Opto- Compressor Questions

i'll be starting my Dual Analag Opto Compressor build soon. a year and a half maybe 2 years ago i did not know what a resistor was. while i'm a paint by numbers electronics DIYer i do try to understand what is happening in the circuit i am building. my questions about this build can apply to any opto compressor i would think.

from mercenary audio:"An optical compressor performs gain reduction control via a light source into a photo sensitive cell... as the light source gets brighter, the photo sensitive cell tells the amplifiers to turn down the volume... hence less dynamic range, or what is known as a compressed signal."
my questions are:How do the photo sensitive cells "tell" the amplifiers to turn down in electrical terms? what occurs in the circuit to make this happen?
what occurs in the circuit when the threshold attenuator(or pot) is rotated that "sets " the threshold and what occurs when the threshold is reached?
any help is appreciated. thanks

There is some good reading on the Silonex site. They manufacture opto couplers. http://www1.silonex.com/audiohm/principles.html

i'll try:
the photocell can act like an automated gain pot in a couple ways--look at the scheme for a simple opamp mic pre like the 312 or NYDs 1 bottle. they have a gain control that feeds the output back to the input in reversed polarity thereby subtractivly combining and reducing output level; the lower the resistance of the feedback pot: the lower the output. put the LDR of an opto in place of the pot and drive the light with a suitably buffered signal from the output pin... and you gots some affect on dynamic range.

look at the scheme for the la2a it works in a similar way in that the signal is picked-off after the LDR, buffered and fed back as a control voltage to the light panel. the difference with this topology is that the gain of the active amplification stages remains steady but the signal input to the first stage is reduced/shunted to ground as the LDR becomes less resistive in the presence of light

there are other types of automatic gain control elements but they all need an active element to change gain or reduce signal, and a control voltage/signal to impart the change

Most use a cell that is essentially a light sensitive resistor. As the amount of light that shines on it goes up, the resistance of the cell goes down. If you have this cell connected between the audio path and ground, then in "off" mode it is high impedance and audio is not affected(much). When in various states of "on" the cell shorts audio to ground in an amount proportional to it's resistance at that time.

Since it's usually driven by a LED, that means using a direct current. We amplify and rectify a portion of audio signal then run it through a driver stage and then drive the LED. Adjusting various parameters of this sidechain gives us the threshold, ratio, attack and release that you already know about.

Optos are really pretty simple since there are constraints on how fast they can act and on the actual attack/release times. Those constraints are pretty much what gives this type of compressor that "opto" sound.

:thumb:

> from mercenary audio:"An optical compressor performs gain reduction control via a light source into a photo sensitive cell... as the light source gets brighter, the photo sensitive cell tells the amplifiers to turn down the volume..."
> How do the photo sensitive cells "tell" the amplifiers to turn down ...?

Uh, that "explanation" is for people who just want to buy, not really understand.

> 2 years ago i did not know what a resistor was.

So now you probably know how to use an ohmmeter. Go to Radio Shack for "CdS Photoresistors (5-Pack) # 276-1657 $2.99". Hook one to your ohmmeter. Put it in total dark, it goes up past 1 Meg. Put it near a 100W lamp, it goes down toward 1 K.(*)

That's all it is.

If you want a phyzical explanation: in dark, the electrons are bound to their atoms. But light "loosens" the electrons so they may go with the flow. There is more elaborate theory available, bands and holes and groundstates. However commercial LDRs are developed by trial and error guided by experience, mixes of impurities to bridge the bandgap.

How would you use this? Most audio outputs can't be loaded-down easily. You build an L-pad, with a fixed resistor and a variable (photo) resistor. Since we will probably represent "too loud" with "more light", and the Photo-R goes low-ohms in light, we put the 50K in series and the photo-R in shunt to ground.



For proof: wire a 1.5V battery to the input of the L-pad. Measure the voltage on the photo resistor. In total darkness it will be over 1.4V, 90% of signal goes through. In bright light it will be near 0.1V, less than 10% of signal gets through.

Now wire the L-pad between a source and a beefy power amp's input. Put a car tail-lamp on the speaker output, and shine it at the photo-R. Obviously the photo-R and lamp should be in a light-tight box. For small output, the lamp stays dark, full signal passes. Around 7V-8V the lamp will glow enuff to reduce photo-R resistance, reduce signal into the amp. Large increase of signal into the L-pad will make the lamp slightly brighter, photo-R resistance goes down and down, you probably can't find enough signal to bring the lamp past 11V.

I have done exactly this scheme, except with LEDs, as a concert-hall PA amp protector. It is pretty rude, but effective.

The trick is doing a good signal-sensitive lamp without a power amplifier. Also incandescents are very slow. In 1960 the most efficient fast lamp was the electroluminescent cell, still occasionally sold as nightlights. That's what the LA2a used, but it still takes a 6V6-size driver.

The photo-R is slow. That is good and bad. If it were perfectly fast, it would clip the signal. We want it to drop quickly and hold for many audio cycles. The LA2a used specified and selected LDRs, which give a semi-fast attack and a lingering decay. They got lucky. It is hard to find a happy time-curve, and you can not change it.

The next step is to do what tube and FET limiters do. Peak-detect and decay with R-C networks. It is possible to over-drive the LDR and shorten a too-slow attack time, and your other time constants are simple R-C nets. I have built good dual-curve small-fault recording limiters this way, using the RS 5/$3 LDR pack. I spent most of a month in a dark room getting it to work well.

(*) These are made-up typical numbers. What RS really sells for 5/$3 is "Various styles and ratings".... whatever they can get cheap. In the past it was often assorted scrap: some low-R some hi-R some fast some slow. However about 9 out of 10 packs (I bought a lot of these!) had the same assortment year after year. I saw a pack recently and it looked like 5 of the same part. Even so, LDRs vary a LOT and you won't find two-the-same even if they have the same part#. Unless you can buy dozens, you will need to trim your L-pad resistor to get any sort of "match"... between channels or even just to get a mirror-voltage for a GR meter.

whats an ohm meter?

just kidding. thanks for the replies. i dont understand it all on first read but i did learn somethings i didnt know. ive pasted the replies to a word document to printout and read . give me time to digest it all. and look at some schematics while reading it.