AUDIO
OUT
AUDIO OUT
L
R
By Jake Rothman
Low-noise germanium transistor power supply
Positive earthing
Setting up the LM317 voltage regulator to
give plus 9V was described last month.
One way to get a negative 9V rail would
be to use the LM337 negative regulator.
This would require a PCB redesign changing the regulator’s pin-out and reversing
all the polarised components. Assuming
the power supply is only going to power
positive-earth devices, a much simpler
method is to connect the mains earth to
the 9V positive output, redefining this
rail as 0V, as shown in Fig.3. The old
negative rail now becomes minus 9V and
the positive rail is earth. This is akin to
changing the battery round in a car to
make it positive earth rather than negative. Some early (1964) VW Beetles had
6V positive-earth electrics, where the
metal work was positive.
Fig.1. A Bush TR91 radio from 1961, a year older than me. It has seven PNP germanium
transistors and is positive earth. The power supply replaced the expensive (£4.45) PP9
battery. It’s still going strong, beautifully playing the Shipping Forecast.
L
ast month’s theremin power
supply board can easily be
adapted to make a power unit
for old germanium technology, such as
Colorsound fuzz pedals, Deacy guitar
amps and 1960s Bush radios (illustrated
in Fig.1). These were described in my
Noise reduction
Ge-mania series of articles in EPE/PE
from April 2015.
A good 95% of germanium transistors
are PNP types, such as the OC44 shown
in Fig.2; thus the circuits are normally
positive earth with a negative supply
rail. Most circuits are also 9V battery
powered with a low current consumption of around 120mA max. Germanium
circuits were generally simple due to the
relatively high cost of early transistors,
which resulted in poor power-supply
rejection ratio. This means a low-noise
power supply is essential.
Last month, we talked about using capacitors with a damping resistance (a
Zobel network) to reduce rectifier-induced ringing and spikes. These occur
when the transformer is unloaded for
the period during which the rectifier
diodes are turned off, and is thus free
to ring for this period. The old Roberts
RM30 radio in Fig.4 damped the ringing
by having an extra transformer winding supplying a 6V filament lamp for
illuminating the tuning scale. A nice
1970s trick of providing two solutions
with one addition.
R ectif ier
9V regulator
Transf ormer
L
Fig.2. The Mullard OC44, regarded as the
best transistor for fuzz boxes and also
used in the mixer oscillator stage of the
above radio. These are now an expensive
antique, treat them to a decent power
supply and remember to get the polarity
right. The Texas 2G306 and Toshiba
2SA12,44,53 are good alternatives.
68
–
+
15
Vin
+
Mains
input
Adj
N
Mains earthing
LM317
S moothing
capacitor
Vout
0V reference
ground rail
O utput
Negative 9V
pow er rail
E
Fig.3. Redefining the 0V rail: connect the regulator’s positive output terminal to mains earth.
Practical Electronics | October | 2020
Construction
Only one rail is constructed on the board,
so the Zobel components can easily be
fitted with a bit of jiggery-pokery in the
vacated space and pads shown in Fig.6.
The full overlay is shown in Fig.7. Note
the earth link across pad A to what was
the positive rail. This is also shown in
a photo of the board in Fig.8.
Putting resistors in series with capacitors can be tricky with radial types, as
shown in Fig.9. However, it’s easy with
a vertically mounted axial component
which then becomes a ‘composite radial’ component, dropping into the existing
pads. This is shown in Fig.10. Note that
Cz is quite tall, so it’s a good idea to insulate the vertical wires with sleeving, as
illustrated in Fig.11. This is good practice
with all vertically mounted axial components to prevent short circuits. Remember
to insulate all mains wiring, including the
tracks under the board, to avoid shocks.
n
Fig.4. Old Roberts RM30 radio from the 70s, still going strong. In this example the tuning
scale lamp damps the mains transformer. I only noticed this when the PSU hash level
rose when the bulb failed.
The surprising benefit of significant
capacitor equivalent series resistance
(ESR) for regulator transient load stability was also described last month. I
saw the effects of this when an old 1980s
Harrison mixing desk was ‘recapped’ by
a self-proclaimed audiophile ‘guru’ at
huge expense. The replacement of numerous tantalum capacitors by low-ESR
electrolytic types resulted in a myriad
of instabilities.
Circuit overview
The power supply circuit we will build
is shown in Fig.5. The main difference
with the versions discussed last month
is that the mains earth link has been connected to the positive output pin of the
regulator, and not to what would normally
Mains
earth
Parts list
be the 0V rail. Electrically speaking, it is
still just a voltage source – but reversed
relative to ‘0V’.
