OT: Dynacomp

[seanm]

So I am trying to fix an old MXR Dynacomp. I have never used one and it basically seems to work, except on the first note.

Setup: I have my P bass into the Dynacomp into a mixer. The mixer has a LED meter that reads -24 -6 0 +6 and clip. For the tests I hit the open low E at a slow 1/8 note rate with some changes in strength to give the compressor something to compress Compressor set with both controls maxed out.

Effect off: Initial note goes to a very brief 0, then settles back to -24 being sold and -6 coming on briefly on some notes and even gettting to 0 very briefly once if I hit the note a bit harder.

Effect on: Intial note goes to a solid +6 and then the following notes stay at -24 with very brief flickers to -6. The results are reproducable. As long as I keep playing, everything is ok. But pause, and the first note seems to get amplified.

This also is noticable at lower sensitivity settings, just not as extreme.

So, is this normal for a dynacomp? Or have one of the caps gone bad and no longer "compress" fast enough?

[Mark Hammer]

Absolutely normal, Sean.  The circuit reduces gain in response to transients.  It will keep the gain reduced until the storage cap in the rectifier circuit gets charged again.  This is the basis for the famed "attack/recovery" mod.

Check the schematic and you will see a 150k resistor tied to V+ and one end of a 10uf cap.  The resistor value slows down the recharging of the cap so that gain stays reduced for a while, making the unit unresponsive to any subsequent notes played soon after.  If you wait a bit, the cap will be fully rechargd and the volume will be up again, allowing that first note to be pegging the meter high once more.  Play a bunch of notes fast and the gain will stay low.

The stock Dynacomp circuit is intended to provide supersquish and a slow recovery so as to make instruments seem like they sustain longer.  If you reduce the value of the 150k resistor, the cap will charge up faster and the circuit will once again allow notes to be initially at full volume and THEN reduced in level, even when they are played quickly.

A quick survey of all relevant compressor circuits using the same rectifier with a variable recovery time, or any of the 3rd party mods for compressors, indicates that one doesn't want to dip below 10k and much above 150k-160k to alter the time it take for the circuit to recover, charge up the cap, and have full gain once again.

Consider either installing a variable control or if that is too annoying, simply tack a 15k-22k resistor in parallel with the existing 150k unit by soldering it to the relevant pads on the copper side.  That will chop down the recovery time so that you can play faster and still hear the start of notes adequately.

If you run into trouble, give me a shout tomorrow and bring the thing over.  I should be home most of the day, except for a brief grocery run in the morning.

[seanm]

Thanks for the explaination! So what I am seeing is normal. I find that bass tends to bring out these issues in compressors more than guitar.

And now that you mention it, I saw the same effect on the flatline compressor with the bass. To solve that one, I reduced the 100uF cap in the bridge to 10uF and I believe even 1uF.

I am fixing this for a guitarist and can't find anything wrong with it. I think I will have to get him to show me what he thinks is wrong. The effect is over 25 years old, and I don't want to mess with it unless he wants me to.