VOX repeat percussion clone

[Frances Rhodes]

hi everyone

I recently built a vox repeat percussion with a couple mods (added a depth pot, raised the gain a little and added a volume pot), and I'm hearing the LFO tick when I don't play.
The clicking sound goes off when I roll the volume pot down, so I'm guessing it's not a ground parasite, but I have no clue.

Here is the schematic for reference


So I'm asking you guys, do you think it is normal behaviour or is it my PCB that is poorly designed?

cheers

[patrick398]

I've build a few of these on stripboard and never experienced any ticking at all really. I think it can be quite common with this circuit though, i seem to remember a lot of people complaining about it on the tagboard fx page.

Just found this in the comments section:

'ozzy11 November 2017 at 01:26
How to get rid of the loud and permanent ticking:
- lift the 33R resistor leg off the board's ground row and connect it to the 10u capacitor negative.
- then run a wire from that junction to the power supply negative.

Basically, no part of the ciruit can share the trace between the 10u cap negative and the 33R resistor. Otherwise it will tick.
I hope it helps.'

Is it also ticking in bypass?

[Frances Rhodes]

hi!

Thanks a lot for answering!
Ok, so, when I plug it into the amp with nothing at its input, it ticks when bypassed, but when I plug a guitar, it stops ticking.
I tried to connect the cap and resistor together then directly to the DC jack negative side (see picture) but it still ticks.


I tried to roll the volume back and crank my amp to get an even volume and the tick is much quieter that way than when the volume is maxed and the amp is set to normal.

[patrick398]

Have you wired the switch to ground the effect when bypassed? I do this as standard now and stops a lot of problems with bleed through.

http://www.generalguitargadgets.com/pdf/ggg_sw_3pdt_tb_gi_dcj.pdf

Obviously you can ignore the battery snap/stereo jack set up if you're just using an wall wart.

That should solve any ticking in bypass problems but i can't tell from what you've said whether it's still ticking with the pedal engaged and a guitar plugged in?

[Frances Rhodes]

that is the way I always wire my switches, with the input grounded when bypassed, except i use the right row for the LED instead of the middle one.
I know there are other ways to ground the output instead, or even ground both input and output... maybe that may change something.

and yes, it's still ticking when engaged, whether the guitar is plugged in or not.
the only difference is when the effect is bypassed and nothing is plugged into, it ticks

I read on fuzz dog that replacing the 33 ohm resistor with a trimmer may help reduce, or even cancel the ticking, but it may also disturb the oscillator. I'll try to tweak that 33R resistor and see how it goes.
earthquaker devices used a 39R resistor for RB1 (instead 33R) and a 470R for RB2 (instead 1k), that also may be a thing to look at!

[PRR]

> when I plug it into the amp with nothing at its input, it ticks when bypassed

How does the guitar sound when not plugged in? I mean, that is probably not a normal performance setup?

The type of controlled attenuator used here is prone to small DC offsets, a few mV shift when NPN2 Base is wiggled. I have seen a broadcast with several trims and compensations to reduce this. Within a few years they dropped that plan. The form shown here is *simple*; if you can overlook its quirks it is a useful scheme.

[Frances Rhodes]

yes, I know it's not normal performance setup, but isn't it strange that it becomes noisy when the input stays "open" and the effect is bypassed?

also, when I crank my little VOX AC4, and turn the volume pot of the tremolo down to get an even guitar volume, the tick gets quieter. I know that the cutoff frequency (the bandpass) of the output changes along with volume settings on a pedal, is the volume "filtering" more high frequencies when i turn it down? would it be a good idea to add a small cap across the volume pot, like a 1nF to try to filter out some of the high end of the LFO pulses?

since I have a MOSFET booster (Jack Orman) laying somewhere, supposedly rather quiet, I wanted to try to add it after the whole circuit and see if I can notice a difference between my circuit as it is, and the same circuit with the volume turned down and a booster after it. I know in the end it's probably not a good idea to do that in a pedal because of the signal/noise ratio, but I'm curious...