KLUDGE@AGCB8.LARC.NASA.GOV (06/04/90)
I seem to be playing with the sound system at a local theatre, and having a
bit of trouble finding information and parts. I don't know if this equipment
constitutes high-end, but it certainly did when it was new, and perhaps with
a bit of work, it may again.
The speaker system is a set of Altec-Lansing Voice of the Theatre speakers
(just like in every other theatre in the country). Unfortunately a piezo
supertweeter system has been added, which is absolutely awful (and has been
removed until
I can dig up some better tweeters. I will probably wind up
going with horn drivers because of the high volume and wide dispersion needed,
as well as because of the cost problem and my having a box of old compression
drivers in the closet somewhere. God, I hate horn speakers, but they are still
quite impressive when high efficiency and volume are required).
The right and left amps are RCA type SA 352 amplifiers, which I have
recapped. These beasties have 7027 output tubes and big output transformers.
As PA amps go, they are quite respectable, and when the transformer input
coupling was bypassed and capacitors replaced, it cleaned up nicely. If
anyone out there has documentation for this beastie, I would very much
appreciate getting a copy, because there are some changes I'd like to make to
the output circuitry that I'd rather not touch without a schematic. Also, if
anyone out there has an old SA 352 they aren't using, I'd like to buy it from
you to use as the center speaker amplifier.
The center speaker is driven by a 1970's McMartin 100 watt amp. It's awful.
It's really awful. It has internal bandlimiting to reduce damage to the stereo
image by the center speaker, while keeping voices properly centered. What it
does is mess up the whole system's response.
Century projectors with Western Electric sound heads. These heads use the
standard exciter bulb and a #22 phototube, driving a 12AX7 which acts as a
preamp and equalizer. Replacing the phototube and the resistors and capacitors
on the EQ loop flattened the response out considerably. The sound heads look
to have been an upgrade to the projector, as the projector manuals (copyright
1934) show much larger Century sound heads, with mono outputs. I have never
seen a stereo WE head before and have no idea whatsoever about the date on
these beasties. They do look to have had a film fire, however, and I know
all Kodak stuff since '52 has been safety film. More on this bit of
archaeology as it progresses.
The first high quality audio systems were those in theatres. For years,
home audio equipment was designed as a scaled-down system attempting to copy
the commercial theatre installations. In the 40's, theatre units with response
flat to 14 KHz was not uncommon, far better than most hime products and much
better than many recordings. In the recent past, the quality of audio in
theatres has fallen off in many places (as has the quality of the image as
well, sadly to say), due mostly to financial considerations and a change in
the place of the theatre in society. Also the loss of good quality positive
film stock has meant most films are printed on comparatively grainy release
film, which makes 16mm sound abysmal and 35mm quite degraded. Compare an old
print of Ben Hur with most stuff being produced today and you'll know what
I mean.
--scott
(who is always careful to congratulate the projectionist when going into
a cinescope film which is properly shown without cutting the edges off)