This is only a preview of the February 1995 issue of Silicon Chip. You can view 28 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 "50-Watt/Channel Stereo Amplifier Module":
Items relevant to "Digital Effects Unit For Musicians":
Articles in this series:
Articles in this series:
Items relevant to "Build An Oil Change Timer For Your Car":
Articles in this series:
|
Digital Effects Unit
for Musicians
This neat Digital Effects Unit can produce
a wide range of sound effects to enhance a
musical instrument. It can be driven from a
guitar or line source (or from both) & uses the
latest in digital delay technology.
By JOHN CLARKE
Adding effects to musical instruments is very popular amongst musicians since they can create their own
unique sounds. These effects can vary
from the more natural, such as adding
auditorium ambience, to the grossly
exaggerated where the original sound
becomes unrecognisable.
The SILICON CHIP Digital Effects
26 Silicon Chip
Unit is based on a microprocessor and
provides adjustable echo, delay, reverberation and vibrato effects. It can be
used with guitars, electric keyboards
and organs, mixing consoles and other
sources capable of providing a 50mV
to 2V RMS output signal.
By connecting this unit into the
signal path before the amplifier, you
can quickly tailor the sound to your
requirements – from adding some interesting reverberation effects to guitar
work to pulsating vibrato effects for
an electronic keyboard. Alternatively,
you can just add in some echo to make
a room sound more “alive”, or you can
use a combination of effects for some
really way-out sounds.
These combined effects can be instantly switched in or out of circuit
using a single switch (Effects In/Out)
on the rear panel.
Main features
As can be seen from the photos,
the unit is housed in a compact case
with a sloping front panel. This
panel carries a 2-digit LED display
that shows either the delay period in
milliseconds or the vibrato rate in Hz.
The delay period can be varied from
1-64ms, while the vibrato rate can be
varied from 1-20Hz.
Immediately to the right of the display are four pushbutton switches, the
first of which toggles the display mode
between delay and vibrato. Two LED
indicators, one above the switch and
the other below it, are used to show
the current display mode.
The next two switches are labelled
DOWN and UP and these set both the
delay and the vibrato rate, depending
on the display mode selected. Pressing
a button in delay mode, for example,
progressively alters the delay period
in discrete 1ms steps. Conversely, if
vibrato mode is selected, the display
alters in 0.5Hz steps up to 10Hz and
then in 1Hz steps up to 20Hz.
The fourth pushbutton switch is
simply used to toggle the vibrato on or
off. A LED indicator above this switch
lights when the vibrato is on, while
another LED situated immediately
above the 7-segment displays flashes
at the selected rate (1-20Hz).
The remaining controls on the front
panel are the Echo switch and the
Reverberation and Vibrato Depth potentiometers. These controls have no
effect on the LED displays, however.
They simply switch the echo in or out
and vary the amounts of reverberation
and vibrato.
Several controls are also located
on the rear panel and these include a
Power on/off switch, an Attenuation
control and the previously mentioned
Effects In/Out switch. Also on the rear
panel are a power socket, two signal
input sockets (guitar and line) and an
output socket. If necessary, both inputs
can be used together – the two input
signals are simply mixed together
before any effects are added.
An important point here is that the
output signal level remains unchanged
when the Effects In/Out switch is
operated. This prevents the volume
from changing each time the effects
are switched in or out.
Obtaining sound effects
All of the effects are based around
the delay function. In practice, this
simply involves storing the incoming
signal and then replaying it some time
later. This delay is adjustable from
1-64ms. Mixing the delayed signal
with the original signal gives the echo
effect and, when a long delay is select-
Main Features
♦
♦
♦
♦
♦
♦
♦
♦
♦
♦
Echo, delay, reverberation & vibrato effects
Microprocessor controlled delay period & vibrato rate with 2-digit display
Digital delay processing
Low noise and distortion
Delay adjustable from 1-64ms in 1ms steps
Vibrato rate adjustable from 1-10Hz in 0.5Hz steps & from 10-20Hz in
1Hz steps
User presets to select settings on power-up
Input attenuator to prevent signal overload
Click-free switching between effects in & effects out
No change in signal level between effects in & effects out
Specifications
Frequency response ��������������������-3dB at 25Hz and 15kHz
Signal to noise ratio ����������������������78dB wrt 1V unweighted; 82dB wrt 1V
A-weighted
Overload capability �����������������������800mV RMS for guitar input (using attenuator); 4.5V RMS for line input (using
attenuator)
Sensitivity �������������������������������������guitar input 50mV; line input 300mV
Distortion ��������������������������������������1% at 1kHz and 300mV output
Vibrato sinewave distortion ����������7% at 20Hz (mainly switching noise)
Vibrato sinewave level ������������������within 0.6dB from 1Hz to 20Hz
Input impedance ���������������������������10k٠for line input; 47k٠for guitar input
ed, it simulates the effect of a very large
hall or outdoor venue.
