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Old 24th October 2007, 06:25 AM   #11
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Hi Miles,,

Quote:
Yes, gNFB should be as low as possible, and I still say 26db(v) is too damned much. In the final analysis, hearing is believing.
This argument only proves you never had the chance to audition a V69a, which employs well in excess of 30dB gNFB and runs the BPT finals in pure class A to boot

In general, I second your opinion, though.

Tom
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Old 24th October 2007, 07:13 AM   #12
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Quote:
Originally posted by ray_moth
Hi Miles,


Actually, not all of those old designs sounded bad. The Cit II used a lot (30dB?) of NFB but didn't sound too bad. Maybe today's speakers call for something else, though?
Of course, the speeks figure into it. I can only use what I got.

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I think the thing is do design for stability at 26dB of NFB, then if it turns out that this much is unnecessary all well and good - reduce NFB to suit your needs. It's better to have it available and not need it than the other way around.
I already do that. It isn't good enough just to have the amp not make like a QRP SW rig. You could also have a rising amplitude characteristic at the high frequencies that'll make for shrillness if your phase margin is borderline. It's definitely something to be avoided, and if you can get stability at high gNFB, then, in all likelyhood, you won't be seeing this at lower gNFB's. There's no reason that I couldn't pour on huge amounts of gNFB. It's that I don't so it because I don't like how it sounds.
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Old 24th October 2007, 05:08 PM   #13
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It seems that my increase to 9dB of global feedback is still fairly low according to general wisdom. With existing tubes this is about as much as I can do and still have enough gain. The next re-wiring will be to try a variant of the Yves/Gingertube partial feedback scheme, and different driver tubes.
This is an excellent idea. I have had great success with this approach, and have heard a Rogers Cadet III which was converted to the Gingertubes schematic - excellent !!!
The great thing with partial feedback and pentodes is that in theory at least they perform better than the same pentode with triode strapping.

This approach seems to bring all of the benefits of gNFB without most of the penalties. Care needs to be taken to make certain that the proportion of feeback doesn't swamp the current through the driver, and there can be issues with high frequency roll off if overdone.

I messed about with gNFB and found all the classic penalties that are often attributed to it were manifested. Of course I only messed about with it so i wouldn't claim to have got the best from global feedback. My current main amp is PP triode class A with zero feedback.
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Old 24th October 2007, 06:12 PM   #14
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Very educational discussion. Does anyone have a link to the Yves/Gingertube scheme handy?
I have an Eico ST70 integrated amp taken apart for a rebuild. Since the OPT are pentode only, everything discussed here are applicable to it. Why bother to restore per original schematic, let's make it better per diy spirit :-)
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Old 24th October 2007, 06:30 PM   #15
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The original one

http://www.dissident-audio.com/PP_ECL86/Page.html

Yves.
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Old 24th October 2007, 07:04 PM   #16
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Remember in most p-p /tetrode /pentode amps using global nfb greater than 30dB, instability trouble often starts at the HF end not low LF with long LS cables. The capacitive layout of the drive circuit plays a big part and when designing a push pull amp, both phase sides should be as symmetrical as possible. Any layout should have the grid (g1) components well away from the H.V anode and screen.

There is another issue. Perhaps more controversial. The case of the Harmon K Citation2, uses inverse feedback within the driver / o/p tubes circuitry and may account to 12dB, the rest 18dB may be global nfb. I'm only quoting an example. The global nfb has the op tranny included to the input tube...a very big difference and may have more effect on sound quality than the inverse nfb. The designer of the Citation 2 might have had 20dB global nfb in mind as the compromise to best sounding with the o/p tranny in the circuit.
Of all the decades I've been on the tube amp ball, the deciding component has the quality of the wretched o/p tranny.

Question;
Does inverse feedback within the tube stage circuitry i.e as per Citation 2 really effect the sound quality ? THD okay less but ->o.m.o... I'm not convinced. Global nfb is far more influencial.
Views welcome.

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Old 24th October 2007, 10:15 PM   #17
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Quote:
Originally posted by richwalters
Question;
Does inverse feedback within the tube stage circuitry i.e as per Citation 2 really effect the sound quality ? THD okay less but ->o.m.o... I'm not convinced. Global nfb is far more influencial.
Views welcome.

richj
Yes it does. Adding local NFB around the finals reduces the effective r(p) which improves woofer damping, and also serves to make the inductive reactance of the OPT primary a bigger percentage of r(p) which also helps the low end. The improved linearity helps to clear up pentode nastiness. It doesn't get the whole job done, of course, since there is still no error correction for OPT nonlinearities, and nonlinearities in the front end. It does make the job of gNFB error correction easier.
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Old 25th October 2007, 06:25 AM   #18
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Quote:
Originally posted by Miles Prower


It doesn't get the whole job done, of course, since there is still no error correction for OPT nonlinearities, and nonlinearities in the front end. It does make the job of gNFB error correction easier.
The way I see it, by using a triode or two in cascade in the front is the best solution regards linearity. THD is higher but that can be dealt with.
Having sorted this out and TRYING to get to the best configured tube amps, we are bogged down by the o/p tranny. Unfortunately what I'm finding by using tube to tube inverse feedback in p-p parallel output pairs there isn't much to be gained as the r(p) which you mentioned is already low enough. There I shall have to leave it as it is.

thanks for reply richj
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Old 25th October 2007, 04:46 PM   #19
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For my tastes, the more the better (or none!) in terms of clearness of sound when the amp is driven below clipping. Of course, more feedback means more OL gain, so more noises is a compromise. Also, stability is an issue.

I am happy with 50W / channel amps with deep feedbacks driving very sensitive line arrays; they never approach clipping.

Usually amps with 6BQ5 output are not powerful enough so used with soft clipping to compress dynamic range, in such case a negative feedback causes harshness on clipping, and especially on recovery from clipping.

Conclusion: weak amps for loud sounds should contain no global feedback at all.
Powerful amps should contain as more as you can afford (stability and noises).
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