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Expression pedal ohmage?

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Expression pedal ohmage?

Postby MC Deli » Sat Dec 19, 2009 8:23 pm

I have a new Eventide Pitchfactor (it is spiffing BTW) and I got a Moog EP2 expression pedal to go with it.

Problem is the pedal only works from heel to half way, even with the "attenuator knob" fully turned. I think the reason is because the EP2 is a 50 ohm pedal (info buried here) and the Pitchfactor likes 25 ohms (FAQ buried here).

I have found this SOS expression pedal article but I am confused - and not getting much joy out of searching the Eventide users - do I have to find a 25 ohm pedal (like the Ernie Ball - is that the only way to get a full sweep?

And what pedals are 25 ohm, as the manufacturers are shy about the specs?

(I have looked up Ernie Ball VP Jnr 25k, Moog EP2 (50ohm), DOEPFER FP 5 (10 ohm), Line 6 EX1 (10 ohm),
Korg EXP2 (?), Yamaha FC7(?), Mission pedals (20 ohm?), MAudio MGear EXP (is it 25 ohm?), Roland EV5 (10 ohm?))
:?
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Re: Expression pedal ohmage?

Postby lebihanj » Sun Dec 20, 2009 3:25 am

There are several factors that impact exp pedal compatibility. Potentiometer resistance, wiring, and polarity. Resistance is commonly in the 10K Ohm - 50K Ohm range but it depnds on the manufacturer exactly what they expect to see on an expression pedal and how tolerant they are. The Eventide should work fine with anything from 10k Ohm - 25K Ohm. Wiring determines if the pedal should be wired TS (mono) and use a regular guitar cable, or TRS (stereo) and require a TRS cable. Most are the latter including the Eventide. Line 6 is the most well known exception and most Line 6 equipment uses a mono TS expression pedal. Last is polarity and this determines in a TRS connection whether the stereo pins are wired one way or the other. Most including the Eventide are wired tip to wiper (wiper is the center pin on a potentiometer) but some are the reverse (ring to wiper). Some Rocktron equipment is wired this way as well as a few others.

First thing I would suggest is make sure you have a stereo (TRS - TRS cable). A regular mono TS guitar cable won't work. If this doesn't fix it, look for an exp pedal that fits the above requirements. We have tested the Mission EP-1 with the Eventide xxFactor boxes and they work fine.

Lastly, a volume pedal like the Ernie Ball is not the same as an expression pedal but in some cases you can 'fake it' by using a mono - stereo insert cable and plugging the 2 moon ends into both the in and out of the volume pedal. James
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Re: Expression pedal ohmage?

Postby MC Deli » Sun Dec 20, 2009 10:47 am

Thanks for the info.

The Moog EP2 I have has a stereo cable attached. The MAudio MExp has a polarity reverse switch, which explains it popularity, and why it appears in the SOS article to be compatible with so many synth manufacturers.

Apologies for the missing "k"s on the first post by the way.
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Re: Expression pedal ohmage?

Postby MC Deli » Sun Dec 20, 2009 12:44 pm

lebihanj, I have gone back over the info again but I am still confused - and I appreciate your recommendation of the Mission pedals and you disclosing your affiliation.

Is the problem I have (getting the 0-100 range in only half a pedal sweep with the Moog EP2) because it is 50k ohm - or because it is the wrong polarity - or something else---?

(I think I am falsely getting stuck on 25/50 = half a sweep)
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Re: Expression pedal ohmage?

Postby lebihanj » Sun Dec 20, 2009 9:54 pm

The most likely culprit is the polarity is reversed. I don't have a Moog pedal to hand but if you can open it up you should see 3 wires attached to the potentiometer. If you have the facilites to change the wiring, you need to swap two over. Locate the ground wire this will be connected to the sleeve on the output cable. Leave that where it is. Switch over remaining 2 wires which are the wiper (normally the center pin on the pot) and the third wire so they are reversed. Give it a try. The Mission pedals also have an optional polarity switch, similar to the M-Audio, so you can switch the polarity as required. James.
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Re: Expression pedal ohmage?

Postby MC Deli » Mon Dec 21, 2009 7:08 pm

Thanks again. I found a local Roland EV5 today and it works. My guess is that the Moog pedal is the same polarity as the Korg and Yamaha pedals, whereas the Eventide likes the Roland as it has the (less common?) "Wiper of pot connected to plug Tip, top of pot connected to plug Ring, bottom of pot connected to plug Sleeve."

