Best optical compressor?

[edvard]

I have the shell of a Maestro Sustainer, and I want to build a compressor into it.  The original "brick o' goop" guts are long gone, as the former owner blew it up, so it's useless unless I can build something approximate in its place.  I have no idea how good or not the original Maestro Sustainer is/was, and all efforts I've read on the internet to de-goop an original unit have proven unfruitful, but in the last thread I read, a component was discovered that looked like a Vactrol or similar.  So, based on this, I surmise it is optical-based, and from the name I assume it is more compressor than limiter.  So I turn to you, oh wise forum denizens, who have much more experience with the internals of compressors than I do, and ask...
What is (and I know this will invite a plethora of opinions) the best optical compressor circuit for guitar?


P.S. I've built a Craig Anderton compressor/limiter (Project #8 from "Electronic Projects for Musicians") and was underwhelmed.  I did use an LED/LDR pair encased in black shrink-wrap instead of a genuine CLM6000, could this be the problem?  Or is the Craig Anderton unit not the best? 

[Matthew Sanford]

Not sure on the compressor, but a light leaky vactrol is not good. Shrink leaves light either end, you could try filling/covering it with something dark and non-conductive. Duct tape can work if you verify it's not conducive, adhesive side too

[ElectricDruid]

The best optical compressor would be the LA-2A, according to most of the internetz!

Not an easy thing to clone, unfortunately, or at least not as a pedal. There are lots of full-size rack clones. But the need for big transformers and tube power supplies makes a true clone pedal version basically impossible.

[Phend]

Colorsound supa sustain ? 18v

https://youtu.be/ZS8NllWdwIE?si=-h0DCfXdEfn2s6f6

[ElectricDruid]

You could build a Cornish OC-1 into it:

https://aionfx.com/app/files/docs/oceanid_documentation.pdf

[Mark Hammer]

There IS no "best optical compressor".  There are compressors which are good for this or that task (e.g., drums, vocals, bass), but nothing which is an all-around "best".  I like the first compressor I ever bought, the little grey Univox Unicomp, which is THE simplest and cheapest optical compressor imaginable: one op-amp, one transistor, and an LED/LDR.  It is not "transparent", but is an effect, and my goodness how it loves the bridge pickup on a Telecaster. 

[edvard]

Not sure on the compressor, but a light leaky vactrol is not good. Shrink leaves light either end, you could try filling/covering it with something dark and non-conductive. Duct tape can work if you verify it's not conducive, adhesive side too

I made sure to leave some heat shrink hanging over, and clamped the open ends with needle nose pliers as it cooled. I just had a thought that I may have the diode connected backwards, I'll double check.

[edvard]

Colorsound supa sustain ? 18v

https://youtu.be/ZS8NllWdwIE?si=-h0DCfXdEfn2s6f6

I'll check into that, thanks!

[edvard]

You could build a Cornish OC-1 into it:

https://aionfx.com/app/files/docs/oceanid_documentation.pdf

I've heard of that one, but good gravy does that look like a job!  I'm a big fan of "as simple as possible, but no simpler"; like smaller than a Big Muff, bigger than an Dan Armstrong Orange Squeezer.  I don't have enough spare weekends to tackle that Aion unit, but thanks for the suggestion

[edvard]

There IS no "best optical compressor".  There are compressors which are good for this or that task (e.g., drums, vocals, bass), but nothing which is an all-around "best".  I like the first compressor I ever bought, the little grey Univox Unicomp, which is THE simplest and cheapest optical compressor imaginable: one op-amp, one transistor, and an LED/LDR.  It is not "transparent", but is an effect, and my goodness how it loves the bridge pickup on a Telecaster. 

Yeah, I kinda figured that I would get a lot of opinions about what "best" means, but was hoping for somewhat of a consensus.  I also did mention "for guitar" specifically, so that rules out many purpose-built units. I just wanted something I could stomp on that wouldn't require but a handful of the spare parts I can dig out of my salvage bin. The Univox sounds like it fits that bill quite nicely, and a few more bits to maybe tweak it for the better wouldn't be out of line.  And your comment on the Telecaster bridge pickup I find intriguing...

