Jim Lesurf wrote:



Then add in that the pots - particularly the old 'AB' ones - didn't always
get the 'flattest' response when the control showed to be at "12 o'clock"..
This also varied from pot to pot.




** Regular carbon track pots were then and still are a real annoyance when the goal is high reliability with low noise and distortion in a piece of stereo gear. Dual gang types can have large tracking errors and easily develop bad spots where the measured THD is way higher than the rest of the system.

In about 1979, I assembled a stereo pre-amp for my own use that avoided such pots completely. I wanted no tone controls and reckoned a simple switched attenuator in each channel are all one really needs.

So I used a pair of readily available, 12 position rotary switches with resistors fitted to give 2dB steps making a -20dB and OFF attenuator. Adding a 0dB/+20dB gain switch in the NFB of the line input stage made the overall range 40dB, which is plenty. Having 2dB steps means the maximum deviation from some desired gain setting is 1dB.

Channel tracking was near perfect with no measurable THD at any setting - PLUS it stayed that way for almost two decades of daily use. Then I made another pair of attenuators, for about $3 each.


..... Phil
























Precision only really arrived with the
better Alps pots, which arrived on the scene long after the 600 was
launched. (One of the changes I made was to use the better pots. etc, from
Alps.)

The 600s were made via waterfall flow soldering onto boards. But when you
factor in the component selection and that some components would then be
changed 'on test' you could argue they were all hand-made or at least
hand-tweaked.

And then in practice the bulk of any slight 'brightness', etc, could
usually be corrected by a slight tweak of the controls. As shown by the
measurements many reviewers made that found the response could be a *lot*
flatter than 2.6dB. At the time it was assumed that any sensible user would
simply adjust the controls to get the results that they liked best. Alas,
reviewers may not always fall into the "sensible user" category. :-/

TBH I came to the conclusion years ago that the reason 'subjective'
reviewers whined about tone controls "degrading the sound" was that they'd
found that a tiny twitch of the tone controls changed their main
"perceptions' of how amplifiers "sounded different". Thus risking putting
them out of a job. 8-]

Jim

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