Expression knob and its weird behavior

Carmacore

Carmacore

Member
Hi guys,
I didn't find a similar thread here. Sorry if i missed something and this particular problem has already been descussed.
I play in a shoegaze band and sometimes it's good to control some delay, etc. values live to produce some Ambient drone like sounds. I'm already using two expression pedals for volume, wah, whammy etc. So i ordered 2 custom made expression knobs (10k linear) to have the feeling of physical controler of an effect pedal. I've measured the impendances, they are realy linear. Somehow i see more like logaritmic behavior of the controllers... So for example, if i try to adjust the feedback of the delay block the midle position of the curve on the screen of axe is already reached at a quarter rotation of the knob. After that the value increases very slow. Both expression pots are calibrated with my mfc101. Any ideas what's going on? Should i go for 100k?
 
10K is fine I believe. Off the top of my head, I think the spec is 10-25k linear.

Don't forget to check your expression modifiers. (In Axe-Edit, right click the value being modified.) If the midpoint is set off of 50%, it imparts a more logarithmic type of curve to the values. Slope, scale, and offset can also change the shape of the expression curve. For perfectly linear operation, (assuming everything is calibrated) you want to see a straight line from the bottom left to the top right on the curve display.
 
Anywhere between 10k to 100k linear is the recommended value for the pot. The actual pot value doesn't really matter in this application. 10k won't be any better or worse than 100k.

Make sure the pot is correctly wired to the TRS jack. Sound like it might be wired with a reversed polarity. Some expression pedals are wired with the tip and ring connectors swapped. This will cause a weird response with Fractal gear. It should be wired with lug 1 to sleeve, lug 2 to tip, and lug 3 to ring. With the pot shaft pointed towards you and the lugs pointed down, the left lug is lug 1 (CCW), middle one is lug 2 (Wiper), and the right one is lug 3 (CW).

Like this:
potentiometers-front-back-png.193158
 
Hellbat said:
10K is fine I believe. Off the top of my head, I think the spec is 10-25k linear.

Don't forget to check your expression modifiers. (In Axe-Edit, right click the value being modified.) If the midpoint is set off of 50%, it imparts a more logarithmic type of curve to the values. Slope, scale, and offset can also change the shape of the expression curve. For perfectly linear operation, (assuming everything is calibrated) you want to see a straight line from the bottom left to the top right on the curve display.
Click to expand...

The modifier is set to a linear curve if you mean it. Of course I can change the modifier curve to counteract the issue of the knob but I don´t want to do it every time I want to assign some parameter to the knob. It has to be another solution.
 
mr_fender said:
Anywhere between 10k to 100k linear is the recommended value for the pot. The actual pot value doesn't really matter in this application. 10k won't be any better or worse than 100k.

Make sure the pot is correctly wired to the TRS jack. Sound like it might be wired with a reversed polarity. Some expression pedals are wired with the tip and ring connectors swapped. This will cause a weird response with Fractal gear. It should be wired with lug 1 to sleeve, lug 2 to tip, and lug 3 to ring. With the pot shaft pointed towards you and the lugs pointed down, the left lug is lug 1 (CCW), middle one is lug 2 (Wiper), and the right one is lug 3 (CW).

Like this:
potentiometers-front-back-png.193158
Click to expand...


The taper is connected ti the TRS jack according to Strymon instructions http://www.strymon.net/strymon-tech-corner-2-build-your-own-expression-pedal/ and it suits to your description.
Even if the ring and tip connections are swapped it can only reverse the min and max positions of the taper, not it´s behavior. The only thing that differs from the Strymon link ist the 1k resistor on the tip. I donßt have one. Could it cause the problem?
 
Carmacore said:
The only thing that differs from the Strymon link ist the 1k resistor on the tip. I donßt have one. Could it cause the problem?
Click to expand...
A resistor would change things. Not sure how to interpret "donßt."
 
Carmacore said:
Even if the ring and tip connections are swapped it can only reverse the min and max positions of the taper, not it´s behavior.
Click to expand...
No, it will change it's behaviour indeed. To reverse the min and max positions you'd need to swap sleeve and ring (not tip and ring). Make sure it's wired how mr_fender wrote
 
The 1k resitor is for current limiting and is not needed. I'm pretty sure Fractal has built in current limiting on their expression pedal inputs. It won't affect the taper.

How did you verify the taper is linear?
 
Rex said:
A resistor would change things. Not sure how to interpret "donßt."
Click to expand...
German keyboard glitch, didn't doublecheck, sorry:)

brokenvail said:
did you calibrate it?
Click to expand...
Yes

mr_fender said:
The 1k resitor is for current limiting and is not needed. I'm pretty sure Fractal has built in current limiting on their expression pedal inputs. It won't affect the taper.

How did you verify the taper is linear?
Click to expand...

I measured the resistance with a multimeter at certain positions of the knob in both directions. Its 2,5k at the quarter turn, 5k at the middle and so on.
 
DLC86 said:
No, it will change it's behaviour indeed. To reverse the min and max positions you'd need to swap sleeve and ring (not tip and ring). Make sure it's wired how mr_fender wrote
Click to expand...
Ok, I will just reconnect all cables and see if it words. Did not know the direction has anything to do with sleeve. Thanks a lot!
 
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