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Old 21st February 2009, 05:18 AM   #11
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Trill, you seem to have come across the "melody" collection..
I have a dozen or so of these and besides being written in
either greek or russian , they are basic "schoolbook" designs
and should be used accordingly . Even the melody BJT designs
are unproven and prone to thermal runaway / and
or oscillations. quasi or ESP designs have been "built to death"
with support...better in the long run (for your speakers).




Quote:
Secondly, I strongly believe that it is foolish to try to drive output MOSFETs directly from the VAS. An EF driver/buffer stage should be put in between. Some people will disagree with me on this point, however. Just because MOSFETs have "infinite" DC beta does not give the designer license to drive them directly from the VAS
The gate capacitance of the OP stage will degrade/affect the Cob
of the VAS...an MJE340/350 EF after the VAS will be highly
beneficial, distortion/stability wise.
OS

OS
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Old 21st February 2009, 08:24 AM   #12
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Hi Richard,

I am surprised at some of the comments you received. Firstly if your LPT is well matched, degeneration resistors increases distortion.

There is nothing uncommon about driving lateral mosfets from the VAS, it works fine.

The amplifier performs well at high frequency and at 100kHz range at full power, THD is a not too shabby 0.12% and at 1 kHz it is around 0.006% better than most.

TIM is not lurking around the corner waiting to mug you. The phase response is ruler flat throughout the audio band with only -8 degrees at 100 kHz there are many high class designs that is three to four times this high.

The slew rate is > 28V/us and better than most. There is no notable over or undershoot using square wave.

In my opinion there is absolutely nothing wrong with your simple amp and if you like what you hear then that is what it is all about enjoying your music.

Kind regards

Nico
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Old 21st February 2009, 08:43 AM   #13
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Hi Richard,

I e-mailed you a few component changes that improve both harmonic and phase distortion.

Nico
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Old 21st February 2009, 03:03 PM   #14
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I agree driving vertical mosfets from the VAS isn't a good idea.

But the laterals have only 10pf or so of Ggd

The Cgs is the limiting factor - limiting current slew rate. But aren't speakers inductive at high frequencies? Current slew rate needed for a certain voltage slew rate will be lower than that needed by a totally resistive load.
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Old 22nd February 2009, 03:48 AM   #15
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Maybe this topology is inspired by Jean Hiraga's amp http://www.tcaas.btinternet.co.uk/monster27f.htm Revert PNP to NPN, it's diagram is similiar to the diagram in pic.1 lower left
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Old 22nd February 2009, 05:58 AM   #16
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Nico,
you don`t need no sophisticated, sumptuous instruments to come up with hardly measurable data without even rolling up your sleeves. A truly enviable ability.
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Old 22nd February 2009, 07:10 AM   #17
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Quote:
Originally posted by Bob Cordell
Sadly, this is the kind of simpleton design that some well-intentioned audiophile designer will fill with very expensive capacitors and think it then sounds better. Oh, I forgot the fancy milled aluminum front panel, the blue LED and the $5000 price tag.

Cheers,
Bob

This is why mosfet amps 'sound bad' to many audio enthusiasts. But, of course with that fancy case and pretty blue LED in there......WOW! Awsome.


Quote:
You might find a few pointer s in my amp cct.
One thing about using a pot for emitter resistors to set the DC output bias at the LTP is that if it is not centered, then the degeneration (local feedback) of each element in the LTP is different. It is important, IMHO, to have not only the DC components of the LTP equal, but also the AC parts as well. IMO, it would be better to have fixed emitter resistors and a pot in series with the emitter resistor of Q3 to adjust the LTP tail current for DC offset.





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Old 22nd February 2009, 09:27 AM   #18
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Quote:
Originally posted by megajocke
I agree driving vertical mosfets from the VAS isn't a good idea.

But the laterals have only 10pf or so of Ggd

The Cgs is the limiting factor - limiting current slew rate. But aren't speakers inductive at high frequencies? Current slew rate needed for a certain voltage slew rate will be lower than that needed by a totally resistive load.
Cgs might be substantially greater, but it is essentially bootstapted because the outputs are followers and G-S voltage has little variation.
However, the output stage presents a nonlinear loading to the VAS(especially in the crossover region) and that should be minimized IMHO.
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Old 22nd February 2009, 12:39 PM   #19
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This must be a really BAD amp.

Click the image to open in full size.
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Old 22nd February 2009, 04:38 PM   #20
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Efficency will be BAD...

Distortion might not be as it's pure class A.





The originally discussed circuit can be seen in an application note from Hitachi that accompanied the lateral mosfets. (could have been upside down with NPN input though, I don't remember) Performance is surprisingly good for such a simple circuit!
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