Really Cheap Compressor  [documentation]

Original effects with schematics, layouts and instructions, freely contributed by members or found in publications. Cannot be used for commercial purposes without the consent of the owners of the copyright.
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mictester
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Post by mictester »

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.
Really Cheap Compressor.GIF
Really_Cheap_Compressor.zip
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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!
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Post by borislavgajic »

really NICE Compressor :thumbsup

thank you

borislav

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mictester
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Post by mictester »

borislavgajic wrote:really NICE Compressor :thumbsup

thank you

borislav
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.
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Post by CRBMoA »

I am waiting on some LDRs. I can't wait to try this!

Looks great!!!
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Post by Brink »

I designed a PCB. At 1.5"x1.5" it would easily fit in a 1590B.
Comments? How does the LED/LDR spacing look?
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Mictester Optical Opamp Compressor.GIF (13.43 KiB) Viewed 17745 times

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Post by Brink »

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?

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Post by mictester »

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?
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).
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Post by mictester »

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?
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.

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!
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Post by dune2k »

If I want to use this comp for bass guitar, would I need to change something?
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Post by mictester »

dune2k wrote:If I want to use this comp for bass guitar, would I need to change something?
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:

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!
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Post by dune2k »

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.)
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Post by dune2k »

Haven't checked, nor built it yet. So everything is still unverified.
(Sorry for the load of jumpers+possibly wired backwards pot, it's my second perfboard layout.)
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Post by viggersster »

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?

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Post by Hides-His-Eyes »

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? :)
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Post by YuGi »


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Post by Hides-His-Eyes »

looks spot on to me.
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Post by YuGi »

Hides-His-Eyes wrote:looks spot on to me.
Sorry, what do you mean? My english is not very good :oops: :slap:

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Post by mictester »

YuGi wrote:
Hides-His-Eyes wrote:looks spot on to me.
Sorry, what do you mean? My english is not very good :oops: :slap:
He means that the part looks exactly right.

You're going to learn a lot of colloquial English on this board! :) ("Spot On" = "Exactly Right") :thumbsup
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Post by YuGi »

mictester wrote:You're going to learn a lot of colloquial English on this board! :)
Yeah I like this board! :applause:
Thanks for the compressor, I will try it as soon as possible :D

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Post by sevinisthenumber »

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?
You have a PNP trace image?
"The man who says he knows everything will never know the truth"
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