# BOSS FV50 Volume to pan mod



## studiodog (Sep 10, 2010)

HI folks,

I'm back with another pedal project. This time I'm looking to rewire an old BOSS FV50L stereo volume pedal into a stereo L/R pan pedal a la Ernie Ball 6165. I simply want the panning/balance function, no volume control. This is to go with an archtop electric guitar with piezo pickup installed. The piezo and electric pickups run from the guitar via a TRS cable on separate channels, so the pan pedal will allow me hands-free balancing of relative volumes. 

Based on the EB6165 schematic here, I've come up with a simple plan using a 500K blend pot, three 1/4" jacks (one TRS, two TS), and a couple of output resistors. 








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Have I missed anything here? Seems like a dead simple setup, but I thought I'd pass it by the circuit boffins here before accidentally blowing anything up! 

FWIW, Ibanez used to make a pedal similar to the EB6165 called the VL10 Volume Balancer. Pics here show a stereo pedal in a very FV50 type case with a pan/volume switch added in. Extra bonus point for anyone who can provide a diagram showing how to add that switch to my existing FV50!


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## Guest (Feb 7, 2011)

That doesn't look quite right to me. You want the ring to be common on all the jacks. Just tie those together Ring in -> Ring Out 1 -> Ring Out 2.

Then you want Tip In -> center lug on both modules on the pot. Then you want the opposite wiper terminals to go to tip out on each of the output jacks. You leave the other wiper terminals unconnected. You're only using half the pot in this case.

Then you've got two volume pots working in opposite directions.

I _think._

Wait for Mark to weigh in. 

Edit: yea, I took a look at that schematic and rings are all tied together on that schematic. They say "to ground" but really I think that's just there to say "the ring is ground in this circuit" -- not that you need to find some other ground point.


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## mhammer (Nov 30, 2007)

Huh? What? Jeez, did I sleep in? Okay, lemme get my pants on first.

Okay. There* may *be a problem with the intended conversion.

Normally, one uses a logarithmic pot for volume, because the way in which human hearing works prefers it. You CAN use a linear pot, but it tends to bunch up all the volume change at the lower end of the pot. Many amps have used linear volume pots over the years, and the cynical among us usually construed that as a plot to get unwitting consumers to think "Man, if it's THAT loud at 3, imagine what it'll be like at 10!".

If a stereo volume pot is intended to be used ONLY as a volume pot, then the chances are pretty dang good (though not 100%) that it will use commonly available dual-ganged log pots.

If a stereo pedal is intended for use as either dual simultaneous volume *or* panning, then the chances are pretty good that it will use dual-ganged *linear* pots. Why? Because if one flips the "direction" of the attenuation around, the two pots should observe the same degree of change, such that in the exact middle of their rotation, they have each introduced the same amount of attenuation. In other words, in the middle of rotation, they should be equal loudnesses. There ARE fancy-schmancy taper pots that are log-antilog, changing from one to the other at the midpoint, but those tend to be VERY special order and not found very commonly. And of course, assuring that the two halves of the dual-ganged pot are matched is a trick and a half.

Bottom line? I'm willing to put money down that your FV50L uses a dual-ganged pot which is fabulous for stereo volume, but irritatingly mismatched for panning. Ideally, it would need to be replaced with a dual-ganged linear pot of the same value to be usable.

Having said that, there IS an outside chance that Boss went "Meh, what do _they_ know?" and used a linear pot as was probably used in the EB6165. If you are unsure, and you have a meter, set the pot so that it is about in the middle of its rotation, and measure the resistance from the wiper to each outside lug. If it is linear then it should be approximately equal.

If it IS a linear dual-ganged pot, it is a trivial matter to install a DPDT toggle, and use it to simply "flip" the connections to the outside lugs of one of the pots, such that what used to be ground is now hot and vice versa. If they have soldered a ground wire to the pot chassis from one of the pot lugs, you will have to remove it for one of the pot halves.

EDIT: Just noticed that the EB6165 schematic specifies 500k*S* pots, which I gather provides a symmetrical log-antilog taper.


