Another Optical Compressor

[duck_arse]

samhay, over at perkins-elmer, they say the spectral response of their ldrs is most closely matched by orange leds.

and it's funny you should say foot shadow, because it came from an alarm install co, which used them attached to a "burgoguard" on a bank vault, looking for the torchlight of safe crackers. also got some peizos they used as microphones/vibration detectors, but they less use in a compressor.

[samhay]

I like the idea of using an LDR for burglar flashlight detection. I trust you didn't take them as a souvenir on your last (bank) job.

To be honest, I am not sure if the LEDs I used were yellow or orange. When I bought them, the pack they were in said they were yellow, but they look orange to me. There is little difference between the two anyway, so I am sure either would work. Actually, it will work with red and green LEDs too. The resistance under light will be higher, but this could be compensated for by tweaking the gain resistor values.

[samhay]

I posted the design on the other form and mictester had a couple of suggestions. One of them was to put a cap across the feedback loop of the second op-amp to stop the LEDS causing spikes in the supply rail when they go into conduction. I had tried this on the breadboard and could not hear anything. But, had another listen tonight - with the compression maxed and the volume high into headphones, you can hear a bit of fizz. A 10n cap seemed to tame this a lot, so the schematic and vero layout have been revised.

[deadastronaut]

@sam: cool, funny, i knocked up a pcb layout for this last night,(not a vero fan )  i need to try it on breadboard today first.

i need a comp for guitar/bass so i thought i'd try yours out....my first dip into the world of comps...

look ok?...i'll try it out and see if its worth using pots instead of trims.

[duck_arse]

sam, I've had my big ldr on the dmm recently, after giving it a couple of days in the light. in the dark of a 35mm film can, it over-ranged on the ohms range. then it counted out and over-ranged the 20M range.

but it reads ~170R under the desk lamp. lookout burglars, this thing's a beast.

[samhay]

Rob - layout looks good. I can't tell which way the pots are orientated though, so make sure the compression knob will increase the resistance between IC pins 1 and 2 as you turn CW.
After a bit more of a play, I don't think there is much point in having the compensation pot external, so I would move the pad over 1 so that you have the option of replacing it with an in-line trimmer and/or add another pad to the right so you can put a resistor in flat.
If you want a mod, a switch to short the 1k trimmer would probably be more useful.

duck_arse - sounds like that will work. My LDRs are >2M dark (limit of DMM) and work fine. It is paralleled with the compression pot, so the max value is not a problem. The lower the min value the better, so you are all good there too. Because the size of your LDR is so large, you will probably need to use two largish LEDs to try and light as much of it as you can. Perhaps you should go all-out and get 10 mm LEDs.

Edit: Compression pot lugs 1 and 2 are to  IC pins 1 and 2 (not IC 2 and 3)

[deadastronaut]

@sam: yep good idea, i have it on breadboard now, i'll tinker with it today...i only gave it a brief go last night, definately getting comp, but like i say i need to tinker led wise i think.

ldr is 1M dark.

what leds did you use yellow standard diffused, or yellow superbrights ?...3mm/5mm?

[samhay]

Rob - I used a diffused 3mm LED. Super bright LEDs have a very fast turn on and off that can lead to quite large glitches in the supply rail, which you can hear. Are you using 1 or both? Oh, and it will not work if you do not have the LDR covered - ok if its in shrink wrap or insulation tape, but if you have just pointed the LED(s) at the LDR, you will need to either work in the dark or cover the circuit e.g. with a black cloth. This can be a PITA.

I spent some more time playing with the trimmers last night - it needed readjusting after I added the 10n cap.
Set up is with guitar plugged in, but no playing: Start by setting both trimmers to 0. The LED should get progressively brighter as you increase the compression. At about 95% compression, dial the 1k trimmer up until the LED is quite dim. You should find a sweet spot where it changes quite quickly - set it at the dim edge of this. Now set compression at noon and increase the 10k trimmer unit the LED is essentially out. It shouldn't take much. This should set a reasonable range of compression over the knobs range. Now tweak to taste.

Basically what is happening is that this is acting like a soft limiter. It shouldn't clip (if it does, decrease the 1k and/or 10k trimmers), but the max volume will be similar(ish) regardless of the compression setting. As you increase the compression, something like the threshold is decreased. You should be able to play a very gentle single note and hear it get progressively louder with the compression knob.