For delay settings of say less than
20ms, we obtain a phase shifting effect.
This occurs because the delay time
is now similar in magnitude to the
period of the input signal. What happens is that the original and delayed
signals are added together when they
are in phase and are subtracted when
they are out of phase. The resulting
sound effect is similar to the Doppler
shift effect that occurs with a rotating
loudspeaker.
By contrast, reverberation occurs
when the delayed signal is fed back to
the input to produce multiple echoes.
With a short delay time, the results
can be similar to the phase shifting
effect described above but the extra
multiple echoes make the effect more
powerful.
The vibrato effect is obtained by
continuously varying the delay above
and below a preset period at a rate
somewhere between 1Hz and 20Hz.
This effect can warble a voice or
make a normal guitar sound like an
Hawaiian guitar. A small amount of
vibrato can also improve the phase
shifting effect.
Digital delay chip
One advantage that this Digital Effects Unit has over many other designs
is its very low noise and distortion.
This has been made possible by basing
the design on the M65830P digital
delay IC from Mitsubishi – a device
designed mainly for use in surround
sound decoders. It is a very versatile
device and can provide 64 separate
delay periods, as selected by a serial
code applied to its data pin.
The device works by first converting the incoming analog signal to a
digital format which is then clocked
into a memory. This digital signal is
then clocked out at the end of the delay period and converted back to an
analog form. In addition, the M65830P
also contains several op amps so that
input and output filters can be added
to the circuit without the need for
additional ICs.
About the only drawback to using
February 1995 27
LINE
INPUT
15kHz LP
FILTER
IC3
MIXER
IC1c
AMP
IC1b
GUITAR
INPUT
ECHO
S2
MIXER
IC1a
ATTENUATION
VR1
IN
DELAY
IC3
15kHz LP
FILTER
IC3
OUT
MIXER
IC1d
RELAY
OUTPUT
OSCILLATOR
CONTROL
INPUT
x5.7
REVERBERATION
VR2
18Hz LP FILTER
IC2
VOLTAGE
CONTROLLED
OSCILLATOR
IC4
VIBRATO
LEVEL
VR3
PC1-PC3
PC4
DIP SWITCH
POWER-ON
RESET
PD3-5,PD7
PC0
MICROPROCESSOR
IC5
(PD0-PD2)(PA0-PA7)(PB0-PB7)(PC7-PC5)
LED1
VIBRATO
MODULATION
LED3
DELAY(ms)
S4
DISP1
DISP2
the M65830P is that it is not easy to
drive using standard counters and
gates. This is because the control signal must be in serial form and must
include various identification, sleep
and mute codes. This means that
some form of programmed device (an
EPROM or a microprocessor) should be
used in order to simplify the circuitry
involved.
We decided to use a microprocessor
to do the job because this could also be
used to perform a range of other tasks
without increasing circuit complexity. In fact, the displays, switches and
LEDs are all driven directly driven
from the microprocessor lines. The
microprocessor is also used to generate
the vibrato waveform.
Block diagram
Fig.1 shows the block diagram of
the unit. At the heart of the circuit is
IC3, the M65830P digital delay chip
mentioned above. To simplify matters,
28 Silicon Chip
LED2
S6
DOWN
S5
UP
S7
VIBRATO
ONOFF
LED4
VIBRATO
RATE(Hz)
it is shown with just four connections
to the outside world: the signal input
and output lines, the oscillator input
and the control input (which actually
consists of three lines).
The control lines come from outputs
PC1-PC3 on the microprocessor IC5,
with the data on PC1 setting the delay
number (1-64). Another output from
the microprocessor, PC4, is fed to low
pass filter stage IC2. Its output in turn
is fed via VR3 to voltage controlled
oscillator (VCO) stage IC4.
By this means, the VCO varies its
output frequency according to the data
from PC4 and the setting of VR3. This
frequency sets the basic minimum
delay period (ie, to 1ms when there
is no vibrato).
Let’s look now at the signal inputs.
The guitar signal is first amplified by
5.7 in IC1b to boost it to line level and
then mixed with the line input signal
using IC1a. From there, the signal passes via attenuation control VR1 to IC1c
Fig.1: block diagram of the
Digital Effects Unit. The
incoming signals are amplified,
mixed & filtered before being
fed to a digital delay line based
on IC3. Microprocessor IC5
controls the delay line (via PC1PC3) & also provides a control
signal (via PC4 & IC2) for IC4
which, in turn, provides the
clock signal.
where it is mixed with the delayed
signal fed back via the reverberation
control (VR2).
