Electro-optical compressor

New revisions as of 05-26-05 with new schematic diagram

Way back when I was a high school student there was a device the peaked my
interest. It was the photocell that was used to turn lights on after dusk, and
off at dawn. I was kind of a hi-fi nut back then. The surface noise found on my
record collection got worse with repeated playings. Scratch filters found on
stereos of the time obliterated the highs. Somehow it came to me to try to use
the photocell to turn off the surface noise between songs on a album, when it
was most apparent. I devised a circuit that put the photocells in line with the
tape monitor loop. I used the tape out audio as the control signal to drive a
pair of green leds that shined on the photocells when the music was at normal
level. Between songs the leds would not glow, and therefore caused the
photocells to turn the audio down, therefore gating it. I did not know of gating
or compressors, or any of that studio stuff at that time. This system worked
reasonably well for me. I thought of the possibility of the photocells working
to do the opposite by keeping loud passages from getting too loud for cassette
tapes made for the car. I could get the overall audio level much louder so I
could hear it over the road noise. Many years later I was shocked to discover
that just such a device had been in service since the mid sixties, and used
extensively on many of my favorite records. After wiping the egg from my face, I
pondered the relative ignorance that existed in the 1970’s due to the lack of
the fountain of information that is now available to all on the internet.

After reading about people that are trying to build their own opto
compressors to get that renowned sound of that universally accepted standby that
exists in just about every major studio in the world, I decided to try building
one for myself. I searched the web to find countless examples of
electro-optical implementations. Some used Vactrols such as those that are use
in many Fender amps for the channel switching and tremelo circuitry. Many
commercially available compressors also use these devices. They have a certain
sound, but not the legendary sound original. Some have used electro-luminescent
panels with varied results. The circuitry required to sufficiently drive the
panel is tricky and usually requires transformers or somewhat high voltage power
supplies.

While on the subject of these electro-luminescent panels, It is known by the
designers that these panels have a rather short life, shockingly so. They lose
half their brightness in around 2000 hours of use depending hard you push them.
Buy two panel nite lites use one for a month or so, then plug them both in next
to each other. You will see the difference. Imagine what is happening in your
studio.

Then I spent alot of time trying many different photocells. There are really
only four photocell formulations in use. Some compressor manufacturers have the
vendor create a special part number for them based on their requirements. You
can do this selection yourself, however. I designed a test rig to select and
match the photocells. Once I settled on the correct photocell for this project,
I went on to search out the led that would posess the right linearity
charicteristics needed for the sound I was after.

Once I was satisfied with the results, I designed a totally op amp based
circuit that used the leds I found to be perfect in replacing the
electro-luminescent panel and it's associated tube circuitry. I then added some
control over the actual decay time external to the led. This is primarily to
duplicate the natural decay time of the phosphors used in the
electro-luminescent panel. I have conducted tests on several photocells and
led/photocell combinations. The Vactrol VTL5CL exhibits far too fast a natural
decay time to be suitable without some degree of external decay time to make
them sonically pleasing. Many other photocells exhibit this problem. The
Silonex NSL 5910 does have the correct decay characteristic, but must be tested
and matched. The external decay circuitry gives the user the option to "fine
tune" the essential charicteristics.

If you intend to build this compressor please note:

The 5k trimpot and adjusted to 2.82k to ensure the best compression slope
when using the Silonex photocell.

Clairex no longer manufactures photocells as of this writing. I like the
results I have had with the silonex photocells. I was told that these photocells
are the actual ones used in today's re-issue of a very famous optical
compressor.

Controls:

R1....input gain.............adjusts the gain of the input circuit.

R11...compression............adjusts the gain of the light amp that drives
the led.

R8....output
level...........adjusts the amount of make up gain after the compression
circuitry.

R18...decay time.............fine tunes the decay time to optimize for
different instruments.

R4....Ratio slope............adjusts compression ratio.

The special green LED is catalog # 25-377 from www.hosfelt.com. The all
important photocell is the Silonex NSL-5910 specially selected by matching for
stereo pairs if desired. They are available from Allied electronics. The red
front panel indicator leds are radio shack 276-307 or just about any general
purpose led.

Notes for this circuit:

An all tube signal path can be implemented with this compressor. Observe all
precautions regarding dangerous high voltages inherent in tube circuitry. (see
disclaimers)

I have found that the use of tube circuitry for this compressor is not worth
the effort because the change of sound is not really a factor in this
implementation.

Power supply is a split supply of +/- 15 volts.

Any decent quality op-amps can be used.

All diodes are 1N914 except Led's

Parts list:

R1....100K AUDIO TAPER
R2....1K
R3....10K
R4....5K TRIMPOT
R5....SILONEX NSL-5910 PHOTOCELL
R6....10K
R7....10K
R8....100K AUDIO TAPER
R9....1K
R10...22K
R11...100K AUDIO TAPER
R12...1K
R13...10K
R14...10K
R15...10K
R16...10K
R17...10K
R18...1MEG LINEAR TAPER
R19...4.7K
R20...4.7K
R21...1MEG
R21...1MEG
C1....0.47UF
C2....0.1UF
C3....0.47UF
C4....1UF FILM
C5....0.47UF
C6....2UF FILM

More notes:

R4 can be mounted on the front panel if desired to provide a ratio control.
In my prototype I chose to set it to 2.82k internally.

C2 can be reduced in value to make the detector less responsive to low
frequencies. Some find this desirable. Experiment with smaller values.

R10 can be increased to 56k if you need more output gain.