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Why is an M Audio EX-P expression pedal incompatible?

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First of all, loving my M9. Probably the wisest gear decision I've made thus far. It's straddling across two parallel/true stereo fx chains so I'm even considering getting a second one for more flexibility...


But I digress.



So yes, when I bought my M9 I already knew that the M Audio EX-P doesn't play well with M-Series pedals... but I don't understand why this is so, and whether I can fix the issue without having to buy yet another expression pedal.


Is the EX-P's design unusual? Does it need to be TS instead of TRS? Would it work if I rewired it to TS (mono) or added an adapter on the end of the cable to convert TRS to TS? Has anyone experimented with this?



Here is what I've discovered thus far about using an EX-P with an M9:


1) The Line6 M9 Pilot's Handbook states that "Expression Pedal 1 + 2 - Compatible with standard expression pedals including the Line6 EX-1 ... The Line6 EX-1 expression pedal is a passive 10k linear taper pot, using a standard 1/4 inch mono instrument cable."


2) The M Audio EX-P is a popular passive 10k linear taper pot expression pedal with a TRS output (ie. not mono) that I've had pleasant experiences with prior to buying the M9. Measured it on a scope to have an actual range from heel-down T-R @ 12.9k, toe-down T-R @ 1.0k  (ie. 11.9k range). Also just in case it matters - heel T-S @ 53.9k to toe T-S @ 65.7k and heel R-S @ 64.7k to toe R-S @ 64.7k.


3) The M Audio EX-P doesn't work properly with the Line6 M9, as mentioned here:

http://line6.com/supportarchivenew/thread/19057/ and verified by my testing. The EX-P can be assigned to actuate parameters in the M9 but tops out/bottoms out early. It won't move the parameter for its full range. Eg. If I assign it to a wah FREQ it stops well short of maximum upper freq. If I assign it to MIX then it mixes neither fully wet nor fully dry. If I switch the EX-P polarity I have to re-assign the parameters but it behaves exactly the same.




Any ideas?



Cheers H

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You kind of answered you own question... Yes, the Line 6 expression pedal jacks is expecting a TS connection, not a TRS one, and because of that, pedals that are designed with TRS connections typically won't work for Line 6 products. This Strymon post kind of describes it:




This page has a simple schematic of what is needed for a Line 6 expression pedal: http://circuits-from-my-shed.blogspot.com/2012/02/first-build-line-6-ex-1-expression.html

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Thanks @phil_m

By the looks of the strymon link, the most common implementation of an expression pedal is to compare the reference voltage on the Ring with whatever voltage is coming back via the Tip. The Line6 implementation instead seems to compare the Tip with the ground/Ring. 

In my mind TRS should still work then, because the voltage between the Tip and the Ground would still be changing (heel T-S @ 53.9k to toe T-S @ 65.7k). Perhaps the maximum pull-down resistance isn't enough though... Or the minimum resistance is too high... Does that sound kind of plausible?

I should try testing it out. If I add, say, a 10k resistor in series (instead of parallel like the DIY guy did) it should shift the range up to a 
63.9k-75.7k range I think. If that's the wrong direction I'll soon find out...


Does that sound sort correct-ish logic? Truthy?

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Ok so I've also tested TS vs TRS connections - same behaviour. I think I've narrowed it down to a toe-down problem (although yesterday the MIX knob wouldn't go fully wet or fully dry).


The Roland EV-5 is TRS but is also compatible... the plot thickens!

I tested my mate's EV-5 which behaves correctly when the range knob is set to max:


                              Toe Down          Heel Down        = Range

Roland EV-5

T-S                           61.1k                   50k                  11.1k

T-R                           0k                        10.4k               10.4k

R-S                          61.0k                   61.0k                0


M Audio EX-P 

T-S                           65.7kOhms          53.9k             11.8k

T-R                           1k                        12.9k              11.9k

R-S                          64.7k                   64.7k                0

I'm no electronics expert, but to me the measurements don't look vastly different. The weirdest thing is that if I adjust the knob on the side of the EX-P I can get pretty close to the EV-5:


M Audio EX-P (range knob set at about 2 o'clock)

T-S                           61.2k                   49.4k              11.8k

T-R                           1k                        12.9k              11.9k

R-S                          60.2k                   60.2k                0

...but it still doesn't act correctly!
One thought that occurs to me is that perhaps the socket inside the M9 has a contact for Ring instead of the Sleeve (so 'TR'). A mono TS jack plugged into this would connect 'TS' and behave normally, but a TRS would connect 'TR' with nothing connected to the jack's sleeve. If this is the case - and the EV-5 at toe down has a T-R reading of zero Ohms but the EX-P reads 1k - then the 1k might be the problem. This would be fairly consistent with my experience of missing the last 10%ish of the range.
OTOH, if the R and S are shorted together within the M9 socket (some devices do this) then I think parallel resistance could come into play, in which case I'll prob get lost and confused and end up staring vacantly out the window hahahahahmm....
My next move will be to hook up a TRS cable to an M9 exp socket and do a couple of tests with the meter. Who knows what I'll find!
I'll also open up the EX-P and try to establish what's causing the 1k min T-R resistance at toe down. There might be a 1k series resistor in there or something.

Any thoughts? Am I barking up the wrong tree?
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I can say, it's definitely worth it to run your M9 with two expression pedals! I am using the Mission L6 exp pedals; they are awesome!


Also, quick note - I just added a small MIDI controller to my M9 board, and it's really cool! Allows scrolling between the 24 scenes with dedicated bank up/down switches, and it's very small box size. Has a third button that I have set up to do all FX bypass, but can also be set to change mode to control looper, then the other two buttons can do record, play, overdub, stop, etc.


Love the M9 - adding the exp pedals and MIDI box just seriously enhanced the functionality for me!


I've got the gen2 version; there is a newer version and a four knob that can be set to call up the tuner:








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The M Audio EX-P actually works !!! Don't throw it out just yet !


    After buying this expression pedal without actually checking if it's compatible with my Line6 M5 or not (I know, not too smart), I was a bit sad to find out it was not fully compatible. For me it kind of worked - I could get it to go all the way up but I had to flip the pedal (so heel was the effect all the way up and toe was the effect all the way down) or it just wouldn't go fully wet or fully dry for other effects.

    I read a few forum posts about this and in them I saw a couple of comments here and there that said exactly what I'm about to say : if you remove the 1k resistor and add a wire in its place it works properly !!!

   Yes, you do have to open the pedal up, desolder the resistor (it's really tiny and located on the potentiometer circuit) and solder a wire in its place. Even though the resistor is tiny, the process of desoldering it is fairly easy if you have soldered before and if not, just watch a quick youtube video on how to do it. I think for the price (I got it for 14 GBP) it's worth trying out this mod. Wish I would have taken a picture but was to enthusiastic to try it out and forgot. I will add some if somebody needs instructions.


Hope this helps !




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