[cspar]

Mictester's Really Cheap Compressor is a small parts count optical compressor that works rather well.

[Fancy Lime]

Terminology caveat:
The following devices are all technically limiters, like almost all classic guitar "compressors".

This one is pretty simple:
https://www.diystompboxes.com/smfforum/index.php?msg=1125364

Slightly more parts would be the DOD 280, a classic for a reason:



Sounds fantastic. Use a TL072 and any ol' NPN transistors for the DOD. However, this one can be improved (imo) by adding diodes from the bases of the transistors to ground, so that the cathode touches the base and the anode is at ground. Exactly like the Ross compressor (not optical but two of the transistors, Q3 and 4, in the side chain work thebsame way as in the DOD) does at Q3 and 4:




One thing to remember is that the attack and release times of vactrols vary considerably and different vactrols may sound substantially different in the same design. Also, the minimum and maximum resistance and LED current expectations of the vactrol need to be in a suitable range for a given design. These things can be adjusted for but in my experience, the DOD 280 is fairly good even with pretty bad vactrols.

All that said, my favorite optical compressor is a FET compressor with the attack and release times slowed down to get that "optical sound". Simplest example and easy to tune for this purpose would be the Love Squeeze:



Increase R9 and R11 to slow the attack and release, respectively. I would use a J113 as the FET and possibly leave out the LEDs.

HTH,
Andy

[fryingpan]

The best optical compressor would be the LA-2A, according to most of the internetz!

Not an easy thing to clone, unfortunately, or at least not as a pedal. There are lots of full-size rack clones. But the need for big transformers and tube power supplies makes a true clone pedal version basically impossible.

Well, I think you can achieve something similar with transistors in a stompbox format. IIRC, it's basically an attenuator controlled by a photoresistor (normally high resistance, so little attenuation when not compressing, larger when compressing), then you have two parallel lines, one to the sidechain and the other to the gain. The sidechain lights up the light for the photoresistor, and basically the speed of the compression is almost completely dictated by the optocoupler (there is like a 5ms added attack time by the filtering, but the rest is just the optocoupler).

All the colour is provided both by the optic element (which reacts in a very characteristic way) and the inline gain stages. Without actually sourcing the original lamp (I think it's more like a regular lamp, so it has slower turn-on time than an LED) I believe you could use a low voltage filament lamp and couple it in a black sealed plastic tube with the photoresistor. (A filament lamp doesn't even need signal rectification, it lights up both polarities). As for the gain stages, if you want it to be somewhat "dirty" you could use a discrete opamp or whatever. Maybe even a relatively low-gain CMOS linear amp. It won't sound like an LA-2A but it might get close enough for guitar or bass through a guitar/bass amp.

[ElectricDruid]

Well, I think you can achieve something similar with transistors in a stompbox format. IIRC, it's basically an attenuator controlled by a photoresistor (normally high resistance, so little attenuation when not compressing, larger when compressing), then you have two parallel lines, one to the sidechain and the other to the gain. The sidechain lights up the light for the photoresistor, and basically the speed of the compression is almost completely dictated by the optocoupler (there is like a 5ms added attack time by the filtering, but the rest is just the optocoupler).

All the colour is provided both by the optic element (which reacts in a very characteristic way) and the inline gain stages. Without actually sourcing the original lamp (I think it's more like a regular lamp, so it has slower turn-on time than an LED) I believe you could use a low voltage filament lamp and couple it in a black sealed plastic tube with the photoresistor. (A filament lamp doesn't even need signal rectification, it lights up both polarities). As for the gain stages, if you want it to be somewhat "dirty" you could use a discrete opamp or whatever. Maybe even a relatively low-gain CMOS linear amp. It won't sound like an LA-2A but it might get close enough for guitar or bass through a guitar/bass amp.

+1 agree completely. There's nothing "magic" going on, and it's a pretty simple design. As you say, the specific attack/release times are a happy accident of the materials they used, and the tone colouring caused by the tube circuitry and audio transformers give it a particular character.

So...perhaps "the best optical compressor" is really a little design challenge for DIYStompboxes?! Anyone fancy it?

