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Old 26th April 2013, 12:40 AM   #11
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Sounds like you're pretty far down the road of diminishing returns already, but for grins you might want to also investigate a Zobel or "snubber" on your supply line. I suspect that there's an optimum Q for a particular application, and it's probably lower than usually seen.

All good fortune,
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Old 26th April 2013, 10:14 AM   #12
DF96 is offline DF96  England
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Quote:
Originally Posted by glassandlight
voodoo and heresay
Do you mean 'hearsay' or 'heresy'? Either could apply to the many stories told about caps.

Having a series string of low voltage caps is bound to add unwanted inductance. Much better to have a few parallel high voltage caps. You might find that electrolytics are fine when used properly! Don't worry too much about component brands while your amp has more basic errors in the PSU.
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Old 26th April 2013, 11:05 AM   #13
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Hi Chris and DF96,

both good points...Chris there may well be a Q problem in pentode mode as hz above 6khz or so has a edge resonance as if riding on top or beside the true frequency and mudding it up (it is not there in triode). My first, and current, impression was the power supply was just to small from the get go for 6 output tubes (and it may be) and thus this grungie (but subtle) artifact was there because of that. The amp pulls twice the power plus in pentode as it does in triode.

And DF96, you point also makes sense to this same issue, in that the 'grunge' or lower quality sound I hear is quite dull compared to the triode mode or mixed triode/pentode mode. The higher frequencies seem muted and rolled off a bit and the subtle stage clues vanish, but boy can it play loud!!!



Actually, I have looked at electrolytic cap brands data sheets and find most to be lacking and no set standard as to what they reveal (Vishay is the worst on details). I found that CDE has the best specs especially for ERS and ESL, low DF and good ripple. Most don't list ESL and few list ripple, or leakage. This is the main reason I began looking at the DC LINK MKP types as they have numbers so low as to be near meaningless, 100K life, etc.

I think the reason this amp has 6 caps in series is the year 1995, and the low profile design of the chassis 'forced' the manufacturer into using this string (that and expense of high voltage caps of quality then and their size compared to now).

So DF96, I can go with one 860uf at 630vdc electrolytic, mess with the layout to have it lay sideways to fit, but I fear I will still have most if not all of the same complaints in pentode. If I go to a higher uf (be it one or two in parallel) then the size begins to compete with the MKP caps and I am right back to one major redesign for fitting (anything other than replacing all six with like kind, means remaking or removing the PCB and going point to point).

I had even wondered if some parafeed idea would work to get the power supply out of the signal path altogether.
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Old 26th April 2013, 11:30 AM   #14
DF96 is offline DF96  England
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With no circuit and no measurements it is impossible to comment on how the amp is behaving or how to improve it.
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Old 26th April 2013, 09:21 PM   #15
tomchr is offline tomchr  United States
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Quote:
Originally Posted by glassandlight View Post
I need to sit and chew on it.
Sounds like a plan.

Quote:
Originally Posted by glassandlight View Post
Redesigning a power supply is not an easy task...that is what this is about is essence.
True. It's often at least as complicated as the amp design.

Quote:
Originally Posted by glassandlight View Post
I can not think of any component that has more voodoo and heresay linked to it than caps.
There's a lot of snake oil and folklore (snakelore?) in the audio business, that's for sure. And often I find that the "audiophile" capacitors come with no meaningful datasheets or measurements. You're supposed to take their "word of mouth" advertising at face value. Sorry... I don't work that way. It's very easy to stick a capacitor on an impedance analyzer and get the ESR and ESL. You can calculate the loss tangent from there. Dielectric absorption is relatively straight-forward to measure as well. If the manufacturer isn't willing to do that much, then I'm not going to buy their products.

Once you know the parasitic components of your parts, you can construct an equivalent circuit for your power supply. Throw it at a circuit simulator and see how it behaves. I'd look at the transient response and output impedance as a minimum. The simulator is nice as it allows you to try different components (say an electrolytic cap instead of a polypropylene cap) or model the effects of an umbelical cord by changing a few values on a schematic. To model the effects of wiring, I use an inductance of 1 nH per mm of wire for the ESL and get the resistance from calculations based on the wire gauge. Add, say 100 mOhm for contact resistance and such.

