Re: Groundplane in poweramplifier PCB design

Wiebe Cazemier wrote:
First, you may want to read this:
http://sound.westhost.com/phpBB2/viewtopic.php?p=6542#6542. I hope
you don't mind I posted your message to the forum of the website
where that amplifier design comes from. The ESP website has a good
community. I was quite surprised to read in your message that the P3a
might be a bad amp, so I posted it there for comments, for people who
have a great deal of experience with that particular amp to read.

On Tuesday 18 April 2006 20:23, Ban wrote:

Wiebe, I just had a short look at the webpage. Even if the author
says it is an excellent amp, it doesn't necessarily need to be so.
This construction invites any instability and there are a few very
weak points.
To bring good performance, the input pair has to be operated with
equal currents, which is almost impossible here. There will always
be some offset voltage at the output. When you trimm it away it will
be again there when the temperature has changed.

I don't follow this. The trim is not for setting DC offset, it's for
setting the quiescent current. DC ofset should be minimal because of
C3, which makes sure all DC feedback goes to the inverting input of
the long tailed pair.

you could trim the offset with the SIM connections or by adjusting the base
resistor of Q4

This is due to the current source made with Q4.
which relies only on its Vbe. It is also dependent on the beta of the
transistor and the forward voltage of the green LED.
C3 will be always operated with changing polarity, this calls for
trouble too.

C3 won't see any (significant) DC unless in fault conditions. By your
argument, you could say the input cap will also be destroyed, because
it also sees changing polaraties of the input signal.


No, it is bipolar. A normal electrolytic cap can be destroyed when its
voltage is reversed and more than 1.6V.

The worst is when there is an overload on the input and all the
current goes through Q1 and saturates Q4, especially without a load.
The output goes high and C5 will be pulled down by R10. Thus the
current through Q9 increases, and the voltage drop across it. The
moment Q6 starts conducting it turns on Q8 and the current will
increase until the fuses blow, but by then it is too late. No
current limit. In this kind of topology it is impossible to
incorporate a simple current limit, because when you pull down the
base in the usual way, the other side starts conducting and shorts
the supply rails. So it is not the gnd plane, it is the principle of
operation which results in self-destruction, whatever you do.
There are many more details that can be improved, but IMHO it
wouldn't make sense.

I can't really comment on the circuit behaviour you describe here,
because it's somewhat beyond me, but I can tell you that this amp is
very stable. Not only is it my experience (I have already made one,
but now I'm redesigning the PCB), but it has been built by many
others. It's predecessor (http://sound.westhost.com/project03.htm)
has been built hunderds of times, as you can read in the article,
without stability problems. In fact, it's so stable, he was able to
run it with 1 meter supply wires and no decoupling.

The lack of a current limiter is intentional BTW. The author hates
the reduced (near) clipping performance caused by such a thing. This
is an amp for DIY, not mass production, so a short circuit protection
is not necessary. And, as you can read in the forum thread I linked
to, the overload situation certainly does not happen according to one
of the posters.


See my other post. I answered everything. Maybe for your home use it is OK,
but certainly *not* for stage or studio monitoring as claimed.
I would recommend an IC called TDA7293 which will have a guaranteed sound
quality and protection incorporated and is in the same ballpark of power. In
fact to drive lower impedance speakers, you can even parallel several. They
are around 5 Euro.

Also, the ESP website is not just made by a random electronics
tinkerer. Rod Elliott really knows what he is doing, and has a lot of
experience in audio electronics. There are over a 100 projects on
that website he designed. And, they all work as stated in the
articles.

It's outdated technology. If you like to understand more about power
amplifier design, read some scientific books about it.
Actually IMHO all competent amps sound the same if operated in their
intended range, better invest the money into speakers.
--
ciao Ban
Apricale, Italy


.

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