"Pooh Bear" wrote
in message
Arny Krueger wrote:

When the first generation SS audio test equipment like
the HP 331-334 first hit the market, residuals
*instantly* improved by like an order of magnitude.

http://mywebpages.comcast.net/mnowlen/hp334a.htm

Heath made a cheap clone of this box (IM5258) that I
was able to improve so that it had mid-band residuals in
the 0.001-0.003% range.

Neat !

Do you fancy posting any more info about that ?

Once upon a time I wrote an outline of an article about it for Ed Dell, but
lost interest in his ragazine due to his negative stance on ABX before
getting much furhter with it.

I still have the modded 5258 packed away someplace. I fired it up about a
year ago and it still worked. It was a real POS compared to what I now do
with PCs. Hard to operate, limited reporting, relatively high residuals.

The mod development work revealed some interesting stuff.

One of the more significant zero-cost enhanments involved taking a grounding
problem out of the power supply circuit card that flooded the whole box with
low-level ripple. Bad land pattern design around the power supply caps.

Part of the mod involved replacing one or two discrete transistor stages
with 5534s.

I had to up the gain of the measurement circuits by about 20 dB to make the
lower residuals useful. There was a 10/20 dB gain boost switch that the mod
added.

I had to work over some time constants in the nulling circuit, and make them
change for the lower frequency ranges. That was controlled by a second added
front-panel switch.

The box was noisy because it had immense bandwidth - something like 4-5 MHz
at -3 dB. To get the lowest residuals a switchable low-pass filter was added
to the metering circuit.