NovaPA
Well-known member
A little preface: back in 2017 (8 years ago, hard to believe!), I designed this circuit. Well—"designed" might be a stretch, as it was based on the well-known and popular "optical compressor" structure using an op-amp and a photoresistor.
Essentially, what I developed back then was a variation featuring a transistor buffer and an ultra-bright LED to exert more "influence" on the photoresistor.
Recently, I saw someone publish a very similar circuit (most likely based on the same source), and it inspired me to finally refine my own design.
And so, after 8 years, I finally built and tested it—and it worked right away!
The circuit is very simple. It uses a dual operational amplifier, which can be a TL072, NE5532, or any other op-amp with the same pinout. Power - bilopar +-15VEssentially, what I developed back then was a variation featuring a transistor buffer and an ultra-bright LED to exert more "influence" on the photoresistor.
Recently, I saw someone publish a very similar circuit (most likely based on the same source), and it inspired me to finally refine my own design.
And so, after 8 years, I finally built and tested it—and it worked right away!
The first op-amp acts as a controlled amplifier. A photoresistor (VR1) is placed in its feedback loop—its resistance decreases as the light shining on it becomes brighter. In series with it is a potentiometer (R7), which will be mounted on the front panel and will determine the "compression" level: at high resistance, the op-amp's gain will be at its maximum. When R7 is set to 0 resistance, the gain will be entirely dependent on the signal amplitude.
The trimmer resistor (R5) is optional (that's why it's marked in blue), but if installed, it can be used to limit the maximum gain.
Additionally, you can see the input diodes marked in blue—they are also optional and primarily serve a protective function against excessive signal overload.
The second operational amplifier further amplifies the signal for the LED. After it, there is a buffer on a complementary transistor pair—this is necessary to allow the use of an ultra-bright LED. Following that, there is a diode bridge.
You may have noticed that the transistors are connected without biasing, meaning they operate in pure Class B. This is intentional. Unlike an amplifier output stage, where such a configuration would cause distortion at low amplitudes, here it establishes a threshold. In other words, until the signal reaches a certain amplitude (0.6V voltage drop across the transistor's PN junction + 0.6V drop across the diode bridge, totaling 1.2V), the compressor will not "intervene" in the signal.
After the diode bridge, there is a single smoothing capacitor (1µF). There is no dedicated soft release time control, but an additional 10µF capacitor can be connected if needed.
A few words about the optocoupler construction. It’s quite simple: take a piece of heat shrink tubing, insert an ultra-bright LED on one side and a photoresistor on the other. Then, fill it with glue—and that’s it!
The PCB has a designated spot for this component.
The tubing is essential; otherwise, ambient light will interfere with the operation!
Colleagues, let’s discuss!
If anyone is interested in replicating the design, I’ll prepare and share the Gerber files a bit later.
Boards avialable on BlackMarket.
And of course, if you like it, we can refine and improve it in the future!
Attachments
Last edited:
...more important than the specific (dark/light) resistances is the time component, how long the LDR resistor takes to regenerate (edit: and attack time, too!). That's audible.
![Soldering Iron Kit, 120W LED Digital Advanced Solder Iron Soldering Gun kit, 110V Welding Tools, Smart Temperature Control [356℉-932℉], Extra 5pcs Tips, Auto Sleep, Temp Calibration, Orange](https://m.media-amazon.com/images/I/51sFKu9SdeL._SL500_.jpg)
