This is only a preview of the March 1998 issue of Silicon Chip. You can view 43 of the 96 pages in the full issue, including the advertisments. For full access, purchase the issue for $10.00 or subscribe for access to the latest issues. Articles in this series:
Items relevant to "Sustain Unit For Electric Guitars":
Items relevant to "Multi-Purpose Fast Battery Charger; Pt.2":
Items relevant to "Command Control For Model Railways; Pt.3":
Items relevant to "PC-Controlled Liquid Crystal Display Board":
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Sustain
Unit for
electric guitars
Are you playing electric guitar without a
sustain pedal? What? In this day of
electronics and technology, you don’t have
a sustain pedal? Add one now and have a
more fulfilling musical life and set out on the
path to fame and fortune.
By JOHN CLARKE
If you’re a guitar player without
a sustain pedal you must be feeling
pretty deprived. But now you can fix
all that by building this new design. It
can give you really long sustain notes
and will help make the sound much
more live.
18 Silicon Chip
Sustain is just one of many effects
that can enhance the sound from a
guitar. Some effects produce deliber
ate distortion – eg, distortion pedals
(previously called fuzz pedals) – so
that they sound harsh, while others
are more subtle and add in frequency
response shaping or envelope modula
tion. The sustain effect works by con
trolling the signal from your guitar so as
to maintain a constant level of sound.
When you pluck the string on your
guitar, it initially produces a high level
of oscillation which ultimately dies
away to nothing. This is reflected in
the volume of the note – quite loud
when initially plucked and then decay
ing away quite fast. A sustain circuit
modifies this natural decay so that the
volume remains fairly constant as the
sound from the string itself dies away
to nothing.
Some guitar sustain pedals are rel
atively crude and provide the sustain
function by amplifying the guitar
signal and clipping the waveform
Fig.1: some guitar sustain pedals work simply by
clipping the waveform when the level becomes too
high. The top waveform is a sinewave signal with
12dB higher bursts occurring every 125ms. The lower
trace is the waveform with a clipping circuit added.
This degree of clipping can sound pretty awful.
Main features
• Low distortion
• Separate Attack
and Decay
controls
• Sustain/Bypass (In/Out) switch
• Matched in and out levels
(adjustable)
when the level becomes too high. The
oscilloscope waveforms of Fig.1 show
this type of sustain. The top waveform
is a sinewave signal with 12dB higher
bursts occur
ring every 125ms. The
lower trace is the waveform with a
clipping circuit added. Note how the
higher signal bursts are clipped hard
to provide flat top limiting which con
stitutes considerable distortion – it can
sound pretty awful.
Such a clipping circuit cannot be
considered to be a pure sustain unit
since it adds in very high distortion. If
distortion is wanted, this can be added
in with a distortion pedal.
The Sustain pedal circuit described
here produces much lower distortion
than the clipping circuit but still
maintains the output level over a wide
signal range. The oscilloscope wave
Fig.2: these waveforms show the response of the
sustain circuit with the same input waveform as in
Fig.1. Note how the lower waveform remains
sinusoidal over its full duration.
forms of Fig.2 show the response of
the sustain unit under the same type of
pulse waveform as the clipping circuit
referred to in Fig.1. The upper trace is
the pulsed input.
Note how the lower waveform
remains sinusoidal over its full dura
tion. Note that the initial attack of the
waveform is more or less preserved
and that the waveform does still decay
away eventually. These times can be
adjusted with the Attack and Decay
controls.
How it works
Fig.3 shows the block diagram of the
circuit. The guitar signal is applied to
a gain-controlled amplifier which
provides the output signal. The sig
nal produced at the amplifier output
is full-wave rectified and filtered to
produce a DC level which is depend
ent on the signal level at the gain
controlled amplifier output. This DC
level is compared against a reference
level set by VR3 in the error amplifier
IC2a. The error signal is then fed back
to the gain controlled amplifier in
order to maintain a constant output.
Fig.4 shows the full circuit. It
comprises six op amps, four in IC1 (a
TL074) and two in IC2 (an LM358).
Q1 is a 2N5484 JFET which provides
the variable gain feature for IC1.
Fig.3: the block diagram of the circuit. The guitar signal is applied to a
gain-controlled amplifier which acts to provide a more constant output
signal.
March 1998 19
Fig.4: the circuit employs op amp IC1a and Mosfet Q1 as the gain controlled
amplifier while IC1b, IC1c and diodes D2 & D3 act as a precision full-wave
rectifier.
Signal input from the guitar is
AC-coupled via a 1µF capacitor to
pin 3, the non-inverting input of op
amp IC1a. The 22kΩ resistor biases
pin 3 to +5V while the 10Ω series
resistor acts as a “stopper” to reduce
the possibility of RF breakthrough.
