Synthesis Technology MOTM-850, Руководство пользователя

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SYNTHESIS TECHNOLOGY PAGE 14
MOTM-850 ASSEMBLY 10/25/01
WWW.SYNTHTECH.COM
From this the maximum voltage out is when the pedal is all the way down, and the
resistance is 10K, or:
Vout = 15 * (10,000/26,500) = 5.66V
In reality, we measured 3 different EV-5 pedals and found the pot to be closer to 9200
ohms, which limits the output to almost exactly 5.0V. Close enough!
The ‘offset trimmer’ simply adds resistance to the
ground side of the pedal pot. If the trim is
set to ’10’, or 50K, the output will ‘lift off ground’ and be from 9.6V to about 11.4V. This is
out of the ‘850s operating range. So, be sure to keep that trim set below ‘3’ or so!!
INTERFACING TO THE YAMAHA FC PEDAL
The Yamaha pedals do not have the additional trimmer. It’s just a single 50K pot. BUT, as
we said earlier, they switched the tip and ring connections around! See Figure 2.
Since the pedal pot is 50K (they must use better pots, because the 4 we measured were all
within 2% of 50K!), we need about 100K of ‘series’ resistance to get out 0 to +5V output. The
voltage output in the Yamaha configuration is complicated, because we are ‘driving’ the
wiper of the pot! The series combination of R17 and R18 is 99K. So, the maximum voltage
out is:
Vmax = 15 * (50/(99+50)) = 5.03V
If you really want to go through the math of the actual voltage out versus pedal position,
please go ahead :)
Now, lets talk about R24. The clever reader will plot these out and see that the voltage out
is non-linear with pedal position. This is because the voltage divider is based on the x/(1-x)
curve. So, since we are perfectionists, R24 is added. The value is quite large, compared to
the pedal resistances AND to the RANGE pot VR1. This adds it’s own ‘series resistance’ to
the wiper of VR1. Since the voltage output from the pedal is not buffered, and has a large
Y A M A H A F C P E D A L
D W N
U P
P E D A L
50K