[Mark Hammer]

Here's the schematic and a vero layout for the Unicomp.  The diode pair on the output is an interesting element.  I forget who here told me (was it Paul, or one of the other "old-timers"?), but such diode pairs were a frequent feature on broadcast compressors.  Their role is to clamp the level immediately, while the LDR "wakes up".  Because it's a difference of milliseconds, in terms of the response of the diodes-vs-LDR, one doesn't hear it as "distortion", just maybe a wee bit of bite to the initial pick attack.  Maybe that's why I like it.  Naturally, the LDR/LED pair selected is something one does by trial and error, with a wee bit of theory thrown in.

[fryingpan]

Well, I think you can achieve something similar with transistors in a stompbox format. IIRC, it's basically an attenuator controlled by a photoresistor (normally high resistance, so little attenuation when not compressing, larger when compressing), then you have two parallel lines, one to the sidechain and the other to the gain. The sidechain lights up the light for the photoresistor, and basically the speed of the compression is almost completely dictated by the optocoupler (there is like a 5ms added attack time by the filtering, but the rest is just the optocoupler).

All the colour is provided both by the optic element (which reacts in a very characteristic way) and the inline gain stages. Without actually sourcing the original lamp (I think it's more like a regular lamp, so it has slower turn-on time than an LED) I believe you could use a low voltage filament lamp and couple it in a black sealed plastic tube with the photoresistor. (A filament lamp doesn't even need signal rectification, it lights up both polarities). As for the gain stages, if you want it to be somewhat "dirty" you could use a discrete opamp or whatever. Maybe even a relatively low-gain CMOS linear amp. It won't sound like an LA-2A but it might get close enough for guitar or bass through a guitar/bass amp.

+1 agree completely. There's nothing "magic" going on, and it's a pretty simple design. As you say, the specific attack/release times are a happy accident of the materials they used, and the tone colouring caused by the tube circuitry and audio transformers give it a particular character.

So...perhaps "the best optical compressor" is really a little design challenge for DIYStompboxes?! Anyone fancy it?

I'll give it a try these days.

[ElectricDruid]

That Unicomp circuit does look good. Dead simple, decent input impedance, one chip, etc etc.

There's a better schematic over on this thread at the other site.

[snk]

You could build a Cornish OC-1 into it:

https://aionfx.com/app/files/docs/oceanid_documentation.pdf

I've heard of that one, but good gravy does that look like a job!  I'm a big fan of "as simple as possible, but no simpler"; like smaller than a Big Muff, bigger than an Dan Armstrong Orange Squeezer.  I don't have enough spare weekends to tackle that Aion unit, but thanks for the suggestion

Hello
Just chiming in 

As you mentionned the Orange Squeezer, I can suggest the Emma Transmorgrifier. It's based on the Orange Squeezer, but imho much more interesting. It's an easy build, and it can be a very colourful compressor, but it can also act as a gentle dynamic smoother and leveller.

I see the La2A mentioned above : if you want an opto compressor inspired by the La2A, I can suggest the Baja Opto Limiter. You really get the opto behaviour of some rack units from the past.

The Sam Hay Light Speed Compressor is also well worth a shot : https://samdump.wordpress.com/welcome/projects/light-speed-compressor/

[fryingpan]

By the way, these are essentially the specs of interest for an LA-2A stompbox adaptation:

- Gain: up to 40dB (I feel that 20dB more than suffice though).
- Frequency response: 30-15kHz (+/-1dB)
- Ratio: approx. 3:1 in compress mode, approx. 10-20:1 in limit mode (little audible difference)
- Attack: approx. 10ms (program dependent)
- Release: "multi-stage", approx. 60ms for 50% release, program dependent for 100% release

I have a couple of free hours, I might sketch up something.

[SprinkleSpraycan]

These aren't on the smaller side but I can't say enough about how much I love the Dinosaural OTC 201. Controls are simple. General tone brightener but also has a lot of push when needed.

https://aionfx.com/project/convex-parallel-compressor/

The thorpy version is probably a little easier and simpler controls still.

https://www.pedalpcb.com/product/generaltso/