292c LPG build question -- switching vactrols

[jumunius]

I'm finally finishing up my 292c Lo Pass Gate, but in testing I've run into a snag. I had added a switch (4PDT) to switch between VTL5C3s and VTL5C4s. My build mostly works, but in LPF mode with resonance set to max and Offset sweeping up from 0, I experience some noise/static sound towards the top of the Offset setting. What appears to happen is when Offset hits 6 or so, the sound cuts out and comes back as noise.

It has been suggested to me that this might be ring modulation between the self-oscillations and some interference, and my switching scheme (which involves a lot of wiring between the vactrols and the board) might be the cause. In moving the deep switch around (as in physically moving the wires) I have made this problem go away, and other times I can play the self-oscillation like a theremin.

So I'm wondering if anyone else made a switch for their vactrols like this, and if so how did you get it to work. I thought I saw someone in the 30+ page 292c thread mention a desire to do this. Did anyone succeed? Suggestions?

I'm seriously considering changing my wiring approach, but I've killed some pads already desoldering this board and I am a bit afraid of doing too much desoldering and resoldering, given that these boards are hard to come by.

For what it's worth, this may or may not be a good idea. Already I can tell the 2 pairs of vactrols seem to respond to offset range very differently, and in the end it means I'll probably have to optimize the trimpot and other build settings for one pair of vactrols (the 3's) at the expense of good range settings for the 4's.

[The Peasant]

I did this very thing with my 292c build using a 3PDT switch, it seems to work fine. Yes the response is quite different between the two vactrols, and the trimmer setting ends up being a compromise between the two, but I found that the offset control has sufficient range to work fine with both. At extreme settings the sound does drop out, but this is outside of the usable range anyways. The number of connections that you are switching may make a difference, you only really need to switch 3 of them.

Details of the changes that I made can be found about 1/2 way down the page on this Muff Wiggler thread.

http://www.muffwiggler.com/forum/topic-8540-100.html

Take care,
Doug

[jumunius]

[negativspace]

I did the 3PDT switch on my build as well and I can report no quirky behavior. It's definitely worth doing. I mounted my vactrols on the board, piggybacked. Soldering the unswitched leads on the top part to the bottom part's leads has proven sufficient to keep the 2nd pair mounted, no need for glue.

[jumunius]

[The Peasant]

[jumunius]

[quote="The Peasant"]

[The Peasant]

Sorry for the delay, here is a picture of the circuit board with the extra two vactrols mounted on the right hand side, as indicated by the arrows. Also, there is a picture of the board wired up to the front panel. The mixer circuits on the pcb were wired as two independant mixer modules.

Take care,
Doug

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[jumunius]

Thanks Doug. Well the good news is your pics came in about the same time I was starting to rewire things. The bad news is that in the process of rewiring, one of my VTL5C4s lost a leg. (It had been mounted and unmounted from the pcb, then mounted and unmounted from a daughterboard, and in moving everything back to the pcb it gave up the ghost.) Not having a spare I'm finishing the wiring with just the VTLSC3's mounted to the switch for now and deal with the 4's next time I order parts.

The other bad news is that now that I board-mounted the VTL5C3s, redoing the switch to be a 3PDT, etc, I test it out again and essentially nothing has changed, except maybe even for the worse. The thing sounds good for most of the offset range when resonance is at a lower setting, but once resonance crosses a (lowish) threshhold the output levels skyrocket, and with resonance above this point, the upper regions of offsetl turn the signal into something that sounds like a radio between stations.

It's good to see how you crammed the extra ones in there. I have some headers along the edge of the board which may or may not get in the way, but I like to have some ideas of how to proceed.

Also in the process of doing this I learned that I fundamentally misunderstood the layout of vactrols. Since my board rev and my vactrols only had a + symbol, I somehow got the idea that the "pinout" of a vactrol was 90 degrees rotated from what it actually is (the LED and resistor running length-wise, not width-wise). So my original switch concept was not only inelegant, but flawed. Sadly now that my wiring is corrected, and far shorter, nothing has changed.

Well, onto more troubleshooting.

[jumunius]

Ok, I'm having more luck with this build, and was able to resuscitate my amputee VTLSC4 and fix up this switch. I ended up mounting my 2nd set of vactrols *beneath* the board, which seems to work ok too. Since they'd been remounted a few times the legs were short and obviously not very sturdy (as one had broke) so this seemed safest physically.

Now for a question and a comment. Firstly, do you notice a pronounced difference in the resonance character between VTLSC3 and 4? My 3 has a very nice, cutting resonance. My 4 is pleasant, but seems tame by comparison, and doesn't self-oscillate. Messing around with the value of R18 helped but the resonance generally sounded more clipped, less precise.

Secondly, it struck me that since I'm using a 4PDT to switch only 3 legs on the vactrols, I can use the 4th pole to switch a resistor either parallel or in series with R19 (the deep switch resistor). Adding 100k of resistance total seems to balance the offset levels such that the vactrols' cutoff is complimentary when you switch back and forth. Now if I could only get the resonance to match.

[emdot_ambient]

[jumunius]

[quote="emdot_ambient"]

[The Peasant]

On my build there is a definitely a difference between the response of the two different vactrols, but they both resonate similarly, just at different settings. Their character is a little different, but not by very much.

Take care,
Doug