n
AUDIO
OUT
AUDIO OUT
L
R
By Jake Rothman
Theremin Audio Amplifier – Part 1
Pesky presets
The Bootstrap Amplifier – an ideal partner for the PE Theremin.
T
his design was first presented
as part of the PE Theremin project
in August 2019. Back then, it was
only a circuit diagram (Fig.34 p.54) and
built on a prototyping board. Now that
our PCB designer, Mike Grindle has
done a board for it, it makes a useful
project in its own right. It has a low
total quiescent (idle) current of 3mA,
which allows it to be used with a cheap
Fig.1. For long battery life using a 9V PP3;
a high impedance (25Ω) speaker is used.
Practical Electronics | November | 2020
IC designers have the great advantage
that all their transistors are on the same
piece of silicon made in one go, giving
inherent matching. They can also use as
many transistors as they want for current
sources, mirrors and bias circuits. This
means few or even no adjustments are
required. With most discrete circuits,
the necessary adjustments are accomplished by preset resistors or ‘Trimpots’
(a trademark popularised by my favourite pot-maker, Bourns). Presets are a
contentious point. There’s no doubt that
open ‘skeleton’ presets (shown in Fig.3)
become unreliable as dirt and oxidation
take their toll. The Art of Electronics says,
‘don’t use them, design your circuit properly’. A lecturer of mine in 1983, told
me ‘use circuit analysis, don’t just put
trimmers in’. Unfortunately, I’ve always
loved ‘knob-twiddling’, getting that sweetspot of tuning, minimum distortion or
symmetrical clipping. For me, the art of
electronic music technology has always
PP3 9V zinc-carbon battery using a
high-impedance 25Ω speaker (Fig.1).
The amplifier is ideal for a portable
theremin, radio and teaching, hence
all the detail I’ll be providing. It gives
275mW and the current consumption is
50mA on peaks of full volume.
Discrete design
Most of my commercial theremin designs
have used the LM386, TB820 and LM384
power amplifier chips (Fig.2) for ease of
production. They do have some disadvantages, however, mainly a high fixed
quiescent current, resulting in shorter battery life. They also have excessive voltage
gain and multiple input transistors, resulting in a significant hiss at minimum
volume, which is a disadvantage when
recording. The advantage of a discrete
design (using individual transistors) is
that every parameter can be optimised
by hand. This approach is rarely allowed
into a commercial environment, but it’s
perfect for education, fun and for those
with a little time on their hands.
Fig.2. Small power amplifier chips such
as this 8-pin DIL LM386 have longreplaced discrete designs in commercial
electronics. However, everyone should
make a small discrete circuit before being
unleashed on expensive Hi-Fi designs.
This is the classic National Semiconductor
LM386, as used in my Eclipse Theremin
(see: http://theremin.co.uk).
63
Fig.4. Bourns-style sealed presets are
reliable, but can cost more than a poweramplifier chip.
Fig.3. Open presets are a component to
avoid if long term reliability is wanted. The
Art of Electronics simply says, ‘no’.
V +
+
R
C
× 1
D C
ca
A u d io
in p u t
been the feedback loop of human senses
controlling electronic variables.
Apart from SMT devices and chips,
component price inflation has now taken off. This is especially true where
there has been a reduction in demand
in analogue parts, for example JFETs or
mechanical latching switches. Presets
are no exception, the standard Bourns
3329H TO5 part, shown in Fig.4, has risen from around 20p to almost £1 since
2000. My ‘new old stock’ and the Chinese
Suntan and Truohm copies from Rapid
cover this problem. Note that there are
two different physical outlines for different shapes of presets available on the
board. Anyway, enough moaning, this
circuit provides the experience of preset
adjustments, which makes it worth it.
B u ffe r a m p lifie r
( e g , e m itte r - fo llo w e r )
b l o cki n g
p a ci t o r
A u d io
o u tp u t
( B i a si n g
a rra n g e m e n ts
o m itte d )
0 V
Fig.5. Bootstrapping employed on a
common-emitter amplifier. This greatly
increases voltage gain by making Rc
appear much bigger than it is.
V +
A u d io
in p u t
Bootstrapping, an old but
useful trick
A u d io
o u tp u t
R
+
V o lta g e fo llo w e r
0 V
Fig.6. Bootstrapping an emitter-follower
increases output by eliminating the loading
effect of R. Multiple bootstraps can
increase the output voltage swing in small
power amplifiers by a couple of volts.
V +
V +
I
C u rre n t
so u r ce
A u d io
in p u t
A u d io
o u tp u t
0 V
In the days when transistors were expensive and a lot of electronics ran on cheap
low-power zinc-carbon batteries, bootstrapping was widely employed to get
extra open-loop gain and voltage swing.
This amp uses it in two locations and the
PCB is labelled ‘bootstrap amp’, which
was its working title during development. The name comes from the phrase
‘pulling oneself up by one’s bootstraps’.
In electronic terms, this means pulling
up the voltage of one end of a component (usually a resistor) while the other
B o o t st r a p
A u d io
o u tp u t
A u d io
in p u t
I
C u rre n t
si n k
V b e
b ia s
A u d io
in p u t
In p u t
r e si st o r
T R 1
T R 2
C o m m o n e m itte r
vo l t a g e
a m p lifie r
E m itte r
fo llo w e r
0 V
Fig.7. Current sources and sinks give the same gain
and current boosting effect of bootstrapping, maintain
the action down to DC and avoid the use of electrolytic
capacitors. However, there is a voltage loss of 1 to 2V.
