Light Speed Compressor - another experiment in optical compression

[tubegeek]

UPDATE: gifted to my buddy Christien on the occasion of his 50th birthday this week. He's VERY happy with his present. As well he might be: such a nice design, Sam. I'm really amazed at how easy/quick it was to take a verified vero layout to a competed pedal, remarkable, even with the mistake it was super quick.

[samhay]

Excellent, and happy birthday to Christien.

[snk]

Hello,
I am reviving this thread,
First, to say thank you to Sam Hay for an interesting design, a very nicely laid out veroboard layout, and sharing its knowledge on the forum 

Then, because I built it today, and sound passes through but no compression occurs...

I will describe my build, hoping that I can get some directions for troubleshooting / debugging :
- I followed the "3 knob version" veroboard layout done by Sam Hay in April 2016.
- I used a DIY vactrol (5mm orange led + GL5516. I have seen some people mentioning that combo above in the thread).
- I know DIY vactrols can be tricky, so I also tried with VTL5C4, and a LCR0202, without any success...
- I have read that if using a DIY vactrol, we should scale up the ratio pot, the 4.7k resistor (R3) and scale back C2 and C3. So, I used 15k (instead of 4.7k) for R3, 250k for the ratio pot (instead of 100K), and 470pF (instead of 1nF) and 1µF (instead of 2.2µF) for C2 and C3.
- I do not use the daugherboard (for compression indicator);
- The ATTACK switch toggles between 100nF and 10µF.
- I have used a "sidechain" switch, by adding a 100nF cap in parallel with C6 (in one position, it is the stock circuit, with no additional cap, and in the other position, it adds 100nF).
- The diodes are 1N60 Schotty.
- I haven't used yet an Output Volume pot (10kA), so the output goes straight from the circuit to the jack socket.
- On my vero build, the vactrol is placed with the LDR at the left, and the LED at the right, with negative pin at the bottom and positive pin at the top.
- The opamp used is a NE5532.


I understand that compression occurs when the vactrol led begins to light up... and I think it doesn't light up (it's wrapped in heatshrink tube, but when i put my eye close to the little hole, I don't see any light whatsoever).
The 2 red leds do not light up either, even if i feed the circuit very loud levels.
Turning the pots or switching the switches do not produce any change in sound.
If I remove the vactrol (it is socketted), the sound is the same.

I am going to measure the voltage at the 5532 pins, but hearing the dry signal passing through the circuit, I don't think it is an opamp issue.
The veroboard layout by Sam Hay is very well done, there are few cuts and few jumpers, so i will check again, but I don't think there is a mistake there.
My guess is that the signal that should trigger the compression action is too low : either my vactrol is not sensitive enough (but i was hoping to trigger a reaction from the vtl5c4 or the lcr0202), or the tweaks i made to R3, C2 and C3 are too much (or still not enough)?
What I don't get is why the red leds do not light up when i feed the circuit some loud signal? Is there a way to check if something is wrong here?

If anyone has any suggestion, I'd be glad to try it

[snk]

Re-
Note to self : when you already double-checked, then triple-check : the 10µF cap at the bottom right was soldered one row too low 

- Good news : I can see the vactrol led light react to the signal.
- Bad news : I can't hear a change to the signal. When I turn the THRESHOLD pot, I can see the vactrol led lighting more, but I don't hear any change sound-wise. And moving the RATIO, ATTACK or SIDE-CHAIN pot/switches does nothing either.

[samhay]

If the ratio knob doesn't make it louder, you have another error to find. You won't get any compression if the ratio is shorted for example.

[snk]

Hi, Sam
Thank you for your circuit design, and for your help.

I will check the ratio knob and around.
[edit] I changed the ratio pot for a 500K, and did nothing.
All i get is the dry signal at unity gain.

Did the modified values for R3, C2, C3 seem sensible to you? I increased R3 by 3 and decreased C2 & C3 by half, but I have no clue is the increase/decrease scale is right, or if it could cause issues...

[snk]

Soooo... when you have triple-checked a veroboard... always quadruple-check it!
I think I deserve an award : on a 5 electro cap circuit, I managed to mess 3 of them 
For the 2.2µF at the bottom left, I had read that I may reduce its value. I didn't had a 1µF, so i used 2 x 0.47µF caps, and didn't realized I had to solder them at row K (and not J).
A closer and more accurate look at the schematic made me see the mistake, and now... the compressor is spanking! 

First impressions are very good : the ratio & threshold pot are very efficient, and the compressor action can go from quite subtle and transparent to really spanky, just the way I like !

So I can confirm that the circuit works fine with a home-made vactrol (using 5mm orange led + GL5516), and that the values I used for R3, C2 & C3(quoted a couple posts above) allows the circuit to behave really well (the way I think it should).