Note that the line level is defined
as 285mV. However, signals up to
4.5V RMS can be accommodated
by using VR1 to attenuate signals
above 1.1V RMS. This attenuation is
necessary to prevent signal clipping
within IC3.
Following IC1c, the mixed signal is
applied to the digital delay stage via a
15kHz low-pass filter. This filter keeps
unwanted high frequencies out of the
delay line to avoid spurious effects.
Note that the op amp used for this lowpass filter stage is actually contained
inside IC3. A similar 15kHz low-pass
filter stage is also used at the output
of the delay line.
The filtered output from the delay
line is fed to mixer stage IC1d and
finally to the output via a set of relay
contacts. The relay itself is controlled
by the PC0 output of IC5; this keeps the
relay contacts open for a few seconds
after power is applied to eliminate
switch-on “plops”.
Echo is added to the output by closing switch S2, so that the signal from
IC1c is mixed with the delayed signal
inside IC1d. VR2 sets the level of the
delayed signal that’s fed back to IC1c
and thus controls the reverberation,
while VR3 sets the vibrato level (or
depth).
Vibrato is produced by applying a
varying frequency (from the VCO) to
the oscillator input of IC3. To provide
a natural vibrato sound, this variation
should be sinusoidal in nature. This
is achieved by first producing a pulse
width modulated signal at the PC4
output of IC5 and then filtering it to
produce a smooth sine wave. This is
then fed through low-pass filter stage
IC2 and used to modulate the VCO.
If the vibrato effect is switched out,
however, IC2’s output sits at a constant
level and so the VCO’s output frequency remains fixed.
As well as controlling the effects
circuitry, the micropro
cessor also
drives the LED displays and accepts
inputs from the four pushbutton
switches (S4-S7). Finally, the PD3,
PD4, PD5 & PD7 lines of IC5 connect
to a 4-way DIP switch. These switches set the initial configuration of the
effects unit at power up and can be
configured to suit your requirements
– see Table 1.
For example, the DIP switches can
be set so that the vibrato function is
either on or off at power up and there
are options for setting the initial delay
and vibrato rate.
Circuit details
Refer now to Fig.2 for the full circuit
details.
IC1b, the guitar preamplifier, functions as an inverting stage with a gain
of 4.68, as set by the ratio of the 220kΩ
and 47kΩ feedback resistors. Its output is fed to mixer stage IC1a which
functions with a gain of -1.2 for guitar
signals (thus giving an overall gain of
5.7) and -1 for line level signals.
Note that the guitar socket shorts
the input to ground when no plug is
inserted. This is done to reduce hum
pickup when this input is unused.
The output from IC1a is coupled to
VR1 via a 10µF capacitor and then fed
to pin 23 of IC3 via IC1c, as described
previously. The RC network connected to pins 22 and 23 forms part of
PARTS LIST
1 PC board, code 01301951, 141
x 131mm
1 PC board, code 01301952, 41 x
146mm
1 desk console case, 170 x 143 x
31 x 55 (Jaycar Cat. HB6092 or
equivalent)
1 front panel label, 168 x 143mm
1 12VAC 300mA plugpack
3 6.5mm mono panel mount
sockets with SP switch
1 SPDT toggle switch (S1)
1 SPST rocker switch (S2)
1 DPDT toggle switch (S3)
4 grey momentary click action PC
board switches (S4-S7)
1 4-way DIL switch (DIP1)
1 10kΩ 16mm log pot (VR1)
2 10kΩ 16mm linear pots (VR2-3)
1 10kΩ horizontal trimpot (VR4)
1 DC panel mount power socket
plus 2.5mm screws & nuts
3 15mm knobs
1 micro U heatsink (18 x 19 x
10mm) plus 3mm screw & nut
1 5V reed relay (RLY1) (Jaycar
Cat. SY-4036 or equivalent)
1 40-pin IC socket
4 9mm tapped standoffs
4 3 x 6mm screws
4 3 x 6mm countersunk screws
1 solder lug
