23 step AVC

discussion of magnetic volume control design for both line and speaker levels.

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bigfunhorns
Posts: 10
Joined: Thu Aug 23, 2012 3:33 am
Location: New Zealand

23 step AVC

Post by bigfunhorns »

I've been using a pair of the 12 step, PCB mounted, AVCs with the added 3 position +/- switches for some years now. I recently changed my speakers from a pair of (100 db/1 watt) Lowther Fidelios to a pair of, back-horn-loaded, (105 db/1 watt!) Lowther TP1s with Lowther PM5A drivers. My lovely Yamamoto A010 uses Pentode wired C3Ms as input/driver tubes, R/C coupled to VT52 DHT power tubes. The tiny, less than 1 mV noise, on the A010 speaker terminals, that didn't used to be audible, now produces a quiet hum through the TP1s. Lots of experimentation with adding caps and inductors gave no improvement. Finally I have a 'fix'. I've connected My, inky-black quiet, Tom Evans Mastergroove directly to the A010 input. I have the 'Slagle' AVCs wired between the C3M coupling cap and the grids of the VT52s!! The coupling cap blocks the B+ and I have the 'out' terminal, of each AVC connected to the VT52 grids. 100 k ohm 'grid leak' resistors minimise 'thumps' when changing volume. The noise is greatly reduced and now reduces even more as the volume is reduced. Is this the best AVC to use in this position? might a different design or value of AVC work better? Suggestions please.
dave slagle
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Joined: Sat Jan 22, 2005 3:54 am
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Post by dave slagle »

Hello,

All of the AVC's have the same basic geometry and sound and the only thing that chances between models is the taper and switch type. The only real possible "upgrade" is to go with a silver wire version

dave
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