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AUDIO
&VIDEO
Tube Clipper
audio processor — with valves!
By Dr. H. Friedli
The Tube Clipper is an effects unit which can be inserted
in the audio path of a stereo system. It provides that ‘valve
sound’, but without the use of high voltages.
There are many audiophiles who like ‘valve
sound’ but who are put off buying a valve
amplifier by the prohibitive cost and who are
put off building their own by the dangerous
voltages involved.
The Tube Clipper described here is a kind
of effect/preamplifier which gives the valve
sound but which uses low-voltage valves
and which needs no voltage higher than
±12 V. It is an ideal introduction to valve
technology.
The Tube Clipper has the follow-
ing distinctive features:
– Limit setting for both stereo chan-
nels via a single potentiometer;
– No coupling capacitors are
required on the output;
– Symmetry is individually
adjustable for each valve;
– No voltages higher than
– Visible indication of when the
input signal is being clipped.
Principles
It is of course an idea abhorrent to
any audio freak that an audio signal
might not be copied faithfully from
sound source to loudspeaker. So
what is a sound processor like this
doing in a hi-fi system? Why is valve
±
12 V are
required;
– Gives that authentic ‘valve sound’;
22
Elektor Electronics
10/2001
AUDIO
&VIDEO
technology so popular at the
moment? The reason can be found in
the studio: CDs are in general not
particularly carefully recorded and
mixed, and voltage peaks in a
recording are routinely clipped at the
expense of signal-to-noise ratio. You
will probably remember the discus-
sion (initiated in
Elektor Electronics
)
of overdriven CDs, the reasons they
are produced and their undesirable
consequences. We will not, however,
continue that theme here.
The idea behind the Tube Clipper
is to limit the audio signal at a given
preset maximum level. Valves do this
in a rather gentle way: they do not
limit excessive voltages as ‘hard’ as
transistors do when driven into sat-
uration, but rather as the voltage
rises towards the limit value, the
valve begins to clip gradually. Hence
we obtain the desired smooth curve.
When a signal clipped in this way
is analysed, we find that only har-
monics at integer multiples of the
fundamental frequency have been
introduced. To a musician it is now
obvious: when the harmonics of a
sound are mixed in this way, no dis-
sonance can arise. In contrast the
non-integer multiples that are intro-
duced by transistor clipping have a
very adverse effect on the sound. The
Tube Clipper does modify the sound
of a CD, but in such a way that no
sounds disturbing to the ear are intro-
duced. Indeed, the added integer har-
monics fill out the sound considerably
to make it very similar to the original.
Ask a guitarist why he swears by his
valve amplifier! Overdriving helps
considerably to improve the sound of
the electric guitar.
The circuit described here can be
inserted directly in a high-imped-
ance audio chain. The Tube Clipper
then comes into its own when the
original sound already exhibits some
undesired clipping.
converter IC1.A. The buffered output
signal is taken to the grid of the first
triode B1.A. The triode is connected
in cathode-follower configuration
with the anode being connected
directly to the +12 V supply without
a series resistor. The voltage on the 33 k
Ω
cathode resistor is in phase with the input
voltage.
The part of the circuit containing the
potentiometer and IC3.D forms a buffered
voltage source that offsets the cathode DC
+12V
1
0V2...–0V6
1
2
3
0V...–7V4
–0V2...0V7
2
B1.A
0V
1
2
39k
IC1.A
6
4
0V1...0V04
3
3
6
B1.B
47
µ
25V
33k
B
49
7
39k
7
IC1.B
4
5
A
1
3
8
9
33k
8
9
5
IC1.C
10
0V
2
220n
470n
E
13
1
25V
U
D
14
10k
IC1.D
12
12
15k
14
10k
IC3.D
U
UD
= 0V65...0V7
D
100k
lin
13
12
14
IC2.D
13
470n
6
–12V
0V
10
8
IC2.C
9
5
9
33k
8
7
5
C
5
8
7
IC2.B
25V
49
3
7
39k
6
B2.B
47
µ
D
33k
3
6
5
0V25...–0V55
1
IC2.A
2
0V
2
39k
B2.A
6
0V...–7V4
1
7
8
–0V4...0V5
0V06...–0V1
+12V
0V18
0V
-10V7
2
A
33k
1
IC3.A
2k2
3
B
9
8V6
8
IC3.C
0V
0V
10
green
220n
220n
E
33k
0V
0V
220n
220n
red
-10V2
5
C
7
IC3.B
2k2
6
–12V
D
-10V7
0V
0V18
IC1, IC2, IC3 = TL074
B1, B2 = ECC83
7812
+12V
+12V
The circuit stage by stage
The circuit in
Figure 1
shows the
components that make up the Tube
Clipper. At the heart of each of the
two stereo channels is an ECC83
double triode and a quad operational
amplifier type TL074. A further
TL074 is used to control the grid
voltage and the LED indicators. The
audio signal goes first to impedance
B1
B2
5
5
4
4
4
9
9
IC1
IC2
IC3
1000
µ
22
µ
300mA T
4
4
100n
100n
100n
11
11
11
15V
1000
µ
22
µ
100n
100n
100n
–12V
7912
–12V
010018 - 11
Figure 1. Circuit of the Tube Clipper with low-voltage valves.
