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Restoring the QUAD 303
Power Amplifier
and Preamplifier
A vintage hifi article by Jim Greig
The QUAD 303 amplifier and associated QUAD 33 preamplifier (that they call a “control
unit”) were introduced in 1967 and sold until 1985. These units belong to fellow HRSA
member Ray Thomas. I was very happy to exchange some time refurbishing for a chance
to listen to a classic QUAD amplifier.
T
he specifications of this equipment
are ordinary by today’s standards
but compare very well with the valve
amplifiers of the time.
While the output impedance is specified as the emitter resistance (0.3W),
the negative feedback across these
resistors will reduce it. However, the
output filter and series capacitor will
increase it somewhat. Power to the
amplifier is from a single-ended, regulated 67V supply.
Amplifier circuitry
The amplifier circuit is broadly similar to a modern ‘blameless’ amplifier
circuit in many ways, with a complementary emitter-follower output buffer
and a ‘voltage amplification stage’ or
VAS based on NPN transistor TR102
– see Fig.1. The main difference is in
the input stage and feedback system,
which doesn’t use the balanced, symmetrical two-transistor input that’s
common today.
The amplifier has quasi complementary symmetry output with transistor
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Silicon Chip
triples (TR104, TR106, TR2) to simulate a PNP transistor and provide a
linear (through local feedback R120)
transistor equivalent for both the PNP
and NPN ‘transistors’. With this technique, ordinary 2N3055 transistors
act as superior PNP and NPN devices.
Diodes MR105 (NPN) and MR106
(PNP) protect the output transistors
from overcurrent. On the NPN (upper)
side, when the current through R123
approaches 4.3A, MR105 conducts,
driving the base of TR103 more positive. As a result, the current through
it decreases, cutting off TR105 and
decreasing the drive to the output
transistor, TR1.
The VAS transistor, TR103, has a
quasi constant-current load based on
resistors R116 and R117 plus capacitor C106. The current available to the
base of TR103 would decrease as the
Any work on this unit should be done
with the mains disconnected, as there
are exposed mains connections when
the outside cover is removed.
Australia's electronics magazine
output moved towards the supply
voltage if not for C106. As the output
moves positive, C106 also takes the
junction of R116 and R117 positive,
ensuring there is voltage across R117
to drive TR103. This is known as bootstrapping.
The output signal from the junction
of R124 and R125 idles at half the supply voltage, so a coupling capacitor
(C1) is required for the speaker output. That is somewhat frowned upon
today as capacitors constitute a significant source of distortion. Still, it
simplifies the design and would have
resulted in a relatively large cost saving at the time.
The amplifier is stabilised with a
Zobel network (R128/C108) and series
filter (R129/L100). This must have
been a very early appearance of the
Zobel network in a hifi amplifier to
ensure a primarily resistive load to the
amplifier, regardless of loudspeaker
impedance changes with frequency.
The driver stages are DC-coupled
common-emitter singles where
siliconchip.com.au
Fig.1: one channel of the power
amplifier circuit from the QUAD
303 Power Amplifier Service Data
manual.
differential or long-tailed pairs would
be utilised today. Overall DC negative
feedback is through R113 and R108,
with R130, R110 and RV100 forming a voltage divider for setting the
output to half the supply voltage. AC
feedback follows the same path, but
the gain is limited by C104 shunting
R111 to ground.
The overall AC gain is 82kW/2.2kW
or 37 times. So 0.5V RMS at the input
is amplified to 18.5V RMS at the output, giving 43W into 8W.
Trimmers allow the output idle DC
voltage (RV100) and standby current
(RV101) to be adjusted. The standby/
quiescent current is set using the same
Vbe multiplier circuit still in use today,
based around TR107.
Power supply circuitry
The power supply (Fig.2) is interesting because the regulator is in the negative rail. The cans of filter capacitors
C2 and C3 must not touch ground. The
supply is referenced to zener diode
MR201 (16V). The zener and associated resistor R204 are connected across
the stable 67V output to keep the current through it constant, for a more
stable reference voltage.
A fraction of the output from the
divider formed by RV200, R202 and
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R203 is compared with the reference
voltage. TR200 and TR201 amplify the
difference; the result is applied to the
emitter-follower regulator, TR3. Trimmer RV200 adjusts the output voltage.
R201/MR200 ensure that TR200 is
conducting at switch-on, while R200
ensures that MR200 is back-biased and
not active during regular operation.
The power supply can be configured for 110/120/220/240V AC mains
supplies using the external selector
switch. A neon indicator glows when
power is applied.
The chassis
The amplifier is elegantly crafted
with a pressed steel chassis that has
the Power, Input and Output connectors on one end and a heatsink
on the other.
