AUDIO
OUT
AUDIO OUT
L
R
By Jake Rothman
Theremin Audio Amplifier – Part 3
L
ast month, we completed a
silicon-transistor-based amplifier
fo the PE Theremin, but I concluded
the article with a promise to look at
a germanium-transistor version. This
actually turned into quite a fascinating
project, as you will see.
Lost values
When I get a break from eyeball-stressing
SMT (surface-mount technology) industrial jobs, it’s a relief to get back to some
old-fashioned electronics. I recently got
really fed up with unmarked SMT components. It started with capacitors, but
now even resistors are being supplied
unmarked, as shown in Fig.1. This makes
fault finding on pre-production prototypes very difficult. When the unit fails
a few years later it can’t be fixed without full service data, and who supplies
that today?
delivery of components rendered obsolete by RoHS directives, my childhood
electronics excitement came back. Behold! – there were unopened packs of
1974 Siemens germanium transistors.
There was also a bundle of Siemens
books, including one called Design Examples of Semiconductor Circuits, from
1969. The section titled, ‘Audio-frequency Amplifiers’ on p.7 compared the use
of germanium output transistors with
the then relatively new silicon planar
transistors. It said the germanium types
were superior for low voltage, which
is to be expected, since their turn-on
voltage is around 0.1V as opposed to silicon’s 0.6V. What intrigued me, however,
was the statement that they had better
linearity, which should mean lower
distortion. The amplifier designer and
Bradford University academic Arthur
Bailey also said the same in his Wireless
World article, ‘High Performance Transistor Amplifier’ (see p.543, November
1966). I’m always a bit wary about ‘linearity’ and other spurious audio claims,
but there is only one way to be sure –
build and compare.
On pages 14 and 15, the Siemens book
had two almost identical four-transistor
amplifier circuits for a 9V supply and 8Ω
speakers, see Fig.3. The main difference
was one used the germanium output transistors (AC153K and AC176K) in the box
and the other used silicon (BC140 and
BC160). Biasing requirements accounted for the other circuit differences. The
claimed output power was 1.1W for the
germanium and 0.7W for the silicon.
This sort of germanium amplifier was
used in Robert’s Radios, such as the R505,
until the early 1970s. They still sound good
today – and they don’t eat batteries. Maybe their designers were onto something.
Nice old junk
Sifting through some musty boxes (Fig.2)
courtesy of yet another lock-down
Fig.1. It started with ceramic chip
capacitors. Now, even some resistors
have no markings, such as these Vishay
CRCW080522K0FKEA resistors from RS.
Just black and blue rectangles (R84 and
R15). This is what I have to put up with in
my professional life – I am a human, not a
pick-and-place machine!
46
Fig.2. A good dollop of old ‘proper’ components with wires and markings. Siemens
germanium output transistors and Philips ‘Minipoco’ 1% tolerance lead-foil polystyrene
capacitors. Still 1% accurate after 35 years.
Practical Electronics | January | 2021
A udio
input
– 9 V
Note: A C 153K and A C 17 6 K transistors are
germ anium , the rest are silicon.
Ω
0Ω
O utput 1. 1W
15m A total I q
Max supply
current 19 0 m A
A C 153K
0Ω
Ω
+
B A 10 3 or
1N4 14 8
. kΩ
0kΩ
1µ F
50 0 µ F
0. Ω
NT C
1 0Ω
0. Ω
10Ω
B C 16 8
330 pF
+
n
Comparison circuit
Fig.3. The Siemens
recommended circuit
for their transistors.
It’s very efficient but
needs matched
output devices. Also,
the signal reference
0V and speaker
reference 0V are
different, which gives
poor power
supply rejection
ratio. We won’t
build these.
A C 17 6 K
10 µ F +
100kΩ
B C 258
1. kΩ
+
250 µ F
0 V
kΩ
10 µ F
+ 9 V
+
Ω
0Ω
Note: all transistors
are silicon.
0Ω
B C 14 0
3x
+
O utput 0 . 7 W
22m A total I q
Max supply
current 150 m A
50 0 µ F
Power supply
Since germanium NPN transistors are
rare, the circuit was inverted to use a
negative power rail and positive earth.