Once again, thank you for the help (and, of course, for sharing your circuit design), and sorry for the dumb troubleshooting

[samhay]

No problem, glad you got it working.

[snk]

A couple more questions, if you don't mind :

"C6 is a HP filter in this Lightspeed Comp."

Do you mean it is HPF in the side-chain filter (to remove some of the bass energy before triggering the compressor), or on the whole signal (affecting the whole signal)?

optionally switch additional caps parallel to 100n

Would you mind elaborating a bit about the timing switch (attack)?
You write "There is some distortion from envelope ripple, especially with smaller values of C8-10, but if you are looking for a 'character' compressor then you might find this quite pleasant – I like it." So, I guess that smaller values for the caps means shorter timings, right?
With the 100nF alone, we get the shortest attack & decay timing, that's it?
Is it much, much longer with 10µF (I tried with 2.2µF and didn't noticed a drastic change in the attack or release)?

[samhay]

C6 forms a high pass filter with the threshold pot + R8. This will only affect the signal going into IC1B (i.e. not the audio). Also, the corner frequency is ~14 Hz, so it won't be filtering much unless you are playing bass.

The C8-C10 caps set the attack and release time - longer attack comes with longer release and vice versa.
You might find a noticeable difference increasing this capacitance above 2u. If you feel like it would benefit from a longer attack/decay, temporarily rig out e.g. crocodile clips and try different capacitor values.
I think I usually use values of about 1-10u, but I the largest in mine is, I think, 100u - I used a 12 pole rotary switch and neither can remember what I used, nor which position the knob gives a specific value.

[snk]

That's perfectly clear, thank you.

C6 forms a high pass filter with the threshold pot + R8. This will only affect the signal going into IC1B (i.e. not the audio). Also, the corner frequency is ~14 Hz, so it won't be filtering much unless you are playing bass.

So it's in the side-chain, great
I'll experiment a bit, and try a value setting it around 80-100Hz.
Am I right assuming that the HPF is formed by C6 on one side, and R8+the threshold pot value? And, if so, that turning the threshold pot will change the side-chain corner frequency?

The C8-C10 caps set the attack and release time - longer attack comes with longer release and vice versa.
You might find a noticeable difference increasing this capacitance above 2u. If you feel like it would benefit from a longer attack/decay, temporarily rig out e.g. crocodile clips and try different capacitor values.
I think I usually use values of about 1-10u, but I the largest in mine is, I think, 100u - I used a 12 pole rotary switch and neither can remember what I used, nor which position the knob gives a specific value.

Thank you for the tip. I will try with 47µF, see what it gives and if I like it, and move down from there.
So far, I like the "stock" timings, but I am curious and willing to experiment a bit

[samhay]

>Am I right assuming that the HPF is formed by C6 on one side, and R8+the threshold pot value?
Yes.

>And, if so, that turning the threshold pot will change the side-chain corner frequency?
No*. The frequency should not change with the threshold setting. This is like a volume control into the sidechain.

* With op-amps like this NE5532, which do not have FET inputs, the input impedance is low enough that it will have some effect on the frequency. The op-amp input impedance is parallel to R8+threshold pot total resistance.
However, this effect will be quite subtle, so I would ignore it to start with.

[snk]

ok, that you for the information.
I'm really liking this compressor!

[snk]

I'm willing to try the reduction indicator daughterboard, but i don't have either BS170 or 2N7000 : is there any transistor suitable for this task (bending legs is ok if required) ?

[samhay]

I need to revisit the indicator as I became convinced it was causing noise.
You need something high impedance. If you don't have a MOSFET, it can probably be made to work with a JFET, but this will require some changes. I'll get back to you.

[snk]

Thank you, Sam.
Reduction indicator is something handy to have (at least for quick checks in an eyeblink, even if we should use our ears first).
Of course, it shouldn't affect the sound quality
I think that Madbean Holy Roller features such an indicator led, and it's an opto design too, if it's of any help...?

[snk]

I'm really having fun with this compressor.
I tried various cap values for the timing and the sidechain, but I found that the stock values were already great as is
I have built en enclosure for it, and will box it soon.

By any chance, did you look at the indicator led daughterboard design yet?

[samhay]

Glad your enjoying it.
Do you have any JETs you could use for the indicator?

[snk]

Yes, I think I must have some BS170 or BSS138 (I need to check).

[samhay]

BS170 is a MOSFET and should work in the original indicator circuit as-is.
BSS138 is also a MOSFET and may work - I haven't played with them before nor liked at datasheet. I would much something up on a breadboard, or socket the FET and see if either work.