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    Building, troubleshooting and testing of these amplifiers should only be
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Active Volume Control in tube preamp?

poolorpond

Member
2005-08-01 7:12 pm
I have seen only a few instances of global feedback as the sole method of volume control for a tube amp. Any reason it isn't used more often? What about local feedback?

I know that feedack is used to tune the frequency response in some amp, but care needs to be taken with poles, phase, etc- none of which I could really account for in any of my projects. Anything I did would need to be tuned by ear, which might be OK.

FYI- I have a post about an "ARC" tube preamp if you are interested in where I'm going with this.
 
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poolorpond

Member
2005-08-01 7:12 pm
I only need attenuation.

Yes, but all I need is attenuation. Distortion etc, will only get better as I turn the volume down. Even as a buffer, there is too much gain from my sources. That is the issue with any active device added when it isn't needed. I do need attenuation, with robust drive current, however. I could reduce the gain of all of my sources. I would rather do this.

Assuming the circuit in my other post, which I will repost here, could I use local feedback from the first tstage effectively?
 

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poolorpond

Member
2005-08-01 7:12 pm
I have a valve amp where I chose a pot for the feedback resistor to experiment with feedback. Didn't bother with the cap, however. So, I don't really know what is happening to phase, frequency, etc. I'll need to read up on it. I added a switch to lift it as well for zero feedback. Second-order harmonics are obvious. Too much doesn't sound clean enough for HIfi, IMHO. I could actually live with zero feedback on this amp if I didn't know any better, but there is obviously much less distortion with feedback. A/B comparisons make feedback an obvious choice in this case. I'll need to try it in the pre.
 
Yes, you could apply feedback to that circuit.

I would be more inclined to replace the 600:150 input transformers on that one input with 15K:150 transformers to give you a lower sensitivity input.

This is a relatively low gain preamp, my suspicions would be that you have a big solid state power amp or some kind of vintage tube amp (or copy) with sensitive speakers, and this is the real root of the issue.
 
This circuit has absurdly high gain for a line stage. Choose a lower mu valve, a step-down transformer or just burn the excess gain with a resistor divider as ARC do.

A volume control which provides a different sound character for different gain settings appears less than ideal.
 
This circuit has absurdly high gain for a line stage. Choose a lower mu valve, a step-down transformer or just burn the excess gain with a resistor divider as ARC do.
How do you figure?

I see a differential 12AU7 feeding a cathode follower. That's about the lowest mu double triode available other than some of the high gm regulator tubes. There's even a transformer coupled input with some step-down built in.
 

egellings

Member
2010-08-21 7:59 pm
CA
One problem that might occur in using the feedback resistor as a volume control is that the feedback resistor usually has a phase compensation capacitor of small value across it, and changing the resistor to change the volume would mean that this capacitor value needs to change too in order to optimize feedback signal. Going too low in the volume setting may also destabilize the amplifier because of too much feedback.