Pedal whistles when bypassed

[Bishop Vogue]

Hey all,

I have a problem with a pedal I have never experienced before.  I built the Freakish Blues Alpha Drive, boxed it up and discovered that the pedal makes a dog-whistle sound, only when the gain is set between 9:00 and 3:00 and only when bypassed.  The enclosure is a standard aluminum 1590 so it isn't radio.  Problem persists with both batter and 9v adaptor.  This is a new one for me... any ideas? 

[kaycee]

Does you switching scheme ground the input in bypass? If not, try that.

[PRR]

Shorten or shield wire to gain pot Wiper.

That's a stray guess. A link to the schematic would encourage better guessing.

[karbomusic]

If you want to find the root cause of the squeal, see PRR's response. If you want to just stop it when bypassed, see Kaycee's response. However if all you do is ground it when bypassed, chances are even when engaged, if you unplug the cable from the guitar instead of the pedal, it may squeal then as well since that causes the input to float too.

[LiLFX]

If you want to find the root cause of the squeal, see PRR's response. If you want to just stop it when bypassed, see Kaycee's response. However if all you do is ground it when bypassed, chances are even when engaged, if you unplug the cable from the guitar instead of the pedal, it may squeal then as well since that causes the input to float too.

This. Schematic and pics of your build will help. Could be a simple thing to chase down.

[PBE6]

This happened to me before with a high-gain build. Adding a large amount of filtering to the power supply (100R in series with 1000uF parallel going to ground) was a quick fix. I would also recommend grounding the input when the unit is bypassed as noted by kaycee.


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[Bishop Vogue]

First off, please just let me say that this DIY community rocks - so many answers in only 24 hours.  Thank you so much.  I don't have a schematic, but I do have the vero layout.  Maybe that helps some.  Also, though I am embarrassed to ask, I understand what it means to ground the input when bypassed but I don't know how this is accomplished.  I use the tonepad 3pdt method to wire my switch.  I'd be very interested to know which lug needs to go to ground.

[Bishop Vogue]

[Kipper4]

http://www.diystompboxes.com/smfforum/index.php?topic=75778.20

[Cozybuilder]

Heres what I come up with for a schematic from the Vero layout- I hope this helps.

[karbomusic]

Also, though I am embarrassed to ask, I understand what it means to ground the input when bypassed but I don't know how this is accomplished. 

Nothing to be embarrassed about, I only know from having it happen. I'll let those who know the finer points better to chime in but basically if the input isn't completing the circuit (guitar plugged in) or grounded it is considered to be 'floating' or rather has the ability to sort of electrically flop around randomly. With a hi gain circuit, even with proper shielding this can be enough to allow it to go into self oscillation.

I had this happen on a tube screamer type design I did, I never found a way to prevent it at the source other than reduce the available gain (it only occurred when volume, gain and tone were maxed). Several people here helped me troubleshoot but never found anything other than it being a byproduct of amount of gain combined with a floating input. Thusly, I fixed it by causing the FX input to short to ground via bypass switch. I still build and sell that design to a handful of players around the country, never had a complaint because the conditions to cause it after I added the workaround are extremely rare which is..

Max volume/tone/gain, unplug guitar cable from guitar with amp active and pedal bypassed.

[Bishop Vogue]

Thanks for all your input - it led me to discover a problem I didn't even know I had: I was wrong before when I said I used the tonepad wiring - I use that 95% of the time - but on this build I used a 2PDT and millenium bypass wiring.  So I switched to the Tonepad 3PDT and the problem vanished.  Tanks again all - and now I will know how to avoid this problem in the future.