 
|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
|
|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
96.25.74.167
In Reply to: RE: Actually, valve amps PSU's can store a great deal of energy, far more than most SS amps can. posted by morricab on June 27, 2012 at 07:37:16
"So, even with a big tube amp that can swing a huge voltage you can still make it sound not very powerful by poor circuit design choices."
Out of curiosity, have you ever tried just adding a little resistance in series with the speaker? Or using thinner speaker cables?
My power amp is about the purest voltage source I've ever measured and simply was damping the bass more than I liked. I happened to have some of the same speaker cables but with half the wires (Kimber) and found that to be a far more enjoyable compromise. But it may have been a lot worse in a larger room?
Our thinking is roughly that all the magic resonances in the original venue are captured in in the recording so our playback should be essentially anechoic. But experientially we all know that doesn't sound good with just two channels of data so we use some bizarre combination of captured and local resonances.
It seems to me that the only fair way to compare amplifiers would be to try and match their output impedances, at least to some extent, to try and ameliorate this issue which is really mostly a function of the speakers and the room.
Regards, Rick
Follow Ups:
There is also something that happens with feedback amplifiers, which is analogous to resonance if you think about it. It is back EMF getting injected back into the input of the amplifier and then being reamplified. Now, depending on how reactive the speaker is the better or worse the problem will be. I think this is why often a high feedback amp can sound pretty bad with some speakers and not so bad with others.
I have experienced this firsthand with an OTL amp from Transcendent Sound. I built a couple of The Beast monoblocks and then hooked them up to my Acoustats to try them out. Above a couple watts they oscillated like crazy, clearly unstable with the highly reactive load.
"There is also something that happens with feedback amplifiers, which is analogous to resonance if you think about it."
Yea, too right, and that makes them trickier to design and implement. Without proper compensation they can turn on you and, as you found out, become oscillators. And even if they don't get there they can tend to which apparently can have audible effects.
The key in my book is that the underlying amplifier needs enough GBW and input filtering so that it doesn't cause artifacts in-band. Well, not audible artifacts anyhow.
I noticed that the things you focus on in measurements are mostly artifacts of "global" feedback and that you seem to be agin it in general. I wonder how much the artifacts that you are looking for in the measurements directly contribute to the sonic problems that you hear and how much they are just indicators to the probable presence of other things that actually do the dirty deed.
Since you can't usually optimize everything at once in this cruel, real world, it would sure be handy to know what really affects our enjoyment...
Fun hobby.
Rick
"I wonder how much the artifacts that you are looking for in the measurements directly contribute to the sonic problems that you hear and how much they are just indicators to the probable presence of other things that actually do the dirty deed."
Difficult to say because, not having the devices on hand and the tools to make further measurements, I have to look at the evidence available. In that case it can usually be seen if an amp has feedback or not from the THD vs. power and the damping factor vs. frequency plot. Often, you can also see it with the THD vs. frequency plot. Finally, you can often see it in the FFT spectrum as well but there are lots of different looking plots even if an amp doesn't have feedback but usually low order dominates those spectra.
The power supply noise is a potential underlying factor, especially when you see in the FFT spectrum IM products of the power supply harmonics and the signal harmonic distortion. I think that this could effect low level resolution.
I am against it in general simply because the amps that have it don't usually sound good and their meausrements don't usually meet the criteria I have arrived at empirically through listening and then cross-checking the measurements of those I thought sounded good and those that didn't. Also, feedback is often used to lock down something so that it is essentially unmovable or drifts only slightly from a setpoint (like keeping a turntable at EXACTLY 33 1/3rd RPM)and a music signal just isn't like that and as a result some weird effects occur that are often audible. This is why static measurements of fundamentally very non-linear circuits can be made to look amazing (as in the THD wars of the 70s and 80s)with feedback.
Well, I think it's cool that you have been able figure out some of the correlations between measurements and sound!
I've got the tools, but I have a pretty limited supply of amplifiers on hand. However I've found your posts inspiring and now think that I should build a small power amp that sounds bad. How's that for a goal? Something to play with that won't cook too easily and see if I can learn to recognize errors by ear. My sense of it is that folks that have been designing this stuff for a long time can largely do that. You have too but by a tougher route than I want to take!
As far as feedback limiting changes, that shouldn't be an issue if you've scoped the bandwidth adequately. And it's really easy to underestimate it. Most of the casualties of the THD wars were caused by that very thing as no feedback system can accept unlimited inputs without gagging, it's the nature of the finite bandwidth beasts. The invention of the "TIM" concept really helped raise awareness of the underlying problems and how to see them.
I don't care what them whippersnappers say, it helps your perspective to be an older goat!
Regards, Rick
Cool idea to build your own amp and "play" with it. Just make sure you make it stable with 0 feedback first so that you can run the gamut of options on it. Also, if you are skilled also in tube design, do the same thing with a tube amp and/or hybrid. Maybe make a SS amp and then replace the input stage with tubes later on to see how that affects the sound.
If you read the stuff from Nelson Pass, he has been playing like that for a long time and even though his bread and butter is big Class A or AB amps that are somewhat complicated and do use feedback, his pet projects have all been super minimalist and often without feedback. Now, his Firstwatt stuff, which apparently sounds superb but I haven't heard them, is super simple and often with no or little feedback.
Thanks for the encouragement.
I decided long ago that I wasn't going to homebrew power amps because the parts themselves tended to be big and expensive making the commercial ones a good deal not even counting the engineering expertise. But doing small ones are a different matter as are preamps.
