Wah pedal hiss

[tjdracz]

Hi, I've got newish Crybaby wah and I've started modding it using Stuart Castledine page 

http://www.wah-wah.co.uk/bypass.html
http://www.wah-wah.co.uk/diy.html

I did all the mods, using true bypass with no buffer, leaving sweep range cap as it was and using 56k instead of 47k for volume mod.
However now, when in toe down position, it's hissing like crazy. II've expected some noise but this is just ridiculous, even with battery. It seems like  the noise just appears in the treble range and is so huge that sweeping makes kind of like scratchy noise. Thought it was the pot but the hiss just stays there at the toe down. It dies  after a while but touching the string makes it come again. Any suggestions on what could it be? Unshielded cable, some grounding thing? Traces on pcb to be cut?

[GibsonGM]

"Scratchy" suggests to me that maybe you have DC on your pot? Can you measure from ground to each pot lug with your meter set on DC volts?  Just a thought.   

Another thing  could be that it's time to replace or at least clean the pot, but let's see what you come up with first.   And yes, unshielded cable could add noise...let's see what you get for the DC check first, tho!

[R.G.]

You're getting RF or ultrasonic oscillation. Clean/bypass the power supply.

[tjdracz]

I think it's not the pot as the noise stays there at the toe down and it does not seem to be connected to pot wear.

RG, I thought that battery is as clean as you can get. No?

[R.G.]

I think it's not the pot as the noise stays there at the toe down and it does not seem to be connected to pot wear.

RG, I thought that battery is as clean as you can get. No?

No. The problem with many power supplies is that they are high impedance, and sometimes non-resistive. Even if the power is clean, other parts of the circuit can insert impedance. And that makes it easy for circuits with gain to oscillate. Batteries increase in impedance as they wear.

Then there's modding. Mods by their nature change things, and can introduce enough unfortunate feedback to let a non-perfect power setup allow oscillation.

Best advice? UN-mod it and see if it still works. Then do things from there a step at a time.

[jimilee]

Well the true bypass no buffer entails cutting a trace, so good luck there, but did you use the small of caps to remove any interference as it suggested? Also like RG said un mod as much as you can and have a go step by step. How's your soldering look?

[tjdracz]

Well the true bypass no buffer entails cutting a trace, so good luck there, but did you use the small of caps to remove any interference as it suggested? Also like RG said un mod as much as you can and have a go step by step. How's your soldering look?

Does it involve cutting? Looking at Stuart's website it looks like the cut is only there when you're retaining the buffer. Tried the caps but no dice, still the same.

Tried different batteries and couple of power supplies, also no difference.

Additionally, no DC on the pot.

Any more thoughts apart from rebuilding it again?

https://soundcloud.com/tomasz-dracz/record-0001 - I made a little recording with my phone to better show you the noise I'm talking about. It's a little quiet but turn up your volume and you'll hear what I mean. Just quite annoying noise at the toe down position that is bit annoying when sweeping the range as it makes kind of a scratch noise if you sweep abruptly

[GibsonGM]

Could be exactly what R.G. said, he is good at 'seeing' where oscillation you can't hear much of may be occurring.  When it's oscillating at high frequency, you experience it as hiss (or sometimes brittleness etc).

Personally, I'd call that "a noisy kind of amplification" even if it IS oscillation....a high noise floor - there are lots of ways to express that hiss, and I'm probably not using the right terms.     If I were trying to fix it, I'd think about trying some pretty small-value caps in strategic locations.    I'd get the schematic out and see where I could trim some very high freq. stuff out to lower the bandwidth, around the bypass cap in parallel with the inductor, for example.

Or, I'd start undoing the mods I made  *shrug*

[jimilee]

Well the true bypass no buffer entails cutting a trace, so good luck there, but did you use the small of caps to remove any interference as it suggested? Also like RG said un mod as much as you can and have a go step by step. How's your soldering look?

Does it involve cutting? Looking at Stuart's website it looks like the cut is only there when you're retaining the buffer. Tried the caps but no dice, still the same.

Tried different batteries and couple of power supplies, also no difference.

Additionally, no DC on the pot.

Any more thoughts apart from rebuilding it again?

https://soundcloud.com/tomasz-dracz/record-0001 - I made a little recording with my phone to better show you the noise I'm talking about. It's a little quiet but turn up your volume and you'll hear what I mean. Just quite annoying noise at the toe down position that is bit annoying when sweeping the range as it makes kind of a scratch noise if you sweep abruptly

I looked at Dunlop rev e f and g diagram, it's says for removing buffer cut here, solder here on the picture. Which one are you using?

[tjdracz]

Quoting the site:

Alternatively, you can just ditch the buffer circuit (the components ringed in green) and run the new wire to one of the two holes circled in green. Obviously, there's no need to cut the track if you remove the buffer circuit. It's not necessary to remove all the buffer components; but at least the transistor and the 10K resistor should be removed.

[tjdracz]

OK, fixed it in the end. Redone the switch to usual non-grounded input one true bypass and replaced the wire from the board with a shielded cable.Now it quacks fine and no excessive noise. There is still some noise at the toe down position but nothing too serious and alarming, guess it's just the nature of the circuit amplifying the noise in that position.

Thanks for all the suggestions!