lateral mosfets vs. bjts in audio amplifier design

[Kevin Aylward]

So, in terms of THD, s/n ratio and response flatness where would you
draw the line where it becomes overkill ?

Graham

0.01% IMD @ 20Khz/19Khz at all power levels. 1db flatness to 50Khz power BW.

I don't claim that this is necessary, but my take is that it is sufficient.
I won't buy a PA amp if it is say, 0.1% at 20khz.

As far as what I am interested in as a personal achievement goal, its
probably 0.001% IMD at 20khz, 1db flatness to 1Mhz power BW.

Kevin Aylward
www.kevinaylward.co.uk

 

[Kevin Aylward]

"Pro" audio design seems like an infinite chain of stolen circuits.

Lets not kid ourselves, "all" circuits are stolen, whatever the field. Its
that simple. Show me *any* design, and I will show you a bit of is in a
prior design. Like diff pairs, current mirrors, cascodes, are all the same
building blocks we all use. However, designing say a cmos opamp, even with a
standard topology, can take considerable time in getting just the right
combination of W, L and M to satisfy a particular specification.

The only way to make production designs reliable, is to use what is already
known to work, and only add the minimal of new additions.That's what being
an engineer is. Maximising profit with the minimum of RISK. There are no
brownie points for a novel circuit that achieves no net advantage. All new
design is based on modifications of existing design. That's just the way it
is. Like, how do you proposes a new car engine is "designed"..Like some new
law of thermodynamics is discovered? I don't think so mate...

Anyone that says that they don't use say, 95% of existing work is either a
liar, or seriously deluding themselves.

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

 

[Eeyore]

Note, as in the mosfet 1000, basically a stolen idea from Hitachi. The
second stage is a diff pair. This diff pair has less distortion inherently.
Don't know why D.Self don't use it in his Blameless amp. Keep everything
differential is one of my mottos.

Ditto. It's crazy to throw away that advantage.

The current mirror load gives the prior
post mentioned push/pull drive to the output buffer. The current source
loads (CMFB) on the 1st stage, rather than resisters, makes the total LF
loop gain, truly huge, making LF distortion, vanishing small.

Indeed so. Mind you, I actually *limited* my LF gain because it was so huge
already ! It didn't need any more. THD didn't start to climb on the 1200B
design until about 2-3 kHz. Knowing what I do now, I'm sure I could do better
than that. This was 20 years ago you know.

The Matti Otala idea of 3 voltage gain stages attracts me. Ever hear of the
Electro-Companiet amplifier ?
http://www.google.com/search?hl=en&rls=en&hs=3lm&q=Companiet+amplifier&btnG=Search

They were hopelessly built (I ended up fixing an early one) but the technical
idea was sound.

Graham

 

[Eeyore]

I am *really* loving my new tascam digital multitrack. Its a joy to record
and randomly seek to any point in the song. The digital saving and operation
of tuneable tone controls are magic. Now I want a digital desk, with
infinite rotate pots with position lights on each channel. Like, there is no
going back once one has the digital controllability.

Horses for courses. The control surface and GUI of a digital desk can make or
break it.

For live situations where many engineers may use the desk, analogue is still
unbeatable for its simplicity and anyone experienced can master one in 5 minutes
even if they never saw that model before (and they're not short on performance
these days either) but even in live, digital is making increasing inroads.

Check out Cadac's S-Digital for example. Just out and I think they hit the sweet
spot.

Graham

 

[Eeyore]

Choosing the right co-efficient could take a month of Sundays though, so not
surprising one don't want to tell anyone what they are.

Like, the basics of a nuclear bomp are pretty trivial, but its the final
details that matter.

I have tried quite a lot of digital delays, and it is somewhat surprising
that many, many of them, still sound like shit. Not one usable setting.
Trust me.

Oh so true. Read the reviews.

Like, do you know the one about the Coke A Cola formula....that's not rocket
science either.

I don't have the time to read most of this thread, Graham, but...

'Designing' those reverbs was a fascinating task, not least because they had to
sound as near identical as possible to some on a withdrawn OEM module we'd used.
I managed a replica as good as anyone could tell. Why ? Because people *loved*
those reverbs and wanted us to keep them.

I read no less than 3 long papers on the subject in the process. And I threw out
some 'holy cows' about how to make them sound right that have always been
assumed - because they actually sound shit.

Just as I do with EQ, I only sign off for production when it *sounds* right as
well as measures right.

Graham

 

[Eeyore]

Indeed. Its just about impossible to make a profit in "pro audio" now adays.
Too much stuff on the market.

Thankfully there are still a few areas with profit left in them.

Graham

 

[Eeyore]

No its doesn't. Its a push/pull class AB current drive to the mosfets, and
secondly, you don't need much. 20ma class drive is way more than enough for
at least 500w at 200Khz power BW.

I thought he was wrong about that. I confess I didn't look it up again at the
time but my 1200B does exactly the same as your approach Kevin.

Graham

 

[Eeyore]

Yes. Wafer probe.

That must be some fine machinery !

Grham

 

[Eeyore]

"Pro" audio design seems like an infinite chain of stolen circuits.

Only in China.

To reseach the issue fully would take you months full time.

