Moog CP-251 User Manual

Understanding and Using Your

moogerfooger®

CP-251 Control Processor

Moog Music Inc. Asheville, NC USA
©2000, 2003 by Moog Music Inc.

Welcome to the world of moogerfooger® Analog Effects Modules! Your Model CP251
Control Processor is a rugged, professional-quality instrument, designed to be equally at
home on stage or in the studio. Its great functionality and classic effects come from its
state-of-the-art all-analog circuitry, designed and built under Bob Moog's personal
direction.

Your CP251 is a direct descendent of the original modular Moog™ synthesizers. It
contains a total of eight independent control signal generating and processing
functions. Used with other moogerfoogers, voltage-controlled analog synthesizers, MIDI­to-CV converters, or other devices that produce or accept analog control signals, the
CP251 enables you to create an amazing variety of dynamic synthesizer effects.

The following pages will first introduce voltage control, which is the basic technical
principle underlying modular analog synthesizers. Next we'll describe each of the
CP251's eight functions. After that, you'll find a few typical patches, which will give you
some good ideas about how you can use the CP251 with your voltage-controlled gear.
At the end of this booklet you'll find technical specifications, service and warranty
information, and information about Moog Music.

CONTENTS

BASIC THEORY - - - - - - - - - - - - - - - - - - - 3

GETTING STARTED - TOUR OF THE MODULES - 6

USING THE CP251 WITH MF-SERIES DEVICES - 15

TECHNICAL INFORMATION - - - - - - - - - - - 24

WARRANTY INFORMATION - - - - - - - - - - - 27

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SOME BASIC THEORY

When you first try out an audio processor like a ring modulator or phaser, you plug
your instrument into the AUDIO IN jack, connect the AUDIO OUT jack to your amplifier,
and immediately hear what it does by playing your instrument.

There are no AUDIO IN or AUDIO OUT jacks on the CP251. This is because it is not
designed to process audio (musical sounds). Rather, it processes control signals, those
electrical voltages which act as 'phantom hands', changing the performance
parameters on your voltage-controlled equipment with speed and precision. The use
of control signals to produce interesting musical effects is called voltage control.

We'll begin with a brief discussion of voltage control, followed by a tour of the CP251's
functions. This will provide enough information for you to explore all the features of your
CP251. Then, after the section where we show some useful patches, we'll provide more
detailed technical information for those who are interested.

WHAT IS VOLTAGE CONTROL?

Imagine an oscillator module. An oscillator produces a signal that repeats regularly.
If it repeats between 20 and 20,000 times a second, then we hear it as a pitched tone.
Figure 1 shows a simple oscillator with a signal output jack and a frequency control
knob. The oscillator signal appears at the output jack, and the oscillator frequency may
be varied by turning the knob. The knob on our oscillator is calibrated in Hertz (one Hz.
= one cycle per second). If you feed the oscillator output to an input on your amplifier,
you'll hear a tone whose pitch goes from low to high as you turn the knob clockwise.

Figure 1 - LEFT: Simple oscillator with a signal output jack and a frequency control knob; RIGHT: Waveform that appears

at the OUTPUT jack.

Now imagine that we add a frequency control input jack to the oscillator. You can
apply an electrical voltage to this jack. As you increase the voltage, say from zero volts
to +5 volts, the oscillator frequency increases (See Figure 2). In other words, increasing
the voltage at the frequency control input of the oscillator has the same effect as
turning the oscillator's frequency knob clockwise. You can use either the knob or the

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control input jack to change the oscillator's frequency, or you can use both together.
We call this kind of oscillator a voltage-controlled oscillator (VCO). The voltage applied
to the frequency control input is called (you guessed it) the frequency control voltage
or, since the pitch that we hear is directly related to the frequency of the oscillator's
signal, the pitch control voltage.

Figure 2 - LEFT: Panel of a simple voltage-controlled oscillator; LOWER RIGHT: Graph showing steps in frequency control

voltage; UPPER RIGHT: Graph showing resulting increase in frequency of output.

In a system of analog synthesizer modules, control signals come from a variety of
sources. For instance, a keyboard produces a series of different voltages, depending
on which key the player depresses. When applied to control a VCO, such a keyboard
pitch control signal produces the pitches of a chromatic scale. Another example is a
low-frequency oscillator (LFO) which produces waveforms with frequencies that extend
below the audio range. When applied to control a VCO, an LFO signal sweeps the
VCO frequency periodically over a range that is determined by how strong the LFO
signal is. If, for instance, we set the LFO frequency to about six times a second, and set
the LFO amplitude (strength) to a small fraction of a volt, the VCO output is a now a
pitched tone with vibrato.

