Poorly implemented EXP-2 expression pedal - CV control

[Piers]

Hello,

I realise I'm coming to this thread 10 years too late, however...

My X3 just died, and I've replaced it with a Trinity Pro V3.

My EXP-2 pedal worked just fine with the X3, it had a nice travel from fully up to fully down. On my Trinity, however, most of the volume change comes in the early part of the travel, i.e. from heel down to about 1/3 of the way.

So, it clearly matters which board you've got it plugged into. One size doesn't seem to fit all, when it comes to the EXP-2. It's odd, but it might explain why some people had no problems with it at all...

When I don't have a gig to practice for I'll work my way through some o the remedies suggested.

Piers.

[voip]

The Assignable Pedal input circuitry of the X3 differs from that found in the Trinity Pro.

In the X3, the signal voltage for the pedal position is derived from a potential divider, formed by an internal resistor in the X3, and the resistance value "returned" by the EXP-2 potentiometer. The X3 basically uses two of the available three terminals from the EXP-2 potentiometer. It will work with a TS, or mono, audio cable.

In the Trinity Pro, all three terminals of the potential divider in the EXP-2 pedal are available to the Trinity Pro. This has the advantage that it makes the Trinity Pro less dependent on the internal potetniometer resistance in the Expression Pedal that is actually used with the keyboard. For the Expression Pedal to work correctly, it has to be connected to the Trinity Pro using a TRS, or a stereo audio cable.

The Trinity Pro Assignable Pedal input is similar to that found on the Kronos.

.

[Piers]

Hello,

OK, well there is clearly a difference between the way the X3 and the Trinity respond to the EXP-2, but for the record, I've always used a TRS connector with this pedal, and it's recommended in the user manuals for both boards, so really ought to be OK...

That said, the Trinity also suggests using the XVP10. Do you think it's likely to have a different position-output curve to the EXP-2?

PIers.

The Assignable Pedal input circuitry of the X3 which is, I believe, similar to that found on the X5D, differs from that found in the Trinity Pro.

In the X3, the signal voltage for the pedal position is derived from a potential divider, formed by an internal resistor in the X3, and the resistance value "returned" by the EXP-2 potentiometer. The X3 basically uses two of the available three terminals from the EXP-2 potentiometer. It will work with a TS, or mono, audio cable.

In the Trinity Pro, all three terminals of the potential divider in the EXP-2 pedal are available to the Trinity Pro. This has the advantage that it makes the Trinity Pro less dependent on the internal potetniometer resistance in the Expression Pedal that is actually used with the keyboard. For the Expression Pedal to work correctly, it has to be connected to the Trinity Pro using a TRS, or a stereo audio cable.

The Trinity Pro Assignable Pedal input is similar to that found on the Kronos.

.

[voip]

Below are the plots of my calculated expression pedal signal voltages going into the synth circuitry (in Volts) shown on the Y-axes for the X3 (first plot) and for the Trinity Pro (second plot, based on the circuits and other info available online. X-axis values are pedal position (in %), with heel down=0 and toe down=100 so, yes, the resultant Expression pedal inputs into the synth are very different, and trend in opposite directions. I'm guessing that, for the X3, the curve is inverted internally so that the resulting #CC value is 0 for pedal heel down, and 127 for the toe down pedal position. This would give a logarithmic characteristic, similar to that of a standard volume potentiometer. For the Trinity Pro, the straight line relationship, between pedal position and output voltage, would result in a greater volume change early on in the pedal's travel.

The XVP-10 Expression Pedal potentiometer resistance is 50kOhms (compared to the published 100kOhms for the EXP-2). This difference would change very little if the two pedals were compared on the Trinity Pro, but would have a different volume characteristic when used on the X3, with the volume increasing more noticeably early on on the pedal movement from heel down to toe down position.

Being as this is the Kronos section of the forum, it's worth mentioning that the Exp-2, XVP-10 and XVP-20 Expression pedals would give a straight line characteristic similar to that shown in the second curve, albeit with a maximum output of 3.3V, since the Kronos' supply to the Expression pedal is 3.3V.

[attachment=0]VdivModeExpressionVoltages.jpg[/attachment][attachment=1]PotModeExpressionVoltages.jpg[/attachment]

[voip]

By way of an update, there seem to be more references to the EXP-2 Expression pedal having a potentiometer of 50kOhms, rather than 100kOhms, so the plot below is from a re-calculation for the Korg X3 synth, assuming a 50kOhm Expression potentiometer value. Compare this to the first plot for the 100kOhm pot on the X3 in my previous posting, above.

The plots for the Kronos and Trinity Pro are almost identical for both 50kOhm and 100kOhm potentiometers, maxing out at slightly less than 5V for the Trinity Pro, and just below 3.3V for the Kronos.

[attachment=0]VdivModeExpressionVoltages50k.jpg[/attachment]