AUDIO
OUT
AUDIO OUT
L
R
By Jake Rothman
Noise about noise – a new slant on analogue
noise with tilt control – Part 2
L
ast month, I introduced my
analogue noise generator. This
month we will build it, examine
its ‘tilt control’ and create a variety of
interesting noise-generator-based synthesiser circuits.
Tilt tone control
I recently described a Baxandall tone control in PE (EPE) April and May 2018 and
I thought this would be a suitable place
to introduce an intriguing variant of this
design, the tilt control. Conceived by R
Ambler (Wireless World, March 1970) and
later adopted in the Quad 34 pre-amp,
this circuit combines Baxandall’s bass
and treble controls into a single control.
This control is flat when the pot is centred and the frequency response pivots
around a mid-point to give boosted treble
and cut bass when rotated fully clockwise. When fully anti-clockwise it has
the reverse effect of maximum bass boost
and maximum treble cut. At intermediate
settings of these positions it can give a
gentle slope or tilt across the whole audio
frequency range. A centre-detent/click
pot is ideal if you can get one. Tayda do a
20kΩ one with a W-law (A-1959). To use
this, re-scaling of the component values
R9
10kΩ
R10
10kΩ
R8
33kΩ
R13
33kΩ
CW
I np ut f rom
C3 on Fig. 8
0V
R7
100kΩ
C5
4 7 0nF
Component list
(for the synthesiser version)
C1
C2
C3
C4, C7
Semiconductors
IC1 TL062 or MCP6002 low-power op
amp. For Hi-Fi, use TL072 or a better
Bi-Fet (dual) op amp
REG1 TL431
C7
2. 7 nF
C5, C6
C8
C9
100µF
47µF
22µF
2.7nF (for Hi-Fi 1kHz turnover
use 6.8nF polyester)
470nF
10µF
4.7µF
0dB
–7.5
20 Hz
50
100
200
500
1k
2k
5k
10k
20k
20 Hz
50
100
200
500
1k
2k
5k
10k
20k
R17
560kΩ
6
–
5
+
7
R14
100Ω
I C1b
B i-Fet (eg, T L06 2)
0V
C9
4 .7 µ F
O utp ut
p in 4
R15
100kΩ
+7.5
0dB
0V
p in 3
Fig.12. Tilt tone control circuit: this can be thought of as crosscoupled Baxandall bass and treble controls combined in one.
58
Capacitors
Dielectric type not critical: ceramic, polyester, electrolytic or tantalum can be used.
All 5mm pitch radial. Scale voltage rating according to power supply. If using
standard 5V use minimum 6V rating.
+7.5
R11
50kΩ
Lin
C6
4 7 0nF
B ias 2. 5V
f rom Fig. 8
Resistors (all 0.25W 5% carbon-film)
R1, R2
2.2kΩ
R3
22kΩ
R4
27kΩ (only for 9V or greater
power supply)
R5
1.5kΩ
R6
56kΩ
R7, R15 100kΩ
R8, R13 33kΩ
R9, R10 10kΩ
R11 (VR1) 50kΩ lin 16mm vertical PCB
mount pot.
R12
560kΩ
R14
100Ω
+
C4
2. 7 nF
would be required: C4 and C7 would be
5.6nF, R9 and R10 become 4.7kΩ and R8
and R13 are set to 16kΩ.
If the control is set to give a downwards
tilt, it can be used to give the white noise
a pink noise approximation. In theory,
this should be –3dB/octave, which needs
a more complicated filter, but this tilt control gives a reasonable sonic impression
for most musical uses. On full treble tilt
it gives a harsh ‘blue’ noise suitable for
high-hats and transient sounds. Turned
the other way, it gives a lowish roar good
for tom-tom sounds. The tilt control circuit is shown in Fig.12; on the PCB this is
combined with the noise source in Fig.8
(last month). The frequency response is
shown in Fig.13a. Note the pivot-point
is 2.5kHz, which gave the best synth effects with the noise. Fig.13b shows the
effect of different pot positions.
–7.5
Fig.13a. (top) Frequency response of tilt tone control; the ‘pivot’
point is 2.5kHz. Fig.13b shows the effect of different pot positions.
