Echo Base LFO Ticking

[Zounds!]

I built an Echo Base on vero using this layout: http://www.sabrotone.com/wp-content/uploads/2011/05/EchoBase.gif
I didn't do any of the mods listed in that layout, just normal buffered bypass/tails and modulation. Here's the schematic too: https://static1.squarespace.com/static/54696a3ee4b05a85cbfe56e5/t/5469759be4b05145d3fd695e/1416197531945/Echo-Base-build-PDF-updated-051810.pdf

It works great but the LFO ticks both with the effect on and in bypass. The rate of the ticking changes in time with the modulation speed pot. It's not extremely loud but it is annoying.

I increased the cap to pin 8 of the LFO opamp (to 220uf up from 47uf) to no avail. I also tried a mod I read on here, which is  putting a 100ohm resistor in series with the 9v+ from the power jack. But that just killed the modulation entirely, which is one way to kill the tick I suppose! Does anyone have any other suggestions?

[Zounds!]

Also tried the pedal after a buffered pedal and tried increasing the value of the cap to ground off pin 7 of the LFO op amp. Still ticking! I can hear now that the tick is repeating with the delay, so it must be getting back to the input of the PT2399 somehow...

[Mark Hammer]

The document shows/recommends a TL072 for the LFO section.  Bad idea.  Ticking results from the sudden current draw that this LFO circuit makes, in order to generate the initial square wave that it turns into a triangle wave.  Look through all the schematics of commercial modulation pedals you can find, and you'll generally see low-current dual op-amp chips like the LM358 or TL022.  Use of one of these (or perhaps a TL062) will begin to address audible ticking.  There are other things one can do on top of this to reduce the audibility, but this is the easiest first step.

[Zounds!]

That all makes sense Mark. I went out and picked up a few LM358s from my local shop this morning. Not sure how, but the tick is actually a little louder with the LM358! So now I'm really confused!

[Mark Hammer]

As am I!  Are you sure you replaced the LFO chip and not the audio chip?

However, strategy 2 is to "decouple" the LFO subcircuit from the power supplied to the audio path.  The traditional approach is to set up a kind of virtual "battery" for the chip providing the LFO.  This can consist of something like a 10uf cap to ground from the V+ pin of that chip, and the insertion of a small-value resistor between the circuit V+ and the V+ pin on the specific chip.  That added cap stores some "backup" charge, such that when the LFO tries to make a square wave, it can pull the current it needs from that cap instead of from the overall power supply.  Think of it like having a slush fund so that you don't have to reach into your savings account or 401K.

The resistor value can't be too big since it would limit the charging up of the cap, and the cap can't be too big since it would take too long to charge up fully.  Generally, for modulation FX, the combination of a 10-100R resistor and 4u7-10uf cap does the trick.

There IS a 3rd strategy for de-ticking, devised by Boss and illustrated on page 2 here: http://moosapotamus.net/files/stompboxology-mo-tremlo.pdf but it involves more modification of the circuit.

[Zounds!]

I tried the LM358 in both positions just to make sure I wasn't swapping the audio chip by mistake. Still ticking!

I attempted strategy 2 - to decouple the LFO power from the audio power. I think I successfully did something similar on a Vox Repeater clone once, but for some reason it didn't help much with this. What did work to some extent was using just a resistor (no cap) between the circuit V+ and the LFO V+. 100R quieted the tick a little, 1K shut the modulation off entirely (and with it the tick). I settled on 330R, which makes the tick all but inaudible through the clean channel of my amp. It's still pretty noticeable if I run it into the gain channel or a dirt pedal, but I only ever use the clean channel with dirt pedals before the delay, so it's really only an issue on paper at this point. I realize I'm limiting the power to the LFO chip which I think seems to be making the modulation a bit less intense, but that's an ok trade off for the time being. I'm going into the studio soon and I really don't want to hear that tick come through on any recordings, so this is workable for now.

Thanks for your help, Mark!

[Mark Hammer]

Well, whatever works.  Any port in a storm, eh?
Congrats on solving your problem.