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Re: lateral mosfets vs. bjts in audio amplifier design

E

Eeyore

Are there any technical advantages that lateral mosfets have over BJTs
in audio amplifier design?

Do lateral mosfets offer lower theoretical THD+N over the entire audio
spectrum than BJTs for instance?
Click to expand...

Lateral mosfets used individually without NFB have poorer THD (in
simplistic overall terms at full rated power ) than BJTs by a factor of
about 10:1. I measured it !

However their vastly wider bandwidth (combined with suitable driver
circuitry - which is very important) means that the extra 20dB of NFB to
compensate can be applied without any great trouble.

Then you get to the real crux, the crossover area where a lot of low
volume listening is done. Quite simply, the transfer characteristics of
complementary lateral mosfets 'join up' to form an almost flat function at
low current whereas BJTs never do regardless of the bias current and how
hard you try. There are some 'fudges' that can help BJTs but they never
actually eliminate the problem.

And that's why lateral mosfets sound superb.

As I may have said before I designed one with < -100dB SINAD @ full power
@ 1kHz @ 350W. I think with the more advanced CAD tools of today I could
better that now. All I had then was MathCad running under DOS.

Graham
 
M

[email protected]

Lateral mosfets used individually without NFB have poorer THD (in
simplistic overall terms at full rated power ) than BJTs by a factor of
about 10:1. I measured it !

However their vastly wider bandwidth (combined with suitable driver
circuitry - which is very important) means that the extra 20dB of NFB to
compensate can be applied without any great trouble.

Then you get to the real crux, the crossover area where a lot of low
volume listening is done. Quite simply, the transfer characteristics of
complementary lateral mosfets 'join up' to form an almost flat function at
low current whereas BJTs never do regardless of the bias current and how
hard you try. There are some 'fudges' that can help BJTs but they never
actually eliminate the problem.

And that's why lateral mosfets sound superb.

As I may have said before I designed one with < -100dB SINAD @ full power
@ 1kHz @ 350W. I think with the more advanced CAD tools of today I could
better that now. All I had then was MathCad running under DOS.

Graham
Click to expand...


MathCAD under DOS... ouch... 20 years ago? In that case, any patent
you or your employer would have had would be expired by now?

Do you have schematics on your website?

Michael
 
M

[email protected]

Lateral mosfets used individually without NFB have poorer THD (in
simplistic overall terms at full rated power ) than BJTs by a factor of
about 10:1. I measured it !

However their vastly wider bandwidth (combined with suitable driver
circuitry - which is very important) means that the extra 20dB of NFB to
compensate can be applied without any great trouble.

Then you get to the real crux, the crossover area where a lot of low
volume listening is done. Quite simply, the transfer characteristics of
complementary lateral mosfets 'join up' to form an almost flat function at
low current whereas BJTs never do regardless of the bias current and how
hard you try. There are some 'fudges' that can help BJTs but they never
actually eliminate the problem.

And that's why lateral mosfets sound superb.

As I may have said before I designed one with < -100dB SINAD @ full power
@ 1kHz @ 350W. I think with the more advanced CAD tools of today I could
better that now. All I had then was MathCad running under DOS.

Graham
Click to expand...

I didn't realize this was an old project. That spec is beyond the
8903, so how did you measure the THD?
 
E

Eeyore

MathCAD under DOS... ouch... 20 years ago? In that case, any patent
you or your employer would have had would be expired by now?
Click to expand...

No-one in their right mind patents circuits that are essentially 'obvious' if
only you're clever / smart enough.

Do you have schematics on your website?
Click to expand...

No. It was DOS Orcad remember. Besides I prefer to keep my circuitry to myself
rather than give it away to every copier.

Graham
 
E

Eeyore

I didn't realize this was an old project. That spec is beyond the
8903, so how did you measure the THD?
Click to expand...

What's an 8903 ?

I used an Audio Precision System One of course. Even then it was barely above
residual.

I back-calcuclated it to 0.0004% ~ -106 SINAD.

Graham
 
E

Eeyore

John said:
---
And have it die with you instead of sharing it with the world if you're
not going to commercialize it?

How mean is that?
Click to expand...

As mean as you are.

I seem to have been on the receiving end of "no good deed should go unpunished" for
so long that I'm very cautious now.

Graham
 
E

Eeyore

Damon said:
Eeyore wrote


The vexation I experienced with low-level distortion measurements
with the few designs I could measure with a Tektronix set was that
noise predominated the measurements. I'm not an engineer, and
design for low noise to match distortion levels is beyond me...
as is the price of an AP system.

But at such low distortion levels, is it worth it?
Click to expand...

Everything is worth it.

Graham
 
A

Allen Bong

No-one in their right mind patents circuits that are essentially 'obvious' if
only you're clever / smart enough.


No. It was DOS Orcad remember. Besides I prefer to keep my circuitry to myself
rather than give it away to every copier.

Graham- Hide quoted text -

- Show quoted text -
Click to expand...