Shame the Roland doesnt have the feel of the Moog or looks of the Missions but it works.

Still confused about the ohmage... but no matter:)
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Re: Expression pedal ohmage?

Postby lebihanj » Tue Dec 22, 2009 1:27 am

OK. Good. The Roland has the right setup work with the Eventide. Some people say that they have dead spots at the beginning and end of the sweep, but others say they work just fine.

The issue with the required resistance is, like wiring, there is no standard, so one processor may require 10K Ohm and another may require something different. There is no way of knowing unless they publish the specs. Additionally, it's not normally a single value but a range. Some processors may work fine within a range of 10K - 50K whereas another may only work 10k - 20K. generally the more sophisticated the filtering on the expression pedal input the greater the range they can support. Some rack processors and MIDI controllers even have programmable inputs that can be calibrated to exactly match an expression pedal. James.
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Re: Expression pedal ohmage?

Postby Martin Walker » Tue Dec 22, 2009 4:58 pm

lebihanj wrote:The issue with the required resistance is, like wiring, there is no standard, so one processor may require 10K Ohm and another may require something different. There is no way of knowing unless they publish the specs. Additionally, it's not normally a single value but a range. Some processors may work fine within a range of 10K - 50K whereas another may only work 10k - 20K. generally the more sophisticated the filtering on the expression pedal input the greater the range they can support. Some rack processors and MIDI controllers even have programmable inputs that can be calibrated to exactly match an expression pedal. James.



Hi lebihanj!

I'm really surprised by this - as far as I know (and as the writer of the expression pedal FAQs mentioned above) most keyboard expression pedal inputs simply rely on the pot acting as a potential divider, so knowing the input voltage supplied by that socket, and then measuring the output voltage should accurately determine the proportion of travel at the current pedal position.

If this is the case there's no real 'processing' going on. As long as the pedal 'ohmage' isn't so low that it loads the output such that its voltage 'droops', it shouldn't really matter too much what value the pot is (which in practice probably means anything over about 1KB and up to 100KB).

I'd be fascinated if you have any links to stuff about such processing so I can add them to my FAQs!

The only reason I can think of for providing 'programmable inputs' is to cope with pedals that don't mechanically manage to provide the full 270 degree pot travel, so that there are dead spots at one end or the other, but I've not had any such problems with a variety of cheap and expensive pedals mentioned in my FAQ.

However, I can understand a parallel capacitor being fitted where the pedal signal goes back into the device, to help filter out any travel noise from dirty or worn pots.


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Re: Expression pedal ohmage?

Postby lebihanj » Wed Dec 23, 2009 5:43 am

Hi Martin,

This is very interesting because it brings up another difference between exp pedal inputs on guitar type processors and those on keyboards. I was coming at this more from experience with guitar processors and I should have mentioned that in my post. On guitar processors (and MIDI floor controllers used with them), some degree of programmable expression pedal control is quite common. One reason is that with guitars, exp pedals are often used for wah. A standalone wah pedal has a fairly restricted sweep and exp pedals which mimic that do not complete the full rotation of the pot exactly as you describe. So a start and stop voltage for the pedal has to be programmed. Also guitar exp ports often double as switch ports and can be programmed to expect latching or momentary switches in the expression pedal ports for IA, tap tempo and similar functions. Most guitar processors I have found do expect an exp pedal to use a 3 wire connection and function as voltage divider although some, Line 6 being the most notable, use a 2 wire variable resistor. There is a chart of MIDI floor controllers herehttp://axefxwiki.guitarlogic.org/im ... lers_8.jpg that lists the models that have at least expression pedal calibration functions. James.
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Re: Expression pedal ohmage?

Postby Martin Walker » Thu Dec 24, 2009 5:40 pm

Thanks for that info - indeed, guitar processors can be a completely different kettle of fish, although my own experience with CryBaby and other Wah pedals suggests that the typical rack and pinion gearing found in them should manage the full 270-degree travel fairly easily.

However, if you want to fine-tune your wah position then programmable parameters do make sense. I'm surprised at Line 6 using a variable resistor - although I suppose this does mean you end up buying their own make of pedal ;)


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