Once you know how the supply behaves depending on the different components chosen, you can try it out to get a correlation between simulation and sound quality. That way you can start predicting sound quality from your supply sims. Or at least have an educated guess rather than relying on trial and error.

I find that a regulated supply is the closest I can get to an ideal voltage source, so that's what I use. The output impedance of my 21st Century Maida Regulator is 50 mOhm or less across the audio band.

~Tom
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Last edited by tomchr; 26th April 2013 at 09:23 PM.
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Old 26th April 2013, 11:11 PM   #16
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Ok Tom....here are the schematics and a photo.

Your website is very interesting, maybe in another project it would be great, I don't know about this one.

Mesa Photos by glassandlight | Photobucket
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Old 27th April 2013, 10:44 AM   #17
DF96 is offline DF96  England
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I'm not surprised you are getting poor sound. That circuit looks like another entry for the 'most creative misuse of a 12AX7' competition.

The phase splitter has far too low anode load resistors. The driver, curiously, has higher anode loads yet is being asked to drive three valves in parallel. Now I am a fan of the 12AX7/ECC83, but even I would not ask one to do that.

The NFB arrangements look unusual (to be polite!). Is this some sort of PA amplifier? It lacks the features which would mark it as either hi-fi or guitar.

Two 12AX7 stages gives too much gain so there is an attenuator between the stages.

No point in putting expensive components into such a poor circuit.
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Old 27th April 2013, 01:05 PM   #18
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My heavens such condemnation, I basically own a door stop then.

One of Fisher's engineers (I believe it was Fisher) said "if it sounds great and measures bad you have won, and if measures great and sound bad you have lost".

Well I can say this much, in triode and even in 1/3 triode 2/3 pentode driving a pair of Soundlab electrostatic speakers, I have had several singing and musican artists that work live at the Met and Kennedy Center say that it was as if they were there 'live' listening to this "PA amp".

It has a problem resolving music in all pentode mode without becoming grainy. That is it in a nut shell. Now some might say you can not serve more than one master, and maybe this amp can not master pentode, and I should forget about pentode and enjoy what it does do in spades.

Feedback is switchable from zero to 6db and has little affect or effect except for stage depth placement. I use zero feedback.

Here ya go, see what Stereophile thought about this PA amp:
Mesa Engineering Baron power amplifier | Stereophile.com
And, here are five pages of component reviews that included this poorly designed amp along with Levinson, Lynn, and some others to test speakers, etc.
http://www.stereophile.com/search/node/mesa%20baron

While Chip Stern gushes, John Atkinson has reservations about pentode as I have...however Mr. Atkinson uses this PA amp right thru 2006 as a reference amp in his collection (including Cary Audio Design CAD-300SEI, dual-mono Mark Levinson No.333, and other high end amps) to judge other components by.

I welcome ideas and suggestions that are constructive and have merit. If you comments and or ideas have such, then print them for others to review and consider as a solution. But throwing rocks does not seem correct for these forums.
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Last edited by glassandlight; 27th April 2013 at 01:14 PM.
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Old 27th April 2013, 08:04 PM   #19
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Well, the fact remains that it's far from the best that could be done with that valve set and general layout. If you post on DIYaudio you'll get honest answers and sometimes no handholding. That's good if you're serious about learning.

All good fortune,
Chris
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Old 27th April 2013, 09:11 PM   #20
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glasandlight this not rock throwing it is a pointed and well stated as well as know short coming of this design. For once I totally agree with DF96 that is 12ax7 does not have the current need to drive 3 pentodes as well as too too much gain in the circuit. A read of classic Mullard 5-20 or other circuits from the 50's will give you a far better amplifier . The pentode triode debate has gone on for 60 plus years. Pentodes are fine if used as a pentode . The eico hf-81 small pentode integrated amp is a vastly better design . For that matter the ampeg has a better front end to the barron .
On this board there is a vast difference of opinion on some things but the basic design and use of tube stages in well documented and to a great extent understood.
Your emotional attachment to some thing you own is understandable . You did not design it you just own it . Sorry if you ask a group of engineers and knowledgeable hobbyist about a circuit as you have you will get honest views of it merits.

Last edited by Triodethom; 27th April 2013 at 09:25 PM.
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