Bias output stage via AVC

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

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Joined: Sun Feb 21, 2021 9:02 am

Bias output stage via AVC

Post by avcLover »

Hi All,

I'm thinking of using fixed bias and kill two birds with one stone, volume and bias. Been experimenting with SIT transistors, a simple 2SK79/2SK60 SET.

2SK79 has 2K rp, loaded with 18K, the AVC at the output helps to lower the impedance. 2uf coupling capacitor for the equivalent 25K reactance of the AVC with a 6V battery at the bottom. I think some microampere of current may be present. Will that be an issue for the AVC nickel core?

Here's a mock-up
avc.PNG (49.5 KiB) Viewed 2496 times
dave slagle
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Joined: Sat Jan 22, 2005 3:54 am
Location: NYC

Re: Bias output stage via AVC

Post by dave slagle »


Your circuit will work fine as drawn and the DC offset is not a problem since as you mentioned, the current will be in the µA range. However introducing a switch to change levels to a situation with DC offset and an inductor (AVC) will open up the possibility of some audible transient "pops" every time you change level. Think of a spark plug coil pack. A small 12V DC pulse can create a several thousand volt spike to fire the plug. This same situation can happen on a much smaller scale when you add a DC voltage potential to the autoformer switching.

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