Modified BOSS Stereo Volume to A/B Blend foot pedal. Question...

[Civvie]

I have modified a Boss FV-60 Stereo Volume pedal to be a one input two output A/B blend pedal by wiring one input to feed both outputs and then reversing the polarity of one output.

It works well but there is a noticeable and annoying dip in volume half way through the transition. As in, the output volumes do not smoothly crossfade.

I can't tell if the stereo pot inside is log or linear. But would changing from one to the other help?

Any suggestions for building a smooth A/B blend very much appreciated.

Thanks

[Processaurus]

It is most likely a log pot, so the reversed one has become anti-log, which would make the taper funny.

There seems to be a whole school of thinking about different crossfade curves, as far as what the ear hears as half as loud vs what actual division of the signal voltage is to get what seems like half as loud.

A classic, single (linear) pot panner can be found in this geofex article, that has pretty good smoothness across its travel.
http://geofex.com/Article_Folders/panner.pdf

[Civvie]

Thanks for the reply and the link.

Yeah, I have always heard that every 3 db is perceived as a double in volume.

All the schems on the link are obviously different beasts to my rudimentary passive single pot circuit.

I will try a linear pot first for simplicities sake. It will probably be the opposite and have a 'bump' in the middle. Which is at least preferable to a 'dip'. Why would anyone want to turn their guitar down???

I wonder if anyone has built a good foot pedal panner-A/B blend pedal? Seems like a popular application to me. I love mine (except for the dip!!)

[Civvie]

I found a product that does exactly what I want, made by Ernie Ball:
http://www.ernieball.com/products/category-534-Volume-Pedals#/product/500K-Stereo-Pan-Volume-Pedal-

The Schem is here:
http://www.ernieball.com/products/volume_pedal/pdf/po6165_vp_stereo-pan_500k_1999-10-12.pdf

As you can see, it's absolutely idiot-proof dead simple design, and an identical concept to how I have rewired my Boss FV-60

Assuming the earnie ball product does not dip in volume in the centre, why does mine?
Is it all down to the type of pot?

The Ernie Ball one uses a 500k Stereo Linear taper pot, I think the one in my pedal is 60k (not sure if it's linear/log)

I guess If I put one of these lin 500k jobs in my boss then it is effectively an ernie ball pan pedal (valued at £240 here in the UK  )

I'll give it a whirl and report back

[Quackzed]

and then reversing the polarity of one output.

... not sure why you'd want the 2 signals to be out of phase with one another by reversing the polarity of one...
i know the splitter blend has a switch to do that, but it's only for when pedals in the loop happen to invert the signal- to get the signal back IN phase...
i'd think that any time you mix 2 out of phase signals in even portions they will cancell each other and you'll get a big volume dip.
try to 'un' reverse the polarity of the 'one output' you reversed and see if that doesn't get rid of the volume dip. 

[Civvie]

Quackzed: I think you have just misinterpreted the situation. There is no phase reversing, just the reversing of the pot so it turns one signal down while turning the other up.

To update this thread. I bought the earnie ball model (found one cheap on ebay) and it does the same thing! 

It dips in the middle when using it in output pan mode.

Looks like I would need to build an active pan circuit to get this to work properly. What a pain.

[joegagan]

based on what you said about yours and the ernie ball, i suspect your pot has some sort of log taper. it is easy to measure, just orient the pot with the shaft facing you and the three lugs pointing toward down ( lugs will be in  approx , 5, 6, 7 oclock positions. mark the shaft end with a point @ 5 oclock when pot is full up, turn it all the way down- it should be symmetrically placed. now take a reading with your marked point rotated to  12 oclock. if it is close to half the pot value, it is likely linear.

now , some tapers can fool you, to be more accurate, place the pot on a piece of paper and make a series of oclock position markings ( these can roughly represent percent of rotation, close enough to get e feel for the taper). take readings at each point, this can then be graphed in the way you see pot tapers graphed.

if you think about it, if the dual pot in your boss pedal is non- linear, it stands to reason that there would be a drop through an area of the crossover. it wasn't noticeable before you reversed because both sides tapered simultaneously, now they do not.

curious to hear about what you find.

here is the sad part. i actually have one of those boss stereo vol pedals here, i could check it myself, but i can't find the damn thing.