression curve. Even this is hardly enought, so you use trimmers on the plate and possibly cathode resistorsI in the vari stage and maybe the output stage also, for even better results. Then trim it for lower thump. If it doen't work, you need another pair of tubes. Paralleling tubes makes it easier, as in the 670. Then use at least 1k as the lower attack time resistor, as it should be low enogh for any transient limiting, and adjust all coupling caps and audio transformers in the unit for 20-30Hz low frequency cut. Oh, interstage transformers helps, as these have near infinit CMRR, and if they are geometrically balanced, even better! There is also some more near-voodoo secrets In the cathode vari mu stage, but are not really necessary.

All that together makes the perfect clean vari mu comp.
Thanks for your comment. I found the extreme instability odd because it is exactly the same in the second channel. I have a balancing circuit in the cathodes of the input valve which usually helps a fair amount but here it does nothing at all.

There is quite a difference in voltages between halves so I will try using different valves. Thanks for the suggestion.

Charlie
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Old 14th February 2013, 08:26 PM   #26
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A little late to this one but however. I have a Vari Mu here in the studio. I have been thinking about doing a side chain mod as it has no side chain at the moment. An external side chain would be what I am looking to put in. Am not looking to put in anything crazy just something clean and SC to an EQ. Is there a schematic that can put me on the right path for an external SC. I wouldn't be installing it myself but an E.E.

Any thoughts?Is this possible?

Any help would be great.

Best
Peter
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