Thanks for the very complete answer. I have the Yamaha pedals and am aware
of their connection. It is amazing that many of the ideas you have listed
below are things I have implemented or thought about. The adapter cord I
thought about since most guys will need an extension anyway. For certain
the normalized CV through a resistor that gets shorted out when a mono plug
is used so a CV input OR audio can be biased (turn AC to pulses).
Think about this and tell me I am crazy.
What if, you ALWAYS put the normalized CV on the ring from 15 V with a
resistor that is 2x the size of the pot in the pedal.
Sure it works great on the Roland. But, I am not certain that is not an
acceptable (but unusual) connection for the Yamaha. Hear me out:
Yes, you put the voltage on the wiper and you take your voltage off the
ungrounded end of the pot. Looks bad. And, the current varies in the
divider because you increase and decrease total R by 50%. But, you still
end up with a voltage that varies at the fixed end of the pot from 0 to 1/3
source V. It is slightly non-linear. I get 60% voltage at 50% pot
position. And, this relies on high input impedance of the first op amp.
The larger the resistor is that is used in series with the pedal pot, the
more linear this gets if I am looking at it correctly. But, you cannot go
too high or you have to make the input impedance of the first amp too high.
So, I decided on a resistor 2X the size of the pot. I want to have a board
jumper that shorts one or two resistors the supply to the pedal to
accommodate 3 different size pots. I was thinking 10K, 50K, and 100K.
Tell me I am crazy for wanting one jack to work for the wiring of either
pedal. Tell me this is a half baked idea. I need to hear that if it is
true. I would be happy to send the schematic for your critique. I am tying
to be creative to fit this into a 1U format that has 2 pedal inputs, 3
outputs, 3 pots and will use MOTM-800 hardware. If the odd pot connection
does not work out for me, then I think the adapter / extension is the way to
go.
The direction of the pedal pot is not an issue as I plan both (+) and (-)
out put for either direction of pedal travel (both outputs for one channel
and a reversing attenuator for the other).
As always your knowledgeable opinion of my crazy ideas would be appreciated
Larry
----- Original Message -----
From: <jhaible@...>
To: <motm@yahoogroups.com>
Sent: Thursday, February 08, 2001 5:43 PM
Subject: Re: [motm] HELP with MOTM pedal interface and voltage source
> My goal is to make the thing compatible with as many different foot
> controllers as possible. I use the Yamaha FC-7 and think it is far
superior
> to ANYthing else out there. But, I know some people may use the Roland
> EV-5(?) and perhaps others.
I'm also using FC-7's and FC-9's a lot, and yes, I like them much better
than any other brand of pedals I've tested.
The problem with these Yamaha pedals is that they are so non-standard
in their connections. Sleeve is GND (well, at least that is standard),
Ring is the wiper of the pot, and Tip is the cw end of the pot. Just
the opposite of every other pedal I know.
This is why I'm making the modulation inputs on all my FX devices
(and on some of "my" MOTM modules as well) compatible to other,
"standard" pedals with the Tip being the output (wiper) of the pot.
(See for the Ring connection, or no ring connection, below.)
For the Yamaha pedals, I'm using a special cable with Ring and
Tip connections crossed.
Why that ? Even when I'm mostly using Yamaha pedals, a pedal
input should also serve as a normal CV input, and here the standard
is doubtless Tip = Input.
That much for Yamaha versus the rest of the world (;->).
Now back to a pedal interface, as I usually do it.
These pedal potentiometers have a medium to high impedance.
(I think 50k nominal for Yamaha - from memory, but it's printed
in the manual or even on the pedal itself, or something between
10k and 100k for other pedals I've seen.
So you can feed the Ring (to "supply" the pedal) with a rather
high resistor. 50k Pot, 100k "feeding" resistor from +15V
results in 5V maximum for the pot. And 100k is high enough
that you can short it to GND with a Mono plug and use the
input for normal control voltages.
But even "better" (depending on the situation): Don't feed the ring
at all. Just use a Mono jack and feed the 100k resistor directly
to the (Tip) input. Use a normalized jack that shorts it to GND
when nothing is plugged in. Now you can connect the pedal,
and the pedal pot will not work as "potentiometer" anymore,
but just as a variable resistor which is one (the lower) half
of a voltage divider. (The 100k resistor is the upper half,
of course.)
And if you want to use the input for normal CVs, no problem
either, the low impedance of the driving module will easily
override the 100k.
This works very well in many of my modules, and I think
I have even included it in some MOTM designs as a
"hidden feature" where I thought it useful, and where
no other criteria were speaking against it. (You won't
want that on precisely scaled 1V/Oct inputs for instance.)
If you have an input that has no such feature you can
feed 5V DC into a standard guitar volume pedal (a cheap
one with just a pot, no active opto stuff) and use the output
for controlling the Module's CV input.
For 5V DC, use the smallest 9V or something wallwart,
and add 2.2kOhm, a 5.1V zener diode and a 10uF electrolytic.
You can either fit that into the pedal, or even into the
wall wart.
JH.