Light Speed Compressor - another experiment in optical compression

[snk]

Oops, error on my side : I only have some MPF102...Do you think it would it work with your daughterboard?

[lykwydchykyn]

Hi, sorry for the necrobump, but this is like the only place on in all the web where there are people who have built this circuit a few times. 

I've built one of these using an LCR0202 and while I'm getting sound through, I just don't hear much compression happening.  There's also a subtle fuzzed sound mixed in way in the background.

With no sound coming through, I measure ~1.6 V across the vactrol LED, and with a loud note played in it jumps to ~1.8 V.  Measuring the resistance on the other side, it goes from ~2.5k with no input to ~1.7k with a loud note (though that measurement may be suspect in a live circuit).  Do these sound like correct numbers?  Seems like not a lot of gain change.

Anyone who has a happy unit care to crack it open and see if my measurements are sane?

[samhay]

The vactrol specs look similar-ish to those I used.
1.6V across the LED sounds ok.
Without any input, the resitance across the LDR should be much higher.
Which version did you build?

[lykwydchykyn]

Thanks for the reply Sam.  I built the version with ratio and threshold knobs, though not from your layout as I was combining it with an EQ circuit and had to adjust for the different polarity on the 0202. 

I think my next step is to disconnect the LDR half of the vactrol and measure its resistance out of circuit to see if maybe I just have a wonky vactrol. 

[samhay]

Edit - Taking the LDR out of circuit is a good idea.
The other thing you could do is to short the 10k resistor from the threshold pot to Vb and turn the threshold to 0. This should null the LED driver and you should then get dark resitance from the LDR (note that the ratio pot is in parallel, so turn this to max to measure the resistance). If that doesn't work then there is probably something wrong with the LDR driver. Perhaps one of the diodes is backwards?

[lykwydchykyn]

So I think the Vactrol is OK, I measured the resistance out of circuit and it goes from about 3k to 1.2k when the input is silent to loud guitar.  That's obviously too low for the circuit to really work right, but it seems to be how this vactrol responds to the voltages being fed into it.

I found I can get more volume and slightly more range by putting some LEDs in parallel with the vactrol LED, sucking up some of that current and dimming all the LEDs a bit, but that seems a hacky solution.  How would I modify the envelope follower circuit to increase the spread of voltage between quiet and loud?  Does R7 control that?

[samhay]

The vactrol should give you 10Meg dark resitance:
https://www.datasheetq.com/datasheet-download/188542/1/ETC/LCR-0202
You can check this by measuring the resistance when the circuit is not powered - you should see the resistance of the ratio pot.

My guess is there is a problem with the LED driver. You should have negligable current through the LED with no input. If you are measuing ~3k, you have 1-10 mA through the LED.
The current is set by R7, which if 1k should limit the current to ~4.5 mA on a 9V supply.
(schematic: https://samdump.wordpress.com/welcome/projects/light-speed-compressor/)
My guess is that you have grounded one of the op-amp inputs (threshold pot?). Can you post voltages on the op-amp pins?

[deadastronaut]

just a quick hi sam....

good to see your still about on here. 

[samhay]

Hi Rob. Got a bit distracted by other things, but seems I still have notifications switched on, and it's nice to dip a toe in on occaision.

[lykwydchykyn]

Well, smack my face and call me late to breakfast.

I had C5 backwards. 

Was originally getting 5.75V on pin 7, after correcting C5 I'm getting a more normal 3.7 (which is around what the rest of the I/O pins are reading.  My Vcc is only 7.5 due to probably over-aggressive power ripple filtering, which is why it's not 4.5V).

Now the vactrol goes from ~90k down to single digits with a loud input, which is what I would expect in parallel with the 100k pot.

We have compression!  And the fizzy sound is gone AFAICT.

Just goes to prove my adage that the longer I look for the bug the dumber the mistake was.

Thanks for your help Sam, and great circuit! 

[samhay]

That'll do it. Glad you got it working.

[Ben N]

Edit: Nevermind.