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## studiodog (Sep 10, 2010)

hi guys, and thanks for joining my modding insanity! I was leaning towards ordering this 500K blend pot from stewmac. They _*claim *_an equal volume from each side at the centre detent position....

I did a quick futzing around with the FV50 last night, swapping pos/neg leads on one of the output pots, but got unusable results - Channel A is fine, while Channel B (the modded one) is only about half volume relative to Channel A. I will check it out again tonight when the after-school madness dies down and I can think without earplugs. Assuming I did something silly with the soldering iron, and can fix it, I'll try adding a DPDT as suggested. Then I'd have both volume and panning available - it's always nice to have options!


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## Guest (Feb 7, 2011)

mhammer said:


> Normally, one uses a logarithmic pot for volume, because the way in which human hearing works prefers it. You CAN use a linear pot, but it tends to bunch up all the volume change at the lower end of the pot.


Mark, I was wondering the exact same thing. I figured the OP had sourced a ganged pot with symmetrical log tapers.


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## mhammer (Nov 30, 2007)

> Mark, I was wondering the exact same thing. I figured the OP had sourced a ganged pot with symmetrical log tapers.


Dual-ganged linear IS symmetrical, albeit the sort of symmetrical that will not provide a pleasing swell if using only one channel.

But the linear dual-ganged pot from Stew-Mac will do what I described.

At the same time, if you happen to be in the vicinity of the downtown U of T campus, you can pick up the same pot for $1.40 at Honson on College.

Just make sure that it is physically compatible with the mechanism (and space) in the Boss pedal. The Stew-Mac is for blending _at the guitar_, not in a footpedal.


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## Guest (Feb 7, 2011)

studiodog said:


> They _*claim *_an equal volume from each side at the centre detent position....


That'll be true of any symmetrical pot arrangement. The problem is with the taper. You may have equal volumes at the center position, but if the taper isn't logrithmic you'll find that the equal volume is kind of low and that it stays that way until you're in to the last few mm of travel in your pedal. It has to do with how you hear and what "louder" means to your ears.


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## mhammer (Nov 30, 2007)

Well, again, the acceptability of the taper depends on what you want to do with it.

If you want to be able to do volume swells with one channel on its own, it won't sound great, or rather, the feel won't be quite right. If you want to simply feed mostly amp B instead of mostly amp A, or if you want to take the dry and wet outputs of a delay pedal and adjust their relative intensity (at the amp) in real time, then it may be fine.

Note that when we talk about "panning", we usually mean a single source that has two possible simultaneous destinations. A stereo volume pedal presumes that there are two separate inputs, each going to their own destination, and that one would make them both louder at the same time, or adjust their individual volumes in a reciprocal fashion.

If a person wants to send the same signal to two different places in varying proportions, one way to do it (albeit with some loading) is via a balance control, which uses only one pot.

Works like this. Your single input jack gets fed to two output jacks, via a 47k-100k fixed resistor. Now, you run a wire from each jack's "hot"/output lug to the outside lug of a 500k-1M linear (or symmetrical log-antilog) pot. The wiper/common of that pot now goes to ground. When centred (assuming a 1M pot and 100k resistor), each output behaves as if it is a 600k volume pot (100k + half the pot value) turned down just a bit (1/6th, actually). Move it in one direction and the resistance to ground from the output increases, while the resistance to ground on the other side decreases. So, if you've moved the pot enough to have 900k between wiper and hot on one side, you'll have 100k between wiper and the other hot on the other side. One pot adjusts the volume/attenuation of each side in complementary fashion.

Ideally, one would use some sort of parallel fixed resistro to adjust the taper and achieve a log-antilog function. See this classic for how to do it. The Secret Life of Pots


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## studiodog (Sep 10, 2010)

mhammer said:


> If you want to simply feed mostly amp B instead of mostly amp A, or if you want to take the dry and wet outputs of a delay pedal and adjust their relative intensity (at the amp) in real time, then it may be fine.