1 22 x 34mm piece of 3mm-thick
red Perspex
1 4MHz parallel resonant crystal
(X1)
2 HDSP-5301 red common anode
7-segment displays (DISP1,
DISP2)
4 3mm red LEDs (LED1-LED4)
22 PC stakes
5 cable ties
Semiconductors
1 TL074 quad op amp (IC1)
1 LM358 dual op amp (IC2)
1 M65830P single chip digital
delay (IC3)
1 ICM7555, LMC555CN or
TLC555 CMOS timer (IC4)
1 programmed MC68HC705C8
microprocessor (IC5)
1 7805 5V 1A 3-terminal regulator
(REG1)
6 1N4004 1A diodes (D1-D6)
1 BC338 NPN transistor (Q1)
1 BB212 varicap diode (VC1)
Wire & cable
1 250mm-length of 6-way rainbow
cable (2.54mm separation)
1 60mm-length of twin shielded
audio cable
4 300mm-lengths of hook-up wire
(red, blue, yellow & green)
1 200mm-length 3-way rainbow
cable
1 300mm-length 0.8mm tinned
copper wire
Miscellaneous
Solder, heatsink compound
the 15kHz low pass filter shown on
the block diagram (Fig.1). This filter
prevents frequencies that are greater
Capacitors
1 1000µF 25VW PC electrolytic
3 100µF 16VW PC electrolytic
1 47µF 16VW PC electrolytic
1 22µF 25VW PC electrolytic
9 10µF 16VW PC electrolytic
2 1µF 16VW PC electrolytic
2 1µF MKT polyester
1 0.27µF MKT polyester
7 0.1µF MKT polyester
1 0.039µF MKT polyester
2 0.022µF MKT polyester
1 0.01µF ceramic
2 560pF ceramic
2 150pF ceramic
2 33pF ceramic
Resistors (0.25W, 1%)
1 4.7MΩ
1 8.2kΩ
2 220kΩ
1 4.7kΩ
3 100kΩ
1 3.3kΩ
2 47kΩ
1 1.8kΩ
4 39kΩ
1 1kΩ
1 27kΩ
1 330Ω
5 22kΩ
1 270Ω
2 18kΩ
1 100Ω
20 10kΩ
1 82Ω
Where to get the
microprocessor
The programmed MC68HC705C8
microprocessor (IC5) is avail
able
from retailers as part of a complete
kit, or can be purchased separately
from SILICON CHIP for $45 plus $6
p&p.
than half the sampling frequency of
the digital delay from causing spurious
conversion products which would
February 1995 29
10k
10k
LINE
285mV
10
10k
+16V
13
IC1a
TLO74
12
8.2k
GUITAR
47k
10
1
10k
ATTENUATION
VR1
10k LOG
220k
10
14
10
4
2
1
IC1c
3
0.27
15kHz
ANTI ALIAS
FILTER
10k
9
IC1b
10
+16V
8
18k
23
150pF
39k
22
39k
560pF
100k
OUT
82 20
10
11
IN
.022
21
100k
100
17
+5V
0.1
MODULATION
FILTER
22k
3
22k
22k
22k
1
.039
22k
4.7k
6
IC2a
5 LM358
1
18Hz LP FILTER
1.8k
1
IC2b
TP1
10k
2
0.1
0.1 18
270
5
7
1
VC1
VOLTAGE
BB212 CONTROLLED
OSCILLATOR
+5V
FREQUENCY
SET
VR4
10k
3
4
100
16VW
6
A1
A2
10k
2
8
0.1
100k K
8
IC4
7555
7
VIBRATO
LEVEL
VR3
10k LIN
27k
+16V
3.3k
4
0.1
.01
+5V
10k
S6
10k
10k
29 PD0
DOWN
S5
30
UP
S7
PD1
31 PD2
VIBRATO
24
PC4
+5V
DIPSW1
POWER
S1
DELAY
PRESET
D1-D4
4x1N4004
V+
VIBRATO
PRESET
+16V
D5
1N4004
12VAC
300mA
IN
22
25VW
1000
25VW
REG1
7805
GND
OUT
10k
10k
10k
10k
4
32
PD3
33
PD4
34
PD5
36 PD7
3
2
1
20
+5V
10
16VW
B
I GO
DIGITAL EFFECTS UNIT
30 Silicon Chip
A2
K
A1
E
VIEWED FROM BELOW
C
A
K
37
+5V
0.1
100
1
24
10k
16
15kHz LP
FILTER
.022
15
ON
39k
IC3
M65830P
14
IN
10k
RLY1
560pF
39k
150pF
OUT
1
10k
13
OUT
7
9
3
10 11 12
10
7
IC1d
5
100
OUTPUT
47k
220k
REVERB
VR2
10k LIN
19
OUT
TP2
6
IN
EFFECTS
S3a
10
XIN REQ SCK DATA
2
4
5
6
ECHO
S2
18k
47
IN
S3b
+5V
VIBRATO
MODULATION
LED1
0.1
VIBRATO
ON
LED2
10k
330
330
25
26
27
PC3 PC2 PC1
V+
22
23
40
PC5
2
PC6
IRQ
3
D6
1N4004
VPP
1
RESET
RLY1
10
PC0
1k
10k
28
B
C
Q1
BC338
E
IC5
MC68HC705C8
+5V
10k
DELAY
LED3
330
PC7
38
4.7M
X1
4MHz
33pF
33pF
39
PA
PA PA PA PA PA PA PA
0
7 6 5 4 3 2 1
4 5 6 7 8 9 10
11
PB PB PB PB PB PB PB PB7
0 1 2 3 4 5 6
12 13 14 15 16 17 18
7x 330
8x 330
10 9 7 6 1 2 4
G F A B E D C 5
A
DP
F
B
G
E
C
D
DISP1 DP
HDSP5301
3,8
MSD
21
19
330
DISPLAY
S4
VIBRATO
RATE (Hz)
DELAY (ms)
VIBRATO
RATE
LED4
1 2 4 10 9 7 6
E D C G F A B
A
F
G
E
B
C
D
DISP2 DP
HDSP5301
3,8
LSD
+5V
Fig.2: IC3 (the digital delay line)
forms the heart of the circuit,
while microprocessor IC5 provides
the control signals & drives the
various LED displays. IC5 also
controls relay RLY1 to briefly mute
the output at switch-on.