Graham

 

[Eeyore]

0.01% IMD @ 20Khz/19Khz at all power levels. 1db flatness to 50Khz power BW.

I don't claim that this is necessary, but my take is that it is sufficient.
I won't buy a PA amp if it is say, 0.1% at 20khz.

As far as what I am interested in as a personal achievement goal, its
probably 0.001% IMD at 20khz, 1db flatness to 1Mhz power BW.

How about flatness in the audio band ?

Graham

 

[Eeyore]

Lets not kid ourselves, "all" circuits are stolen, whatever the field. Its
that simple. Show me *any* design, and I will show you a bit of is in a
prior design. Like diff pairs, current mirrors, cascodes, are all the same
building blocks we all use.

But these are simply 'prior art', not actually 'stolen'.

Coming up with a genuinely new configuration these days is increasingly
difficult, although the USPTO will probably still give you a patent on it !

Graham

 

[Jamie]

Jamie wrote:




You're the fool. You are nothing more than a disgusting piece of pig
excrement.

You're just jealous because you can't do it. Bloody typical of idiots.

LOL!!!

Right... now go back to your room before the hall police see you
wondering around..

 

[Eeyore]

Right... now go back to your room before the hall police see you
wondering around..

WTF are you babbling on about ?

You contribute nothing here except bad / erroneous / stupid and retarded
info.

GET LOST ! And stay lost !

Graham

 

[Eeyore]

Eeyore wrote

JF

Did you have something useful to contribute or were you simply dumbstruck by a very
simple question ?

Graham

 

[Eeyore]

---
Not true.

In the dim recesses of time, an idea flashed into being which was unique
and was fleshed out electromechanically into our being here, now, so
_that_ circuit wasn't stolen.

On a more mundane level, some of us work through the problems of design
without resorting to directly infringing the work of others, whether
that prior work exists or not.

Oh Good Lord, you mean you accept my work as original ?

Graham

 

[Jamie]

---
The hall police are responsible for bringing you to task for your
wandering around.

The mind police are responsible for bringing you to task for your
wondering around.

JF

I must be getting the same illness as you know who. Just hope I
still have some more years to go before I get as bad as him. Or I'll be
the next one wondering why I am wandering around and going no where!.


http://webpages.charter.net/jamie_5"

 

[Eeyore]

advantage.

That's certainly true.

Mostly.


Yours, maybe. Not mine. I hate to copy circuits, even my own.

Likewise. I was really pleased back in 1988 to design a new mic amp topology that
used twin feedback paths in order to handle signals over a 70dB range with a
single rotary control for gain that was close to dB linear. And the only serious
cost plus was dual gang pot in place of a single one.

Graham

 

[Eeyore]

You live in your own little fantasy world where you actually have
some value.

More than just 'some' my friend.

This guy just gave me a reference btw.
http://www.ibd-uk.com/members/jones-martin.htm

Look at his credentials ! A totally charming chap btw and we think very alike. We
worked on 2 successful projects (one of which I turned round from a 'runaway'
into a full blown commercial success) and stay in occasional contact to this day.
They were radar projects btw lest you think I only do audio.

Graham

 

[Eeyore]

More than just 'some' my friend.

This guy just gave me a reference btw.
http://www.ibd-uk.com/members/jones-martin.htm

Look at his credentials ! A totally charming chap btw and we think very alike. We
worked on 2 successful projects (one of which I turned round from a 'runaway'
into a full blown commercial success) and stay in occasional contact to this day.
They were radar projects btw lest you think I only do audio.

---
RADAR, huh?

Here's open-ocean PPI marine collision avoidance I was doing in 1972:

(View in Courier)

TX TRIG>------[MS1]---[MS2]----A
AND Y---A _ _____
RX VIDEO>-----[CLIP]-----------B NOR Y--+-->ALARM
+--B |
| _ A--+
+--Y NOR
B--+---O |
| |<-- SET
[R] O |
| |
GND +V

To start with, the SET switch is made, momentarily, causing the ALARM
signal to go false.

Then, the main bang triggers MS1, the RANGE one-shot, which triggers
MS2, the CAPTURE one-shot, on MS1's trailing edge.

In the meantime, clipped video is applied to one of the inputs of the
AND, the other input being connected to the output of MS2, so that the
AND's output will go true if there is clipped video and a high from MS2
on its inputs simultaneously.

When the output of the AND goes high it resets the RS latch made from
the two NORs, and the ALARM output will go low, causing an audible alarm
to sound, which will alert the RADAR operator (or whoever's on watch) to
the fact that an object has just been detected at the range set by MS1.
JF

Funnily enough, they were working on a new collision avoidance radar system when I was
at Kelvin Hughes. That wasn't my project though. I was working on new spec Rhine River
Radar intended for barges and the like using the Rhine (and doubtless other
waterways).

If you're interested I could explain what their problem was and how I told them the
answer from my bed in 30 seconds. That wasn't good enough though, so we had to make a
MathCad model to prove it.

Then they said - help us ! And I fixed it.

Graham

 

[Eeyore]

So you'd be happy with the -3dB @ 20Hz and 20kHz of the 60s and 70s ?

Graham