Vibra-Tone LFO ground/+ decoupling

[bluesdevil]

Hello again - Since getting my Vibra-Tone build to vibe a while back, I've noticed a "ticking". I would like to try to decouple the LFO's power supply from audio path and separate it's grounds as well. Hate to sound ignorant, but I'm not 100% which parts are the LFO.  I tried cutting the ground trace between R25 and R29 (Q7's emitter resistor) and routed a separate ground wire from the right side board to the output jack ground lug with no luck. Am I missing another LFO ground connection besides that and the speed controls leg 1/2 lugs? Also where would be a good place to decouple it's power from the audio section? I was hoping to use a 100R resistor and 10uf cap to do this.
   Any clue would be greatly appreciated.... I guess I still have a lot to learn!!

Layout:
http://www.aronnelson.com/gallery/main.php/v/MarkMs-Gallery/album15/album76/Vibrato+LAYOUT.png.html
Schematic:
http://www.geofex.com/FX_images/FakeFenderVib3.pdf

thanks!!

[bluesdevil]

One other thing: The ticking stops when the speed control is at max setting... weird.

[gez]

Could be stray capacitance.  Have you used shielded wire for the audio input and output connections?

The LFO is the block at the bottom of the page.  The one directly connected to the battery leads.

[bluesdevil]

Thanks Gez, I appreciate you pointing the LFO section out.  I haven't tried the shielded I/O yet because it isn't in an enclosure... wires aren't crossed or crowded right now, so I was thinking I might need the decoupled power for the LFO. I'm going to try something tomorrow and post back.

[gez]

wires aren't crossed or crowded right now

Doesn't mean anything, they could still be the cause of your problem.  If they're long, try cutting them down a little (if you can).  Also, try shifting them around/separating them from audio to see if it helps.  If any of the LFO wires pass over points on the board used for audio, tick can be picked up that way too.  Something to bear in mind.

If the layout is tried-and-tested, then it's probably a wiring problem.

[bluesdevil]

Well, I shielded the I/O's and it seemed to help a bit..... going to try shielding the speed control wires next. That should do it I hope. If not, I'll try the more drastic measures or just live with it.

[snap]

a sinewave lfo should not tick, should it?

[gez]

a sinewave lfo should not tick, should it?

Good point!

Perhaps the waveform's amplitude is too great?  Either it's slamming into the rails, or it's pinching off the FETs.  Is the tremolo sound choppy and does the tick clean up when you reduce the depth?  If you own a scope you'll have your answer. 

[bluesdevil]

Here's something strange: I switched out the dual 100k pot to 2 separate 50k pots to see how the speed range would be affected and I noticed I could get rid of the ticking by keeping the 50k pot that's connected to R32(on layout) rolled all the way down and moving the other half of the speed control about a quarter of the way up past it. The LFO stalls if the one pot goes to a certain  distance away from the other so I have limited speed capabilities without ticking..... also the effect is not quite as choppy with the two pots spaced apart. BTW, the Depth control does not affect the ticking at all.
     Would you still guess that the LFO is draining too much power at once from the rest of the circuit when the two halves of the speed control are equal? If I can't get rid of the ticking I will just do away with the Depth control and mount the additional speed control in it's place. A damned cool circuit, but I think there are still some bugs to work out, including the volume loss issue.... it's definitely worth perfecting in the future!!!
   
   

[gez]

The LFO stalls if the one pot goes to a certain  distance away from the other so I have limited speed capabilities without ticking..... also the effect is not quite as choppy with the two pots spaced apart. BTW, the Depth control does not affect the ticking at all.     

Phase-shift oscillators don't necessarily churn out a waveform that is consistent from circuit-to-circuit.  Also, amplitude often varies over the course of the LFO's frequency range.  The fact that the LFO craps out just past the point where you get no ticking, suggests that your tinkering has reduced the amplitude.  In which case, the LFO is probably too frisky with stock values and is outputting a waveform that is too large.

I tend to avoid these circuits for the reasons I've mentioned, so suggesting a fix is a little tricky as I don't have the experience.  My intuition says try sticking a low-value trimmer in series with the 100 ohm emitter resistor (R29) of Q7.  If there's no room, just try increasing the resistance a little.  Hopefully, this mod will reduce the gain in the amp and bring the waveform down in size.  Who knows...

[puretube]

Dual-pot tolerance...   ("tracking")

[bluesdevil]

Thanks Gez, that sounds like a good idea and things are starting to make sense. I'll post back within a few days with the results. 

[bluesdevil]

Well alright, here's an update. I found a trimpot that measured 185 ohms and put that in series with r29. I could tweak it to stop the "beating", but it definitely took away a good bit of intensity and the LFO stalls out at some speed settings, including the fast ones that I like.
  After shielding some wires the LFO noise isn't noticeable when playing, except when notes start fading out. I guess I prefer that to a weaker effect. The wobble at full depth is killer on this thing!!
          Thanks for the suggestions, time to add a booster output circuit and box 'er up.

[gez]

After shielding some wires the LFO noise isn't noticeable when playing, except when notes start fading out.

That's common to many modulation effects - there's often a faint click/thump in the background (when not playing) - so it sounds as if your circuit is behaving normally.