February 1995 31
RLY1
100k
47k
47k
10uF 10uF 10uF
10k
560pF
0.27
1k
10k
1
10k
1uF
1000uF
560pF
10k
18k
39k
REG1
18k
Q1
10k
1uF
39k
82
10k
10k
TP
GND
10uF
39k
DIP1
100k
100uF
10k
10uF
10uF
10k
D1
D5
10uF
IC1
TLO74
10k
10k
10k
10k
100
D2
8.2k
10uF
D3
D4
220k
220k
D6
39k
22uF
1 2 3 4
150pF
22k
IC3
M65830P
22k
10uF
22k
100uF
IC5
MC68HC705C8
150pF
0.1
0.1
X1
47uF
22k
100uF
0.1
.022
4.7M
0.1
33PF
.022
33pF
TP2
1
1
.039
20
22
21
7 8 9 101112
K
A
131415161718
S5
20
19
22
21
S7
24
23
Fig.3: mount the parts on the two PC boards as shown here, taking care to
ensure that all polarised parts are correctly oriented. Note particularly the
orientation of the ICs, the 7-segment LED displays & the four pushbutton
switches (S4-S7). An IC socket is recommended for IC5.
subsequently be passed to the output.
After processing inside IC3, the
delayed signal appears at pin 15 and
is then fed back into an internal op
amp via pin 14. This op amp, together with the associated RC network,
forms the 15kHz low-pass output filter
depicted on Fig.1. The output signal
then reappears at pin 13 of IC3 and is
AC-coupled to pin 6 of IC1d via a 1µF
capacitor and switch S3a.
A
10k
330
K
330
S6
LED4
DISP2
32 Silicon Chip
TP1
10k
330
10k
10k
1
1 2 3 4 5 6
10k
23
S4
DISP1
10k
LED2
A
330
K
K
1
24
4.7k
100k
LED3
LED1
A
19
131415161718
VR4
1
0.1
1 2 3 4 5 6
IC2
LM358
27k
0.1
1
7 8 9 101112
1uF
1.8k
IC4
7555
0.1
330
1uF
22k
VC1
330
330
330
330
330
330
330
330
330
330
330
10k
330
330
330
3.3k
270
.01
IC1d functions as the output mixer
stage. It operates with a gain of -1, both
for signals from IC3 and for signals fed
in from IC1c when the Echo switch
(S2) is closed. VR2 sets the reverberation as described previously. It feeds
a sample of the delayed output signal
to the input of IC1c via S3b, a 10µF
capacitor and a 10kΩ resistor.
Switches S3a & S3b are used to
switch the effects in or out. In the
OUT position, the signal from IC1c is
fed directly into IC1d and the digital
delay circuitry is effectively bypassed.
At the same time, the feedback signal
from VR2 is switched out to eliminate
any reverberation effects that would
otherwise occur if VR2 was not set to
its minimum position.
The 220kΩ resistor connected to
pin 7 of IC1d maintains the DC charge
on the associated 1µF input coupling
capacitor when the effects are switch
ed out. This eliminates noise when
S3a is subsequently switched to the
IN position.
The muting relay (RLY1) is controlled by the PC0 output of IC5. This
output goes high several seconds after
power is applied and turns on RLY1
via transistor Q1. Diode D6 protects
Q1 by quenching the back-EMF spikes
generated when RLY1 switches off.
A 4MHz crystal connected between
pins 38 & 39 of IC5 sets the clock
frequency for IC5. This frequency is
internally divided by two, so that the
microprocessor actually runs at 2MHz.