This way only one NPN transistor was
needed, the rest are PNP devices, normal
Ico = grounded em itter
leaka ge current
0 . 15m A <at> 25° C , typically
doub les ev ery 5° C
1Ω
B A 10 3
To perform a practical germanium vs
silicon comparison, I decided to use
the PE Theremin Amplifier circuit and
PCB from November/December 2020’s
Audio Out. I installed transistor sockets
on the board – always a good idea when
trying unusual parts. I used cut strips of
SIL turned-pin IC sockets for this (see
Fig.4), since real transistor sockets are
expensive. I decided to go for a standard
9V, 8Ω set-up to see if I could replicate
the Siemens amplifiers’ results.
There is no point in trying to go for a
micro-power design with germanium,
since their high leakage currents prohibit this. When experimenting with
germanium transistors it is important
to check the leakage current (I cbo) of
old stock devices. This is the current
that flows between collector and emitter with the base open-circuit. I use a
Peak DCA75 Analyser for this (Fig.5),
but a 5V power source and multimeter,
as shown in Fig.6, will also do the job.
. kΩ
0kΩ
A udio
input
1µ F
2. 2nF
+
250 µ F
100kΩ
4 . 5V
O C 7 1
( typical type)
+
B C 10 8
B C 16 0
B ase lef t open
hence the ‘ o’ in Ico
3. 3nF
1. kΩ
–
mA
1Ω
B C 17 8
+
. kΩ
0 V
10Ω
0 V
Fig.6. Circuit for checking leakage current.
Fig.4. When trying out unusual transistors, use SIL sockets for easy
substitution during R&D.
Practical Electronics | January | 2021
Fig.5. It is important to check the leakage currents of
germanium transistors. Anything above 0.25mA is suspect,
except for power types, where the limit is 2mA.
47
1. 7 m A
R 5
1. kΩ
R 6
1. kΩ
C 4
4 7 0 µ F
16 V +
C 3
22µ F
10 V
+
– 5. 4 V
C lip
R 14
Not
used
T R 4 *
B C 14 3
* T R 4 / 5 with T O 5
clip- on heatsink s
7 20 m W
output
R 7
1kΩ
T R 3
B C 327
+
D1
B A T 8 6
VR 2
kΩ
R 3
100kΩ
C 11
10 µ F
6 V
R 11
0. Ω
R 12
0. Ω
C 7
4 7 0 µ F
10 V
– 4 . 2V
+
R 2
47kΩ
R 13
10Ω
I q set
C 9
Not
used
R 4
47Ω
VR 1
00Ω
VR 1: DC
m id- point
adj ust
R 8
1 0Ω
C 2
4 7 µ F
6 V
R 9
47Ω
+
– 9 V
– 4 .8 V
T R 1
B C 557 B
D2
R ed
A udio
input
I q = 29 m A
Ω
C 6
10 0 nF
T R 5*
B C 138
+
+
R 1
4.7kΩ
C 8
10 0 0 µ F
16 V
+
10 m A
T R 2
B C 557 B
C 10
Not used
C 1
10 µ F
10 V
T ant
R 10
10Ω
C 5
10 0 µ F
10 V
0 V
P ositive earth
Fig.7. To start the development of the germanium amplifier, a silicon amplifier was
developed using a negative power rail with positive earth. This way, only one NPN
device was needed, with the rest being PNP types, paving the way for step-by-step
substitution with germanium devices.
creating your own ‘Robert’s Radio sound’
then I’ve even got some old alnico magnet Celestion 6×4-inch speakers for sale).