Suppose we want to use both a keyboard and an LFO to control a VCO. That is,
suppose we want to play a scale in which every tone has a vibrato. To accomplish this,
we need to combine the keyboard and LFO signals so that they both affect the
frequency of the VCO. We do this with a mixer module. A mixer for processing control
signals is similar to an audio mixer. They both combine two or more signals. The main
difference between them is that an audio mixer is designed to process frequencies
within the range of hearing. Generally, an audio mixer is not able to process either an
LFO signal or a keyboard signal, because these vary more slowly than audio signals. A
mixer for control signals, on the other hand, is designed to process signals that vary
slowly, or even remain steady. (Technical note: The term for such a mixer is a Direct­Coupled Mixer. It is called that because there are no capacitors in the mixer's signal
path to limit its low frequency response.)

Figure 3 shows how we might connect a VCO, an LFO, and a keyboard in a modular
system, to enable the player to produce a chromatic scale with vibrato. The mixer has
two inputs, one of which is equipped with an input level control. The LFO signal is
patched to the mixer input that has the level control, and the keyboard signal is
patched to the mixer input without a level control. The output of the mixer is then

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applied to the VCO's pitch control input. The mixer in Figure 3 contains a circle with a
"+" in it. This is a way of showing that the two input signals are combined, or added
together. The player adjusts the LFO Frequency (sometimes called Rate) knob to
change the vibrato rate, the Mixer Input Level control to change the amount of vibrato,
and the VCO Frequency control to transpose the pitch range of the keyboard.

Figure 3 - Using a mixer circuit to combine LFO and KEYBOARD control signals. Output of VCO is a series of pitches with

vibrato.

Varying the frequency of an oscillator is just one way to control a module's operation
by means of an externally-applied control voltage. Some other ways are: varying the
gain of a voltage-controlled amplifier (VCA), varying the cutoff frequency of a voltage­controlled filter (VCF), and varying the attack and decay times of a voltage-controlled
envelope generator (VCEG). In the case of moogerfooger MF-Series Analog Effects
Modules, all performance parameters are voltage-controllable by expression pedals or
externally-applied control voltages.

A wide assortment of signal sources may be used as control voltages. In addition to
LFO's and analog keyboards, commonly-used sources include envelope generators,
envelope followers, sample-and-hold circuits, expression pedals, and MIDI-to-CV
converters. In fact, virtually any source of variable voltage may be used as a control
voltage, including custom-made and homemade control interfaces.

5

GETTING STARTED

We are about to embark on a tour of your CP251's functions. To do this, you'll need
the following items:

• Your CP251,

• A tone generator of some sort, whose frequency is voltage-controlled,

• An expression pedal such as the moogerfooger EP-1,

• Six or eight short (1’-3’) patch cords with ¼” phone plugs,

• A monitor amplifier-speaker.

We’ll feed the CP251’s signals into the tone generator’s frequency control input, and
then listen to how the generator’s pitch changes. This will help us visualize the CP251’s
signals. In the rest of this manual, we’ll use VCTG as an abbreviation for “ Voltage
Controlled Tone Generator”.

What can you use as the VCTG? Here are three candidates:

•Use an analog synthesizer with a pitch control input. The classic Minimoog™ and
Micromoog™ are good examples. Set the instrument so it produces a steady tone at a
pitch of around middle C, and apply the CP251’s signals to the instrument’s pitch
control input.

•Use a moogerfooger MF101 Lowpass Filter. Set MIX to 10, CUTOFF to 250, and
RESONANCE to 10. (These settings cause the filter to oscillate at its cutoff frequency.)
Use the CUTOFF jack as the pitch control input, and patch the filter’s AUDIO OUT to the
input of your amplifier.

•Use the Carrier Oscillator section of a moogerfooger MF102 Ring Modulator. Set the
MF102’s LO-HI switch to HI and the FREQUENCY knob slightly above 120. Use the FREQ.
jack as the pitch control input, and patch the CARRIER OUT jack to the input of your
amplifier.

Setting up for the Tour:

Unpack your CP251. Set it next to your VCTG. Have your supply of short patch cords
handy. Connect an audio cable from the tone generator’s output to your monitor
amplifier’s input. Turn up the amplifier’s gain just enough to verify that you’re getting a
tone.

Verify that your power adapter has a nominal rating of +9 volts. Plug the power
adapter’s cord into either power receptacle on the CP251 (There is a power receptacle
on the back panel, below the moogerfooger trademark, and another receptacle on
the CP251’s bottom panel.) Then plug the power adapter itself into a power voltage

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receptacle. (See Page 28 for more detailed information on power adapters for the
CP251.) Note that the LED in the LFO section lights up.

Before we begin on our tour, let's look at two of the CP251's features.

