Really Cheap Compressor [documentation]
- mictester
- Old Solderhand
Information
Do you remember the old-fashioned way of doing bass compression? It used an LDR across the volume control illuminated by an incandescent lamp across the speaker terminals! It was a simple method back in the 60s....
This compressor is almost that simple! It uses a handful of cheap components (I built one for £9 including hardware!), but works really well. Guitarists who've tried it often ask "Is it on?", but miss it when it's off! It's quiet, and has good "squeeze" without the gain-change artefacts you frequently get with transconductance amplifiers or FETs.
The LDR should be ~1M in the dark, but almost anything will do.
Some of the component values are quite carefully chosen. The input capacitor (15nF - made up of a 10 and a 4.7 if the 15 is unavailable) is chosen to give some frequency shaping, and the interstage capacitor prevents handling noise affecting the compression. Green LEDs are chosen because they most closely match the response of the LDR, but yellow works almost as well. Obviously, the LDR / LED combination should be facing each other, and must be in a light-tight enclosure.
Bypass switching can follow your favourite scheme - my prototype has a "Millennium Bypass" and an externally mounted blue LED to shown when it's on - and the input pull-down resistor prevents switching "pops".
Enjoy!
This compressor is almost that simple! It uses a handful of cheap components (I built one for £9 including hardware!), but works really well. Guitarists who've tried it often ask "Is it on?", but miss it when it's off! It's quiet, and has good "squeeze" without the gain-change artefacts you frequently get with transconductance amplifiers or FETs.
The LDR should be ~1M in the dark, but almost anything will do.
Some of the component values are quite carefully chosen. The input capacitor (15nF - made up of a 10 and a 4.7 if the 15 is unavailable) is chosen to give some frequency shaping, and the interstage capacitor prevents handling noise affecting the compression. Green LEDs are chosen because they most closely match the response of the LDR, but yellow works almost as well. Obviously, the LDR / LED combination should be facing each other, and must be in a light-tight enclosure.
Bypass switching can follow your favourite scheme - my prototype has a "Millennium Bypass" and an externally mounted blue LED to shown when it's on - and the input pull-down resistor prevents switching "pops".
Enjoy!
"Why is it humming?" "Because it doesn't know the words!"
- borislavgajic
- Opamp Operator
Information
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really NICE Compressor
thank you
borislav

thank you
borislav
- mictester
- Old Solderhand
Information
Thank you! I've built three of them, kept one for myself, one for my nephew and sold one to a famous player who tried mine in a studio and said "I WANT ONE!!". I'm thinking of getting some boards etched, as this could become a regular build.borislavgajic wrote:really NICE Compressor![]()
thank you
borislav
"Why is it humming?" "Because it doesn't know the words!"
- Brink
- Degoop Doctor
I designed a PCB. At 1.5"x1.5" it would easily fit in a 1590B.
Comments? How does the LED/LDR spacing look?
Comments? How does the LED/LDR spacing look?
- Attachments
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- Mictester Optical Opamp Compressor.GIF (13.43 KiB) Viewed 58286 times
- mictester
- Old Solderhand
Information
Correct. Basically, you can just connect the wiper ("pin 2") and the end so that as you turn the control clockwise, you increase the resistance (you're actually reducing the amount of negative feedback around that second op-amp so the gain increases).Brink wrote:Mictester, I assume that sustain pot lugs 1 and 2 connect to the opamp output, and the you turn the knob clockwise for more sustain. Is this correct?
"Why is it humming?" "Because it doesn't know the words!"
- mictester
- Old Solderhand
Information
Your board layout looks good. I normally bend the LED legs so that one is "stacked" on top of the other, with both facing the LDR. Your spacing is fine.Brink wrote:I designed a PCB. At 1.5"x1.5" it would easily fit in a 1590B.
Comments? How does the LED/LDR spacing look?
Remember - for it to work properly, it must be dark around the LDR / LED combination. To test it outside its box, I made a temporary tube of black insulating tape. It worked OK, but was better when I put it inside an Eddystone diecast box!
"Why is it humming?" "Because it doesn't know the words!"
- dune2k
- Breadboard Brother
If I want to use this comp for bass guitar, would I need to change something?
JOHNO wrote:P.S Great pedal for playing 70's porn music.
- mictester
- Old Solderhand
Information
Yes. The frequency response is set up for guitar. To make it OK for bass, you need to (basically) double the values of the capacitors, so that the response goes down an octave. The changes you need are:dune2k wrote:If I want to use this comp for bass guitar, would I need to change something?
The input capacitor needs to be a 33n (from 15n), the coupling capacitor to the second op-amp needs to be a 220n (from 100n), the electrolytic feeding the LEDs should be a 2µ2 (from 1µ), and the output electrolytic should be 4µ7 (from 2µ2).
I've just tried it on a breadboard, and it works well with my Fender Precision!
"Why is it humming?" "Because it doesn't know the words!"
- dune2k
- Breadboard Brother
Nice, thanks!
Gonna order the parts soon & will get you posted.
I also made a perfboard layout for the guitar version today and will post it soon. (Gotta check that everything's ok.)
Gonna order the parts soon & will get you posted.
I also made a perfboard layout for the guitar version today and will post it soon. (Gotta check that everything's ok.)
JOHNO wrote:P.S Great pedal for playing 70's porn music.
- dune2k
- Breadboard Brother
hey, i am ordering the components to build this pedal but i am having some trouble finding the LDR with the right value. any suggestions?
- Hides-His-Eyes
- Tube Twister
Most LDRs are ~1M in the dark, where are you ordering from?
And mictester, if you do end up ordering a bunch of PCBs, would you add one to the order for me?
And mictester, if you do end up ordering a bunch of PCBs, would you add one to the order for me?

Testing, testing, won too fwee
- Hides-His-Eyes
- Tube Twister
looks spot on to me.
Testing, testing, won too fwee
- mictester
- Old Solderhand
Information
He means that the part looks exactly right.YuGi wrote:Sorry, what do you mean? My english is not very goodHides-His-Eyes wrote:looks spot on to me.![]()
You're going to learn a lot of colloquial English on this board!


"Why is it humming?" "Because it doesn't know the words!"
- sevinisthenumber
- Cap Cooler
You have a PNP trace image?Brink wrote:I designed a PCB. At 1.5"x1.5" it would easily fit in a 1590B.
Comments? How does the LED/LDR spacing look?
"The man who says he knows everything will never know the truth"
C.S. Lewis
C.S. Lewis