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In Reply to: Re: Low DCR question this possibility for Jeff posted by Dave Davenport on July 8, 2006 at 07:21:41:
Hello Dave,You ask some questions that I will respond to, but maybe not offer you an absolute answer out of my relative inexperience. Also, I am fearful of not representing Dennis accurately. But, please let me try.
- - - - - - - -Q. From memory, you have been recommending 10 Ohms series resistance or less. Is this too low? Does Dennis recommend a lower limit?
- - - - - - - -
A: No, but Dennis likes lower DCR as always sounding better in general. Dennis recommends building the unit as a whole, as a single entity piece, with as SIMPLE a circuit as possible to get the job done with HIGHEST performance as the goal.
With his Serious Stereo 2A3 SE amp, being all-direct coupled, there is a HIGH B+ requirement - to satisfy the final stage, which is elevated in B+ over the first stage. This high B+ requirement allows Dennis some "slack" as far as NOT having super low DCR.
Recall, I recently posted him saying......... "as the B+ gets LOWER, the need for low DCR is more important."
So, in his D.C. amp, you see him using a fairly hefty power trannie, ( but probably not what I would call an "ultra hi fi " power trannie ), and a higher Z single 5U4G rectifier, in combinbation with very low DCR ( "ultra hi fi" ) L1s and L2s . These L's were carefully selected to ALSO be LOW in HY, by ALL conventional thinking and practice. He found this entire approach to work with his amp's design as a WHOLE or an ENTITY, allowing the amp to play unencumbered when reproducing dynamic and complex musical events. He couldn't arrive at this performance level ANY other way - is my honest guess.
Now, lets say you were a choke load fan, in which case you don't have such high B+ requirements. HERE, you need to have low low low DCR in "all" areas of the supply, more so than, say, a 500 VDC supply. You design the choke load amp and its entire supply as a single entity, a WHOLE. What is best for this later amp example, is NOT optimal and best for the initial amp example.
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Q. Does Dennis recommend any apportionment of DC resistance between the power transformer, chokes and rectifiers? So for example, maybe the transformer and chokes should have lowest possible resistance but the rectifiers should not be minimal.
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A. No, not to my knowledge .. on apportionment.
- - - - - - - - -Q. Are you or Dennis concerned about the DC resistance in the primary of the audio output transformer?
- - - - - - - - -
A. Or, maybe the DCR of a plate choke? Well, we are focussing on Power Supplies and not plate chokes, interstage audio transformers, audio output trannies...things of the so called "audio" circuit. Things such as these have numerous design considerations associated with them, of which, DCR is only one criteria. I THINK Dennis pays attention to the DCR criteria in his S.E. 2A3 amp's output trannie, but I betcha' DCR is NOT the only or even the main thing he focussed on in selecting its use. Its a Magnequest BTW.
- - - - - - - - -
Q. I know that Dennis does not like any feedback or capacitors in the signal path. This would imply that he uses fixed bias with the cathode connected to ground. If not, what does he prefer and are you or him concerned with the resistance in the cathode circuit?
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A. When I met Dennis twenty years ago, he used Fixed Bias ( properly engineered !! ) and thought it was superior. Somewheres in the last 10 or 15 years, Dennis has changed his mind and " just loves " cathode bias ( properly engineered. ) He feels that it follows the music better, is apprecaibly more dynamic sounding in his amps, and has a freedom in its presentation.
( There are ALWAYS added problems and complexities in building a suitable fixed bias supply.......... and then lay it "on top of" the sensitive control grid of a triode tube !! Best to be avoided, ) Of course, Dennis uses only the best possible Rks and bypass caps, film caps like DynamiCaps, Hovland, etc. He is adament about self bias now-a-days Dave, from what I can tell !! - My take. ( My original amp mentor, the great Mr. Bob Fulton, also liked self-bias sonically. )
Well Dave, I hope this answered your query OK and we can always hook you up direct if need be. Was fun posting to you.
I was at a retreat in the Ozark Mountains this weekend and in my spare time, ALL I did was stare at my schematic and parts list of my next amp. I never have built an ALL Direct Coupled Push-Pull triode amp and this one will follow Dennis' posted guidelines. I really wanna HEAR it !! ........ So does my old friend Norris!
Jeff Medwin
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Follow Ups
- Re: Low DCR, answer to possibilities.......... - drlowmu 21:07:18 07/09/06 (0)
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