The 10µF capacitor on pin 1 briefly
pulls this input low at switch-on to
provide a reset pulse.
Power supply
The display PC board is mounted on the lid of the case via four 9mm tapped
spacers. After mounting, adjust the height of each indicator LED so that it just
protrudes through the panel. A small piece of red Perspex provides a window
for the two 7-segment LED displays.
Following IC1d, the processed
signal is coupled to the output socket
via a 10µF capacitor, a set of relay contacts and a 100Ω resistor. The latter is
there to prevent IC1d from going into
oscillation when a long output lead
is connected.
Vibrato circuitry
The vibrato function is toggled on
or off using pushbutton switch S7 to
pull the PD2 line of the microprocessor
low. The resulting PWM signal from
PC4 (pin 24) is then AC-coupled to
low-pass filter stage IC2a to derive a
control signal for the VCO.
IC2a’s non-inverting input (and thus
its output) is biased to about +5V by
IC2b. This latter stage functions with
a gain of two and amplifies the voltage
fed to its pin 3 input from the wiper
of trimpot VR4. As a result, IC2a’s
output sits at a constant +5V when the
vibrato is off and varies in sinusoidal
fashion about the +5V level when the
vibrato is on.
This control signal is fed to the
cathode (K) of varicap diode VC1 via
VR3 (the vibrato level control) and a
100kΩ resistor. Note that the +5V bias
level, as set by VR4, determines the
output frequency of the VCO when
the vibrato is off (and when VR3 is set
to minimum).
VC1 and 7555 timer IC4 make up the
VCO. IC4 operates in astable mode and
varies its output frequency according
to the capacitance of VC1. This capac-
itance, in turn, varies according to the
control signal from VR3.
When the vibrato is off, the control
signal remains constant and thus the
VCO output also remains constant at
a nominal 1MHz. Conversely, when
the vibrato is on, the control signal
varies sinusoidally and so the VCO
output varies in similar fashion. This
in turn modulates the delay period
of the digital delay line to produce a
pulsating sound effect.
Microprocessor functions
Not a lot can be gleaned from the
looking at the microprocessor circuitry
since its operation depends mainly
on the software. Basically, its various
I/O (input/output) lines accept inputs
from the various switches (S4-S7 and
DIPSW1) and drive the various LED
indicators and the two 7-segment
displays.
Depending on the settings programmed into it via these switches,
IC5 also controls the digital delay line
via its PC1-PC3 outputs as described
previously. And, as we have just seen,
it also controls the vibrato circuitry via
its PC4 output.
Outputs PA0-PA7 drive the two
7-segment displays via 330Ω current
limiting resistors, while PC5 & PC6
drive the two vibrato LED indicators
(LED 1 & LED 2). PC7 controls LED 3
& LED 4 to indicate the display mode
–when PC7 is low, LED 3 is on and
when PC7 is high LED 4 is on.
Power for the circuit is derived from
a 12VAC 300mA plugpack supply. Its
output is full-wave rectified using D1D4 and the resulting DC filtered using
a 22µF capacitor to provide a nominal
16V rail. This rail supplies RLY1 and is
also fed to 3-terminal regulator REG1
via isolating diode D5.
A 1000µF capacitor provides further
filtering at the output of D5 and the
16V rail at this point is used to power
IC1 and IC2. REG1 provides a regulated
+5V output and this supplies IC3, IC4,
IC5 and the LEDs.
When power is switched off, the
relay supply falls quickly due to the
modest amount of filtering employed.
As a result, the relay switches off well
before the voltage across the 1000µF
capacitor falls by any appreciable
amount. This effectively mutes the
output and eliminates any nasty
switch-off effects.
Software
Although the software programmed
into IC5 is fairly complex, we can describe some of the main features of the
program. The program is divided into
various subroutines and interrupts
and each of these performs a separate
function.
At power up, the RESET program
begins and this sets up the initial conditions for the I/O and monitors the
DIP switch settings. The Delay period
is initially set by the DIP switches on
PD3 & PD4, while the Vibrato Rate is
initially set to one of two values (either
3.5Hz or 8.5Hz) by the DIP switch on
PD5.
The DIP switch on PD7 selects
whether the Vibrato is ini
tially on
or off. The program then waits for
several seconds and then brings PC0
high and updates the delay time in
February 1995 33
This view shows how everything fits together inside the plastic case. Note that
REG1 is mounted with its leads bent at right angles & is bolted to the PC board
along with a small heatsink. The wiring is secured using plastic cable ties.
IC3 in a subroutine called UPDTE. The
display is then driven by a subroutine
called SET.
The program now monitors switch
es S4-S7. If one is pressed, it acts according to the function of the switch.
