ADA flanger makes a ticking sound when bypassed

[frank]

Hi all. I have had an ADA flanger (reissue) sitting unused for a long time, and I feel like putting its cool sound back to work. Problem is, when it's in bypass, I can hear a distinct "tick" which relates to the modulation speed. Strangely, the tick is absent when the pedal is engaged.

I tried to take a look at the schematic, but it's a little scary for someone of my abilities. I am taking a total shot in the dark here that there may be some sort of filtering component gone wrong which should normally filter out the oscillator clock from the audio?

Anybody got any insight?

thanks,

Frank

[Mark Hammer]

Audible ticking tends to happen when the current drain from the LFO provides a sudden glitch on the power lines in the circuit shared by both control/modulation circuitry and the audio path. Not all LFO circuits are identical, but a great many work by providing a square wave and then turning it into a triangle wave. Generating that square wave requires an almost instantaneous burst and corresponding demand for current to do so. That produces a glitch/spike/whatever on the power line.

There are several ways to reduce or eliminate it. One is to use a low-power op-amp for the LFO so that even when it produces the square part, it does not need THAT much current to do so. This is why you'll see a TL022 or TL062 or LM358 frequently used in LFO circuits. A second is to "decouple" the LFO circuit so as to provide a sort of temporary reserve supply for it. This will be a small-value resistor going from the power line to the V+ pin on the chip, with an electrolytic capacitor going from that pin to ground. Think of it like "petty cash" for the chip; enough to get by on for a bit so that it doesn't have to make a big withdrawal and pester the folks in accounting.

The third tactic is a way of redesigning the LFO so that it starts out with a trapezoidal wave, instead of a perfectly square one. That extra millisecond or two that it takes for the leading edge to rise/fall may not seem like much time to you or I, but it is enough to avoid sudden spikes on the power line.

Looking over the schematic and parts layout of several releases/issues of the A/DA, it appears the chip in question is the 8-pin op-amp sitting between the "manual" and "range" pots on the board. It has a 470R resistor and 2.2uf cap to ground. Nothing is likely to happen to the resistor over time, but there is an outside chance the cap is off spec. Try replacing it with a 10uf/25v cap, making sure to observe polarity. There is every reason to expect that the ticking may be reduced to something tolerable as a result.

Good luck. The A/DA is one helluva flanger, the sort you want to be able to show off in its best form.

[frank]

Mark,

Wow! That was one helluva post! Totally understood it and really appreciated your time, effort, and clarity in your description. I'll swap out the capacitor and post the results. Thanks!

-F