for germanium. Of course, when doing
this ‘mirroring’, all the electrolytic capacitor and diodes have to be reversed
too. This is no problem with the silicon
circuit, since the transistors cost peanuts, whatever the polarity. Later I’ll
use this positive-earth amplifier design
with some germanium fuzz circuits and
Mullard LP1171/69 radio modules, as
used in the Robert’s R600. (If you fancy
e b
underside
c
T O 5 B C 138 / 4 0 underside R 6
pin vi ew
1. kΩ
e b c
( case)
+
C 1
10 µ F
10 V
T ant
R 1
4.7kΩ
C 4
+
4 7 0 µ F
16 V
1. 7 m A
R 5
1. kΩ
+
R 14
Not
used
C 9
Not
used
VR 2
kΩ
+
C 11
10 µ F
6 V
R 12
0. Ω
+
C 8
10 0 0 µ F
16 V
+
+ 9 V
C lips at 19 0 m A
supply current
* T R 4 / 5 with T O 5
clip- on heatsink s
R 11
0. Ω
+ 4 . 2V
7 20 m W
output
C 7
4 7 0 µ F
10
R 13
10Ω
C 6
10 0 nF
Ω
+
VR 1
00Ω
R 8
1 0Ω
C 2
4 7 µ F
6 V
R 9
47Ω
T R 5*
B C 14 3
I q = 29 m A
S tandard silicon am plif ier
with negative earth
I q set
R 4
47Ω
VR 1: DC
m id- point
adj ust
R 7
1kΩ
T R 3
B C 337
D1
B A T 8 6
R 3
100kΩ
T R 4 *
B C 138
+ 4 .8 V
T R 1
B C 54 9 C
C lip
R 2
47kΩ
10 m A
T R 2
B C 54 9 C
D2
R ed
A udio
input
R 10
10Ω
C 3
22µ F
? ? V
C 10
Not used
+ 5. 4 V
The first task is to get the silicon version
working. That way any transistor blowups will cost 5p rather than possibly £1.
To do this, we’ll take the original circuit
(Fig.9, p.65, PE November 2020) and
+
T O 9 2 B C 54 9 C
pin vi ew
Germanium gestation
C 5
10 0 µ F
10 V
0 V
Fig.8. The silicon circuit with standard negative earth, a useful medium-power amplifier.
48
scale up the currents by a factor of three
for the lower speaker impedance of 8Ω,
rather than the original 25Ω. This entails
dividing most resistor values by three.
Capacitors will have to be increased
by three to take into account the lower impedances. Finally, the power rails
and all other polarised components are
flipped. There are then resistor tweaks
to bring the biasing into line. This new
circuit is shown in Fig.7.
An interesting high-frequency instability occurred where the frequency
of oscillation was so high my 40MHz
‘scope couldn’t see it, but it was enough
to completely mess-up the DC conditions and drive one output transistor to
partly cut off. The clue was when I put
my finger near R6 and the problem just
cured itself, a sign of VHF oscillations.
Ironically, removing stabilising capacitor C10 fixed it. When the new silicon
circuit was rebuilt using a standard positive rail (Fig.8) the instability vanished
and there was no problem with C10. I
have heard that there is more likely to be
problems with silicon PNPs; they have
more junction capacitance modulation
(Early effect) than NPNs.
The germanium transistors were then
gradually plugged in, checking along
the way with a multimeter, ‘scope and
signal generator. The only significant
changes were to the DC biasing, with
R2 being changed from 47kΩ to 15kΩ.
Of course, the output current bias (Vbe
multiplier) transistor has to be germanium if the output devices are. This can
be an AC153 or a special low-voltage
transistor designed/selected especially for the job, such as an AC169. Using
an NKT214 didn’t work because the Iq
could not be turned down to zero. Do
note that some AC169s only have two
wires, they are essentially just a 0.13V
diode. The metal-cased three-wire ones
have to be checked on a transistor tester
to determine the connections.
Germanium-alloy junction transistors are generally ten-times slower than
silicon planar types, giving inferior
square-wave and high-frequency response. With TR1 and TR2 (in Fig.9)
being 2MHz-Ft (frequency at which gain
falls to unity) NKT214 audio types, the
–3dB point was about 15kHz, fine for
guitar and AM radio. The square-wave
response at 1kHz 2V pk-pk output had
overshoot on it from TR2. Connecting a
33pF capacitor (C10) removed this. The
high-frequency response was brought up
to Hi-Fi (40kHz bandwidth) standards
by putting in germanium 2SA12 RF
transistors, as used in AM radio oscillators and leaving C10 out. These have
an Ft of around 10MHz. Finally, we had
a germanium amplifier with good performance. The circuit is shown in Fig.9.
