From: gstopp@fibermux.com Date: Mon, 31 Jul 95 15:09:10 PST Encoding: 53 Text Subject: Re[2]: Effects Here's a compressor circuit from my early days of screwing around: RED LED | /| 220 ohm ------|< |--\/\/\/\--- | | \| | V || | VV | 100 ohm | Input >----\/\/\/\------o------\/\/\/\----------o-----> Output | Photoresistor | | | | |\ | | 2 | \ | -------| - \ | | \ 6 | | 741 \-------' | / 3 | / -------| + / | | / V |/ This design has a few things that make it stand out as a typical beginner's design, but I remember it worked amazingly well. I plugged an electric bass into it and I could pluck a string, put down the bass, and walk away and come back 15 minutes later with the note still playing. Finger and fret noises jumped out while hard-played notes were crushed into submission. The photoresistor is a typical Radio Shack thing the size of a dime, which has the LED touching the lens and pointing directly into it, with the whole mess sealed in a mass of black heat-shrink tubing pieces to block outside light. The photocell has a dark resistance of about 10 megs and a LED-full-on resistance of a few hundred ohms. It also has a real sluggish time lag which I'm sure has a lot to do with the performance of this circuit. The 100 ohm resistor means that the input impedance is 100 ohms, which is pretty bad for an in-line FX box, and I suspect it's not ideal for connecting to a bass pickup, but hey who cares if it sounds neat, right? p.s. my 1st attempt at ascii CAD, lemme know if it's AFU. From: gstopp@fibermux.com Date: Mon, 31 Jul 95 16:44:23 PST Encoding: 47 Text Cc: analogue@hyperreal.com Subject: Re[4]: Effects Changing the decay time would probably involve adding a peak detector with a variable bleed resistor (to vary the decay time) followed by a buffer to drive the LED. This would be added between the first op-amp and the LED like this: ------------------ RED LED | /| | | / | | | /| 220 ohm | / - |----- ------- ---------|< |---\/\/\/\---o----/ 741 | | | | | \| \ | | V "decay" V \ + |----o-----o---\/\/\/\--- || \ | | | R | VV \| | + | V photons to photoresistor --- | C --- | 1N4448 etc. | | | /| V ---|< |---- | \| | | ^ from 1st op-amp R and C values will determine the max decay time - 1 meg and 1 microfarad will mean 1 second time constant. I have not tried this and in thinking about it I can speculate that the addition of the peak detector will boost the overall output level of the thing compared to the original circuit as it tries to overcome the .6 volt drop across the peak detector diode to turn the LED on. One other change to consider would be to use an op-amp with higher input impedance like a TL-0whatever since the 741 is, in technical terms, "kinda sukky".