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Old 22nd September 2012, 08:05 PM   #891
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890 posts, 45 pages. Wow!



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Old 22nd September 2012, 08:20 PM   #892
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Kenpeter, I thought YvesM had a promising idea, with cathode-follower drivers that were directly coupled to the output tube grids.

Why don't you discuss the pros and cons of that idea?
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Old 23rd September 2012, 02:42 AM   #893
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All pros, no cons.
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Old 23rd September 2012, 02:55 AM   #894
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In #55, I showed how to make impedance equal, and proved
it does nothing useful (except give Mu/2 gains and very low
equal impedance) because you still can't drive grid current...

In #160, Wb showed how to drive grid current without adding
followers, though the unequal impedance is still there in his
circuit, I think....

These two ideas need to be combined and see what happens.
I have a feeling that DC on the plate side of the coupling cap
still wanders, but maybe Mu/2 would keep that in check?

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Old 24th September 2012, 02:34 PM   #895
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Nevermind. Cathode responsible for supplying DC to both output grids
just makes plate voltage pull inward (toward a clipping point at center)
twice as fast.

Combining #55 with #160, only makes both mistakes worse.

A pair of followers still seems the most straightforward answer...
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Old 24th September 2012, 08:01 PM   #896
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I didn't understand how that was supposed to work. I was about to sim it. I still may to better understand it.

I agree about a pair of followers. I thought maybe it would be good for them to be low mu, since this tends to push the grid bias more negative, giving the design more headroom before the followers draw grid current.

Another nice thing about YvesM's circuit is its opportunity for fully differential operation. With a nice current source for the input diff pair, the design should have excellent CM and PSRR. In addition, if the input stage pentode screens are tied together, I suspect that their decoupling cap might not be needed, and if it is, it shouldn't insert a zero/pole pair into the design.

Your thoughts?

By the way, I think there is room here to pursue both discussions on DC bias current-pumping and Cathodyne operation. It's important to set the record straight on the latter, with some still clinging to a model that misleads. (Witness what it did to Dave Slagle's specification.) I'll continue to try to point out problems with the equal-Z model. But that shouldn't stop other discussions.
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Old 24th September 2012, 08:23 PM   #897
Merlinb is offline Merlinb  United Kingdom
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Quote:
Originally Posted by CPaul View Post
With a nice current source for the input diff pair, the design should have excellent CM and PSRR. In addition, if the input stage pentode screens are tied together, I suspect that their decoupling cap might not be needed, and if it is, it shouldn't insert a zero/pole pair into the design.

Your thoughts?
Yes, this was quite a common topology for DC amplifiers- pentode LTP with screens tied together, no need for decoupling, CF drivers, etc.

Various combinations of balanced or push-push feedback were employed.

A constant-current source in the tail was standard practice, although for maximum CMRR the tail needs a finite impedance, not an infinite one, so the first LTP in the circuit may use a simple resistor tail.
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Old 24th September 2012, 08:28 PM   #898
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Why does it need a finite impedance?

Where was your graph with the Rp and Rk values posted? Would you kindly answer my questions about effective impedances?
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Old 24th September 2012, 09:53 PM   #899
Merlinb is offline Merlinb  United Kingdom
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Quote:
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Why does it need a finite impedance?
Because mu is non-linear. I can't remember the algebra off the top of my head, but the result is that you can obtain essentially infinite CMRR from a long-tailed pair, but only when the tail resistance is finite (a few hundred k-ohms usually), which is perhaps counter intuitive. A near-perfect CCS tail gives you perfect AC balance, but only finite CMRR; you can't maximise both at the same time. So in the vintage DC amplifiers you see different sorts of tail depending on what the designer was trying to achieve. Doesn't matter as much for audio.

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Where was your graph with the Rp and Rk values posted?
Post #879. If you can't see an image you should still see a link?

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Would you kindly answer my questions about effective impedances?
They're not REALLY useful. You can use them to find the bandwidth, but only if the capacitive loads are in the same ratio as the resistive loads (e.g., post #786), which is unlikely except for the usual balanced condition.

Last edited by Merlinb; 24th September 2012 at 09:57 PM.
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Old 24th September 2012, 10:06 PM   #900
SY is offline SY  United States
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Quote:
Originally Posted by Merlinb View Post
...unlikely except for the usual balanced condition.
LOL!
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