JFET Q1 is used to dynamically
vary the gain of op amp IC1a; this is
the gain-controlled amplifier referred
to earlier in Fig.3. The gain of IC1a
is set by the 10kΩ feedback resistor
between pins 1 & 2, in conjunction
with the drain-source resistance of
JFET Q1 and the 100Ω source resis
tor. If the JFET is biased on hard, the
drain source resistance is low and the
corresponding gain is high.
Note that the JFET does not pass
DC because of the associated 47µF
capacitor. As well as blocking DC
and effectively setting the DC gain
of the circuit to unity, the 47µF ca
pacitor also sets the low frequency
20 Silicon Chip
rolloff of the circuit. High frequency
rolloff above about 16kHz is provid
ed by the .001µF capacitor between
pins 1 & 2.
Rectifier & envelope control
Op amps IC1b & IC1c, plus diodes
D2 & D3 and associated resistors form
the full wave rectifier. When the sig
nal from IC1a goes negative, the out
put of IC1b goes high, forward biasing
D3. The gain for negative signals is set
by the 20kΩ input and 20kΩ feedback
resistors to a value of -1. The signal
at the cathode of D3 is coupled to the
inverting input, pin 9, of IC1c via the
10kΩ resistor. Gain for IC1c is set at
-10 by this 10kΩ input resistor and the
100kΩ feedback resistor. Overall gain
for the input signal is therefore -1 x
-10 = +10.
However, there is an extra path
for the input signal via the 20kΩ
resistor to pin 9 of IC1c. This path
gives a positive signal at the output
of IC1c with a gain of -5. Adding the
two gains gives us +5. So when the
input signal is negative, the output
at pin 8 of IC1c is negative.
For positive input signals diode D2
conducts and clamps the output of
IC1b to +5V. Signal then passes via
the 20kΩ resistor connecting to pin
9 of IC1c. IC1c inverts the signal and
provides a gain of -5. Therefore, posi
tive input signals result in a negative
output at pin 8 of IC1c.
So, regardless of whether the input
signal swings negative or positive,
the output at pin 8 of IC1c always
swings negative. Thus we have a
full-wave rectifier.
The 10pF capacitor across the
20kΩ feedback resistor for IC1b
prevents instability while the 0.1µF
capacitor across the 10kΩ feedback
resistor of IC1c provides a measure
of filtering. The full-wave rectified
signal is filtered using D4, VR1,
VR2, the 10kΩ resistor and the 10µF
capacitor. Diode D4 allows the 10µF
capacitor to be charged via VR1 but
only discharged via VR2 and the
series 10kΩ resistor. This allows
separate control over the attack and
decay times.
Error amplifier
IC2a is the error amplifier. It
compares the rectified signal from
D4 with the DC voltage (Vadj) at its
pin 2 and it amplifies the difference
between these two signals by a factor
of 5.7, as set by the 10kΩ input and
47kΩ feedback resistors.
Reference. Vadj, the DC reference
fed to the error amplifier, comes
from op amp IC2b and is set using
trimpot VR3.
The error amplifier drives the gate
of JFET Q1 via a 10kΩ resistor and
switch S2. The 10kΩ resistor between
the gates and drain of Q1 has the
effect of linearising the signal and
thereby reducing distortion.
Slide switch S2 is used to select
Sustain (in) or Bypass modes (out).
When Sustain is selected, the voltage
from pin 1 of IC2a controls Q1’s gate.
When S2 is in the out position, the
gate is held at a voltage set by VR4
and the 10kΩ resistor between gate
and drain. In this mode, the drainsource resistance of Q1 is constant
and so the gain does not vary. In use,
trimpot VR4 is adjusted so that the
same volume is experienced whether
the switch is in or out as the guitar
string is first plucked.
Power for the circuit is derived
from a 12V DC source which will usu
ally be a plugpack. Diode D1 protects
against reverse polarity connection,
while the 100µF capacitor decouples
the supply. LED1 indicates power
when S1 is switched on.
Most of the op amps are biased
from a 5V DC supply. This is de
Fig.5: all the components mount on the PC board, including the input and
output jack sockets. Note that IC1 and IC2 are oriented in different directions.
rived with zener diode ZD1 which
is supplied via a 1kΩ resistor from
the 11.4V rail, following D1. The
resulting regu
lated voltage across
ZD1 is filtered with a 10µF capacitor
and then buffered with op amp IC1c.
Construction
Typically, a guitar sustain circuit
such as this would be pedal operated
and perhaps the electronics would
all be mounted in the pedal housing
itself. However, some guitar players
would be just as happy mounting
the circuit board in a simple plastic
utility case and with simple switches
to operate it instead of a pedal.
With those thoughts in mind, we
are presenting this project in the
simplest possible form, as a PC board
with all circuit components mounted
on it. The PC board measures 105 x
60mm and is coded 01302981. It has
been designed to fit into a standard
UB3 plastic utility case measuring
130 x 67 x 43mm (Altronics Cat.
