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 artifacts 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.
I've set up the board with pads for both 10nF and 4.7nF in parallel if you don't have a 15nF cap.
Verified, sounds great.
ReplyDeletedoes the 1m ldr point towards the two LEDs?
ReplyDeleteyes
DeleteAny audio demos online yet?
ReplyDeletehttps://youtu.be/cRa89jE3hv4
DeleteThanks for all the love for this simple design from all those years ago! It was the simplest compressor for guitar that I could come up with, and of all the pedals I've designed over the years, it's been the most successful.
ReplyDeleteThere are a couple of simple modifications that can be done to the basic design to improve the signal to noise ratio slightly. The two changes are to add a capacitor in parallel with the input series resistor (forming a first order treble lift circuit), and another in parallel with the output level control (forming a low-pass filter of the same shape as the input filter). This will win you a 6dB reduction in hiss at high frequencies, but won't affect the flat frequency response through the pedal. The two minor changes are detailed in the FSB thread. However, if you find that the TL072 is quiet enough, don't bother with the modifications!
Have fun with it!
Chris aka Mictester
Hi Chris! Any suggestion for the value of the caps! I'm planning to do this project, is awesome!
DeleteSuper nice design, but mine worked for a couple of minutes and died. Any guesses? What should i look at? Thanks, cheers forma argentina
ReplyDeleteThe compressor unambiguously adds mids and lows to the tone, flattening and making the instrument easier to play.
ReplyDeletehttps://youtu.be/2Q79JujCHow
Hello, I have built this one, and although I liked it a lot, I must say that to use it alone with the amp it is perfect, but to use it together with saturation pedals, it tends to clean excessively. it's like it overrides any overdrive pedal.
ReplyDelete