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Old 27th April 2005, 03:31 AM   #101
dimitri is offline dimitri  United States
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Iíve read another thread and got your point, Greg.
But I think OPA637 and three pairs of output FETS, er, slightly expensive.
Had you used stacked supply for diff pairs and VAS?
Any buffer between VAS and OS?
No cascode in diff stage, yes?
Keep it simple and let NFB to do the job?
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Old 27th April 2005, 04:30 AM   #102
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Hi Dimitri,

Yes I guess a bit hard on the wallet. I'm not sure what you're referring to with OPA637 and 3 pairs of output FETS - a 200W+ power amp?

Had you used stacked supply for diff pairs and VAS?

???

Any buffer between VAS and OS?

No

No cascode in diff stage, yes?

No

Keep it simple and let NFB to do the job?

absolutely
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Old 28th April 2005, 12:15 PM   #103
dimitri is offline dimitri  United States
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Quote:
Had you used stacked supply for diff pairs and VAS? ???
I mean higher voltage clean rails for diff pairs and VAS to achieve full output swing.
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Old 28th April 2005, 02:42 PM   #104
ilimzn is offline ilimzn  Croatia
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Regarding sound quality and distortion figures...

I would tend to agree that the THD figure (as in x%) does not correlate well with percieved sound quality. It is after all based on excess harmonics produced out of one sinewave stimulus, but this holds true for standard published figures. Let me give an example:
0.1% THD at 1kHz 1W sets one single point in a multidimensional continuum of sorts - if you compare percieved sound quality from many amps where measurement gives you that same figure, you will get amps that differ like night and day. That ONE single point gives you little usable data. That being said, it is quite likely that an amp with 100 times less THD under the same conditions, may sound better. And this is really all one can say with any accuracy if one is fixated to one single point of measurement.

Things however change drastically once you start showing distortion spectra, preferably in relation to output power an/or frequency - not to mention intermodulation sweeps etc - correlations between measurements and perception become more evident, though not obvious or absolute. As one poster mentioned, if you cannot measure a difference between amps that sound different, then either your hearing is deceiving you, or more likely, you are using the wrong instruments to measure the wrong things. When this leads to avoiding to admit ones own ignorance by dispensing the dogma that all measurement (or for that matter all listening) is worthess, well... it's called the height of arrogance, to put it mildly.

Still, determining one-point-of-measurement THD from listening tests is as futile as going the other way around. Music is a vector sum of, for all intents and purposes, infinite sinewaves, all of which generate different spectra of harmonics - some of which may reinforce, and some of whic may cancel - and that's excluding IM and TIM effects. Just like measurements need to be speciffically targetted to reveal a problem, so muct material in listeming tests. But unless you find correlation between the two, you are flying blind. Designing amplifiers is as much an art as it is science, but one cannot function without the other in this endevour - overemphasizing one over the other is a path to at best mediocre products if not downright abominations.
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Old 28th April 2005, 03:35 PM   #105
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ilimzn: I think that is a very good way of putting it. This is an ancient discussion, between the theoretic and 'artform' school of amplifier designing. It must exist in 100's of threads here on diyaudio.com

I was just trying to place 'guru' in either of the two schools.
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Old 28th April 2005, 06:04 PM   #106
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some things that are related to the thread...

I don't think so that PCB lay outs can affect so much on sound
quality, for as long as you are not designing a PCB layout
that is a 100 square in. in area

this will only be very true when you are designing amps
for the Radio Frequency spectrum as such that you would always see to it that you are always keeping the standing wave raito low.

most audio hobby kit websites put this to emphassy that one contribution
to a good sounding amp is a better PCB layout. "their" PCB
lay out. very commercial......




best regards,

hienrich
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Old 28th April 2005, 06:35 PM   #107
ilimzn is offline ilimzn  Croatia
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Quote:
Originally posted by hienrich

I don't think so that PCB lay outs can affect so much on sound quality, for as long as you are not designing a PCB layout that is a 100 square in. in area
this will only be very true when you are designing amps
for the Radio Frequency spectrum as such that you would always see to it that you are always keeping the standing wave raito low.
most audio hobby kit websites put this to emphassy that one contribution to a good sounding amp is a better PCB layout. "their" PCB lay out. very commercial......


best regards,

hienrich
Heinrich, you would probably be surprised how wrong your supposition that PCB layout does not affect sound is. I have seen some quite ingenious PCBs as well as others that could be mercifully described as 'attempts' - both on the DIY and commercial side of the fence, including makers of kits. If there is one area where design resembles artform,. that would be PCB design. Improper routing of power and ground lines, or even signal lines in high impedance circuits can reduce even the best topology to literally an interference generator. That being said, once you get to a certain level of competence, as with anything, there is a law of diminishing returns at work. Still, when you are aiming at superlatives, all stops need to be traken out, and that includes VERY careful PCB design.
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Old 28th April 2005, 07:51 PM   #108
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Quote:
Originally posted by ilimzn
Heinrich, you would probably be surprised how wrong your supposition that PCB layout does not affect sound is.
I would go so far as to say that the PCB is an integral part of
the design, and I can introduce or cancel all sorts of effects
through design alone.

As exhibit A, I point to the classic HK Citation 12 which had a
a trace routing which by itself introduced a 2nd harmonic. That
part of the circuit board was altered on the Mosfet Citation 12.
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Old 28th April 2005, 08:44 PM   #109
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We don't doubt about your knowledges, Nelson. But I many time asked personaly, why are you making amps, which have not so low distortion.
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Old 28th April 2005, 09:57 PM   #110
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And I reply again: I like very simple amps with low
feedback.

Your statement presumes that I have not or do not
offer amplifiers with low distortion figures. Much
most of what I do is well less than .1% at ordinary levels
and frequencies and some of it is less than .01%.
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