Radio Shack FX2 User Manual download pdf (Page 10)

Build a Control Voltage

I n t e r f a c e / S e l e c t o r f o r t h e E P S

Intro

Back in my article, "Much More than a Volume Pedal" (July

'91), I discussed ways of using the CV/pedal jack on most En-

soniq instruments with extemal control voltage sources. Thanks

to a few cards and letters from a few interested readers, I'm

gonna discuss some more intriguing uses of this feature and

suggest a few do-it-yourself construction projects.

In this installment, I'll present a very simple electronic circuit

that is designed for use with the original EPS sampler. Due to

its design, the EPS requires a negative-going control voltage —

unlike the remainder of Ensoniq instruments (and 98% of

music synthesizers ever made).

The circuit described in this article allows you to use the typi

cal positive-going control voltage with your EPS; it also allows

you to switch between a regular footpedal, and the CV input,

without having to repatch things. Best of all, this circuit is so

simple and cheap, that almost anyone should be able to wire it

up without much trouble!

How It Works

ACHTUNG! While every reasonable effort has been made to

verify the accuracy of this article, neither the author, or this

magazine will be held responsible for any damages that result

J2 - Footpedal

Tip

Shield

J1 - CV In

Tip

Shield

Pedal

Ch arles R . F is che r

by the actions of readers. This project should not be attempted

by persons lacking basic electronics skills. An excellent book

for the uninitiated is fellow Hacker, Craig Anderton's,

"Electronic Projects for Musicians" (available from the Mix

Bookshelf, 6400 Hollis St., #12, Emeryville, CA 94608).

The schematic diagram of the interface circuit is illustrated in

Fig. 1. Incoming 0-to- 4-lOV control voltages are connected to

Jl. Rl, R2, and ICl form the heart of our circuit, which is

known in the electronics world as an "inverting ampliﬁer."

As the name implies, the inverting amp has the interesting

talent of producing output signals that are 180 degrees out of

phase with the input signal — negative input voltages become

positive output voltages, and vice versa. This is exactly what

we need to convert positive control voltages into something

that your EPS can use.

In order to do its job, ICl needs a regulated bipolar power

supply that can furnish -i- and - 12 to 15 Volts at 100 mA. Be

cause this circuit handles both positive and negative voltages at

its inputs and outputs, we need to have a power supply that can

provide both positive and negative voltages. I do not recom

mend using batteries for this application, as they have the nasty

tendency to drift with age (which affects the circuits accuracy),

and they have a knack for going dead — usually just before an

important gig. Spend a few bucks more for a decent power

supply, and you won't be sorry. CI and C2 are provided to ﬁl

ter out any transients or noise on the power supply lines; this

helps to keep them from showing up in the output signal.

By adding switch SI and an extra phone jack, we can switch

between the control voltage, or the footpedal. The pedal plugs

into J2, and the interface is connected to the EPS via J3. SI is

used to connect either the CV output, or the footpedal, to J3.

f q

6 5

3 4

Figure 1. Schematic diagram of the CV pedal interface.

ICl

+12to+I5V

——<

Ground

-12 to-15 V

1 2 ... 5 6 7 8 9 10 11 12 13 14 15 ... 31 32

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Radio Shack FX2 User Manual Page 10

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