This program is called POLL and is the
background program that runs continuously until the power is switched
off or it is interrupted by an internal
interrupt program called TIMER.
This interrupt program generates
the PWM code for PC4 and LED 1
when Vibrato is selected. When vibrato is off, PC4’s output is a 1kHz
square wave.
Construction
Most of the circuitry is contained
on two PC boards: a main board coded
01301951 and a display board coded
01301952. Fig.3 shows the parts layout
on these two boards.
Before installing any of the parts,
check the boards carefully for shorts
and open circuit tracks by comparing
The rear panel carries (from left to right) the Effects In/Out switch, the input &
output sockets, the Attenuator control, the Power switch & the DC power socket.
34 Silicon Chip
them with the published artworks. If
all is correct, begin the assembly by installing PC stakes at all external wiring
points on the main board, excluding
points 1-24. PC stakes should also be
installed at TP1, TP2 and TP GND.
Next, install the links, resistors, diodes and ICs 1-4, taking care to ensure
that the semiconductors are all correctly oriented. Table 3 lists the resistor
colour codes but it is also a good idea
to check them using a multimeter, as
some colours can be difficult to decipher. Note particularly the row of
resistors below IC5 – the resistor on
the extreme left has a value of 10kΩ,
while the rest are all 330Ω.
The MKT capacitors can now be
installed (see Table 2), followed by the
electrolytic types. This done, install
RLY1, VC1, Q1, VR4, X1, DIP1 REG1
and a 40-pin socket for IC5. The latter
is mounted with its leads bent at right
angles and its metal tab fitted with a
small heatsink. Smear the metal tab of
the regulator with heatsink compound
before bolting the assembly (regulator
plus heatsink) to the PC board using a
screw and nut.
Finally, complete the main board
assembly by installing IC5 in its socket.
Take care to ensure that it is correctly
oriented.
By contrast with the main board,
the display board carries relatively
few components and should only
take about 10 minutes to assemble.
Begin by installing the resistors, then
mount the two 7-segment displays and
the pushbutton switches (S4-S7). The
displays must be oriented with their
decimal points at bottom right, while
the flat sides on the switch bodies go
towards the bottom of the board – see
Fig.3. Push the switches down onto
the board as far as they will go before
soldering their leads.
The four indicator LEDs can now be
installed. For the time being, mount
them so that they sit about 10mm
▲
Fig.4 (right): figure-8 shielded cable is
used for the connections between the
two input sockets (Guitar & Line) &
the PC board, while the remainder of
the wiring shown on this diagram can
be run using medium-duty hook-up
wire. Points 1-24 of the two PC boards
are connected together using four
60mm lengths of 6-way rainbow cable.
S3
S1
GUITAR
LINE
OUTPUT
28
POWER SOCKET
26
VR1
25
TP
GND
29
27
MAIN PCB
1 2 3 4 5 6
20
7 8 9 101112
131415161718
19
22
21
25
S2
24
23
VR2
26
VR3
29
28
27
1 2 3 4 5 6
7 8 9 101112
131415161718
20
19
22
21
24
23
SOLDER
LUG
DISPLAY PCB
February 1995 35
Table 1: DIP Switch Settings
1
2
3
4
DIP Switch
Vibrato Off
on
x
x
x
Vibrato On
off
x
x
x
3.5Hz Vibrato
x
on
x
x
8.5Hz Vibrato
x
off
x
x
1ms Delay
x
x
on
on
17ms Delay
x
x
on
off
33ms Delay
x
x
off
on
49ms Delay
x
x
off
off
TABLE 2: CAPACITOR CODES
❏
❏
❏
❏
❏
❏
❏
❏
❏
❏
Value
1µF
0.27µF
0.1µF
0.039µF
0.022µF
0.01µF
560pF
150pF
33pF
IEC
1u
270n
100n
39n
22n
10n
560p
150p
33p
EIA
105
274
104
393
223
103
561
151
33
above the surface of the board and
secure each LED by just soldering one
lead. Do not trim the leads at this stage,
as the LEDs are adjusted for height later
on when the display board is attached
to the front panel.
Take care with the orientation of the
LEDs; the anode lead of each LED is
the longer of the two (see Fig.2).
Refer now to Fig.4 for the wiring
details. The first step is to connect
points 1-24 of the two PC boards
together using four 60mm lengths of
6-way rainbow cable. The main board
is then mounted on integral plastic
pillars in the base of the case and
secured using the small self-tapping
screws supplied.
The display board mounts on 9mm
tapped spacers which are secured to
the lid of the case using countersunk
screws. Attach the four spacers to the
lid, then fit the red Perspex window
for the LED displays.
