Minimizing hiss in wah pedals?

[Mark Abbott]

This is a tough question but what areas of a wah pedal are most sensitive to hiss. The hiss occurs when the pedal is depressed.

Mark

[artifus]

wah is (usually) a band pass filter. treadle is connected to pot that changes centre frequency of filter. heel down = low frequency, toe down = high frequency. hiss is high frequency (from upper mid to beyond hearing). when toe down you are boosting hiss.

[Seljer]

I redid the wiring in my wah with shielded cables to and form the bypass switch and potentiometer. I believe it lowered the noise floor enough that the difference was noticeable.

[joegagan]

customers who have come over to try out my custom wah builds have commented that my wahs have less hiss than the commercial counterparts they are comparing them to. i don't have any solid facts to suggest what is causing this, except a few possible things
- some of my wahs are tuned a wee bit lower voiced, they don't go as far into ear bleed treble.
- i use transistors in the 180 - 200 hfe range. is it possible that the lower gain transistor, even when boosted via a lower Q1 E to ground R are quieter overall?
-some of my builds also use shielded in/out wires to and from the switch with common ground at the switch side.
-short wire runs
-minimal parts count, ie; no buffer.

[Paul Marossy]

It has been my experience that it has to do with transistors you use. Generally, the higher Hfe = the more hiss you have. It really comes out when you have a high gain distortion in conjunction with high Hfe transistors in the wah pedal.

[Mark Abbott]

The transistors in the wah are BC547C's, I had thought of using 2N3904's as they have a gain of 100 and they sound pretty good in most effects.

http://www.fairchildsemi.com/ds/2N/2N3904.pdf

http://www.mouser.com/ds/2/149/BC547-190204.pdf

Good to hear from you again Joe, it has been a while.

Mark

[merlinb]

Most popular DIY wahs use BJTs, which have high current noise. They're a fundamentally poor choice for use with guitar pickups; it's the price you pay for using 1960s technology. The frequency response of the wah will make the hiss sound even more noticable.
The answer is to use a JFET buffer (or JFET input opamp buffer), which have very low current noise. This converts the high source impedance of the guitar to a low source impedance suitable for driving the BJTs, so their current noise is no longer significant. Amplifying the signal a bit first too, will help, if you can sacrifice the headroom.
You can also minimise the BJT current noise by choosing a high gain transistor with a high Ic(max) rating, such as the BC337 or ZTX653.

[joegagan]

doug h may not toot his own horn on this thread, but he was on the jfet buffer for wah many years back. great minds think alike, thank you merlin b.

[Keppy]

I have a Vox wah that only hisses with a wall wart, not a battery. If you haven't checked yours on battery power, you may discover that power supply filtering is what you need.

[Mark Abbott]

Thanks for the replies Merlin and Joe. It's funny as I use to pull out the first transistor buffer from a Crybaby wah pedal ( the buffer to prevent the tone sucking effect.) I found FET followers worked much better and I increased the value of the input cap so the wah sounded fatter to boot. I'll try implementing a FET follower. I've the annoying noise was the pot starting to sound scratchy.

Mark

[joegagan]

waht? a scratchy pot in a wah?

[zombiwoof]

The transistors in the wah are BC547C's, I had thought of using 2N3904's as they have a gain of 100 and they sound pretty good in most effects.

http://www.fairchildsemi.com/ds/2N/2N3904.pdf

http://www.mouser.com/ds/2/149/BC547-190204.pdf

Good to hear from you again Joe, it has been a while.

Mark

According to the Fuzz Central page on the Clyde McCoy wahs, he says it's best to use transistors in the 300-400 hfe range with the stock Vox/Cry Baby circuit to achieve the vintage sound (along with the right inductor and component values).  Dunlop changes a resistor value to make up for their use of the high gain transistor they usually use (MSA18 I think), they have something like 800 hfe or so as I recall.  BC109B's are suggested often, as their gains usually run in the recommended gain range.

Al

[Mark Abbott]

Yeah Joe, it wasn't your typical scratchy pot noise, the noise wasn't apparent until the note died right down. It also sounded more like an induced noise rather than a scratchy pot, I suspect the hiss may have been a little louder when the pot was dirty or perhaps it didn't seem so loud once the scratchiness was gone? I'm a little surprised as I hadn't used it a lot. I've cleaned it with De Oxit a couple of times and I'll see how I go. The wah sounded like it was distorting too, very much like the Jeff Beck track You shook me.

Mark

P.S. Merlin I'd love to see your take on a wah pedal, I'm building your compressor now.