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Old 12th January 2018, 06:06 PM   #11
smoking-amp is offline smoking-amp  United States
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Interesting! So that's just using the attenuator pot. for the bottom bank drive?

I wonder if that is very load impedance specific, without an explicit cathode follower bank.
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Old 12th January 2018, 06:17 PM   #12
Koonw is offline Koonw
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Yes, that is AC balance pot for both banks!

The current plot for different load (say 24, 32, 64..) should stay symmetical, this is the main target, that AC balance comes in handy: say you adjust it for 32 Ohms and then change the load to see whether symmetry is still there.

Remember you still have GNFB, it can now better handle the correction when the correction range has been much narrow down to a more manageable level :-).

Last edited by Koonw; 12th January 2018 at 06:19 PM.
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Old 12th January 2018, 07:57 PM   #13
smoking-amp is offline smoking-amp  United States
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Quote:
Originally Posted by Koonw View Post
The latest version of MP mods has out performed even Fig1 and others in the SIM test
Quote:
Originally Posted by Koonw View Post
Remember you still have GNFB, it can now better handle the correction when the correction range has been much narrow down to a more manageable level
OH, I guess I mis-interpreted the earlier remark above. So Fig. 1 was referring to your earlier post on the Sim tests, not the Fig. 1 posted by Banat.

I certainly would expect balanced gains to outperform the original MP design.
But, how does it compare now to the Futterman Fdbk design(s)?

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Tubecad articles, for reference on some earlier posts on OTL:

The Tube CAD Journal: E-Mail, Transcendent OTL amplifier

Circlotron Hybrid OTL: Circlotron Amplifiers Once Again

Last edited by smoking-amp; 12th January 2018 at 08:16 PM.
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Old 12th January 2018, 08:38 PM   #14
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"But, how does it compare now to the Futterman Fdbk design(s)?"

MP/Fut
THD% 0.202/0.227 @44.8V RMS both
Output Z 0.35/0.4 Ohms
My option:
In Futterman amp, the cathode of the phase splitter is fully sitting on output, this is 100% NFB for bottom and 100% PFB for top bank, this can be overkilled, and hence slight inferior performance.
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Old 12th January 2018, 08:44 PM   #15
smoking-amp is offline smoking-amp  United States
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So maybe the Concertina splitter tube is the limiting factor there. A better splitter might do better, like a LTP/CCS. Or more voltage span for the Concertina. Even a small HV Mosfet for the splitter maybe.

Last edited by smoking-amp; 12th January 2018 at 08:48 PM.
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Old 12th January 2018, 08:57 PM   #16
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Yes, I think so. I change to LTP from Concertina sometimes ago, and I think have gotten ride of the "congestion" in the sound. But my most favorate is seperate Class A driver and feedback to each, of course with LTP phase splitter in the front end. This sounds more SET to me, dispite all, that is why you don't really have to worry but just trust your ear. Btw the 2 class A tube is asymmetry, from triode/strapped from 6H12P. This is to minmise any sonic difference due unequal driver level.
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Old 12th January 2018, 08:57 PM   #17
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IMO by Futterman design (Fig.1) power triode V3 grid also gets local NFB via Rk of V1(is that Shade Fb?) dynamically lowering V3 anode gain close as possible to 1, in the same time V1 anode is dynamically boosted up (by bootstrap action) forcing V2 to come close as possible again to gain of 1.
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Old 12th January 2018, 09:19 PM   #18
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Go by the SIM:
Futterman

When it's full output the bottom is 20V, the top is 80, the input is 27. The gain is 20/27(0.74) for bottom and 80/27 (3).

MP mod
Same story, except the top gain is 82.5/27 (3.05)

The output stage has a combined gain of 3, the top would need that because it is cathode output but the bottom is common cathode less gain (0.74) is needed.

Last edited by Koonw; 12th January 2018 at 09:30 PM.
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Old 12th January 2018, 09:37 PM   #19
banat is offline banat  Serbia
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I think your SIM results is pretty accurate
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Old 12th January 2018, 09:51 PM   #20
smoking-amp is offline smoking-amp  United States
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Quote:
Originally Posted by Koonw View Post
But my most favorate is seperate Class A driver and feedback to each, of course with LTP phase splitter in the front end.
Doesn't it need feedback to just one of the class A drivers? Or maybe you are altering the feedback to look like a 50% CFB for each bank?
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