What do people think of the SETH 2A3 PP? - Page 2 - diyAudio
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Old 16th May 2007, 01:45 PM   #11
rdf is offline rdf  Canada
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Quote:
Originally posted by Shoog
.... stick with the magnaquests this shouldn't be an issue. I have heard people rave about this particular splitter arrangement. The impression I got was it sounds wonderful - but measures less so.
Shoog
I brought this up in one forum or another, discussing bench test results of common split chokes or unloaded transformer windings used as splitters. The answer came back that Magnequest and such optimize the windings of their splitter autotransformers specifically for this application, compensating for the fact that one winding is effectively unloaded and the other terminated by the driver output impedance. The balance probably still goes awry at ultrasonics, however no common music source (ie. CD) will excite that error.
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Old 16th May 2007, 05:11 PM   #12
kevinkr is offline kevinkr  United States
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The worst flaw is the 2A3 bias circuit. Filament xfmrs aren't designed to pass audio frequencies, and so I'd replace this arrangement with filament balancing resistors and connect the bias RC network at the center point. (See attached)
I have tried both approaches and have not been able to measure any discernible difference in distortion, noise, frequency response or sound quality in a number of amplifiers where I have center tapped filament transformers. The secondary winding inductance is generally quite small and since the audio appears common mode across the winding most of what there is cancels out leaving just the small dcr of the windings to contend with. I prefer omitting unnecessary components where possible, and have done this in a number of 45 SE, 2A3 SE and PP amplifiers with no issues so far.

I have not heard this specific amplifier, but have repaired and modified similar amplifiers using the MQ phase splitting choke, and without exception these amplifiers sounded pretty good.
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Old 16th May 2007, 06:39 PM   #13
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Quote:
Originally posted by arend-jan
If you agree with me that the audio currents are not going through that transformer, then I don't see how fidelity is compromised. Anyway, it should be easy enough to try so I'll give it a spin in my amp.
I didn't say that. There is no such thing as a perfectly balanced OPT. The electrical neutral is never the half-way point since the coefficient of coupling between windings is never 100%. Nor is it likely that each 2A3 used there will be perfectly matched. There will be AF currents flowing in the heater xfmr. If there weren't, then why bypass the cathode resistor in the first place? That bias method is a quick and dirty that I'd use only for modulators, PAs, or anywhere else fidelity wasn't a high priority.

I don't expect anyone else to agree, and I don't particularly care. It's just not how I design.

My perference would be for fixed bias, with a bias control for each final, so that the currents can be brought into better balance.
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Old 16th May 2007, 06:56 PM   #14
Gluca is offline Gluca  Italy
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I built it ... something pretty similar to seth with the same MQ splitter and a LL1660. I preferred the MQ's iron (wound on a nickel core).

My suggestion is to replace the driver stage with a 6H30 SRPP (or a CCS loaded 6H30). You will not be disappointed.

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Old 16th May 2007, 07:27 PM   #15
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The use of a grid choke and the split choke suggest that this amp should behave very well under overdrive conditions - and so should produce more watts than you might anticipate.

Shoog
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Old 16th May 2007, 07:36 PM   #16
kevinkr is offline kevinkr  United States
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Quote:
Originally posted by Miles Prower


<snip>
There will be AF currents flowing in the heater xfmr. If there weren't, then why bypass the cathode resistor in the first place? That bias method is a quick and dirty that I'd use only for modulators, PAs, or anywhere else fidelity wasn't a high priority.
<snip>
My perference would be for fixed bias, with a bias control for each final, so that the currents can be brought into better balance.
I never said there weren't audio currents flowing through the filament transformer, just that the effective winding inductance was close to nil because those audio currents are common mode in the winding. Obviously another (unrelated) issue would be the coupling of noise from the primary into the filament supply - an issue addressed only through the inclusion of common mode chokes, but in practice the leakage inductance in EI types seems adequate in my environment at least to prevent problems - such may not be the case with toroids or in noisier electrical environments.

Also as I indicated I have not been able to measure any difference in performance using either technique. Incidentally on dc I always use the technique you mention, on 2.5Vac I use either depending on whether or not the filament transformer has a tap, and how good the actual winding symmetry is.. (Hum)

I totally agree with you on the fixed bias issue, virtually all of my amplifiers use it, imo nothing worse than a lousy cathode bypass capacitor...
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