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Old 19th July 2003, 09:49 AM   #1
Gunders is offline Gunders  Norway
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Default How much feedback is too much feedback

well, the subject says it all.
I'm just wondering how much feedback that it's normal to use in tube amplifiers.
I have no sense with how much feedback I should use in my poweramp, of course I could experiment a little bit and see what I find best but I would also like to hear what you have to say about it(I know that a lot of you prefer no feedback at all ).

The first amp I made I used variable feedback, and I could even turn it on and off, and for that amp I often preferred to have little or no feedback at all. I felt that the music became more alive with low amounts of feedback, but for larger amounts of feedback the amp became more "correct".

The only thing I can say I know about the amount of feedback applied is about opamps (and a bit for transistor stages, although not used for audio). Opamps have a large open-loop gain so when resistors is applied the amount of feedback will often be pretty large.
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Old 19th July 2003, 10:03 AM   #2
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Yes, SS amps typically use more than 60dB (dBV I presume.. but no one ever specifies, damnit! ), but they have to cover their high distortion butts with it and get ungodly damping factors for why-I-don't-know...

Triode amps don't need much global because of the internal NFB in triodes. Pentodes need it to reduce distortion and increase D.F. The tradeoff, pentodes vs. triodes, is you get more power and less distortion from pentodes. People seem to prefer distortion for some reason.
In any case, more than 40dB should never be necessary for tubes. Typical is 6-10dB for a triode amp, and 20-30dB for pentode.

Tim

P.S. Frank: what's a penthode?
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Old 19th July 2003, 10:08 AM   #3
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Hi,

A very old rule says 10 dB of “overall” feedback is the limit. And this rule still holds for tube power amps, whether it is a triode or a pent(h)ode amp. Using more feedback does reduce distortion but blows out all the music of your amp, not to mention the risk of instability and peaking at the frequency ends (oscillations, ringing and/or “motorboating” ).

I myself stick without overall feedback for my UL pp amp.

Cheers
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Old 19th July 2003, 10:10 AM   #4
Gunders is offline Gunders  Norway
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Quote:
Originally posted by Sch3mat1c
(dBV I presume.. but no one ever specifies, damnit!
hmm.. I also wondered about that, if the reduction of gain where measured in terms of voltage or power, but I think it's normally voltage.
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Old 19th July 2003, 10:14 AM   #5
Gunders is offline Gunders  Norway
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A feedback of 10dB will give a reduction of gain of about 3 times or so... Thats fits fine with my current plans where I'm going to use about 6 to 10dB feedback
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Old 19th July 2003, 10:30 AM   #6
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Quote:
Originally posted by Gunderz


hmm.. I also wondered about that, if the reduction of gain where measured in terms of voltage or power, but I think it's normally voltage.
When we are talking about gain concerning audio power amps we are always talking about “voltage gain”. For Rf amps “power gain” is more common.
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Old 19th July 2003, 10:56 AM   #7
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This question is too general to make any sense. If you use triodes/triode-strapped penthodes they all sound better (to my ears) with no feedback at all, not even local. If your speakers enjoy being driven by a low DF amp of course.

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peter
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Old 19th July 2003, 11:51 AM   #8
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In general I agree with you Peter, but as always it is largely a matter of taste. The local feedback of the Ultra Linear topology suits me best. It has some lower DF than real triodes, but impedance correction of the speakers with a zobel sweeps out much of the importance of DF. My speakers have a 16 ohms hump at 1.8 kHz, which makes them quite forward sounding with most tube amps. The zobel corrects for this.

I am planning to use the Plytron/Amplimo 4070-CFB OTF’s for a new amp. It uses cathode feedback for the output tubes. Let’s see what this will bring. Maybe you are right and do I gain nothing from it.

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Old 19th July 2003, 12:00 PM   #9
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Cool I'LL GIVE YOU SOME FEEDBACK...

Hi,

Quote:
P.S. Frank: what's a penthode?
Just a stubborn remainder of my linguistic education, Tim.

The noun penthode is derived from the old greek composite words:

Penta + Hodos : Five elements.

Simarly:

Hepthode: Hepta + Hodos : Seven elements.

And so on...

Cheers,
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Old 19th July 2003, 12:33 PM   #10
SY is offline SY  United States
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Peter's answer is pretty close to mine. It depends on the amp and also on how you add up the feedback. For example, do you count cathode degeneration? I've got source followers driving my output tubes, do I count their degeneration? After all, that's feedback.

If you've ever had a chance to listen to some Futterman OTLs, preferably with Quads, you've listened to some of the highest feedback tube amps ever made. Yet I don't hear too many complaints about their sound.

What I personally would avoid is amps with moderately low feedback, say 6-10 dB; that's the point at which you've extended the harmonic spectrum (see, for example, Baxandall's series of articles in Wireless World), but not really knocked down the distortion to super-low levels.
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