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

--
Please use the address on the audiomisc page if you wish to email me.
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html
Audio Misc http://www.audiomisc.co.uk/index.html

I have a denon here which has a motorised volume control. Near the bottom
end the tracking is rubbish. I'm sure the electronic devices now used are
cleaner and less prone to nasties.
Talking of pots, in an old Akai tape recorder the dolby usnits fet gate
adjustment was via carbon presets and they were always prone to wandering
values. Bit of a design flaw.
Changing the subject slightly. The Armstrong 600 series were the ones with
medium and long wave in one long band as I recall. a bit unusual for the
time and I wondered how they did it.
Brian

--
From the Sofa of Brian Gaff Reply address is active
Remember, if you don't like where I post
or what I say, you don't have to
read my posts! :-)
"Phil Allison" wrote in message
...
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

--
Please use the address on the audiomisc page if you wish to email me.
Electronics
http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html
Audio Misc http://www.audiomisc.co.uk/index.html

Double IF / conversion.

The long and medium wave input was upconverted to about 3.1 MHz using a
varactor-tuned oscillator. Balanced mixer that used Shottky diodes in the
later versions. This was followed with some 3.1 IF stages/filters before a
second mixer and fixed LO converted back down to the more conventional
circa 455 kHz for a second IF.

The use of the unusually high first IF allowed the first oscillator's
varactor to be tuned enough to cover the range.

It was an unusually capable AM tuner for a Hi-Fi set. Came from Ted Rule's
background as a radio amateur, RAF erk, and the era when people took AM
seriously because the bands were less crowded and the modulation bandwidths
wider. Alas, by the 1970s it was largely wasted as the AM broadcasts by
then weren't what they'd been. Helped with interference rejection though.

If you removed the front end lowpass filter that blocked 3 MHz from
getting to the mixer you could use it as a shortwave tuner. 8-]

Jim


In article , Brian-Gaff
wrote:
The Armstrong 600 series were the ones with medium and long wave in one
long band as I recall. a bit unusual for the time and I wondered how
they did it.

--
Please use the address on the audiomisc page if you wish to email me.
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html
Audio Misc http://www.audiomisc.co.uk/index.html

Been looking though old magazines and been able to add an interesting P.S.
to http://www.audiomisc.co.uk/Armstrong...es/1975-8.html

If you have a copy of the relevant mags I recommend having a read. Barry
Fox went into more cases and problems that I dealt with.

Jim

--
Please use the address on the audiomisc page if you wish to email me.
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html
Audio Misc http://www.audiomisc.co.uk/index.html