H-0153 or equivalent).
Fig.5 shows the component layout.
Before you install any parts on the
board, check it thoroughly against
the PC artwork shown in Fig.6 and
make sure that all holes have been
drilled. That done, install the two
Capacitor Codes
❏ Value IEC Code EIA Code
❏ 0.47µF 470n 474
❏ 0.001µF 1n0 102
❏ 10pF 10p 10
Resistor Colour Codes
❏
No.
❏ 1
❏ 1
❏ 3
❏ 3
❏ 6
❏ 1
❏ 1
❏ 2
❏ 1
Value
100kΩ
47kΩ
22kΩ
20kΩ
10kΩ
2.2kΩ
1kΩ
100Ω
10Ω
4-Band Code (1%)
brown black yellow brown
yellow violet orange brown
red red orange brown
red black orange brown
brown black orange brown
red red red brown
brown black red brown
brown black brown brown
brown black black brown
5-Band Code (1%)
brown black black orange brown
yellow violet black red brown
red red black red brown
red black black red brown
brown black black red brown
red red black brown brown
brown black black brown brown
brown black black black brown
brown black black gold brown
March 1998 21
Parts List
Specifications
1 PC board, code 01302981,
105 x 60mm
2 DPDT slider switches (S1,S2)
2 6.35mm PC mount mono
unswitched sockets
1 10kΩ linear PC mount pot
(VR1)
1 100kΩ linear PC mount pot
(VR2)
2 knobs
1 5mm red LED (LED1)
2 PC stakes
1 40mm length of 0.8mm tinned
copper wire
Total harmonic distortion (1kHz) ................... 0.7% at 10mV input, 2% <at>
20mV, 0.02% <at> 200mV
Semiconductors
1 TL074, LF347 quad op amp
(IC1)
1 LM358 dual op amp (IC2)
1 2N5484 N-channel JFET (Q1)
1 1N4004 1A 400V diode (D1)
3 1N914, 1N4148 signal diodes
(D2-D4)
1 5.1V 400mW zener diode
(ZD1)
Signal to noise ratio at maximum
gain (with respect to 100mV)............................-60dB with 20Hz to 20kHz
filter (better noise figure at lower gains)
Output level versus input level............................. flat from about 10mV to
170mV input
Attack time ................................................................................. 5ms (max)
Decay time ................................................................................ 25ms (min)
Maximum gain ...............................................................................18 times
Frequency response .....................................-1dB at 20Hz; -3dB at 16kHz
Capacitors
1 100µF 16VW PC electrolytic
1 47µF 16VW PC electrolytic
2 10µF 16VW PC electrolytic
2 1µF 16VW PC electrolytic
1 0.47µF 63V MKT polyester
2 .001µF 63V MKT polyester
1 10pF ceramic
Resistors (0.25W, 1%)
1 100kΩ
1 2.2kΩ
1 47kΩ
1 1kΩ
3 22kΩ
2 100Ω
3 20kΩ
1 10Ω
6 10kΩ
PC stakes, two links, resistors and
diodes, followed by the capacitors,
the LED and the two trimpots.
Make sure that the diodes and
electrolytic capacitors are installed
with correct polarity.
Next, install the two ICs and note
that they are oriented differently. Pin
1 of IC1 is adjacent to the two jack
sockets while pin 1 of IC2 faces the
other end of the board.
Slide switches S1 and S2 are in
stalled by inserting the switch pins
into the PC board and soldering in
place. If the pins are difficult to in
sert, crimp them with pliers first or
use tinned copper wire through the
22 Silicon Chip
Fig.6 actual size artwork for the PC board. It has been designed to slip
into a standard plastic utility box (UB3).
switch pins which then insert into
the PC board.
The JFET (Q1) is mounted with its
package oriented as shown on Fig.5.
The Attack and Decay potentiome
ters, VR1 and VR2, are PC types and
are soldered directly into the board.
Note that they have different values
so don’t get them swapped around
by mistake.
Finally, mount the two 6.35mm
PC sockets in position. They are PC
types too and solder directly into
the board.
Testing, testing
Connect up a 12V DC power supply
to the PC stakes on the board and
check that there is about +11.4V at
pin 4 of IC1 and pin 8 of IC2 when
S1 is on. Pin 7, pin 10 and pin 12 of
IC1 should have about +5V present.
You are now ready to test the sus
tain unit with your guitar. Switch
S2 to the Sustain setting (towards
S1) and play a few notes. Adjust
VR3 for best effect on the sustain.
You may also need to adjust the
volume level from your guitar to suit
the input range of the sustain unit
which operates best between 10mV
and 200mV.
Adjust the Attack control to set
the rate at which the note is reduced
in volume when the string is first
plucked. Then adjust the Delay control
to ensure that the note’s volume is
maintained as much as possible. SC
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