From smithj9@hotmail.com Fri Jan 10 16:04:57 2003
Subject:Possible Expression Pedal Solution (technical)


Hi,

First, let me apologize if this is too techie for you.
I know many of you won't be interested, but I hope this
may inspire a few folks to pursue this idea.

If you're a knowledgeable tinkerer who uses a Hammond chop
or a clone that doesn't have good expression pedal response,
and have been looking for a better solution for an
expression pedal than just using a guitar volume pedal,
you might want to investigate possible use of the
National LM1036 Dual DC Operated Tone/Volume/Balance
integrated circuit, available for $2.82 from DigiKey.

From the National datasheet (available at the www.national.com
website):
"The LM1036 is a DC controlled tone (bass/treble), volume, and
balance circuit for stereo applications in... audio systems.
An additional control input allows loudness compensation to
be simply effected.
"Four control inputs provide control of the bass, treble,
balance and volume functions through application of DC voltages
from a remote control system or, alternatively, from four
potentiometers which may be biased from a zener regulated supply
provided with the circuit.
"Each tone response is defined by a single capacitor chosen to
give the desired characteristic."

Some interesting features of this IC include:
- it's pretty low noise and low distortion.
- has high input resistance and low output resistance (easy to
interface).
- Only needs a small handful of capacitors and resistors to use.
- is designed to run off of a single 9 to 16 volt DC supply.
- The frequency response behaviors of the tone and loudness parts are
well-documented and easy to modify.

And here's the clincher:
The loudness behavior of the standard "application circuit" in the
datasheet
appear to be a close match to the behavior of the standard B3
expression
pedal.

In addition, the standard tone circuit is very close to the behavior
of
Bob Schleicher's tone control circuit, which is highly regarded among
Hammond players.

What does this mean?
Without too much effort, a Hammond enthusiast with a moderate amount
of
electronics knowledge could create a little circuit (with maybe
nothing
more than the LM1036 and a small handful of capacitors and resistors)
that plugs into a standard synth pedal like the Yamaha FC-7 and
creates
a Hammond expression pedal response (instead of a pure volume pedal
response).
And, as a bonus, you would also get treble and bass controls that are
similar to the ones that Hammond folks like.
Since the circuit is stereo, you would actually be able to control two
keyboard's expression (or a stereo keyboard) with one pedal and one
IC.

I had been planning on playing with this IC soon for the expression
pedal of a spinet chop, but I purchased a Nord Electro instead.
I played around with SPICE models of the AO-28 preamp expression
circuit
response and looked at some other solutions to this problem, and I
really
think that this IC may be the simplest good-sounding solution.

There were earlier-generation chips from Philips (TDA1537?) that sort
of did the
same thing, but they were noisier, harder to use, and their loudness
functions
weren't deep enough to emulate the AO-28 response. This new chip can
easily
scale the loudness with respect to the volume, so the response can be
tailored
to more closely match the AO-28. I believe that with a moderate
amount of
playing with capacitor and resistor values, you should be able to get
really close (like maybe indistinguishably close) to the AO-28
expression
behavior.

I wouldn't recommend attempting this for the pure electronics novice,
but
anyone with a moderate amount of op-amp tinkering experience
shouldn't have
a problem messing with this. A volume-to-stereo-expression-plus-tone
box
or kit might even make a nice cottage product!

If anyone plays with this idea, I'd be very interested in what they
find out!

Again, sorry for the length and techiness of this post; I hope it
inpires someone
to check it out! Thanks for bearing with me.

All the Best,

- Jim Smith