"Stewart Pinkerton" wrote in message

On Tue, 17 Aug 2004 23:13:16 -0400, "Arny Krueger"
wrote:

"Chris Morriss" wrote in message


An LM3875 makes a useful little amp. It's basically a power op-amp
with just the five active pins, so it's easy to wire up a neat
design.

I've got a three channel LM3875 amp I built to try out active
crossover networks to drive the bass, middle and tweeter of
speakers. I've used stabilised +/- 35V rails and it sounds and
measures pretty well.
(Actually, the wiring structure is important, I've heard of
marginally stable 3875 amps where people don't know how to return
signals to the 0v line in the right place.)

The latest version of this part appears to be the LM4780 - reference
http://www.national.com/ds/LM/LM4780.pdf

Note that *with careful PSU design*, this amplifier is capable of
being made into a low-cost 'monobloc' using parallel operation, which
will equal or exceed the performance of any 'high end' 50 watt
amplifier.

The Stereophile review does not seem to support this claim. Nor do the spec
sheets. Many power amps, even the so-called PA Amps that golden ear
audiophiles like to despise have far better performance, an order of
magnitude lower nonlinear distortion at decent power levels, for example.

As the excellent article by Hennessy notes (as do the
application notes in the data sheet above), it is absolutely critical
to get the PSU design and layout correct.

It seems pretty clear that he didn't.

"Unfortunately, because of this, low-level 100Hz charging pulses find their
way onto the output when the two amplifier channels are connected together
(by the preamp). "

His circuit card design varies significantly from the National "reference
design" posted at http://www.national.com/ds/LM/LM4780.pdf page 22.

It's just good basic 'star earth' engineering practice, but it *is*
important.

I agree that power supply circuit card design is important, as I've debugged
that area of some commerical designs. You also have to be careful how you
pick off the feedback path, as I've corrected that in some commercial
designs.