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In Reply to: RE: Actually... posted by Tre' on July 03, 2012 at 13:57:35
No, it is not CCS loaded, it is resistor loaded, sorry I should have clarified that. I'm not sure the Pimm CCSs will work with such low current (ca. 1ma) but it might be interesting to try.
Follow Ups:
Thanks anyway.It would have been a good test.
The output impedance of that 6sl7 stage is not low enough to fully drive the Miller capacitance of the 2a3.
The result is a small loss of high frequencies and a phase shift that extends well into the audio band.
If you had a CCS loading the 6sl7 plate you could use the mu output and have a drive impedance that would be low enough to fully drive the Miller capacitance of the 2a3. (but you still wouldn't have the necessary drive current so maybe the whole point is moot)
Oh well, maybe next time.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 07/03/12
Tre': You sound like a broken record, saying the same thing over and over. While I don't disagree with what you are saying, I don't believe that such designs are sonic junk because they don't have zero phase shift within the audible range. Have you seen the phase response on loudspeakers? 99.9% have excess phase shift and group delay that are MUCH worse than suspect amplifiers. And yet, many of these speakers sound good. The human ear and hearing mechanism is not the same as an AP or o'scope looking at square waves.
Donald North
I just said it would have been a good test.
Comparing that same circuit with the only change being the Miller fully driven vs. not.
If one likes that circuit they might like it more with the Miller fully driven.
"While I don't disagree with what you are saying, I don't believe that such designs are sonic junk because they don't have zero phase shift within the audible range."
Neither do I but I'm not buying into the opposite claims made so many times we've lost count.
I'm looking at one now.
http://www.audioasylum.com/forums/set/messages/7/71278.html
"Just design your equipment so it does what it's supposed to do-- AND NOTHING ELSE."
Delusions of grandeur when the thing doesn't even overcome Miller?
That's my point, not that the thing is junk and nobody should like the sound of it. Everybody likes different things.
That OK, I just wish Dennis would stop saying "it's the best amp ever made" and "everyone who doesn't understand that from a technical (common sense) point of view has no common sense".
From a technical point of view, the thing is flawed.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
I'm sorry, are Steve and Dennis the same person?
As for the Miller issues and testing, does running the mu-out of a CCS alter current loops versus not using mu-out. Does the load line change with CCS versus R? Personally, I am not sure as I have not investigated CCS in detail.
Point is, reductionist thinking and experiments are helpful only when we use them effectively and draw appropriate (non)conclusions. Even then, they get us only so far before we need to return to other methods of investigation...
Cheers.
“As long as we have any intention to be right… we should be wary. So long as words have the slightest ego attachment, they are dishonest.” Charlotte Joko Beck
"As for the Miller issues and testing, does running the mu-out of a CCS alter current loops versus not using mu-out."Yes.
"Does the load line change with CCS versus R? "
Yes.
Even a poor CCS will represent a very high impedance. Gary Pimms Self Biased CCS represents more than 2 billion ohms mid band.
The grid resistor (if there is one) of the following stage will determine the load line.
In a direct coupled setup, a driver tube loaded with a CCS will operate into a absolute horizontal load line except where the reactance of the Miller capacitance dictates otherwise.
Because of the high impedance of the CCS, the power supply is not part of the (AC) current loop.
If the mu output is used, 1/2 of any current needed to drive the Miller of the following stage will have to come from the power supply in addition to and above the set current that is already being delivered to the tube through the CCS.
This places the power supply back in the (AC) current loop.
This is why Gary uses a 'CCS feed VR tube shunt regulated power supply' in these instances. (as do I)
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 07/05/12 07/05/12 07/05/12
Horses for courses....Miller means squat.
Be careful, don't go out at night, the BOOGIE MAN may get you.
Cheers.
Jeff
Edits: 07/05/12
Jeff, You quoted your own post, above, wherein you first wrote "Miller means squat". That hardly suffices for citing an independent source to corroborate your blanket statement. When you wrote it the first time, I assumed you meant with reference to a Loftin-White topology. Now, when you repeat it here, it seems more another "rule" that cannot be questioned. Can you in any way amplify on your statement? Why does "Miller mean squat", in your opinion?
Lew,
Well, by all means, Miller must be considered in a design. I just like to be dramatic at times, picks up the Forum from the summer time blahs.
But in terms of Loftin White, a half of a 12AX7 will beautifully drive a 2A3 if you design the amp the way Dennis does, as a total design.
I have had several others on this Forum build L-W 2A3 amps, and when they, or their audiophile buddies hear the amps, there is NEVER ever a discussion of Miller. The amps kick butt and they are very pleased.
Dennis is optimizing the first zero to one Watt...where the high efficiency speaker is operating 99.9 percent of the time, and he is not worried at all about how well it drives the finals at three point five watts.
One other thing, I was privy to what the EEs measured as far as frequency response of Dennis' amps at the University, and Lew, it was spectacularly wide band and linear. One can hear that at the RMAF show if one listens carefully, or, in private installations.
IF the amp is fed really good source materials, it responds and is exceedingly honest and neutral. In the last six years, I think Dennis has only had two instances when his digital (barf) source was up to snuff, 2005 when S. Harrell flipped-out in his Six Moons review, and in 2011, third day, Sunday, when the Berkeley and new computer server settled down.
This year, Dennis will bring a good digital source again, Dennis redid the computer's power supplies, and he'll tote his Berkeley to the show. With that, his amps seem to have no limits on good material. By Sunday, its FUN Lew, really FUN. Ask Dave Davenport privately, he took in 2011.
Jeff Medwin
(1) I had a feeling you were playing the gadfly (and it worked).
(2) My query was not implicitly meant to cast a slur on Dennis' amplifier. I have never heard it.
I try to learn from this forum. To do so, I have also to figure out what is hyperbole (e.g., your statement about Miller capacitance), what is just crap, and what it is that I should retain. It's not always easy. Thx for your more tempered explanation.
My own lifelong approach to audio has always been to start with the speakers I like best and work backwards from there. Choosing the speaker first places some constraints on the choice of amplifier. A one-Watt amplifier just will not cut it with my speakers of choice, even though I/we recently made a huge advancement in improving their efficiency via a mod to the crossover which in fact allows one to do away with the crossover entirely. High efficiency is a worthy goal in speaker design, for sure.
The mu out of the CCS cannot provide any more current even though it might be lower impedance so what makes you think it will drive the 2A3 any better?
cheers,
Stephen
It won't, that why I added "(but you still wouldn't have the necessary drive current so maybe the whole point is moot)" in my post to Steve Brown.http://www.audioasylum.com/forums/tubediy/messages/20/208642.html
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 07/04/12
Sorry, should have read better.
And yet Steve prefers such an amp over several that do not 'suffer' such issues. Perhaps the advantages outweigh the disadvantages, even if they are not all quantified (yet).
Interesting.
“As long as we have any intention to be right… we should be wary. So long as words have the slightest ego attachment, they are dishonest.” Charlotte Joko Beck
Then why is a resistor loaded 12AX7 the perfect tube to drive a 2A3?
sorry...
Chris
Because it isn't.
I much prefer the 6GK5 CCS loaded.
A 200H plate choke is better yet.
CB and others prefer the 6GK5.
DanL
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