Practical Electronics | May | 2020
n
NB, VR1
is IC2
Fig.14. PCB overlay for combined noise
generator and tilt control Fig.8 and Fig.12).
Fig.15. PCB track layout. This is singlesided, so easy to etch.
Miscellaneous
PCB and 4-pin Molex 0.1-inch connector assembly.
the noise generator components and constructing the circuit shown in Fig.17. The
pivot point should be moved to the standard Baxandall value of 1kHz, as shown
in Fig.18. This is done by multiplying the
2.7nF frequency-determining capacitors
C4 and C7 by 2.5 to give 6.8nF. If you are
using the 20kΩ centre-click pot, use 15nF
and change resistors R8, R9, R10 and R13
as previously mentioned. Note the noise
generator part of the circuit has to be rejigged to become a bias generator. Also,
the first op amp stage is reconfigured to
be inverting to ensure there is no overall phase inversion. (This is because the
tilt circuit is inverting). The op amp used
should be changed to an audio-quality
device, such as a TL072 and the resistors
to metal-film types. The resistors, pot
and capacitors could all be scaled to a
lower impedance (capacitors multiplied
by 10 and resistors divided by 10) and a
PCB
The PCB overlay is shown in Fig.14 and
the track layout in Fig.15. Since the design
is single-sided it is suitable for home etching. Eagle files and PCBway ready-made
PCBs will be available from the PE site and
shop as usual. I still get my single-sided
boards made at PCBway because of the
greater joint reliability afforded by plated-through holes. The finished board is
shown in Fig.16. It is small enough to be
mounted by means of the pot shaft, but
support at the back is needed if the unit
is to be moved around.
Hi-Fi tilt control
Here’s a bonus circuit! This design can
be adapted for Hi-Fi use by leaving off
V+
24 V
R1
100kΩ
R10
. kΩ
R8
16kΩ
R13
16kΩ
The 1980s, the decade of
analogue noise-based music
synthesis
The classic analogue drum sound with
noise was popularised by the Simmons
SDS-3 syndrum, familiar to many UK
readers as the tom-tom beats at the beginning of the old theme tune to the
BBC soap East Enders. This sound was
generated by adding a short duration
Fig.17. Circuit for
Hi-Fi version of the
tilt control with noise
source removed.
C7
15nF
C8
C1
10µ F
C2
R5
22µ F 10kΩ
8
3 +
I C1a
T L07 2
2 –
C3
+ 100µ F 22µ F
+
+
CW
1
4
R6
10kΩ
C6
4 7 0nF
R7
100kΩ
C5
4 7 0nF
N otes f or P CB – see Fig. 19
1. Link R3 on P CB
2. Mounting of C2: V– lead to
p ad nearest R3 on I C2.
3. I np ut p in is p ad f or R2
R11
20kΩ
W -law
Centre d etent
R17
560kΩ
6
–
5
+
R14
100Ω
7
I C1b
B i-Fet (eg, T L06 2)
C9
4 .7 µ F
O utp ut
p in 4
+
I np ut
R9
. kΩ
NE5532 op amp used to further reduce
noise and distortion. The power supply
should be upped to say 24V to give reasonable headroom. Finally, Fig.19 shows
the PCB constructed as a tilt-control-only
circuit. The pot can be replaced with a
dual-gang version for stereo use.
1/ 2 V+
bias
+
R4
100kΩ
C4
15nF
Fig.16. The finished noise module. Oops I
forgot the mounting holes at the back!
R15
100kΩ
0V
p ins 2, 3
0V
Practical Electronics | May | 2020
59
A mp
+7.5
Enve lop e
0dB
T rigger
(p iezo
p icku p )
Click
leve l
Decay
–7.5
20 Hz
50
100
200
500
1k
2k
5k
10k
20k
P itch
bend
amount
VCO
VCA
VCA *
P itch (f req )
+7.5
O utp ut
* Could also be
a VCF (f ilter)
N oise
0dB
N oise
leve l
–7.5
20 Hz
50
100
200
500
1k
2k
5k
10k
20k
Fig.18a. (top) Frequency response of the Hi-Fi tilt control with
almost all possible pot positions. Pivot point now the standard
1kHz, using a 20kΩ W-law pot with 15nF frequency-determining
capacitors (C4 and 7). Fig.18b shows the response with a 50kΩ
lin pot and 6.8nF capacitors, a little higher at 1.2kHz. If 7.5nF caps
can be obtained it would be nearer the desired 1kHz.
