Grateful Dead Music Forum

A place to talk about the music of the Grateful Dead 

 #119255  by milobender
 Sun Sep 30, 2012 11:15 am
Hi all,

I've been trying to implement Scott Walker's Blend Pot mod idea and I'm having problems... there is a very dramatic (and unacceptable) volume loss throughout the majority of the pot's travel, with only the very ends being somewhat useful. I asked Scott about it and he said it was a 'phase issue' and that swaping lead positions on the pot would fix it. I've tried every variation with no sucess. If it is indeed a phase issue, which I can see, it would seem that this issue is inevitable given the design, that is, the output of the buffer (send to fx) is split with one side going to the fx and the other going, in one level or another, directly back to the return (most certainly out of phase with the portion traveling through the loop); and I can't see how it's actually going to work.... if phase is indeed the issue.

Has anyone actually done this mod, and had sucess? Any input would be appreciated, perhaps I'm just blind at this point... See the link below for the schematic.


 #119284  by milobender
 Mon Oct 01, 2012 9:17 am
After spending quite some time thinking about this I've concluded that it is not a phase problem... rather a basic design problem. At the middle of the rotation there is basically 250K volume reduction, for both sides, like two volume controls... it can't help but be very quiet in the center. As the pot rotates, it gets louder for whichever side until the end of the rotation where it's the original signal. In that vein, thinking of the situation as just using two volume controls and combining the outputs, and trying to minimize the volume reduction, I used two 25K audio taper pots, one for the dry signal split off the OBEL send, and one for the wet/return. Turning both pots at the same time (to mimic a dual/ganged pot) it worked much better, but still not up to quality standards in my opinion. The dry signal is quite alot hotter than the return and in the 50/50 blend position the volume is still considerably lower than the dry signal, and of course, the by-pass mode, making a big master volume adjustment necessary... which means you'd have to have the master volume no higher than 75% at any given time, to save the compensation headroom. (I also tried one of the specialized blend pots from Stew Mac, and that didn't work at all in this application... it was either all dry, or all wet)

I'd be happy to find out I'm wrong here, as I like the idea of an onboard blend option, but it's not working for me, and I don't see how it can in this configuration. Since there seems to be a good blend pedal option available, that seems to be the way to go.
 #119326  by ScottWalkerGuitars
 Tue Oct 02, 2012 8:36 am
Hey! Sorry it didnt work out for you. Like I said, I have done this probably 10 times and each time had to tweek it until it was right. It is still not perfect though, but it works. Sometimes I have had to invert the polarity of the guitars pickups to solve the problem you have described. Barry Sless had me put it in his guitars and he loves it, very dynamic and expressive. He gigs with them all the time and uses the loop exclusively.
A couple years ago, John Cutler, Peter Miller and myself designed and built a Pedal Version of this idea, a box with expression pedal. The box allows either a FX loop type guitar or regular guitar to be plugged in, as well as the FX. And expression pedal used to mix the wet and dry. It works great . Very amazing! Barry has one we built for him, but he prefers the Blend in the guitar, over the pedal. We ended up abandoning the project, figuring for us to build this the right way it would be to high of a retail price. At that same time this company (xotic effects) came out with the same product. Although theirs doesn't have the Expression Pedal.

(edit) To cure the problem your talking about, we added the Phase switch on the network box, because certain FX will invert the signals phase. The symptom is that in the middle of the rotation there is significant volume loss. SO mattering on which buffer im using, which FX are being used, and which pickups, I sometimes end up inverting the phase at the pickups (swapping the leads).

Heres a couple pics of our prototype -



Cheers and sorry again for the troubles, i wish I had a straight forward answer for you, other than swapping the phase of the guitar at the pickups. Or, try inverting the phase at the buffer, like using a different buffer. I find that stuff like this really needs tweeking until it works, thats all I can say!!
 #119533  by milobender
 Sun Oct 07, 2012 2:09 pm
Yep, this is the direction I've been trying the past few days... but based on FETs instead of opamps... I've got the FETs here to try "o)

I concluded early on, that the passive use of one pot wasn't going to work for me, for one thing, it defeats the idea of the OBEL (which is to send the full signal to the effects consistantly) by putting a 'volume control' in series with the signal...

I'm ordering parts today for a bit more experimentation... I'm planning to include ordering some opamps and compare them with the FET designs.

 #120303  by milobender
 Tue Oct 30, 2012 10:19 am
Sorry for the delay... After trying many variations with both FETs and the Opamps, I came to the conclusion that the full blown opamp design sounded/performed the best, although I would replace the 25K blend pot with a 10K... which minimized the volume dip in the center of the rotation. However, I ended up using a very simple 2 FET circuit instead (it sounded nearly as good as the opamp design) because of space limitations. I simply couldn't make the opamp circuit small enough to fit into the cavity along with the already present UGB and Stratoblaster boards, and battery/clip... the cavity is PACKED!