Another Optical Compressor

[samhay]

While we need another dual op-amp LED/LDR compressor like we need a hole in the head, after struggling to find one I could get to do what I wanted I figured I would cobble one together anyway.

Like similar designs - DOD 280, Hollis' Flatline, Mictester's Really Cheap Compressor, etc. -  it is a one-knob (+ level) design. What does this do differently? The LED driver is a little different (increasing the resistance into the second op-amp (-) input decreases the current in the LEDs; idea thanks to an old post by R.G.) and it is somewhat self compensating - when you crank up the compression (and it gets pretty squishy) the volume stays reasonably constant. This latter idea came about after a recent post on the GGG Guv'nor, which does something similar to adjust the gain of two adjacent op-amps.
It is currently sitting on my breadboard and sounds pretty good - I will build it a some stage and there will be a vero layout in the works. Before I build it, I thought I would see if any of the wiser members  of the forum (pretty much everybody else) can suggest any improvements.

Edit: seeing as these things tend to get derailed by the choice of LDR, this is what I used: http://www.bitsbox.co.uk/sensors.html (half way down the page). 8k-24k at 10 lux & 1M dark resistance.

Cheers,
Sam

[SmoothAction]

Awesome work. Gimme a bit to put it on BB and report back. I've been a compressor hound lately. Thanks!

[samhay]

Thanks for taking a look at it. If you have a vactrol to try with it (I don't), I would imagine it would play nice too.

[midwayfair]

Can't believe I missed this!

This is a great idea for the LED driver. I love how few parts it uses, too.

I'll breadboard it as soon as I can clear the BB, maybe whip up a perf layout. It's a 1590A contender for sure.

[wavley]

Very nice, I was looking for a simple compressor build for a friend!

[jimilee]

Well i love the Idea, seeing as how I'm a solder by numbers guy, I'll watch this thread anxiously!

[samhay]

Thanks guys. Probably best to breadboard first (it has a low part count after all), but I should get a vero layout done tomorrow.
Jon - mine will be replacing another optical compressor in a 1590a. I'm not sure if it can't be improved first though. I'm a little worried about the very low impedance into the second op-amp - it feels like bad practice.

[samhay]

Well it took a bit longer than I thought, but here is a vero layout for the compressor. It is verified in as far as i have built it and it works. The layout was designed to replace something else in a 1590A so there were a number of space/layout constraints. A blurry gut shot is included to show how it all fits. Note that I 'rolled my own' vactrol (LED/LDR combo) for this one. I think a bona fide vactrol would fit, but I don't have one to try. I used one LED for this and moved the other one off board as a signal indicator - this works quite well and you can see a regular 3mm LED under quite bright light.

I have also updated the schematic a little. Nothing major, I just replaced a couple of resistors with trim pots - they are not in the signal path and allow a bit more tweaking. I also increased the potential gain a bit by dropping the value of R3. If it gets too noisy on high compression in between notes and/or you get clipping then try 10k here.

It doesn't have a name yet. If anyone can do better than 'Another Optical Compressor', please feel free to share.

[ACS]

Nice!  Can't wait to try...

Name: A-OK (Another Optical Kompressor  )

[therecordingart]

I really want to understand that first IC stage. As the signal gets hotter the LEDs glow brighter which lowers the LDR resistance which decreases the gain of the first IC stage. Turning up the compression pot increases the gain which increases the the LED brightness which decreases the LDR resistance and thus reducing the gain of that stage. Turning down the compression pot bypass the LDR entirely. The part I'm having trouble with is Vb feeding the inverting input via the 5k6 resistor. Does this make the IC act as a comparator? I don't get how to figure out what is going on with Vb being applied.

Help?

[SmoothAction]

Hey, just built one of these and it sounds pretty good. Not as "squishy" as I want, but it's really nice sounding. I'll probably be using it from now on. Thanks dude.

[SmoothAction]

 I've got the 10k trim set in the middle, I'll see if messing with it will give me better squish. Does sound good though, no clipping at all.

[SmoothAction]

Sorry, it was the 1k trim that needed tweaking. Great compressor.

[PRR]

> The part I'm having trouble with is Vb feeding the inverting input via the 5k6 resistor. Does this make the IC act as a comparator?

No. It's just the gain-set network. The gain is roughly 100K/5.6K, or less when the LDR sucks.

The 5.6K goes to signal ground.

If it went to DC ground, while the opamp's other input was biased to 4.5V, then the opamp would gain-up the 4.5V times 18.8, try to go to 85V, get stuck solid at 8V or so.

You could instead run the 5.6K through a cap, say 1uFd, to chassis and DC ground. You often see that.

In many cases it works all the same to return the 5.6K to the bias voltage. And it saves a cap. Which, looking at that intense gut-shot, is critical for this build.

[samhay]

ACS - love the name.

therecordingart - looks like PRR gave a better description than I could have done. I usually think of op-amp circuits in terms of bi-polar power supplies. In this case, you can think of Vb as being the apparent ground in between the bypass caps.

SmoothAction - glad you like it. Now that you have tweaked the 1k trimmer, do you find it gives enough compression? At least with mine, there is enough noise being amplified that the LEDs light up with max compression and 100R between the op-amps. The way I would suggest you set the trimmer is it to max the compression knob, plug the guitar in, but don't play anything, then to increase the trimmer until the LEDs just goes out.

PRR - I didn't add a cap from R3 to ground (or Vb; either works) as I was thinking in bipolar mode and I didn't think it needed any more high-pass filtering. Is there any advantage to doing so? I didn't hear any great improvements on the bread board. However, I could move the protection diode of-board and there would then be enough room in the vero layout for a cap to ground. I would suggest a 10u as this will keep the bass roll-off as is.

[therecordingart]

PRR - Thank you. I need to pound into my head that when looking at the AC action of a circuit that all voltage sources must become shorts.

[duck_arse]

samhay, what colour leds are you using? I have a whacking great (20mm diameter window) nsl-447 laying about doing nothing, about time I used it for something.

[samhay]

duck_arese - I used 3mm yellow LEDs. These were chosen because the LDR I used seemed to be most sensitive to this colour and you seem to get more bang for your buck (brighness/mA) with 3mm over 5mm LEDs.

If you want to use a Silonex NSL-4470, it should work to some degree but you will need to tweak a few things. The dark resistance is quite low (200k or 500k depending on whether it is hermetically sealed?) so you may need to reduce the value of the compression pot and/or R3. The 470p cap may need tweaking too.
Personally, I would probably mount one of these on the outside of a box and use my foot (shadow) to play havoc with an LFO or something. Either way, good luck.

[SmoothAction]

Tweaked the 1k as you suggested. Getting some decent squish now. It's more than a "is it on?" compressor, I'm getting some very obvious compression, which I prefer because I use comps as more of an "effect" than an "always on" type deal. Kicked the Flatline off my board (too subtle) and towers aside my over-engineered ibanez comp (too obvious). Again, this probably my favorite comp I've built, great job.

Oh yea, vero is 100% verified. You already said it was but now it's uber verified. Fab some nice PCBs and sell those bad boys Sam!

[samhay]

SmoothAction - glad you like it. Will leave the PCB to someone else though - don't want to ruin a perfectly good hobby by trying to make it turn a buck.

It occurred to me that it might suck a bit more power than you would expect. If you are using a battery, keep an eye on it.