64
end is being driven in the same direction. The effect of this is to reduce the
current through the resistor, making its
apparent impedance much higher. The
upper end has to be driven by a buffered
voltage source, such as an emitter follower, since there has to be an additional
energy source. A single transistor can’t
bootstrap itself. The technique applied
to a common-emitter amp is shown in
Fig.5. It can also be applied to an emitter follower as well, as shown in Fig.6.
The impedance boost effect can also be
provided by replacing the resistor with
a current source, which is used in most
chip amplifiers. However, bootstrapping provides an additional advantage,
it increases the effective supply voltage,
boosting the maximum output swing,
which is very useful for battery-powered
circuits. Conversely, current sources and
sinks reduce the available output swing
by around 1.5V, which is needed for its
own operation (see Fig.7). A bootstrapped
amplifier will give a few volts more than
a current-source amplifier.
The disadvantage is that bootstraps
generally have to be AC coupled with
large-value capacitors, typically 1 to
220µF to get the voltage boosting effect.
These capacitors operate in a way similar
to the capacitors in a voltage-doubling
circuit, where the capacitor’s voltage is
added to the supply voltage. Electrolytic capacitors have to be used, which
are cheap, but they are large and dry-up
over time. If solid-aluminium and tantalum capacitors are used, the cost is five
times more.
Bootstrapping is out of favour in modern
Hi-Fi amplifiers because the effect does
not operate down to DC, giving an increase
in distortion as the frequency decreases.
Another problem is recovery from bursts
of clipping, where the bias point can shift,
although anyone who clips a Hi-Fi amp
must be driving it too hard. Of course, it
is possible to bootstrap a current source/
sink to get both advantages. This was done
in the TBA820 amplifier chip. Inductor
loading is another way and is the most
efficient of all, but audio inductors are
expensive and difficult to obtain.
F e e d b a ck
A u d io
o u tp u t
T R 4
T R 3
T R 5
B o o t st r a p
P u sh - p u l l
co m p l e m e n t a r y
e m itte r fo llo w e r
r e si st o r
Fig.8. Block diagram of the Bootstrap Amplifier. Note the class-A emitterfollower TR2 driving the output stage and the bootstrap for TR1.
Practical Electronics | November | 2020
C 4
+
1 0 0 µ F
1 6 V
R 5
3.3kΩ
C 3
6 .8 µ F
6 V
+
T R 2
B C 5 4 9 C
C 1 0
8 .2 p F
+
C 1
4 7 0 n F
6 V
T a n t
R 1
12kΩ
+ 5 .4 V
R 7
1kΩ
T R 3
B C 5 4 9 C
D 2
R e d
R 2
100kΩ
R 1 4
10kΩ
D 1
B A T 8 5
V R 2
5kΩ
R 3
270kΩ
C 9
1 5 p F
I q se
R 4
150Ω
V +
9 V
*LS1: 25Ω/35Ω delivering
275mW/200mW
T R 4
B C 3 3 7 -4 0
+
C 1 1
1 0 µ F
6 V
R 1 2
1Ω
t
R 8
1kΩ
C 2
2 2 µ F
6 V
R 9
620Ω
T R 5
B C 3 2 7 -4 0
R 1 1
1Ω
+ 4 .2 V
C 7
2 2 0 µ F
1 0 V
R 1 3
22Ω
C 6
2 2 n F
L S 1 *
+
+
Iq = 3 .6 m A
+ 4 .8 V
T R 1
B C 5 4 9 C
C lip
A u d io
in p u t
C 8
+
4 7 0 µ F
1 6 V
+
R 6
3.3kΩ
R 1 0
68Ω
provides bootstrapping drive to the
VAS load resistor to increase the openloop gain. A class-A follower is used
because there is no point in using the
output follower, as done in many designs, since it is class B. There would
then be no bootstrap action around the
crossover point, which is just where it
is needed to reduce this nasty sounding distortion. To improve efficiency
for battery operation, TR2’s load resistor is bootstrapped by C5. TR3 is the
usual Vbe-multiplier stage to bias the
output stage into class AB. Capacitors
C6, C9 and C10 prevent high-frequency oscillations. The voltage gain is 22,
half the normal minimum for IC amplifiers. The full circuit is shown in Fig.9.
Circuit overload
An overload indicator is provided
which indicates when clipping occurs.
LED1 turns on when the driver voltage
0 V
at the collector of TR1 exceeds that of
the output by 1.85V, which occurs at
clipping. It can also provide a soft clip
Fig.9. The full circuit of the Bootstrap Amplifier. Five transistors are about the minimum for a
on the positive output cycle if the sesonically acceptable performance with low current consumption.
ries Schottky diode D1 is shorted out.
The rounded waveform top sounds nice,
Circuit design
as shown in Fig.8. The difference is
like a fuzzy guitar practice amp.
that an emitter-follower stage (TR2) is
The circuit follows a standard invertinserted between the voltage amplifiing power-amplifier pattern of input
er and output. This increases the drive
voltage amplifier stage (VAS) comNext month
current to the output stage and reducmon-emitter stage (TR1) and push-pull
That’s a nice overview of the Bootstrap
es the load on the VAS. This stage also
emitter-follower output (TR4 and TR5),
Amplifier – next month, we’ll build it.
V R 1 : D C
m id - p o in t
a d j u st
V R 1
1kΩ
C 5
2 2 µ F
1 0 V
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