[deadastronaut]

hi sam:


i'm using 1 yellow 5mm standard led for the ldr , heatshrinked, and 1x5mm led yellow as the indicator.

definately getting compression, cool little circuit, ..nice one, ...i'll tinker a bit more, 

[bassmannate]

Finally got this one completed the other day and I've been messing with it. Love it on bass but i have a question about adjusting the trim pots. I'm trying to follow Sam's instructions but putting the trimmers at zero never gives me lighted led with compression knob at max and no playing. I know I've turned them the right way because i have a pretty solid light when playing light to moderate. Any thoughts? Possible just the type of led i have? Using 3mm standard yellow. Would really like to adjust this for maximum range off compression available.

[maiko]

the link doesnt work anymore

Can you repost the image.  Im interested to see it

[samhay]

^sorry. I've fallen foul of dropbox's decision to disable the public folder.

Here are the links to the schematic and layout:

https://drive.google.com/open?id=0B-0Ui8eNhljBMk5pR3JMRVNzdk0
https://drive.google.com/open?id=0B-0Ui8eNhljBZ2dqUXJtOEhYdjg

[samhay]

Finally got this one completed the other day and I've been messing with it. Love it on bass but i have a question about adjusting the trim pots. I'm trying to follow Sam's instructions but putting the trimmers at zero never gives me lighted led with compression knob at max and no playing. I know I've turned them the right way because i have a pretty solid light when playing light to moderate. Any thoughts? Possible just the type of led i have? Using 3mm standard yellow. Would really like to adjust this for maximum range off compression available.

If you turn the R2 trimmer all the way both CW and CCW, can you get the LED to go (almost) out? If not, you will need a higher-resistance trimmer and/or to increase the value of the 100R resistor.

[maiko]

Thanks for the links

Will try it out seems simple enough and low parts count too

[highwater]

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.

Emphasis added.

I'm trying to follow Sam's instructions but putting the trimmers at zero never gives me lighted led with compression knob at max and no playing.

It's possible that your bass rig is simply too quiet for Sam's instructions to work. Do you have a noisy pedal that you can run into it?

It's also possible that you accidentally used a 1k resistor in place of the 100ohm resistor. I keep my resistors in labeled bags and I've still managed to do that several times.

[samhay]

>Finally got this one completed the other day ...

Sorry, I clearly didn't read your post properly.
The R2 trimmer is there to set the current through the LEDs such that you get maximum change in brightness (actually max swing in the LDR resistance) when some signal hits the input of the circuit.

As you seem to be getting good response from the LEDs 'because i have a pretty solid light when playing light to moderate' and the LEDs must be going dark when you aren't playing, then it must be working ok and you can adjust the trimmer by ear.

It is some years since I played with this circuit (although the compressor was on my board until quite recently) and I have forgotten a few details. I expect that my suggestion was based on setup using a poorly shielded telecaster with single coils. If you are using an active bass (?) then it will be much quieter than this, and as highwater pointed out, this approach doesn't seem to work.

[bassmannate]

>Finally got this one completed the other day ...

Sorry, I clearly didn't read your post properly.
The R2 trimmer is there to set the current through the LEDs such that you get maximum change in brightness (actually max swing in the LDR resistance) when some signal hits the input of the circuit.

As you seem to be getting good response from the LEDs 'because i have a pretty solid light when playing light to moderate' and the LEDs must be going dark when you aren't playing, then it must be working ok and you can adjust the trimmer by ear.

It is some years since I played with this circuit (although the compressor was on my board until quite recently) and I have forgotten a few details. I expect that my suggestion was based on setup using a poorly shielded telecaster with single coils. If you are using an active bass (?) then it will be much quieter than this, and as highwater pointed out, this approach doesn't seem to work.

Ah, thanks for the tips! Not active but I am running a couple of EMG soapbars. They're incredibly quiet. I'm probably just not getting enough noise to trigger it when not playing. Once again, really loving this circuit! At the right settings, it can really fatten and tighten up the tone of my bass!

[samhay]

An EMG - the 'I can't believe they're not active' pickups.
Glad you like it.

[samhay]

Here is, hopefully a more permanent link to, the schematic and vero layout:

Edit - let's try something else:

[duck_arse]

Here is, hopefully a more permanent link to, the schematic and vero layout:

"my client" does not have permission to peep at those https addys. so says googoo.