getting a bit more output and "Squish" from Mictester's Really Cheap Compressor

[mordechai]

I built up a vero layout of this cool little circuit:




With the sustain all the way up, it does subtly level-out my tone, but I'm wondering if there's a way to increase its compression/squishiness and sustain.  I'd also like to increase the output a bit -- I have to turn the 10K volume pot up to about 2:00 to get to unity volume with the bypassed signal.

[thermionix]

I'd also like to increase the output a bit -- I have to turn the 10K volume pot up to about 2:00 to get to unity volume with the bypassed signal.

Turn it up past 2:00?

[Mark Hammer]

Hi from Scotland, Mark.

The design is vey close to Craig Anderton's EPFM compressor.  Where it differs is that Anderton used 3 op-amps.  The third one was simply a variable gain stage after the output of the first op-amp.  The circuit you posted has a variable gain stage for increasing the sensitivity of the compression, but contributes nothing to the output level.  If the combined parallel resistance of the 2M2 feedback resistor and LDR drops below 220k, you end up with *less* than unity gain.  And of course, the more sensitive the LED/LDR pair is made, the more likely the feedback resistance WILL drop below 220k.

One solution is to reduce the 220k input resistor to 100k or so, to increase the overall gain of that stage.  The LDR will still yield a drop in gain, and audible compression, but probably produce a little more output level.  Whether it yields enough to constitute a discernible "boost" is your decision.  And since gain is multiplicative, any gain added to stage one will increase the combined gain of the two op-amp stages driving the LED/LDR.

More "squish" can likely be provided by making those LEDs red...for two reasons: 1) red will be more responsive to lesser input signals, 2) most readily-available LDRs respond maximally to red-ish wavelengths.

What I don't see in the circuit is any sort of means to adjust gain-recovery time.  Indeed, gain recovery is pretty much set entirely by the characteristics of the LDR.  To be fair,  Anderton's circuit didn't include any gain-recovery control either.  Fast gain recovery increases what folks call "transparency", but reduces audible squish, because the gain bounces right back between pick strokes.  If you add something like a 10uf cap to ground, from the junction of the 1uf output cap and the diode pair, that should "hang onto" the average sensed signal for a bit longer, driving the LEDs for a few moments more, and keeping the gain from bouncing back instantly.

There's always the possibility that you have the perfect LDR characteristics.  But if not, try out the additional capacitance for a llittle more lag.

[PRR]

> gain recovery is pretty much set entirely by the characteristics of the LDR

LA2a was and IS a highly respected studio limiter, time constants only in the LDR. (True, they started with a magic batch, went crazy trying to select more from the very variable LDR processing...)

> 10uf cap to ground, from the junction of the 1uf output cap and the diode pair

That's still AC. No rectifier, until current is turned to light.

Frankly: do you want Really Cheap or do you want "good" for not much more loot?

[Ben Lyman]

Paul is right, I'm thinking spend the extra $5 on a CA3080 and build yourself a Dynacomp.
That said, my R.C.Comp is very squishy and has lots of hang time on the release.
For volume, ya I have to turn it up more as I turn the comp knob up but never seems to lack in volume.
I do my vactrols the way cozy builder taught us in one of his posts and I do believe it's important to take your time and maximize the potential of your vactrol when making it.
http://www.diystompboxes.com/smfforum/index.php?topic=110831.msg1018078#msg1018078

[Mark Hammer]

Paul is right on both counts (not unexpected).  I *thought* my suggestion of averaging cap before the LED pair didn't look exactly right, but the buffet we went to prior to posting compromised my reasoning.   

As for the LDR,  if one can select the response characteristics judiciously, there's nothing wrong at all with relying on them for the "feel" of the compressor.  Anderton certainly relied on the characteristics of the CLM6000 for his compressor.  When we rely on home-made vactrols, using garden variety parts, though, I suspect some additional control is needed.

Given my suggestion of red LEDs, rectifying the op-amp output before the LEDs with a schottky, and then tacking on an averaging cap, could work.  AND it would still be really cheap.

[pinkjimiphoton]

funny, was dicking around with this circuit and saw this while researching to squash a bug (i had the cathode of my 10u cap off by one hole, so instead of being grounded it was connecting to the input cap <slaps head>)
what i found with the 220k resistor is you can go as low as about 47k, and it will get louder, but the lower ya go the noisier it tends to get. adding a second 220k in parallel works out to a reasonable compromise, but i just added a 220p treble peaker cap across it.
also went with a 100k output pot in search of a bit more balls. seemed to do the trick.
didn't try the shotke trick, but i tried a couple led/ldr combos and settled finally on an ldr reading about 1.3m in full darkness and a pair of water clear yellow 5mm leds. get a really good squish, and a crazy long sustain. very quiet.
i adapted the tagboard vero slightly and got it down to an 11x12 piece of vero, and got it to fit nicely in a 1590a.
this is a surprisingly fun little circuit. i don't care for compressors. this is more of a sustainer. think big muff without the noise and filth and you'd be close. very clear, very transparent.
always reccomend chris's circuits

my two cents.. ymmv of course

[merlinb]

Replace the TL072 with an NE5532 and lower the LED limiting resistor to 470R; this will allow more current to be driven into the LEDs. You coudl also put in higher-efficiency LEDs. The brighter you can get those LEDs, the more gain reduction (squish) you get.

[Boba7]

With the sustain all the way up, it does subtly level-out my tone, but I'm wondering if there's a way to increase its compression/squishiness and sustain.  I'd also like to increase the output a bit -- I have to turn the 10K volume pot up to about 2:00 to get to unity volume with the bypassed signal.

If you used tagboard's layout, the sustain pot is reversed, so full CCW is maximum sustain.
If not, well there's nothing I can add to this topic! Interesting read, thanks.

[pinkjimiphoton]

guys,
if you, like i , had a problem with distortion on the compressed signal and rolled your own vactrol like i did... i actually put both leds on one side and the ldr right against 'em and shrink wrapped it... originally. there was always an edge of distortion to it i couldn't get rid of.
last nite i remembered in my fuzzy out hippy brain having a similar problem with the wobbletron/magnavibe
so i cut open the shrink wrap and dragged one of the led's out of the vactrol i'd made. bit of an improvement.
as i started dragging out the other one i noticed the distortion was going away as i backed the led from the ldr... i'd used golden/orange leds on this build for some reason, clear 5mm ones. at about 1/4 of an inch or so separation, i lost the distortion and gained all the compression i'd never realized was on tap, as well as several db more headroom. i used to have to dime the volume just about to get unity gain, now unity is about 9:00-ish.
getting some great squish and got the whole thing stuffed in a 1590a. works great.
so if you're getting distortion, try increasing the distance between led and ldr. you don't need to wrap them once its in the box... but make sure leakage from any indicator lamps is blocked off, cuz that WILL affect it.