"Robert Morein" said:

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.


An experiment I sometimes do with a just finished amp: connect a dummy
load consisting of a resistor and capacitor in parallel, and playing a
CD.

Then another amp of known reputation connected to the feedback signal,
reproduces the error signal that feedback loop nr. 1 creates.

Very revealing!

Of course, a double trace oscilliscope and a low distortion generator
will yield results that are better suited to produce on paper.

The Hafler box does the same IIRC, but with a HF compensation that has
to be nulled out or something (have to go through my stash of
magazines, I know it was described somewhere).

--

"Audio as a serious hobby is going down the tubes."
- Howard Ferstler, 25/4/2005