True Bypassing A Vintage Wah

[Paul Marossy]

If I true bypass a vintage wah, will it change the way the circuit sounds? Anyone have experience in this? I thought I remember reading somewhere that it will mess with the sound when you do that. Would it be better to put a JFET buffer circuit at the input to the circuit instead?

[Peter Snowberg]

True bypass only changes the tone when the circuit is bypassed to eliminate tone sucking so no worries there. If you're not convertng it to true bypass, the next best thing is to address the main reason for adding the bypass which the high impedance of the JFET buffer does.

Bypass away.

Take care,
-Peter

[Davefx]

You will experience a slight volume drop with true bypass. You can lower the value of the 68k input resistor (try 47k) to compensate.  I wouldn't go any lower than that.  In mine I also lowered the value of Q1's emitter resistor from 470ohm to 330ohm and helps to compensate as well.  

But of course, go to www.geofex.com and check out the "Technology Of" series about wah pedals.  All the poop is here on the subject.

[Paul Marossy]

Yeah, that's what I was remembering - a slight drop in volume. I did the true bypass part last night, I just have to test it out and see how it sounds...

That's weird, though. Why would there be a drop in volume? Is that because some un-effected signal is also making it to the output jack, too?

[Davefx]

I'm definitely no theory genius by any sence of the word, but it would seem to me that if you were running the old SPDT method, when bypassed your signal is being degraded to a point that when you engage the wah, there seems to be a little boost in contrast to the degraded bypass. So with the DPDT method, in bypass you have your pure signal, and when the wah is engaged the "true gain" of the wah shines through and you notice a drop.

I'm glad you asked the question, but I'm not completely clear on this either. Any local gurus are welcome to chime in...

[Paul Marossy]

Thanks Dave, what you say makes sense. It really does make a difference when you do the true bypass.

I replaced  the 6.8K input resistor (which measured 7.5K) with a carbon comp I had lying around, which measured 5.3K. A very noticeable change in the volume level, maybe now it's a little too loud. But, that's easily fixed.
Oh, and I had to add a pulldown resistor, got a pop when switching. Now I think I am happy with it.