Placing your CP-251:

Unlike the moogerfooger MF-Series effects modules, the CP251 is housed in a low
rectangular enclosure. It's designed to be used in conjunction with one or more MF­Series modules and/or other voltage-controlled gear, and to be placed where it is most
convenient. Where you place it in your setup will depend primarily on how you arrange
the rest of your gear. One possibility is to place the CP251 behind your effects
modules, so the patch cords to the modules are short. A second possibility is to place
the CP251 on a table or stand, and run longer patch cords to your voltage-controlled
gear. In this way, you can "play" your CP251 with your hands during performance. A
third possibility is that you can custom-mount your CP251 in an equipment rack. With its
wood end-caps removed, your CP251 is exactly the right size for a 3-unit high, half-rack
space. Note that there is a second +9V power input jack on the CP251's bottom plate,
which can be used when the unit is rack-mounted.

The "Red Jacks"

Eight of the CP251's jacks are held with red nuts. These input jacks are equipped with
power for expression pedals. You can plug an expression pedal directly into any of
these jacks - or use the jacks as conventional inputs. The CP251 jacks with black nuts do
not supply power for expression pedals.

Let's now begin our CP251 tour. We'll begin with the LFO module, which is a control
signal source.

LOW FREQUENCY OSCILLATOR

An LFO (Low Frequency Oscillator) provides repeating waveforms that may be used
for control and timing in voltage-controlled systems. The LFO in your CP251 provides two
waveforms: triangular and square. The output jacks are labeled with graphic symbols
that indicate the waveforms.

The RATE knob varies the speed of the LFO's waveforms over the range of one cycle
every five seconds (0.2 Hz.), to 50 cycles per second (50 Hz.). The LED brightness follows
the triangular waveform, thereby providing a visual indication of the rate. The PEDAL IN
jack is a control input which may be used to voltage-control the LFO rate.

•Connect a patch cord from the LFO's triangular output, to the pitch control input of
your VCTG.

•Set the RATE knob to mid position.

Note that the resulting pitch fluctuation is very wide, usually covering as much as five

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octaves. Turn the RATE knob slowly to hear the LFO's full speed range.

•Turn the RATE knob counterclockwise.

•Plug an expression pedal into the PEDAL IN jack.

Vary the setting of the pedal to hear its effect on the LFO speed.

Figure 4 - Setup for trying out the LFO module.

ATTENUATORS

An ATTENUATOR cuts down the strength of a signal. Your CP251 has two identical
attenuators, each of which has an input jack, an output jack, and a knob. When the
knob is fully clockwise, the full input signal is available at the output.

•Connect patch cords as follows:

a) From the triangular output of the LFO to the input of one ATTENUATOR,

b) From the ATTENUATOR output to the VCTG pitch control input.

• Set the knobs as follows:

a) LFO RATE to mid position, and

b) ATTENUATOR full clockwise.

You should hear the pitch vary over several octaves. Now turn the ATTENUATOR knob
counterclockwise. Note that the width of the pitch variation decreases. When the
ATTENUATOR knob is fully counterclockwise, you will not hear any pitch variation from the
LFO signal.

Next, remove the first patch cord from the LFO's triangular output, and plug it into the

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LFO's square wave output. Turn the ATTENUATOR knob back and forth. Note that the
shape of the effect is determined by the signal that is fed to the ATTENUATOR's input,
and the amount or intensity of the effect is determined by the setting of the
ATTENUATOR knob. This is generally true for all control signal sources and all control
destinations on voltage-controlled devices.

Now we'll illustrate a patch that uses both attenuators.

• Connect more patch cords as follows:

a) from the OUT jack of the second ATTENUATOR, to the PEDAL IN of the LFO.

b) Plug the expression pedal into the IN jack of the second ATTENUATOR.

Set the second ATTENUATOR knob to about "9 o'clock". Now the first ATTENUATOR knob
is varying the strength of the LFO control signal, and the second ATTENUATOR knob is
varying the amount that the expression pedal is changing the LFO rate.

Figure 5 - Setup for trying out the ATTENUATOR modules.

FOUR-INPUT MIXER

A MIXER combines, or adds together, two or more signals. Your CP251's Four-Input
Mixer has four input jacks, labeled 1 through 4. Inputs 1 and 2 have input attenuator
knobs, while inputs 3 and 4 are mixed in without attenuation. The OFFSET knob raises or
lowers the voltage of the mix, while the MASTER knob determines the amplitude of the
entire mix. There are two output jacks, labeled "+" and "-". The "- "output is the negative
of the "+" output. That is, the voltage at the "-" output goes down when the voltage at
the "+" output goes up, and vice versa.

•Connect patch cords as follows:

a) from the LFO triangular output to MIXER input 1,

b) from the LFO square output to MIXER input 2,

c) from the MIXER "+" output to the VCTG pitch control input.

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