That is indeed what I'm after. For my application, the piezo signal (Tip on the TRS input) from my guitar will go either to an acoustic amp or a DI to the board. The electric signal (Ring on the TRS) will go to an electric amp/pod/whatever. The pan/balance pedal will allow me to feed more of one signal than the other to the respective amps. 

Getting back to my original post, assuming I use the correct dual pot, is my schematic acceptable ie Will it work? Can anyone see a way to improve the circuit?


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## mhammer (Nov 30, 2007)

With that out of the way, the principle of the EB6165 schematic is perfectly acceptable.

Is the pot value appropriate for the application? There may be values that result in less loading, but if a 500k volume pedal is fine, then a dual-ganged 500k pot is fine too.


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## studiodog (Sep 10, 2010)

The pot currently in the FV50L is 20K - designed for use after effects chain/buffer. I've used it as a straight volume pedal in this position and it works fine. I know from a colleague with a similar setup that the EB6165 works fine too, so I'll try using the stock 20K pot since I have it in my hand. 

That said, anyone in the UofT area of Toronto want to grab me a couple 500K dual pots from Honson? I'm happy to pay postage to Kitchener!


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## Guest (Feb 7, 2011)

I completely missed that the input was a TRS. Sorry about that. I thought it was a mono source. Your schematic is correct for the TRS -> TR + TR case.


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## studiodog (Sep 10, 2010)

hi gents, 

Just a quick follow-up - is THIS the famous log/antilog pot?


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## mhammer (Nov 30, 2007)

I hate for this to get cryptic, but "yes".....and "no". It's close, but no cigar. The log/anti-log I was alluding to is _within each pot half_. That is, the transition is logarithmic as you rotate, then once you get past the midpoint, it switches over to anti-log. The one you linked to (and hopefully didn't spend money on) has one pot section log, and the other pot section anti-log.

Yes, it is absolutely the one you'd want if ALL you wanted to do was use it for panning, since it is two pot halves that function exactly as a volume pot should, except in opposite directions.

The "no" part/problem arises when you want to flick the switch and use it as a dual-parallel volume pedal _going in the same direction_. At that point, it becomes absolutely the wrong pot to have.

This is why I recommend checking out that document at GEOFEX. It is possible to play with the taper of any pot by straddling two adjacent lugs with a fixed resistor (something explained and illustrated in the document). If one uses a dual-ganged linear pot, it then becomes possible to have something close to a log taper, by means of parallel fixed resistors, going in the same direction. Then, when you want to use it as a panning control, you can either lift the parallel resistors, or simply flip the outside connection of one of them so that it goes from lug 2-to-3 now instead of 2-to-1, thus creating something like the one illustrated in the link.

Ideally, this would require a 3PDT toggle, slide, or rotary switch; two poles to flip the outside lugs around, and one to flip the parallel resistor over.

Alternatively, you just stick with a linear dual-ganged pot, and live with the taper as is. I suggest trying that out first, and seeing if it provides something acceptable. If not, then you can always add the parallel resistors.


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## studiodog (Sep 10, 2010)

Thanks mhammer. I'll stick to the dual-ganged pot and take it from there! 

FYI, I've come across what I think is a better option for a case to mod - I picked up a Dunlop HighGain mono volume pedal for $35. Much easier to swap out the pot in this one, and lots of room inside the case to add extra jacks. 

I plan to drill new A/B output jacks into the top end of the pedal (right through the HighGain logo sticker - sorry, Dunlop!), and use the original input hole for a TRS input jack. Maybe even wire one feed from the input to a tuner out jack at the original output point if I'm feeling creative. 

I'll hold onto the BOSS FV50 as a stereo volume pedal, perhaps for a secondary or practice rig, expression pedal or whatever. I think the cast steel chassis on the dunlop will hold up better to gigging anyway.


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## mhammer (Nov 30, 2007)

Sorry to make it more complicated. I'm sure you and many others will agree, though: when it comes to things you adjust with your feet, "feel" is very important.


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