" wrote in message
ink.net...

"Robert Morein" wrote in message
...

"Kalman Rubinson" wrote in message
...
On Tue, 11 Oct 2005 22:53:15 GMT, "
wrote:

Years ago I read an article by Larry Klein IIRC, that describe a way
to
find
differences in amplifiers. It was called the null speaker test, again
IIRC.

Essentially it was a way to hook up 2 amplifiers and a single
loudspeaker
so
that the speaker would generate sound if there were any differences
between
the 2.

Has anyone heard of this test, and if so, can you point me towards a
place
that describes the setup in detail?

Cannot recall that but I do recall something similar that was advanced
by Hafler. Probably involves inverted polarity feeds to two amps with
the amp running off the two (+) terminals or a summing of the outputs.

Kal

It consists of a bridge. One input to the bridge is the line-level input
to
the amplifier.
The second input to the bridge is the output of the power amplifier,
passed
through a voltage divider.
The bridge output consists of the difference signal between the input to
the
amplifier, and the output.

Hafler claimed that the XL series of amplifiers exhibited a level of
distortion, when measured in this fashion, 20dB below other extant
amplifiers. The box was to be loaned by dealers so that XL owners could
make
a nulling adjustment in the field. The nulling adjustment is a capacitor
that adjusts an ultrasonic peak in the frequency response of the
amplifier,
thus providing an open loop compensation for load reactance at high
frequency. The XL design also claims an extremely low phase shift: 1/4
degree from 20Hz to 20kHz.

Reviewers seemed to ignore this feature, preferring to test the
amplifier
only at the factory default setting. But if Hafler's claims are to be
believed, his method picked up, as an undifferentiated power, types of
distortion that were or are generally ignored by HM and IM measurements.
This is one reason I am skeptical of claims that amplifier distortion
has
been adequately quantified.

I don't know if this is the process I was trying to remember or not, but
the
main reason I mention it is that it would be a very good way to find out
if
there's any differnce between two amps. No sound, no difference, no
difference, no upgrade. And you don't need an ABX box.

The Hafler box does not compare two amps. However, it does compare the
amplifier to the input, ie., to the mythical "straight wire with gain." It
will extract every change the amplifier has made to the input, and display
just the change.

One can hook a scope or spectrum analyzer up to the output of the box, and
see a representation of the distortion which is entirely different from the
description of TM and IM specs. Unlike those numbers, this test misses
nothing. Hafler claimed that his amplifiers were 20 dB cleaner than other
amplifiers, as of 1997, when the Excelinear design debuted. I use these
amplifiers extensively, and find them to be extremely revealing of low level
detail. They are a good match for metal domes. However, they are not nearly
as dynamic as amplifiers with othe topologies, such as the Transnova, or
bipolar amplifiers, because. This is because the design of conventional
MOSFET amplifiers prevents makes it impossible to drive them all the way
"on". For fabric domes, I recommend other amplifiers, such as your Acoustat.

Do not try to use this box with your Acoustat. The Hafler box cannot be used
with an amplifier that requires that both output terminals float from
ground.

Truely, Mikey, setting aside our jousting for a moment, I am sorry that you
are not receptive to these kinds of observations with respect to amps. You
are missing a world of perception, as well as fun, by sticking with your
notion that amplifiers sound the same. I have a very large personal
collection of power amps. Every time I go through the collection, I hear the
music in a different way.