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In Reply to: RE: Innerstage vs R/C coupling. posted by Michael Samra on February 25, 2017 at 10:47:01
Mikey,
Lets talk Class A1 triode amps, for high efficiency speakers.
Think about what the prior stage has to work into, and you can prioritize the three ways of coupling one tube stage to another :
WORST : Cap Couple, tube has to power a cap !!
LESS WORSE : Transformer Couple, not too bad at all
BEST - by FAR : Direct Couple, tube gets connected to next stage grid by a short length of high quality wire.
As far as transferring of the signal, the Direct Couple is many many times superior to ANY Cap Couple, in transferring and driving the next stage, in general terms.
Yes, there are engineering exceptions, but if you have a choice, ( for example, a two stage ONLY triode Class A1 amp ), it is hard to beat a DC !!
Jeff Medwin
Follow Ups:
The problem with direct coupling is that you commonly end up using a cathode bypass cap on the second stage because of the larger cathode resistor.
Because I use DHTs like 26, 01A and 4P1L which can all be used in filament bias with no cathode bypass cap, I prefer using a small teflon coupling cap (or an interstage) to a large cathode bypass cap - however fancy it is.
As said, there's no free lunch. A cathode bypass capacitor is a capacitor, period.
It takes MONEY and brains and listening to execute a proper cathode bypass cap ( a cap bank ). Especially important in SE applications, and in speaker crossovers.Most everyone I meet is unaware. Dennis Fraker, on the other hand, singularly, is not !! He knows what he wants, and gets it, performance wise.
Jeff Medwin
Edits: 02/27/17 02/27/17
I would have to hear an AB comparison with DHTs in filament bias to make my own mind up. Until then there's absolutely no way of telling. I'm not going to believe that any kind of cathode bypass using any kind of combination is superior unless I actually hear it. I tried several combinations of good caps myself and they were all worse, and by a significant margin. I haven't heard your solution but I'm not going to just take your word for it, I'm afraid.
My friend, Dennis Fraker knows how to do all of this, and he's singularly, the best there is, in taste and experience, to implement this. His amps, since 1989,
always out perform all other SETs I hear at shows, on an A-B, so I am pretty sure he knows what he is doing, and knows how to get it done.
It is not universally accepted.
I think I'd really like Dennis and his amps, but can differentiate between my opinion and Truth.
91.
"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein
Of course its my opinion, formed by a lifetime doing audio, and going to RMAF the last nine years -spending much of each three days in the Serious Stereo room.
I know I am experienced enough in audio, and honest enough as a human being, to voice my opinion up here.
Yes, I think you would like Dennis, and his amps, and his GPA 604 MLTL implementation, but Australia is a far way to come from, for an audio show !!
Cheers,
Jeff Medwin
Did not mean to question your experience Jeff - just trying to say different strokes for different folks.
Yeah, we can be pretty isolated down here, for both better and worse...
91.
"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein
I have met God, and when I brought up cathode bypass caps, he gently told me to use fixed bias.
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
But Douglas when you talked to God what did he say about SE vs Push Pull??? I am staying out of these theory debates. My phono stage a lone has 6 interstage trans with some of Dave's lcr riaa inductors.
I just have to suffer with my phase shifted, band limited, shit riaa curve phono stage. Someone has to keep these iron guys in business.
Enjoy the ride
Tom
I don't see any point to delivering His side of that discussion...LOL
I have not built many SE amps, and certainly none for any purpose other than to take their part of an instrument's overall performance( as in geetar amps ).
I'll stick with PP when it matters; I have yet to hear a SE amp worth getting up to turn on, save perhaps low level stuff for the wife in the Kitchen...
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
Then you haven't been listening to the right SE amps.
yeah, sure...I have heard *MANY* SE amps, from *MANY* builders. I'll spare you the List, several frequent this board, and none are religious fanatics...just solid, well respected builders.
And I'll hand that one right back to you, you have not heard the right PP amps.
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
I only have like 6 IT in my 845/211 LV amp. The more IT I add, the better it sounded. It must be the earwax that is masking all the phase shift in my amp while adding the high extensions with the more immediate bass that had my tin ears fooled.
.
.
.Thou shall not stand where I type for I carry a bottle of Certified Audiophile Air and a Pure Silver Whip.
If you were able to hear his amps, then you would understand exactly where he comes from. Theory doesn't matter much, listening results do !!
Jeff Medwin
Jeff met you and Dennis at Denver about 5 years ago. You and Dennis were a pleasure to talk to. The amps were good but magical well you did not have a turntable and quad speakers in the room. I know the quads will not do polka music as well but a tenor sax.
Just having some fun. You pick your speakers 1st. Then you build modular so you can change everything then you listen change listen etc. Etc. Etc. As far as power coating goes I did that on my 510 Datsun.
Enjoy the ride
Tom
Thanks !!
At the 2016 RMAF, Dennis had a turntable, first time. Next year, there will be a turntable and plenty of Polka music, fun stuff to experience on high efficiency rigs.
Right on, "speakers dictate the system." I am so happy to be back to ALTECS, after decades of not having them.
Jeff
Jeff wrote:
"WORST : Cap Couple, tube has to power a cap !!
LESS WORSE : Transformer Couple, not too bad at all
BEST - by FAR : Direct Couple"
consider that when driving an IT your generator (the tube driving the primary) has to also charge the capacitances of the transformer as well as the tube must drive all of the losses (core and copper losses) of the transformer.
Measure the exciting current necessary to drive the IT you have in mind and then compare it to cap coupling (RC or LC).
And in direct coupling if the power supply is in series your front end tube must drive that reactive impedance.
I'm afraid that there is no fee lunch.
The "gods are not kind" to us :=)
MSL
MSL
Builder of MagneQuest & Peerless transformers since 1989
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