lateral mosfets vs. bjts in audio amplifier design

[[email protected]]

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?

Thanks,

Michael

 

[Kevin Aylward]

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?

Thanks,

Michael

So...is this a flame or what...?

Anyway....

Both have advantages and disadvantages
THD/IMD depends on both the linearity of the intrinsic device (with load)
and how much feedback is applied.

Getting silly distortion figures at silly bandwidths is easier with mosfets,
imo...

Kevin Aylward
www.blonddee.co.uk
www.kevinaylward.co.uk
www.anasoft.co.uk

 

[[email protected]]

So...is this a flame or what...?

No, just trying to determine whether the higher price for lateral
mosfets is justified *at all*...

M

 

[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?

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

 

[Eeyore]

Getting silly distortion figures at silly bandwidths is easier with mosfets,
imo...

You're not kidding ! I had < -103dB SINAD back in 1989.

THD gets confusing at those numbers because you can never remember how many
zeroes there are after the decimal point.

Graham

 

[Eeyore]

No, just trying to determine whether the higher price for lateral
mosfets is justified *at all*...

In spades. Not least because of the total absence of secondary breakdown.

Graham

 

[[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

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

 

[Kevin Aylward]

You're not kidding ! I had < -103dB SINAD back in 1989.

THD gets confusing at those numbers because you can never remember
how many zeroes there are after the decimal point.

I don't remember if I mentioned this not not..but, I measured 0.0018% at the
input of the mosfet 1000 on the bench, and 0.0015% at the output

Kevin Aylward
www.kevinaylward.co.uk

 

[Kevin Aylward]

In spades. Not least because of the total absence of secondary
breakdown.

Yes.

Kevin Aylward
[email protected]
www.kevinaylward.co.uk

 

[[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

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

 

[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?

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?

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

Graham

 

[Eeyore]

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

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

 

[Eeyore]

I don't remember if I mentioned this not not..but, I measured 0.0018% at the
input of the mosfet 1000 on the bench, and 0.0015% at the output

Not with that Radford shit by any chance ?

Graham

 

[Eeyore]

---
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?

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

 

[Eeyore]

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?

Everything is worth it.

Graham

 

[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 -

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

 

[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.

** 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?

** 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

 

[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

 

[[email protected]]

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

The 8903 was HPs distortion analyzer.
http://www.telford-electronics.com/hp_8903b_spec.htm

I don't recall the AP coming out in the DOS era. I think I first saw
one in 1983. Hey, wait, that was the DOS era. ;-) Never mind.

 

[[email protected]]

Everything is worth it.

Graham

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.