burst of white noise to the start of a filtered decay of a triangle waveform with a bit of downwards pitch bend as the
sound decays. Occasionally, the noise had its own envelope
and VCA for snare sounds, but it was often sufficient to just
mix the noise with the main waveform. The leading edge of
the trigger pulse was often also mixed in to give the initial
transient hit. Usually, the trigger signal came from a piezo
ceramic disc glued to a piece of wood which was struck.
Maplin offered some kits called the Syntom and Synwave
(published in Electronic and Music Maker in April and
July 1981) which used these tricks. Putting the two together made a plausible Simmonds imitator. Building loads of
them helped fund my student digs at the time. The basic
block diagram is shown below in Fig.21. I suspect it’s only
a matter of time before the Syntom is resurrected to replace
the Theremin as the new hipster musical toy.
Vinyl scratch simulator
Fig.21. Basic syndrum block diagram.
+ 5V
V+
2V
3V k k
2.2 Ω
noise
J1 7 6
1nF
k
+
T ime
T L06 1
T h resh old
leve l
0V
–
1 Ω
+ 2. 2V
2.2
1N 4 14 8
2.2 Ω
1 6
Fig.22. ‘Vinyl scratch’ generator.
vital part, but there is also anti-phase, low-frequency rumble; wow and flutter; and also distortion effects to add. The
JFET circuit shown in Fig.22 produces the short-duration
random spikes to produce ‘scratches’ when fed with random
noise. It also makes a good Geiger-counter effect – great for
dispersing any eco-warriors who wander into the workshop.
Cymbals and high-hats
These sounds are often generated most simply by putting the
white noise though a resonant 10kHz tuned LC circuit filter
(Fig.23) often heard in old 1970s electronic organs (Klaus
Wunderlich anyone? https://youtu.be/29RU-3q_p9U). A more
metallic effect is achieved by ring-modulating white noise with a high-frequency
tone, which generates a whole range of
dissonant sidebands. The simplest way
of doing this is to use exclusive-or (XOR)
gates, which provide the square-wave
equivalent of ring modulation. Alternatively, one can just use a hex-inverter
chip to generate six square waves together and mix them. This process was used
in the Roland TR606 drum machine, a
staple of bed-room electronic musicians
in the 1980s. Robert Penfold’s Cymbal
Synthesiser in Electronics Monthly in
December 1984 employed XOR gates.
I created a ‘turbine noise’ generator,
Fig.20. Finished Hi-Fi tone control PCB.
shown in Fig.24 using CD4070 XOR
Note there are not enough pins on the
gates and 40106 inverters. Gated and
Molex connector so a spare pad is used
filtered it was also good for cymbals.
for the input connection. This is the black
Varying the supply voltage (3 to 15V)
and white striped wire. The input capacitor
varies the pitch, a useful side effect of
C2 can be seen jumping to a pad on the
the wide operating voltage of old-fashvoltage regulator outline.
ioned CMOS logic.
I think it’s fun to make pristine digital recordings sound like
vinyl records. This is necessary for giving a vintage sound
effect for films and animations. Of course, scratch noise is a
V+ 0V 0V O/P
Link
Input
–
+
C2
IC2
Fig.19. Note how the components are
re-jigged on the PCB to omit the noise
generator for the Hi-Fi tilt tone control.
60
Practical Electronics | May | 2020
N oise
T win-T
section
V+ 6 V
I np ut
1nF
B C54 9
4 7 mH
T oko
10RB
4 . 7 nF
1 Ω
50kΩ
A nti-log
82kΩ
6 . 8 nF
2. 2nF
+
33µ F
O utp ut
B C54 9
2.2 Ω
. kΩ
V+ 9 V
CW
Freq
10kΩ
10kΩ
4 . 7 nF
68kΩ
H igh -h at
68kΩ
6 8 nF
1 Ω
18 nF
(sets
d ecay)
4 .7 µ F
910kΩ
0V
+
T rigger
4 . 7 nF
910kΩ
O utp ut
Fig.23. Simple high-hat voicing circuit using a tuned inductor.
