Demeter Compulator - unintended vibrato?

[rullywowr]

Hey Guys,

I had a few boards fabbed for the Demeter Compulator using this schematic:



Anyways, the pedal sounds pretty good except I hear a (very) slight vibrato in the effect as a whole.  I am curious if this is normal or perhaps there is a component issue?  It is ever so slight but enough to be annoying.  Perhaps the power rail caps are too small (4u7)?   It is a good sounding compressor, although I feel it could use more bass frequencies.  I am using the stock LF353s for opamps and have tried two vactrols in case that was the issue.  The trim test points are biased to 1k1 as recommended.  Thanks in advance.

As a side note, I do have a couple extra boards available if anyone is interested.

[Droogie]

I have built three of these on vero (obsessive compressor disorder) and don't experience any vibrato-like effects. I used 4u7 as per the schemo in each build. Not sure what might be causing the wobble.

[rullywowr]

Thanks for the info.  I might add that the LED for the compression is dimly lit all the time and doesn't seem to pulse as it should.  Does yours glow when you attack the strings? 

I am using a regular diffused red led.

[Droogie]

I used a Vactrol 5C10 for all of my builds. I believe it is somewhat particular in this regard—not sure where I read that though.

[rullywowr]

That must be it!  I just checked and I am using a VTL5C2.  I have read that there is something special about the VTL5C10.  I would love to get my hands on one to try out.  It's not like the pedal is bad, it just has a bit of annoying tremolo/vibrato going on.

This thread over at DIYSB has some great information about Vactrols:
http://www.diystompboxes.com/smfforum/index.php?topic=76376.0;prev_next=prev

From their suggestion, I may try socketing R12 (10k resistor that works on the schematic's bottom right LF353) to my decade resistor box and see if that can make it work any better.  Otherwise, I have to find a VTL5C10.

[Droogie]

Excellent! I believe it has something to do with how fast the LDR reacts to the LED and specific light/dark resistance.

[derevaun]

I recently populated a board that was etched from Baja's layout (thanks Ian!), and I ran into the common potential errors of mixing up the 4k7 and 47k resistors that are close on his layout, and the nearby diode orientation. But these are probably not an issue on your layout. It seems like the action around that part of that LF353 are critical, so measuring actual resistor values might be a good idea. But I'll bet it's the vactrol flavor. Did you try it with a battery?

In any case, I'd gladly trade or pay for an extra board, if they're not priorly committed!

[RobA]

There's an application note out there, probably on the Vactrol website, that breaks down the differences in each of the Vactrols according to their characteristics of dark resistance, light history, response speed, dynamic range(dB) and slope of R(i). The 2 and the 10 are about as far apart as pair of Vactrols can be in every regard.

There is a lot of overlap between the various vactrols, but the one very different characteristic of the VTL5C10 is the slope of the R(i) curve. It is by far the most shallow of all the Vactrols and is six times smaller than the VTL5C2. If this is the characteristic that a circuit is principally designed around, it would be very hard to adjust the circuit to fit one of the other vactrols.

If what you are hearing is a tremolo (volume oscillation, pumping), I'd guess it's because the VTL5C2 has a small hysteresis and a very steep slope compared to the VTL5C10 causing it to overshoot and then respond to the compression too fast.

[rullywowr]

Wow.  Thanks for all the help guys, especially the detailed information RobA!  I am 99% sure its an issue with the type of Vactrol itself.  Can't wait until I try the correct Vactrol. 

Derevaun, I will send you a PM.  I have two boards left currently.

[jkokura]

I can say that I've built the compulator, and offer my own modified version of an opto comp as well. I've never noticed it to act like a vibrato at all, even when I was 'xperimentin' on it. Also, the VC10 is very important. I tried others, and the 10 is the only one that worked well.

Also of note - it's noisier than I like.

Jacob

[Droogie]

Also of note - it's noisier than I like.

I found that using the trimmer to dial out some of the wool was usually necessary on this one.