I'm Building an Optical Compressor pedal and need some help with the signal flow

Thread Starter

dephlated

Joined Feb 14, 2024
9
I am currently studying music production at University and for my final project I'm building an opto compressor pedal for my bass, it is focused around a VCA and is inspired by the compressor designed and built by Moritz Klein. I'm unsure of what the signal flow out of the VCA would be, would it be to route the output signal directly into an LED, have that LED shine on an LDR, and then have that LDR linked to a threshold and voltage comparator then going back into the CV on my VCA or will I need a peak detector and so on? I have attached a diagram I drew of potential signal flow but I don't know if it is the correct signal flow or if the peak detector is even necessary.
 

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Thread Starter

dephlated

Joined Feb 14, 2024
9
What is the purpose of an "Optical Compressor Pedal"?
It is a type of analog audio compressor. to quote izotpe "In an optical compressor, the resistors are light-dependent: the audio signal feeds a lighting element (such as an LED), which shines upon a light-sensitive resistor. The resistance of this light sensitive element informs the compression circuit how much and how quickly to attenuate the audio signal."
 

BobTPH

Joined Jun 5, 2013
8,996
I think it is basically using the slow response of the LDR as a peak detector. The same can be done with a diode, capacitor and resistor.

An advantage of this approach is that the resistance is isolated from the rest of the circuit, so it can be used anywhere in the circuit.
 

Papabravo

Joined Feb 24, 2006
21,226
So are you saying that compression is the same thing as attenuation? If so, it seems like a lot of fiddle-farking around for a straightforward proposition. Attenuation can be done with ordinary resistors.
 

Thread Starter

dephlated

Joined Feb 14, 2024
9
So are you saying that compression is the same thing as attenuation? If so, it seems like a lot of fiddle-farking around for a straightforward proposition. Attenuation can be done with ordinary resistors.
yes it is. I agree, way easier with a standard VCA compression set up that uses OP-Amps and resistors, however this is for my University final project so it needs to be more interesting. Opto compressors are already very popular within music with the LA-2A, LA-3A and many others but Im trying to design my own on a smaller scale because these are very large old school outboard effects units and don't make for easy portable pedals that i can stick into my effects chain when playing my bass
 

Papabravo

Joined Feb 24, 2006
21,226
yes it is. I agree, way easier with a standard VCA compression set up that uses OP-Amps and resistors, however this is for my University final project so it needs to be more interesting. Opto compressors are already very popular within music with the LA-2A, LA-3A and many others but Im trying to design my own on a smaller scale because these are very large old school outboard effects units and don't make for easy portable pedals that i can stick into my effects chain when playing my bass
OK, but I'm still having difficulty wrapping my head around what is actually happening and what is the connection to peak detection which was also mentioned. Oh, and what does VCA stand for? I'm not familiar with that particular TLA (Three Letter Acronym) despite having worked for a digital audio company once upon a time.
 

Ian0

Joined Aug 7, 2020
9,835
The whole point of the LED/LDR arrangement is that it needs no VCA.
the LED illuminates an LDR. The LDR is the feedback resistor for an op-amp in inverting amplifier configuration, so the higher the signal, the brighter the LED, the lower the LDR resistance, the lower the gain, so it acts as a compressor.
Don‘t forget that Cadmium Sulphide LDRs are illegal under RoHS regulations, so I hope you are not thinking of selling the final product.
 

Thread Starter

dephlated

Joined Feb 14, 2024
9
The whole point of the LED/LDR arrangement is that it needs no VCA.
the LED illuminates an LDR. The LDR is the feedback resistor for an op-amp in inverting amplifier configuration, so the higher the signal, the brighter the LED, the lower the LDR resistance, the lower the gain, so it acts as a compressor.
Don‘t forget that Cadmium Sulphide LDRs are illegal under RoHS regulations, so I hope you are not thinking of selling the final product.
It wont be for sale its just for University. Would there be any benefit to keeping the VCA for a hybrid design or is it completely redundant? I was also thinking I could do it with a light transistor rather than an LDR if I were to sell it in due course
 
Confusion arises because audiophiles talk about "compression" when they really mean AGC - Automatic Gain Control. Essentially, low volume signals are boosted with higher gain and above a chosen threshold the gain is reduced. For guitars, they help sustain the notes/chords played. For analogue processing I favour the use of an FET but the LDR approach is popular and has a long history, using low voltage light bulbs instead of LEDs. To use an LED you can fullwave rectify the amplified signal or use two in parallel with reverse polarity.

The key point is that the LDR resistance doesn't change instantaneously so if you use it in parallel with the feedback resistor in an op amp gain stage it will gently reduce the gain as the output volume increase. I've used two op amps - for example the TL072 dual op amp which is especially good for audio. The input to output op amp used for the signal (with the LDR in the feedback loop to reduce gain) and the second amp to sniff the output and with a higher gain drive the LED(s).
 

Ian0

Joined Aug 7, 2020
9,835
You can make a current-controlled amplifier using a bipolar transistor.
028005D1-5573-4940-A16A-09507B2C3761.jpegSo, I don’t see why it won’t work with a phototransistor.

The original compressor used a filament lamp not an LED. That meant that no rectification or smoothing was required, as the thermal intertia of the filament did the smoothing for you.
 

Ian0

Joined Aug 7, 2020
9,835
Confusion arises because audiophiles talk about "compression" when they really mean AGC - Automatic Gain Control. Essentially, low volume signals are boosted with higher gain and above a chosen threshold the gain is reduced. For guitars, they help sustain the notes/chords played. For analogue processing I favour the use of an FET but the LDR approach is popular and has a long history, using low voltage light bulbs instead of LEDs. To use an LED you can fullwave rectify the amplified signal or use two in parallel with reverse polarity.

The key point is that the LDR resistance doesn't change instantaneously so if you use it in parallel with the feedback resistor in an op amp gain stage it will gently reduce the gain as the output volume increase. I've used two op amps - for example the TL072 dual op amp which is especially good for audio. The input to output op amp used for the signal (with the LDR in the feedback loop to reduce gain) and the second amp to sniff the output and with a higher gain drive the LED(s).
You made the same point whilst I was occupied doing my (bad) drawing.
I’ll just add one thing. . .the LDR has unequal response times, resistance falls quickly when illuminated and takes a while to change when things go dark. That adds it own unique, but non-adjustable, attack/decay characteristic to the circuit.
 

sghioto

Joined Dec 31, 2017
5,390
The LDR used as an attenuator or volume control was popular back in the 70's.
It's main purpose was to isolate the control potentiometer from the audio signal to provide noiseless operation.
 
I actually built such an audio compressor for a recording studio where I was the engineer. I used an assembly consisting of a miniature incandescent bulb coupled to a CdS photoresistor which the company CP Clare made for the purpose. The photocell’s latency gave it a smooth release time, but most importantly, an AC signal could be fed directly into the lamp and the thermal inertia would provide an intrinsic RMS conversion.
This was in the very early 1980s. You can still find these vintage devices on Ebay, at outrageous prices.

The product line has been acquired by the company Advanced Photonix. But the product is now excited by an LED. I haven’t tested it, but I doubt that you could feed directly an audio signal without some conditioning, rectification and averaging as a bare minimum.
 
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