B C54 9
+ 9 V
68kΩ
S low
10kΩ
Lin
Fast
S p eed
CW
+
10µ F
CW
1kΩ
A nti-log
Volume
6.8kΩ
+
100µ F
CW
1kΩ
100nF
2. 4 nF
Resonance
100Ω
Lin
N oise
inp ut
0V
0V
3.3kΩ
150kΩ
14
1
6 . 8 nF
2
I C1a
4 0106
100kΩ
6 . 8 nF
3
2
1
I C2a
4 07 0
11
T R1*
B C18 2
0V
* T h is stage need ed to
clip signal since lev el
v aries as p ower sup p ly
v oltage to I C1 is v aried .
10
Full-p ower current
d emand = 200mA
I C1c
4 0106
B eat
6
I C1d
4 0106
120kΩ
13
which was so overused in 1980 records, such as the
Kim Carnes song, Bette Davis Eyes – see: https://
youtu.be/wyRosnwO_mg
A reasonable clap simulation can be made from a
string of typically four white-noise bursts triggered
from a pulse-train generator. These can be produced
digitally by a 4017 decade-counter IC, which does a
quick count and then resets. An analogue method is
to use a string of comparators fed by an exponential
decay similar to an LED bargraph. This trick was
used in Harvey Kent’s Hand Clapper in Practical
Electronics in August 1989. Surprisingly, the start
of the ‘clap sound mania’ was acoustic, engineered
by Alan Parsons in the 1975 Pilot song, January –
see: https://youtu.be/WdcrTUcdO0Q (0.36). This
effect was achieved by recording real claps with
Dolby A noise reduction on, then leaving it off on
playback. I feel an urge to design a Clap Trap coming on, oh dear!
10kΩ
W h ole circuit q uiescent
current, I q = 18 mA
100kΩ
Lin
6 . 8 nF
10
9
4
5
I C2b
4 07 0
14
3 8
91kΩ
6 . 8 nF
Fig.25. Twin-T band-pass filter: adds a whistling effect to noise.
T R2
B C337
I C1b
4 0106
6 . 8 nF
35Ω
+ 9 V
3.3kΩ
39kΩ
I C2c
4 07 0
6
5
35Ω
T R4
B C337
4 12
13
I C2d
4 07 0
7
11
10kΩ
T R3
B C18 2
12
Speech synthesis
I C1e
4 0106
68kΩ
100nF
9
4
8
I C1f
4 0106
0V
Noise is an essential component of speech, being
present in the ‘ess’ sound components, sibilance
and fricatives. This is a whole field in itself and
there is insufficient space to dwell on it here (definitely one for the future though), but the circuits
given here will provide hours of fun.
Fig.24. Turbine generator for simulating the four turbochargers used in
a class 40 diesel engine.
Submarine sonar sound
This is one of my favourite sound effects, very prominent in
Thomas Dolby’s track, One of Our Submarines. It consists
of feeding bursts of white noise into a high-Q ringing bandpass filter shown in Fig.25. It can also be used to do breathing
noises, such as in the BBC drama series Casualty theme tune.
It’s also great for doing steam whistles for model railways.
The FET ‘puff’ generator in Fig.26 provides a suitable burst
of white noise with a bit of decay.
Claptrap
The Simmons Clap Trap must be the definitive musical white
noise ‘clap’ sound (see/hear at: https://youtu.be/k2NoL_a4Ulg)
Practical Electronics | May | 2020
T rigger
I np ut
1N 4 14 8
+ 5V
N oise
I np ut
+ 2V to op en
gate and p ass
noise signal
1N 4 14 8
10kΩ
kΩ
10kΩ
Delay
2 Ω
Lin
220nF
B ias
1 Ω 1N 4 14 8
2 kΩ
O utp ut
J1 7 6
P -ch annel
JF ET
(Voltagecontrolled
attenuator)
0V
Fig.26. Using a FET as a voltage-controlled attenuator to
pass a ‘puff’ of white noise.
61