The Zobel network (Cz and Rz) is
connected across the unused secondary
winding of the transformer. The output
capacitor C12 is shown as a ‘perfect’ capacitor, but with its ESR in series. A 47µF
cheap electrolytic will work. Alternatively, a high-cost MnO2 solid-aluminium
capacitor will last forever. If you want
the over-engineered solution for lowest
noise, use a 470µF low-ESR polymer
solid-aluminium type with a real series
resistance of 0.56Ω (see Fig.9). Cheap ‘rubber-bung’ electrolytic capacitors have
short lives. The expensive capacitors
are only required if a 20-year life for the
power supply is expected.
Note: many component positions on the
PCB are unused, so the part numbering is
not consecutive. The PCB and transformer are available from the PE PCB Sevice.
Resistors (All 0.25W except R1)
R1
1kΩ 0.5W
R2
750Ω
R3
120Ω
Rsurge 10Ω
Rz
2.2Ω
Capacitors
C1
10nF ceramic X7R 5mm
C2-5 100nF ceramic X7R 5mm
C10 1500µF or 2200µF 25V radial
electrolytic
C11* 15µF 20V axial tantalum in series with Rsurge (or 100uF 16V
electrolytic)
R1
1kΩ
0.5W
Rz
Earth link
E
D9
1N4001
C1
10nF
D1-4: 1N5817
C2-5: 100nF, X7R
C2
Thermal
cut out 127°C
–
L
Vin
C3
D1 D2
D3 D4
+15V
+
11V
C4
Mains input
230-240V
C5
IC1
LM317
Adj
+
A
C10
2200µF
25V
Vout
R3
120Ω
0.25W
18V
N
T1
Zobel
CZ
4.70µF
100V
Film
0V
D10
1N4148
Rsurge
10Ω
R2
750Ω
+
RZ
2.2Ω
Pad 4
4
+
C11
15µF
20V
Tant
* C12 can also be a 470µF polymer
capacitor in series with a 0.56Ω resistor.
C12*
47µF
16V
1.3Ω ESR
Pad 6
–9V
Pad 7
Fig.5. Full power supply circuit – note the resistor in series with the output capacitor (C12). This
resistance can be a parasitic (within the component) or added externally.
Practical Electronics | October | 2020
Cz
Pad 5
Fig.6. The Zobel network
is mounted in the holes
shown. Rz is placed in D5’s
position and Cz goes from
the left-hand hole of C9 to
the right-hand hole of D7.
69
Mains
input
D9
D10
C2
L
Thermal cutout
C3
D2
C4
Secondary
Primary
N
T1
E
17V
AC
R
1
Rsurge*
D1
IC1
+
D3 C10
C5
D4
Rz
A
0V
C12*
+ C11*
+
R3
–V 9V
R2
Output
B
Cz*
C1
*Mounted vertically
Fig.7. Overlay of the positive-earth power supply.
Link
C
C12* 47µF 16V 123 series solid aluminium (or 100µF 16V electrolytic)
Cz* 4.7µF 160V axial polycarbonate
or other non-polarised capacitor
up to 10µF
*Available from me: 01597 829102
jacob.rothman<at>hotmail.com
Semiconductors
D1-4 1N5817 Schottky diodes 20V 1A
D9
1N4001
D10 1N4148
IC1
LM317T 1.5A voltage regulator
Miscellaneous
Transformer: see last month (or use a
10 to 12V dual 200mA 6VA equivalent)
1mm sleeving for vertical components.
Silicone is best because it doesn’t melt
with soldering.
Thermal cut-out (if plastic case used)
not essential for a die-cast metal box.
250mA time-delay fuse plus holder.
3-pin IEC mains connector
DC power connector 2.1mm. (See
Techno Talk last month for a detailed
description of these connectors).
Installation
If putting the power supply in an old radio,
make sure it is away from the ferrite rod
aerial and volume control to avoid hum
pick-up. I just wire PP9 battery connectors to the power supply, so it can simply
take the place of the battery and be unclipped if needed. This is a good idea if
the equipment is highly collectable.
Unlucky C13
Fig.8. The completed board – view with Fig.11 to get the whole picture.
Fig.9. Putting components in
series for axial components can
be tricky. Here’s one trick I use
to add extra series resistance to
solid polymer capacitors (such as
C12) for damping. In this case, the
resistance is split into two resistors,
0.39Ω and 0.22Ω, to give a total of
0.61Ω. The capacitor’s actual ESR
of 0.01Ω is just too good!
70
Fig.10. It’s simple to put two axial
components in series to make a
‘composite radial’ component.
This is a tantalum capacitor in
series with a surge protection
resistor, as used for C11.
I’m afraid a gremlin crept into last months
PCB overlays: Figs 23, 25 and 28. There
are two C13s. The smaller one is C11.
Fig.11. I used to avoid vertical mounting due to track
breakage in the days of single-sided boards, but platedthrough-hole boards are capable of withstanding the
mechanical stress. It’s good practice though to insulate the
exposed vertical wires with sleeving to prevent shorts. Note
the green/yellow earth reference link from point A (groundlifted mains earth) to the old positive output terminal.
Practical Electronics | October | 2020