Hi Graham,

Is this a good audio amp using HexFET for a starter?

http://users.otenet.gr/~athsam/power_amplifier_45w_hexfet_eng.htm

The driver and output stages use IRF9540 and IRF540 and the opamp is
LF411. THD is 0.2%. Power supply is +/- 30V.

I like it because the PCB design is included.

Allen
 
P

Phil Allison

"Damon Hill"
The vexation I experienced with low-level distortion measurements
with the few designs I could measure with a Tektronix set was that
noise predominated the measurements. I'm not an engineer, and
design for low noise to match distortion levels is beyond me...
as is the price of an AP system.
Click to expand...

** It can be very useful to have a digital storage scope when measuring THD
residuals.

Most digital scopes have an "averaging" feature, so as long as you have the
time-base locked to the fundamental, the harmonic residual signal will add
to itself as many times as the scope allows while noise and any AC supply
harmonics tend to disappear from the trace as they are not correlated to the
fundamental sine wave.

Nowadays, many folk like to use a PC with a 24/96 sound card and FFT to do a
spectrum analysis - makes harmonics stand out like dogs balls.

But at such low distortion levels, is it worth it?
Click to expand...


** Better ask someone like Halcro ( an Aussie manufacturer).

http://www.halcro.com/home.asp

They have been making a fortune selling $40,000 amps with 0.0005 % THD to
New York's fattest & dopiest Jews.



....... Phil
 
P

Phil Allison

"Allen Bong"


Is this a good audio amp using HexFET for a starter?

http://users.otenet.gr/~athsam/power_amplifier_45w_hexfet_eng.htm

The driver and output stages use IRF9540 and IRF540 and the opamp is
LF411. THD is 0.2%. Power supply is +/- 30V.

I like it because the PCB design is included.


** That design is a complete ABORTION !!!!

Every BAD idea ever thought of in amp design has been included.

It will be hopelessly UNSTABLE, both thermally and at HF.

Stay away !!!



....... Phil
 
M

[email protected]

Everything is worth it.

Graham
Click to expand...

What exactly does back-calculate mean?

I have used the AP system 1. It does quite a few tests automatically,
so I would say it is worth it. The 8903 I got was $250 or $300 (I
don't recall exactly) about 6 years ago. APs were substantially more
at the time.

For those that haven't played with the AP, it's source is transformer
coupled. Pretty interesting given the specs. I never cared for the AP
DSP option. It never correlated with ATE, probably due to all the
windowing. The HP DSA would correlate to a few mdB, and it could be
syncronized to a 10MHz reference. But you got the AP for the THD
specs, since nobody else could match them.
 
M

[email protected]

No-one in their right mind patents circuits that are essentially 'obvious' if
only you're clever / smart enough.


No. It was DOS Orcad remember. Besides I prefer to keep my circuitry to myself
rather than give it away to every copier.

Graham
Click to expand...


Oh well. I'll just borrow a copy of Slone's text then.

Douglas Self didn't care for mosfet amps too much. That's why I posed
my original question.

Michael
 
E

Eeyore

What exactly does back-calculate mean?
Click to expand...

To do the RMS addition calculation backwards.

I have used the AP system 1. It does quite a few tests automatically,
so I would say it is worth it. The 8903 I got was $250 or $300 (I
don't recall exactly) about 6 years ago. APs were substantially more
at the time.
Click to expand...

It was an 8903 I probably used once on one project. Clumsy to use and poorer
residuals than AP. I'd have the AP Portable One any day over that. In fact for
simple bench work I prefer the Portable One over the System One because it's
simple 'press button' stuff, no mice to arse about with and it takes up little
room. No other audio measuring set comes close in terms of performance or
convenience.

Graham
 
E

Eeyore

And it was developed under contract for a client so it's not really mine to give
away.

Oh well. I'll just borrow a copy of Slone's text then.

Douglas Self didn't care for mosfet amps too much. That's why I posed
my original question.
Click to expand...

I don't care for Doug Self too much come to that. Long story.

Graham
 
E

Eeyore

John said:
---
Geez, Graham, unlike you, as you well know, over the years I've given
away hundreds, if not thousands, of fully worked out, functional designs
and presented them right here, on these groups, as either formal
drawings roughly conforming to DoD-STD-100C and MIL-HDBK-1006/1 or as
ASCII drawings, when possible.

If some of my designs were commercialized, as I'm sure they were, that
could easily amount to millions of dollars that I've given away.

Are you that mean?
Click to expand...

If you knew me in person you'd know otherwise.

I have actually considered putting up some pages online of design tips for audio to help
counter all the shit out there.

---
Strange, that 'cautious' bit, since it seems that even though you get
flamed, for example, for posting gratuitous insults and professing that
you're better than everyone else (which you seem to think are good
deeds) you throw caution to the wind and continually post that kind of
crap.
Click to expand...

As far as audio goes I AM better than 99.9% of the competition. I was in a meeting on
Monday regarding a project and sadly had to blow one of the originator's ideas (and he
seemed pretty smart) straight out of the water since he hadn't even properly read a data
sheet it seemed. I'm notorious for finding faults before they get built.

Graham
 
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