This window should be a tight fit
into the front panel cutout and can
be secured by applying a thin line
of epoxy resin around the underside
edge. The display board can now be
mounted in position. Adjust the height
of each indicator LED so that it just
protrudes through the front panel
before soldering the remaining leads.
Next, fit the lid to the case and
carefully mark out and drill holes in
the rear panel for the three 6.5mm
sockets, the two switches, the Attenuation pot and the power input socket.
The 6.5mm sockets and the switches
should be mounted 15mm down from
the top edge of the rear panel, while
the pot should be mounted half-way
down so that its lugs can be soldered
directly to the PC stakes on the board
immediately below it. The power
socket mounts directly below the
power switch.
The various items of hardware can
now be mounted in position and the
wiring completed. Shielded cable is
used between the two input sockets
and the main PC board, while the
remaining wiring is run using medium-duty hook-up wire. Run the wiring along one edge of the main board
(see photo) and don’t forget the lead
between TP GND and the solder lug
on the display board.
Once the wiring has been completed, it can be tidied up and secured
using a number of cable ties.
Testing
To test the unit, first connect your
multimeter between TP GND on the
main board and pin 1 of IC3. This
done, set the meter to a low voltage
range, apply power and check that the
meter reads 5V. The display should
initially show two dashes (- -) and
then, after a few seconds, a number
(the value depends on the DIP switch
settings). The Delay LED should be
also be lit.
Now check that pin 8 of IC4 and pin
2 of IC5 are at +5V. Similarly, check for
+16V on pin 4 of IC1 and pin 8 of IC2.
If these voltage measurements are
OK, check that the LED readout toggles
between the Delay and Vibrato Rate
modes each time S4 is pressed. If it
does, check that the display increments when Up is pressed and decrements when Down is pressed. Check
that the display range is 1-64 in Delay
mode and 1-20 in Vibrato Rate mode.
Now check the operation of the
TABLE 3: RESISTOR COLOUR CODES
❏
No.
❏ 1
❏ 2
❏ 3
❏ 2
❏ 4
❏ 1
❏ 5
❏ 2
❏
20
❏ 1
❏ 1
❏ 1
❏ 1
❏ 1
❏ 1
❏ 1
❏ 1
❏ 1
36 Silicon Chip
Value
4.7MΩ
220kΩ
100kΩ
47kΩ
39kΩ
27kΩ
22kΩ
18kΩ
10kΩ
8.2kΩ
4.7kΩ
3.3kΩ
1.8kΩ
1kΩ
330Ω
270Ω
100Ω
82Ω
4-Band Code (1%)
yellow violet green brown
red red yellow brown
brown black yellow brown
yellow violet orange brown
orange white orange brown
red violet orange brown
red red orange brown
brown grey orange brown
brown black orange brown
grey red red brown
yellow violet red brown
orange orange red brown
brown grey red brown
brown black red brown
orange orange brown brown
red violet brown brown
brown black brown brown
grey red black brown
5-Band Code (1%)
yellow violet black yellow brown
red red black orange brown
brown black black orange brown
yellow violet black red brown
orange white black red brown
red violet black red brown
red red black red brown
brown grey black red brown
brown black black red brown
grey red black brown brown
yellow violet black brown brown
orange orange black brown brown
brown grey black brown brown
brown black black brown brown
orange orange black black brown
red violet black black brown
brown black black black brown
grey red black gold brown
Fig.5: check your PC boards against these full-size etching patterns before installing any of the parts.
Vibrato On/Off switch (S7). When the
vibrato is on, its associated LED should
light and the Vibrato Modulation LED
(LED 1) should flash at the selected
rate. Check that the rate at which this
LED flashes alters according to the
vibrato rate.
If you have access to a frequency
meter or an oscilloscope, adjust VR4
for a reading of 1MHz at TP2 when
the vibrato is off. If this equipment is
unavailable, simply adjust VR4 for a
voltage reading at TP1 (note: not TP2)
of 2.5V. This should provide a clock
frequency that’s reasonably close to
the mark.
The unit can now be given a practical test by connecting the output to
an amplifier and feeding a signal into
one of the inputs. Check that the Echo
On, Reverberation and Vibrato Depth
controls all produce the desired effects
on the sound and that the Effects In/
Out switch operates correctly.
Once you become familiar with the
various effects, you can set the DIP
switches on the PC board so that your
normally selected settings appear at
switch on – see Table 1. To see how
this works, let’s assume that you want
the unit to power up with the following settings: (1) vibrato off; (2) vibrato
rate = 3.5Hz; and (3) delay = 33ms. In
this case, the switch settings would
be: DIP1 on; DIP2 on; DIP 3 off; and
SC
DIP 4 on.
February 1995 37
|