Practical Electronics | January | 2021
A C 17 6 K / A C 153K , X O 4 packa ge
R ed dot
( NK T 214 only)
b
Mounting hole
e
c
underside
e
T O 1 packa ge
NK T 214 / 2S A 12/ A C 153
c
b
1. 7 m A
R 5
1. kΩ
17 m A
R 6
1. kΩ
* Use C 10
with NK T 214
f or T R 2
R 1
4.7kΩ
VR 1
00Ω
+
– 9 V
C lips at 19 0 m A
supply current
www.poscope.com/epe
Germ anium am plif ier
with positive earth
8 8 0 m W
output
R 7
C lip
0Ω
T R 3
A C 16 9
+
VR 2
kΩ
C 11
10 µ F
6 V
R 11
0. Ω
R 12
0. Ω
– 4 . 3V
R 8
1 0Ω
C 2
4 7 µ F
6 V
R 9
47Ω
T R 5
A C 17 6 K
on heatsink
C 7
4 7 0 µ F
10 V
R 13
10Ω
I q set
R 4
47Ω
VR 1: DC
m id- point
adj ust
3m A
D1
C G9 2
or
O A 9 1
R 3
100kΩ
C 9
Not
used
T R 4
A C 153K
on heatsink
I q = 22m A
+
R 2
1 kΩ
T R 2
NK T 214 or
2S A 12
C 8
10 0 0 µ F
16 V
+
– 4 . 5V
D2
5. 1V
A udio
input
R 14
Not
used
T R 1
NK T 214
or
2S A 12
R 10
10Ω
C 3
22µ F
10 V
C 10 *
33pF
– 4 .7 V
C 4
4 7 0 µ F
16 V +
C 6
10 0 nF
Ω
+
+
C 1
10 µ F
10 V
T ant
+
c
I ndent
C 5
10 0 µ F
10 V
0 V
P ositive earth
Fig.9. The final circuit using germanium transistors.
Output transistor selection
For the silicon amplifier, standard TO5
metal-can devices were used; in this case
BC138 and BC143 – just because they
were in the drawer. They can comfortably deliver output powers up to 2W with
standard clip-on heatsinks. The germanium equivalents are the AC153K (PNP)
and AC176K (NPN) shown in Fig10. The
type with the K suffix was used because
they have a convenient hole to mount
on a metal bracket for heatsinking. The
XO4 and TO1 germanium cases are electrically isolated, the heat passing from
the junction to the case via a white paste
filling of aluminium oxide and silicone
grease. There are few germanium complementary pairs available. Alternatives
are AC128/176 and AC188/187. For higher powers up to 6W the AD161/162 are
often used (Fig.11). These TO66 devices are the germanium equivalent of the
silicon TO126 BD135 and BD136 types.
Next month
That’s all for this month. In Part 4, we’ll
finish with the component options and
examine the frequency responses.
Fig.10. The Siemens AC176K and
AC153K complementary output
transistors. These are still used today
in the very expensive EMS VCS3
synthesiser for their ‘musical’ sound.
Practical Electronics | January | 2021
Fig.11. The Mullard AD161/2 output
transistors (right). Very popular in the
1970s. Robert’s used them in their RM50
table radio along with the Celestion
speaker in Fig.18. Note their small TO66
case compared to the standard TO3 (left).
- USB
- Ethernet
- Web server
- Modbus
- CNC (Mach3/4)
- IO
- PWM
- Encoders
- LCD
- Analog inputs
- Compact PLC
- up to 256
- up to 32
microsteps
microsteps
- 50 V / 6 A
- 30 V / 2.5 A
- USB configuration
- Isolated
PoScope Mega1+
PoScope Mega50
- up to 50MS/s
- resolution up to 12bit
- Lowest power consumption
- Smallest and lightest
- 7 in 1: Oscilloscope, FFT, X/Y,
Recorder, Logic Analyzer, Protocol
decoder, Signal generator
49