When I was a kid I built everything and even though I worked in the summers I had to save almost every penny for college and so couldn't afford decent plate to VC transformers. Transistors didn't exist, at least not useful ones and everyone was well aware how limited tubes were but they were all we had. The folks that could afford the magnetics that had extra windings to allow feedback to the screen were held in awe by us youths and even relatively "normal" people built Heathkits. Really a different era than now. I do find it a bit touching, that some of my favorite tubes then, such as 6SN7's, now have a cult following. However, manufacturers that make products with tubes on little thrones or peering out of wee windows can count me out. Eye candy for the ignorant.
I guess you can tell I don't long for the technology of the "golden era", been there, done that. I think THIS is the golden era! But still and all, if I need to use tubes in the name of science...
I am quite interested in the Pass stuff, I'm fact the power amp I use in the living room is based on his work, I need to revisit his site. For me the "first watt" is pretty close to the last watt as my ears are old and battered enough that I don't do good with loud. I have a small system in the study and one time I measured how much peak voltage I was driving the speakers (Celestion 3's) with at a pleasant level. It turned out to be, in amplifier terms of equivalent watts into 8ohms resistive, 1.5 W peak. I cranked 'er louder than I ever use and hit 5W. So if I use similar speakers I can do a lot of playing without a lot of thermal or power supply hassles.
Regards, Rick
The only Pass amp that I would be really interested in would be the First Watt F4, which is essentially a Class A, no feedback, no gain output stage! Couple that with a top notch tube preamp or tube DAC with a proper volume control (like the Ayon Skylla) and you might very well have killer sound.
Morricab said:
"Difficult to say because, not having the devices on hand and the tools to make further measurements, I have to look at the evidence available. In that case it can usually be seen if an amp has feedback or not from the THD vs. power and the damping factor vs. frequency plot. Often, you can also see it with the THD vs. frequency plot. Finally, you can often see it in the FFT spectrum as well but there are lots of different looking plots even if an amp doesn't have feedback but usually low order dominates those spectra."
The only reliable and conclusive method for determining the level of feedback employed in any design whether feedback is global or intermediate is to measure and compare open loop gain with closed loop gain. Distortion, bandwidth, and power in an amplifier are affected by a multitude of phenomena. Attempting to ascertain the level of feedback employed by looking at power vs. distortion and frequency attributes is pure folly. The only other reliable test for the presence of feedback is whether or not an amp oscillates. If it is properly designed and compensated, that won't be happening so in the real world, the open loop/closed loop measurements are the only conclusive test worth talking about. Everything else is pure blather.
It was you, right, who is using the Crown amp, which is the poster child for the THD wars and bad sounding SS amps. Sorry, I don't take your comments seriously if you honestly think those are good sounding amps. Clearly your ability to hear doesn't match your ability to rationalize based on what your eyes see on a chart. There is a serious disconnect there between observation and hypothesis and you don't even realize it probably because your detector (i.e. your ear/brain) is broken!!
I don't think the Macro Tech Crowns ever were the "poster child" for prior "distortion wars" among manufacturers - that's a bit of a stretch. My amp measures approximately .05% THD - hardly in the same league as earlier Kenwood, Sansui, or Luxman products that did participate in the distortion races of their day. In fact, it's almost exactly the same figure as some Atma-Sphere tube amps.
And let me be clear once again, I never suggested Crown amps should be the end all be all for everybody. They sound fine to me - as in - I have trouble detecting anything other than gain from the amp. Perhaps my ear/brain is broken or more likely, it isn't wired precisely the way yours is. Does that give other people on this forum the right to drag one of my home system amplifiers into a discussion about Magico speakers in a pathetic attempt to discredit me technically? I have several systems spread throughout my home. Singling out one amp and drawing conclusions about me based on the presence of that one amp is pretty shallow and capricious. But this is a public forum and I expect that kind of behavior from at least 25% of the participants.
Its not any worse than you trying to discredit Magico speakers solely on the principle that they were designed as a sealed box.
Actually, knowing what you find to sound satisfactory is very revealing about you as a listener. Sorry if you feel bad about that but you have basically been drawing your own conclusions about you based on the comments you put out there about your preferrences and biases.
My comments about the Magico speakers were related to actual measured performance and facts pertaining to design - not my personal preferences or biases. The drawbacks to sealed designs are well known and documented. And if you need to build a speaker with a 2ohm impedance to meet your performance goals, there is something wrong with your logic and your design.
Unlike you, I try to stick to subject matter that deals with objective fact. Opinions are like rear ends. Everybody has one. And instead of having yours thrust in my face at every opportunity, I prefer to read about facts and real, verifiable experience. None of your posts convey this. They are all purely subjective, opinionated, and frequently incorrect drivel. Your behavior is consistent with what's described here:
http://www.stereophile.com/content/reflections-audiophile-image
"This brings us to the saddest case of all: overly refined audiophiles who brag about their inability to enjoy most records on most hi-fis, and who love more than anything to explain to you why that is the case, thus establishing their own sophistication while making you miserable, in a single supremely elegant and edifying gesture. But every hobby has its extremists, and audiophiles—as far as I know—have never caused themselves or anyone else real harm. Loud, whiny, and annoying is about as badass as we get."
Post a Followup:
| FAQ |
Post a Message! |
Forgot Password? |
|
||||||||||||||
|
||||||||||||||
This post is made possible by the generous support of people like you and our sponsors:
Top of Page ]
[ Contact Us ]
[ Support/Wish List ]
[ Copyright © 2013, Audio Asylum, all rights reserved ]
[ Take5 ]
