Light Speed Compressor - another experiment in optical compression

[samhay]

Possibly for the best.

[Kipper4]

I see what you mean the addition of the Attack switch is good.
The feedback loop diodes too. Definitly sounds less overdriven at low ratio settings and more squished at high settings.
Again Might just be my ears.
Ozziescope time. Laters.

[Kipper4]

Try a 47k R8 cobber .....

[samhay]

RE: using a roll-your-own vactrol.

The ratio pot sets the gain of IC1A. Remember, this is a non-inverting op-amp, so the gain =  1 + (R(ratio pot) || LDR)/4.7k. This gain in turn determines how much compression you will get.

The dark resistance of the NSL 'vactrols' I used is > 1M* so the LDR's dark resistance is >> any possible ratio pot resistance and can be ignored giving gain from 1  to about 21.

The compression happens when the LED turns on and reduces the LDR's resistance. In this circuit the NSL's light resistance can get to < 1k.* This sets the compression ratio as R(LDR) < 4.7k and the gain can approach unity. As we are using a non-inverting op-amp, the gain can not be < 1, so we don't need a make-up gain stage afterwards (let's call this a feature).

What does this all mean? Most home-rolled LDRs have light resistances much greater than the NSL's used here. To make them work, scale up the values of the ratio pot, the 4.7k resistor (R3) and then scale back the associated caps (C2 and C3). Aim to make R3 a little larger (edit) than the LDR light resistance and the ratio pot 10-20 x larger than R3. However, the LDR's dark resistance will become limiting (to max gain) at some point, so try to pick an LDR with as wide light-dark resistance range as you can.

* datasheet: http://logosfoundation.org/instrum_gwr/tinti/datasheets/Silonex_NSL-32SR2_60721.pdf

[Kipper4]

Thanks Sam I'm far from done with messing with this.
I'll be a few days getting back to this. Cheers.

[bluebunny]

A small necro-bump to post a picture of my build from a couple of weeks ago.

[samhay]

 Very nice.
Which caps did you use for the attack/release control?

[bluebunny]

I went with the 1uF / 10uF combo in the version 1 (March 2016) schematic.  (If there was another version, I don't have it!)  TBH, I didn't get much chance to test it properly (too late, people asleep...), but my layout has plenty of room to tack on other values if I need.  Either way, cool design.  Thanks, Sam.

[samhay]

That should work nicely, and you're welcome.
I think my attack caps go up to 100u, but I almost never use that setting. If 10u isn't quite long enough, particularly for bass, 20 or 30u would be plenty.
Once you've had some time to play with it a bit more, I'd be curious to hear what you think.

[Tonechaser84]

I'm planning on putting together a version of this compressor adapted for a 15-0-15 dual power supply, but I've hit a problem.

The bottom (or top - pin 3) of the Threshold is connected through a 'minimum-squash' stopping resistor (R8) to the mid supply (4.5v) ref voltage, and a capacitor (C7) to earth. This suggests that the side chain signal is positively biased, and that in a dual supply version R8 should (theoretically) be connected to earth, and C7 should be connected to -15v.

Is this right? and if not, what should I change to adapt this circuit for a dual supply? Would appreciate any guidance.

[samhay]

The threshold pot and R8 provides bias to IC1B, so should go to ground on a bipolar supply.
If you use a non-polarised capacitor for C7 (as is likely to be the case for 100n), then I suggest you also connect this to ground. If that's tricky, you can alternatively connect to the negative supply.

[Tonechaser84]

Thanks for the reply Sam. I'll test this out on the prototype.

One more little question if I may, I'm hoping to incorporate it into a solid state practice amp, and would like the compressor to be controlled by one single pot (Threshold (with an internal preset for Ratio)) where minimum compression (maximum threshold) corresponds to no compression. This would, I suppose, mean omitting R8 altogether and connection pin 3 of the Thresh pot to earth?

Is C7 just power supply local de-coupling for the half supply ref (along with C11)?

I'm just guessing hear.

[samhay]

If you want a one-knob compression control that doesn't compress at minimum then the control has to be via the ratio pot. The 'Two Knob' version I designed fits this criteria and would be my recommendation (below).
Make R8 10-22k or a trimmer that you can set so that the volume change is minimised when you vary the compression.

C7 and C11 - yes, this is just to filter the bias and supply voltages, respectively. On second thought, C7 should be unnecessary if you use a bipolar supply, but then you would probably add the equivalent of C11 to filter/decouple the negative rail....

[tubegeek]

Here's my first-ever vero build that came from someone else's layout - thanks to Sam Hay for both the design and the layout. Not tested just yet - it's pretty late.

[samhay]

Looking good.
Hope you like it.

[tubegeek]

Getting oscillation - trying it without being boxed it up. Suspecting too-long leads to the ratio and threshold pots - frequency varies with the setting of one of the pots. More later - will shield the pot leads, ground the pot bodies, and try again. Using this layout.


Question: I can omit the filtered Va lead and reduce the number of antennas by one - that's only intended for the indicator daughterboard right?

[samhay]

Correct - don't need Va if you don't add the indicator (and I don't recommend you do add the indicator).

[tubegeek]

Jeez, Sam, I thought you had my back! How come there isn't a warning on the Vero layout? Perhaps something along the lines of, "Make ALL the trace cuts indicated for best results." (See zoomed-in photo for The Inevitable Vero Rookie Mistake.)

Working great! This thing is very unobtrusive, a nice design indeed. I was drawn in by the full-wave envelope rectification and optical compression - seems like they are definitely a good combination.

THAT IS A VERIFIED (YET AGAIN!) VERO! Thank you for a lovely design and an easy-to follow vero layout.

Build Notes: Attack implemented with 1uF (grey box) and 10 uF (tantalum) mounted on an on/off/on switch. The 10uF caps on the board are also tantalums. Vactrol is a roll-your own, I measured my photoresistor at about ~350 ohms min and >>2M max, with a "reasonably fast" recovery, and I used a bright white 3mm LED, mated the two in some shrink tubing. A little light is getting in so the max ratio will probably be a touch bigger than what I'm hearing now with no enclosure. On the other hand it's reassuring to see the LED flashing a bit as confirmation it's working. I used Schottky diodes in the fullwave bridge, matched tightly around 234 mV drop.

My buddy Christien better like his birthday present - he was hoping for a Ross but I didn't feel like building a Ross once I saw this.

Thank you Sam!

[samhay]

Glad you like it.
Hope you didn't have to do too much debugging before checking the solder side of the board.

[tubegeek]

Glad you like it.
Hope you didn't have to do too much debugging before checking the solder side of the board.

Thanks! Honestly, I found it  surprisingly quickly - I started verifying all the parts positions against the layout and looking at all their solder joints one by one and when I did, I was sort of looking at them relative to the board cuts. So when I got up into that corner it was plain to see.

I like Vero - someone called it "perf board's idiot cousin" but it sure goes together quickly.