10/2001
Elektor Electronics
23
AUDIO
&VIDEO
Figure 2. Example construction
voltage negatively with respect to the grid so
that the triode can be overdriven to a greater
or lesser degree. The first triode, then, clips
only on positive-going peaks in the
signal. IC1.B simply inverts the sig-
nal and the second triode clips the
negative peaks of the input signal in
the same way. The ratio between the
two resistors, 39 k
Ω
and 33 k
Ω
,
Valve characteristics
It is possible to get to grips with the triode valve using only simple equipment. Take three
multimeters and two voltage sources and connect the triode as shown in Figure A. Mea-
sure the anode current as a function of grid voltage at constant anode voltage, and obtain
curves as shown in Figure B (clipping level at 0 V). The three sets of values measured at
different grid voltages exhibit the typical characteristic curve of the valve: as the anode
voltage is gradually raised the anode current rises, linearly at first, and then the curve
bends —gradually, in contrast to the transistor — into a flatter region as the anode current
reaches its maximum level.
If all the grid voltages corre-
sponding to the audio signal
are in the linear range, the
signal will not be distorted;
if the grid voltages go
higher, then the anode cur-
rent will saturate. In the cir-
cuit presented here this
anode current also flows
through a resistor, resulting
in a voltage proportional to
the current.
+12V
A
A
I
a
U
IN
V
B
U
a
2.5
–2V...+2V
U
g
= 1V
V
U
OUT
2
U
g
1.5
U
g
= 500mV
I
a
(mA)
1
0.5
U
g
= 5mV
–6V...0V
(Clipping Level)
0
0
5
10
15
20
25
U
a
(V)
010018 - 14
010018 - 15
24
Elektor Electronics
10/2001
AUDIO
&VIDEO
C and the dual LED.
The LED lights green when the stereo sig-
nal is not being clipped; yellow, when one
channel is being clipped; and red, when both
ECC83s are clipping. The input and output
signals are rectified by diodes and smoothed
by capacitors. Peak voltages are stored briefly
and then decay gradually away. IC3.A/B are
comparators whose outputs are positive or
negative according to whether the corre-
sponding input voltage is higher than the out-
put voltage or not (i.e., whether there is clip-
ping or not). The comparator outputs are
summed by resistors and drive the red part of
the LED. The junction of the resistors is also
connected to an inverter (IC3.C), which in
turn drives the green LED. We therefore get
the following behaviour: if there is no clip-
ping, the green led lights. If just one channel
is clipping, then the red LED lights also,
resulting in a yellowy-orange mixture; if both
channels are clipping, the green LED goes
out and the dual LED appears red. A small
positive voltage on point E ensures that the
green LED lights at very low audio input lev-
els, which gives a pleasant effect.
Other variations on the indicator theme
can be considered: these are left to the whim
of the constructor.
The Tube Clipper circuit can be powered
from a simple regulated ±12 V power supply.
The valve heaters can also be driven from
+12 V DC, since the ECC83 (as well as the
ECC81 or ECC88) do not require an AC sup-
ply. It is, however, important to ensure that
pin 5 is connected to the positive side of the
supply.
We have not produced a printed circuit
board for the Tube Clipper, since the valves
do not have any dangerously high voltages
present on them and there are so few other
components.
Figure 2
shows an example of
how the circuit might be constructed.
After the circuit has been built and the sol-
dered joints checked, the Tube Clipper can be
switched on and the various voltages and tol-
erances shown in the circuit diagram can be
checked. Then apply a sinewave signal to the
input and set the symmetry of each channel
separately using the trimmers. The unit is
now ready for use.
The effect of the Tube Clipper is shown
graphically in
Figure 3a
. The effect of the
‘level’ potentiometer is shown in eight steps
from minimum (unclipped curve) to maxi-
mum. Finally,
Figure 3b
shows the corre-
sponding distortion curves. When the signal
is not clipped, the distortion is at its lowest,
around 0.02 %, while, as might be expected,
at the highest clipping level the distortion is
at its most severe.
2
a
1.5
1
500m
V
0
-500m
-1
-1.5
-2
1m
1.1m
1.2m
1.3m
1.4m
1.5m
1.6m
1.7m
1.8m
1.9m
2m
s
010018 - 12
100
b
50
20
10
5
2
%
1
0.5
0.2
0.1
0.05
0.02
0.01
20
50
100
200
500
1k
2k
5k
10k
20k
Hz
010018 - 13
Figure 3. Signal shape at various clipping levels (a), with corresponding distortion
curves (b).
results in a small gain that compen-
sates for losses elsewhere in the sys-
tem. The output signal (if not
clipped) will then be at about the
same level as the input signal.
IC1.C produces a low-impedance,
and slightly amplified output signal
for the following stage (which might
be a power amplifier). Together with
IC1.D it ensures that the output sig-
nal has no DC component. IC1.D is
connected as an integrator. A DC off-
set at the output will cause a large
voltage to build up on the output of
the integrator, with opposite sign.
This voltage is fed to the non-invert-
ing input of IC1.C and acts so as to
oppose the DC offset. In this way the
average output voltage is kept at
zero, and an electrolytic capacitor at
the output can be avoided. If the
source signal is guaranteed free of
DC offset, the Tube Clipper can be
built entirely without coupling
capacitors in the signal path. The
second channel is built identically to
the first using B2.A and IC2.
Op-amp IC3.D provides a low-
impedance voltage source for the
cathodes that is adjustable via a
potentiometer. Using the two trim-
mers the output signal can be made
symmetrical by compensating for
component tolerances. This also
allows the ECC81 to be used with-
out difficulty. Input and output sig-
nals can be found at points A-D.
These voltages are used to drive an
indicator, for example using the cir-
cuit shown using op-amps IC3.A, B,
(010018-1)
10/2001
Elektor Electronics
25
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