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Old 26th December 2009, 04:50 PM   #281
GK is offline GK  Australia
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Quote:
Originally Posted by Bob Cordell View Post
Hi Glen,

I don't recall Self achieving 0.01% THD-20 with MJ802 devices (which I also don't think were 4 MHz ft). I'll have to go review what he did and what he achieved.

Is suspect if he did it, it was with his beloved CFP output stage, which is very picky with quiescent bias, and in which he actually starves the output transistors to achieve proper crossover.

In any case, I agree that a good design with 21193/4 devices can achieve 0.01% THD-20.

A generously-biased MOSFET amp can also achieve that number without EC, but it will have somewhat higher idle dissipation. Two pair of vertical MOSFETs, each pair biased at 150-200 mA, combined with a 1 MHz gain crossover frequency, will do it. Such a design will not suffer nearly as much trouble with thermal stability and distortions due to junction temperature variations inflicted by program material.

Cheers,
Bob

I'm almost certain that the 0.01% amp was with those old MJ's but I'd have to look it up (all I have on hand ATM is the THD plot). Can't remember if it was a CFP OPS or not. However it's after 4am ATM, so I'm going to bed now instead.

Regarding the MOSFET comparison, my claim is for a pair. If I recall correctly your EC amp did in the vicinity of 0.02% - 0.030% THD20 without the EC? That was with a 2MHz ULGF, right? The EC was more effective than 24.4dB? (0.01%/0.0006%)

What was that biased at?

Cheers,
Glen
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Old 26th December 2009, 05:01 PM   #282
Mooly is offline Mooly  United Kingdom
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I think Dougs figures were well below 0.01% and nearer 0.003% ? at 20khz with the MJ's
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Old 26th December 2009, 05:25 PM   #283
wahab is offline wahab  Algeria
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Originally Posted by Mooly View Post
I think Dougs figures were well below 0.01% and nearer 0.003% ? at 20khz with the MJ's

yes, with a MJ802/3502 pair...
0.004% at 20 KHZ and 0.001% in midrange...
truly very low values...
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Old 26th December 2009, 05:29 PM   #284
PMA is offline PMA  Europe
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Take tons of feedback and you are there. How worthful? Not sure that the THD race has any sense.
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Old 26th December 2009, 05:36 PM   #285
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Quote:
Originally Posted by PMA View Post
Take tons of feedback and you are there. How worthful? Not sure that the THD race has any sense.
Many things in audio are devoid of any sense. But the show must go on... ;-)

Hi
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Old 26th December 2009, 06:22 PM   #286
BV is offline BV  Slovakia
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Quote:
Take tons of feedback and you are there. How worthful? Not sure that the THD race has any sense.
Take tons of feedback and you are there. How worthful? Not sure that the THD race has any sense.
Try it, and post Your results . It has the same sense, as race for SR about 300V/us. Maybee than it is better to use Schmit trigger.
BTW, "tons of feedback" in graph I posted means about 25dB LG in whole band from DC to about 40kHz. Low THD(20) means low IMD also, and this has much more sense,as plain THD.

Last edited by BV; 26th December 2009 at 06:34 PM.
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Old 26th December 2009, 06:24 PM   #287
Telstar is offline Telstar  Italy
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Self was obsessed with THD. Other designers are with other things.
Who can tell who's right?
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Old 26th December 2009, 06:29 PM   #288
Mooly is offline Mooly  United Kingdom
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Doug made an interesting comment years ago
That if you want a "different" sound, then you should introduce that as a device to distort the signal in some way (add even harmonic distortion etc) before the "blameless amp". I think he called it a niceness control lol

Sadly my experience has taught me it's not quite that simple... and no "niceness" circuit was ever forthcoming.

An interesting idea though... don't ya think !
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Old 26th December 2009, 06:34 PM   #289
PMA is offline PMA  Europe
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Self is a very good professional in audio amplifiers design. BUT, he only concentrates on non-linear distortions (THD, IMD). He does not care much about speed and ultimate stability, he does not care about symmetry of rising and falling edges, he does not care about hard clipping. His design has +, but - as well. Nothing is perfect.
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Old 26th December 2009, 08:16 PM   #290
Telstar is offline Telstar  Italy
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Originally Posted by PMA View Post
Self is a very good professional in audio amplifiers design. BUT, he only concentrates on non-linear distortions (THD, IMD). He does not care much about speed and ultimate stability, he does not care about symmetry of rising and falling edges, he does not care about hard clipping.
Very well said.
The biggest problem IMO in his designs is the use of global feedback.
Take it out and those THD figures stop looking so awesome
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