Re: lateral mosfets vs. bjts in audio amplifier design

[Eeyore]

It would be a good idea if those lateral mosfet makers had current sense
transistors so that a current mirrored, push pull, source follower can be
done.

Motorola did a mosfet with a current sense terminal. Doubtless only N-channel
though.

Graham

 

[Eeyore]

I point out two circuits

http://www.kevinaylward.co.uk/ee/circuits/VeryLowDistortionAmp1.jpg
http://www.kevinaylward.co.uk/ee/circuits/VeryLowDistortionAmp2.jpg

They are similar, but one has a local feedback loop around the output
devices, one doesn't.

I'm a great believer in local feedback. Linearises stages nicely and improves HF
and phase response. If you need to, you can make up some lost overall gain with
jellybean transistors (or ICs) at the front end.

Graham

 

[Eeyore]

Exactly. I'm behaving professionally. Nor do I want to let all the Chinese see
how to do it right.


You haven't the skills and experience in this field.

Ain't gonna work.

I'm sure you're very good at other things.

Graham

 

[Eeyore]

You would think they already have a know it all janitor.

That's a bit low Michael. The fact of the matter is that after something of
a drought (partly for health reasons) it's damn monsoon out there right now.
I don't know how I can satisfy both clients who both have very interesting
respectively moderately big and huge projects on right now.

Plus my back's still fucked.

Graham

 

[Eeyore]

But it's FUN !

Where would you place the threshold (and what harmonic structure) of audible
THD ?

Graham

 

[Eeyore]

I wish you every success.

It also has to fit industry standard form factors. A 3u 19" rack is considered big
for a 2kW amplifier for example, must be as lightweight as possible and must have
certain input processing requirements such as anti-clip circuitry, balanced inputs of
course and user selectable high pass filters to name a few.

Graham

 

[Eeyore]

Not if the "additional" pole is at a much higher frequency than the
previous pole inherent to the capacitances of a heap of fets driven by
some wimpy resistive source. Increasing the bandwidth of the output
stage - the serious speed problem - by, say, 20:1 has got to help the
overall loop.

Just buffering Ciss helps a ton.

There's no free lunch here: we're adding GBW, and paying for it. But
not much, since opamps are cheap.

I confess I do that kind of thing. Just not put op-amps round the actual output
devices myself so far but it sounds interesting. They'd have to be damn fast
though.

Graham

 

[Eeyore]

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
that being the case, then it must be shit you're making!
I think any beginner can accomplish that.

As ever you're quite clueless.

What do you a PCB / chassis mount professional audio XLR 3 pin gold-plated
connector should cost for example ?

Or an NE5532 ?


Graham

 

[[email protected]]

"Damon Hill"




** 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.


** 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

Or just feed the residual to a DSA. While it is interesting to see
what harmonics are present, looking at the residual relative to the
source is probably more useful. For instance, if the residual error
gets large at the crossover, you know where to tweak. The residual
could also get large at the peaks if the output stage is not beefy, or
some current source is out of compliance on large swings.

 

[Eeyore]

Just sticking a unity-gain buffer, like an LT1010, in front of the
gate helps a lot.

I've used discrete Class A, with AB capability to get that node really slewing.

But then, you may as well use an opamp and close the
local loop.

I just have this feeling there's a snag here somewhere. Can't place it yet, ah maybe
slew rate from negative cut-off back to conduction would cause a small dead band (in
time terms).

Graham

 

[Eeyore]

its nt as low as the crap you've been spreading in these audio
threads.

I don't do crap, especially not in audio. Do you want some references btw - one
would be from a guy with a PhD and a Chartered Phsysicist, the other from the
Directors of the company who make the best audio to digital and back converters in
the world.

Big deal. I'm sill 100% disabled, and I had a palsy in my good eye
four months ago that left me effectively blind. I still only have
limited use of that eye, after three months of not being able to see out
of it. I have sores on my legs that aren't healing, and I am scheduled
for some surgery to remove something from my right lower eyelid in about
two weeks. It has been growing rather fast, and I have to wait and see
if it will cause more problems. I have tripped, and banged into so many
things while blind that I am too sore to do much of anything.

I have said before that have every sympathy for you. There's hardly anything
practical I can do though.

Graham

 

[Eeyore]

This is what I meant. If you can't stand the heat, quit shoveling on
the burning coal.

I am a true audio expert Michael, John Fields is not. Each to their own.

Graham

 

[Eeyore]

Or just feed the residual to a DSA. While it is interesting to see
what harmonics are present, looking at the residual relative to the
source is probably more useful. For instance, if the residual error
gets large at the crossover, you know where to tweak. The residual
could also get large at the peaks if the output stage is not beefy, or
some current source is out of compliance on large swings.

The crossover area is the critical one and indeed as you say is also the harmonic
element. If it gets nasty near full power, get a bigger amp !

Graham

 

[Eeyore]

---
It depends:

http://www.google.com/search?source...q=harmonic+distortion+threshold+of+audibility

JF

I was interested in YOUR opinion though.

Graham

 

[Eeyore]

But I have given him a helping hand by recommending the very best stage for the
job. The complementary emitter follower biased into full Class A. More efficient
than his resistive load circuit too.

Graham

 

[Eeyore]

---
Should an RFQ for something like that cross my desk I'd probably do the
design just to see whether I wanted to bid it or not.

But a 19" rack?

With the hole spacing still in inches too?

My, oh, my, will wonders never cease! ???

The inch is included in the metric system via a back door. I think 0.1" is called an 'e'
or somesuch. History has to be accepted.

482.6mm does for me though.

Graham

 

[[email protected]]

The crossover area is the critical one and indeed as you say is also the harmonic
element. If it gets nasty near full power, get a bigger amp !

Graham

For a single sine wave test, the amp getting mushy at the top and
bottom doesn't contribute to the THD as badly as the crossover since
the sinewave itself is flattening out at the top and bottom of the
wave. However, I assume though never examined two tone tests, where
the higher frequency sine wave might distort when riding along the
lower frequency near the crests.

 

[Eeyore]

Yes I absolutely was. Don't tell me what I'm thinking.The absence of a repsonse shows how litle you know about the subject.

Congratulations. I wouldn't employ you.

Graham

 

[Eeyore]

---
You're either a liar or you have a short attention span or, more likely,
both.

You didn't "help" until you got beat into it.

Here's your "helpful" first response to his original first post:

"Stop mucking about with stuff you don't understand and simply buy a
modern
hi-fi amp. Power amps are a real speciality (especially if you want them
to be stable). It's no place for a beginner.

Do you understand stabilty criteria for example. What's a Bode plot ?
etc

Graham (designer of stable pro-audio power amps since 1980)"

It's damn practical answer. Like so many posters <sigh> he omitted his real reason
for doing what he wanted. If he'd included that, he'd have got a very different
repsonse.

Graham

 

[Eeyore]

For a single sine wave test, the amp getting mushy at the top and
bottom doesn't contribute to the THD as badly as the crossover since
the sinewave itself is flattening out at the top and bottom of the
wave. However, I assume though never examined two tone tests, where
the higher frequency sine wave might distort when riding along the
lower frequency near the crests.

You shouldn't be pushing the amp that hard ideally. IMD will really show up sloppy slew
rates though.

Graham