Phase 90 variable resonance- LFO tick

[jdub]

Hey all-

I've got a Phase 90 with the variable resonance mod (18k resistor in series w/25k pot), and it works, but I'm getting significant LFO tick when the pot is at max.  I've already changed the LFO IC to an LM358 and decoupled it using a 100R and 100uF cap to ground.  I've also got a .1uF cap in series with the resonance pot.  Anyone else have this prob? Any suggestions?

Thanks!

[Mark Hammer]

The decoupling should be for the audio chips.  The tick occurs when the LFO produces a sudden current draw from the supply that the audio chips are affected by.  If you provide the audio chips with "momentary reserve supply" via something like the 100R/100uf network (though it doesn't have to be as big as 100uf), it should eliminate the  tick.  The use of an low current op-amp for the LFO is a good idea, but you've confused me by saying it's an LM358.  That's a dual op-amp and the LFO only needs one op-amp.  What gives?  Is this a perf/vero build of your own layout?

[jdub]

Hi Mark
It's a perf build based on Francisco's Pez 90 schematic (basic layout (without LM358 and decoupling) here:http://www.aronnelson.com/gallery/main.php/v/jdub/Phase+90+perf.PNG.html; I just subbed the LM358 for the TL072 (half of which is responsible for the LFO).  As for decoupling that IC, I got that from this thread http://www.diystompboxes.com/smfforum/index.php?action=search2, specifically the following advice from yourself:

The third strategy is to create a tiny reservoir of current for the LFO so that it doesn't have to rob the other chips when it needs to produce a square wave (a necessary aspect of the LFO circuit).  The traditional means for doing that is to break the trace leading to pin 8 of that chip (the V+ line) and connect the two sides of the break with a small value resistor such as 100 ohms.  Now connect the + side of a medium-value electrolytic capacitor, like 10-22uf (15V rating or better), to pin 8, and the other end to ground.  That cap becomes your little current reservoir, and is said to "decouple" the LFO from the rest of the circuit.

I interpreted this to mean that the IC that handles the LFO is the one that requires the "reservoir" so that it has something to draw on when producing the square wave (thus not impacting the other two ICs, or at least impacting them less), so I performed the above procedure on pin 8 of the 358.  Did I misinterpret? Should I instead decouple the TL072s?

As always, thanks.

[jdub]

Addendum (random guesswork): Actually, I should mention that this tick lessens as the depth control is turned CCW. Significant?  Also, since the  Phase 90 design has both audio path and LFO sharing one dual IC (TL072 or whatever), would decoupling that IC even work?  Would it be better to have separate single op-amps for the two?  May be way off base...it's late & I'm tired...

[Mark Hammer]

As much convenience as dual and quad op-amps offer, one of the traps they lay for us is that when we use them for both LFO and audio path subcircuits, there is no means to decouple the LFO from the audiop path because the same power supply feeds all op-amps within the chip.  If you are simply using one op-amp in the 358 and the other is lying fallow, then fine.  But if the spare one is in use in the audio path, you may have some trouble there.\

The "reservoir" is principally for the chips in the audio path.  The electrolytic cap between the V+ pin and ground on each chip provides enough reserve current such that when the LFO suddenly draws a lot of current, it doesn't pull so much away that the audio path suffers....because they have "petty cash" to survive on for those few milliseconds.

[jdub]

Thanks, Mark.  I'll give it a try & see what happens...although finding a place to cram in the components on the board might be kinda tricky...

[Sir_Ian]

I had the same problem you are having...but with a Phase 45. I'm guessing you might be having the same problem as I did, because both you and I perfed.

If you use an Audio Probe on the traces or (leads and solder perboards) around the LFO, you will hear a really loud tick. What happened with me, is that my leads for the output ran right next to some of the leads for the lfo. It acted as an antenna and picked up the tick on its way out of the pedal. I fixed it by cleaning up my soldering around that area. I had a few little globs I got rid of and I also spaced the LFO path away from the signal out path as best I could. In the end...it worked for me...and no ticking anymore. I had to spend some time on it. I cleaned it up some at first, and the ticking was alot quieter, but still there. So it took a little more cleaning up and it worked. Even now, if I use my audio-probe, I can hear the tick from the LFO on the LFO leads, but not anywhere else.

I'm not 100% sure this is your problem. But it sounds like you've tried a few other things with no success. Give this a look. I don't mean to give offense, but if your the type to gob solder down in rows along the solder pads for your traces then I'd be willing to bet somebody's life that this is your problem. Not my life....but somebody else's for sure. If you do gob the solder, than try using some clipped resistor leads and just soldering the ends. Once again, I don't mean to give offense, you probably know this already, but I'm just trying to cover all bases. Good luck.

[jdub]

Thanks for the tip, Ian.  It's certainly worth checking- seems like the output path is sufficiently far away from the LFO path, but then again there might be some inadvertent contact or proximity that I didn't catch.  While I'm not a "solder-globber"  (I use PC bus wire or snipped component leads), there's always a chance that I overdid it a bit at some connection or other.  I'll take a look- between your advice and Mark's I should be able to figure something out. 

Again, thanks.