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Old 4th February 2009, 08:49 PM   #31
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Hi Richy,

"DePalma's Audio Amplifier design of the symmetrical phasesplitter diff driver ...."

You mean that Circuit 1 Vacuum Tube amp, dated 1-16-97?
http://depalma.pair.com/Analog/analog.html

Looks to me like each top 6SN7 driver acts as a follower and the bottom one uses its Mu gain, with the splitter pickoffs providing coordinated grid signal amplitudes.

I'm not sure how you plan to put a pentode in there, but one idea would be to put P channel Mosfets for the top tubes (with adjusted - smaller - gate drives) and the pentodes for the bottom driver tubes. (forming complementary matched current drives on each side) Then crossed output plate feedbacks to the screen grids. This would end up as two hi-Z/hi-gain drives controlling the output tube grids, with the plate to driver screen feedback resistors setting the effective gain and effective drive Z.

That configuration requires crossed feedbacks though, which makes me uncomfortable about leaving class A operation. (the feedback signals having to go thru the xfmr then) That could be fixed though by using a CT'd CFB winding on the xfmr. for the feedback pickoffs instead of the plates. The extra output grid drive voltage required should be a breeze for this driver configuration.

Don
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Old 4th February 2009, 09:05 PM   #32
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Hi Jaap,

Say, that Scott Reynold's headphone amp sure looks similar to Bigwill's first post. The feedback scheme seems more necessary for output pentodes than triodes though. As has been mentioned a few times, one can save a tube(s) easily by just routing the feedbacks to the driver cathodes instead of a full diffl. pair for each side.

Don
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Old 5th February 2009, 03:45 PM   #33
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On second thought, my idea in post 33 above (for DePalma circuit mod) isn't working too well, since the screen feedbacks only control the bottom pentode tubes (and not the P Mosfets). The feedbacks would need to go all the way back to the splitter to work that way.

I guess the top driver tubes could be kept as N-followers, and the pentodes (bottom driver tubes) with screen feedbacks could be gain matched to them. Maybe thats what Richy had in mind?

Don
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Old 5th February 2009, 10:31 PM   #34
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OK, back to my idea in post #33 again. The P-channel Mosfets are now set up so as to operate in "near CCS" mode. The drives (Ra, Rd pots) to the Mosfets are adjusted so that the lower driver tubes (V1, V2) continue to operate with constant current WITH their actual loads attached (which would presumeably be some output tube grids and their biasing resistors).

This is slightly different than a CCS load, since the P-chans are adjusted to actually provide the output current so that the drivers still operate at constant current despite the load. The drivers (V1, V2) are low impedance outputs though, so they maintain control of the output voltages.

This allows the driver tubes to operate in CCS constant Mu mode, even with load currents. So this is an improvement over ordinary CCS loading.

I have drawn triode drivers for V1, V2 in the diagram for simplicity, but these could just as well be pentodes with screen feedbacks from the output plates (crossed feedbacks).

The interesting unknown here is what happens if the P-chans are adjusted for a little more current drive than the output requires. Then the V1,V2 drivers would begin to operate with a little less current when pulling down the load.

Since tube distortion increases with more loading generally, this would now provide anti-loading (or neg. resistance loading) for the driver tubes. Maybe one could adjust out some residual distortion from the V1,V2 tubes that way (providing they don't oscillate).

Don
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Old 6th February 2009, 03:03 PM   #35
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Don, interesting info. I've tried the existing DePalma circuit 1 and find the adjustments far to snappy and the big problem is swapping tubes (old habit of mine) all goes haywire/out of trim.
My first instinct is to improve the exisiting circuit and re-examine the levels and avoid the wretched balance adj potty (preset). I have my doubts if the 6SN7 is the optimum tube for the job, but I'm currently hacking at the math.
You mention the CGS an active load. This should give thd reduction advantage even as the impedance at diff cathode junctions is already low and forcing into constant mu mode should nicely linearise the lot. Sounds too easy.. My other intention is to provide some regeneration feedback from cathode tail to grids. All this takes time and some refreshment reading up.
As I see it, the 6SN7 spec falls sim to alot of other tubes but >I'm after an even lower mu. I'm working on the assumtion of the concertina is capable of 30+30V rms on each 1/2drive. I don't need alot of extra gain and may even go for ECC88 types (using parallel sections) as to keep B+ within limits. It should be easily possible to provide 70+70V Rms from this circuit using alot less than 400V B+.
I've got alot of ECL82's, the pentode strapped in triode mode is another alternative.
Someone mentioned class B ? we'll run into this another time.


richy
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Old 6th February 2009, 03:29 PM   #36
Jaap is offline Jaap  Netherlands
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perhaps it is possible to make a nice small amp with this idea if you use ecc40 concertina => ecl82 triode/mosfet => ecl82 pentode (?)
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Old 6th February 2009, 05:33 PM   #37
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Quote:
Originally posted by Jaap
perhaps it is possible to make a nice small amp with this idea if you use ecc40 concertina => ecl82 triode/mosfet => ecl82 pentode (?)
I think with small amps there's no advantage. The o/p stage Miller effects are too small to dump. Circuit 1 with the right tubes should be able to be optimised without tweaking.

Since I'm in the BIG game 150W++ with parallel o/p pairs; Circuit 1 made me bash my skull against the wall why it took me solong to get coined with that split winding UL transformer technique. There's alot going for it although the winding is more complicated. The reason why; snubbers are minimised. And thats a BIG plus as the heat goes upwards.
Incidentally as the subject is about diff drive, I also had another look at the E.F.Worthen (modified New Isodyne). On the bench I wasn't impressed and the article does mention tube selection of the fittest.

richy
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