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On Thu, 13 Oct 2005 00:15:32 GMT, wrote:

"Don Pearce" wrote in message
. ..
On Tue, 11 Oct 2005 22:58:48 GMT, wrote:

Years ago I read about something called a null speaker test.

It was a way to detect differences between amps by connecting 2 amps to a
single speaker. If there were differences between the 2 amps then the
speaker would generate sound.

Does anyone know of this test and can you give details?

Despite what the others have told you, there is a way to do this without
danger. I'm presuming the amplifiers are niminally similar, or you
wouldn't
even be trying this.

The way you do this is to connect the amplifers in a bridged formation.
Ignore the ground terminals at the amplifier outputs, and connect the
speaker between the two live terminals. Now connect the audio source to
both amplifier inputs in parallel. There should be no net audio on the
output if the amps are the same. Use an oscilloscope on one of the outputs
to see just what signal level is present while you are doing this.

I'm not really sure I'll try it in any case, I just thought it would be
agood topic of discussion, particularly if my memory of it only producing
sound when there is a difference is correct.

The output will not be zero. In a good amplifier, the gain is set by a
pair
of resistors - the feedback network. If good 1% resistors are used for
these, you can expect to hear a signal perhaps 40dB below that from a
normally wired amplifier. You can trim the level to one amplifier (the one
with the higher gain) slightly to null out the remaining residue of
signal.
What remains now will be slight frequency response differences and
distortions.

d
Hopefully I can find the issue of the magazine where I saw this setup
mentioned, and reveal the whole thing to one and all.

I thought it might offer some alternative to those who get their panties in
a twist at the metion of ABX.

The method is no good for this. It will reveal all sorts of differences
that heave nothing whatever to do with audible differences between
amplifiers. These would be to do with small phase and amplitude differences
between two perfectly competent amplifiers.

d