The two amplifier and power supply circuit boards fit across the bottom of the chassis, held in place with
QUAD 303 amplifier weighing 8.2kg:
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Power output: 2 × 45W into 8Ω
Frequency response: 30Hz to 35kHz +0,-1dB
Total harmonic distortion at 45W: 0.03% at 70Hz and 700 Hz; 0.1% at 10kHz
Output source impedance 0.3Ω (+ output capacitor & Zobel network reactance)
Hum and noise: 100dB below full output
Inter-channel crosstalk: better than -60dB from 30Hz to 10kHz
Input sensitivity: 0.5V RMS
Speaker load impedance: 4-25Ω
QUAD 33 preamplifier (“control unit”) weighing 3kg:
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Frequency response: 30Hz to 20kHz, ±0.5dB
Total harmonic distortion: 0.02%, 30Hz-10kHz at all controls level; 0.5V RMS out
Input sensitivity (RMS): 2mV (moving magnet), 100mV (ceramic), 100mV (line)
Signal-to-noise ratio: 70dB (moving magnet), 85dB (line)
Tone control: approximately ±16dB at 30Hz and 20kHz
Filter: flat to -20dB per octave at 5kHz, 7.5kHz and 10kHz
Inter-channel crosstalk: better than -40dB, 30Hz to 10kHz
Output level (RMS): 100mV (line), 0.5V (Pre out)
Australia's electronics magazine
January 2024 93
Fig.2: the regulated
DC power supply
circuit, again from
the QUAD 303
Power Amplifier
Service Data
manual.
plastic clips. These are easily opened
to allow the boards to be removed for
service, and still function without
breaking.
This layout is tidy but necessitates
long leads from the PCBs to the output transistors. They are neatly bundled and laced, giving the amplifier a
professional appearance (see Photo 1).
However, compared with today’s
short leads following similar paths, the
layout will limit performance. Still, we
are in 1967, where 0.1% distortion is
considered very good.
QUAD 33 preamplifier
The QUAD 33 preamplifier complements the appearance of the 303 amplifier. Appearance and construction are
clearly design inputs.
The preamplifier is built on a steel
chassis and implemented on five modules plugged into a passive motherboard and filter board (see Photos 2 &
4). The modules are the Disc Adaptor,
two Preamplifiers, Tape Adaptor and
Right/Left Hand Amplifier.
The Disc Adaptor provides matching for Low Output Magnet (M1), High
Output Magnetic (M2), Ceramic (C1)
or Spare (S1) inputs by connecting
different components in the preamplifier input and feedback paths. The
card that plugs in from the back of the
unit provides the four functions, as it
is square and can plug in one of four
ways – see Photo 3.
The preamplifier (one channel
shown in Fig.3) has two DC-coupled
BC109s with R313/R314, R310 and
R302 providing DC feedback to stabilise the operating point. RIAA equalisation is provided by the Disc Adaptor
using connector M2 and components
C104 and R110/C101 from the output
to the emitter of TR301.
Capacitor C308 connected to the
emitter of T301 ensures that the emitter side of R302 closely follows the
AC input to the base. The signal current through R302 is then minimal,
greatly increasing its apparent resistance (another form of bootstrapping).
The amplified disc signal may be
selected along with Radio 1, Radio 2
and Tape as inputs to the Tape Adaptor board. It has an emitter follower
stage, with the full output passed to
the volume control and jumper selectable full or partial output to the Tape
Record connector.
The outputs from the volume control feed the Left and Right Amplifiers,
which drive the tone controls, balance
control and filters. Fig.4 shows the
tone control circuit, with the output
level control circuitry at lower right.
The input stage is an emitter follower
driving the Baxandall tone control circuit. The output passes through the
filter network.
There is a top cut switch with -3db
points around 5kHz, 7.5kHz, or 10kHz
and a slope control (RV8), varying the
response from flat to a steep cut at the
selected frequency. The filters can be
independently set on/off, and a cancel switch bypasses the tone controls
and filters.
The power supply is a simple zener-
regulated 12V configuration. A second
supply connected to the output plug
Photo 1: the internals of
the QUAD 303 amplifier
are very neat, with
multiple modules built
on small PCBs, wired
together very neatly
with loomed wiring.
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Silicon Chip
Australia's electronics magazine
siliconchip.com.au
Fig.3: the preamplifier front-end circuit for one channel, an
extract from the QUAD 33 Control Unit Service Data manual.
The Disc Adaptor shown on the left can be inserted in one of four
ways, effectively acting like a four-way switch to select the S1,
C1, M1 or M2 connections to suit different signal sources.
is not used, other than to power the
indicator.
The QUAD 33’s construction is of the
same high standard as the 303 amplifier. For example, the balance control
is a standard potentiometer operated
by a mechanical link from a slider on
the front panel. The outer cover slides
off on rails attached to the chassis, and
the wiring is neatly loomed.
Restoration
These units are over 50 years old
and, if untouched, require attention.
Several companies offer “upgrade
kits” that contain new electrolytic
capacitors, resistors and transistors.
There are three versions of the power
amplifier, and the upgrade kit must
match your model.
I used a kit from Dada Electronics
(https://dadaelectronics.com.au &
https://dadaelectronics.eu) to restore
the amplifier, while I purchased parts
separately for the preamplifier.
The following steps are listed in the
Dada documentation for QUAD 303
power amplifiers with serial numbers
above 11500, to replace:
1. The two 2000μF filter and the
2000μF output coupling capacitors
with three 4700μF capacitors.
2. All electrolytic capacitors on the
power supply and amplifier boards
with new electrolytic capacitors (plus
one 0.68μF foil type).
3. All trimmer resistors.
4. All resistors on the power supply board.
5. Both diodes on the power supply board.
6. Some power supply cabling.
The existing wires from the power
transformer to the rectifier and on the
filter capacitors are solid 24 gauge
(about 0.5mm diameter & 0.25mm2).
They look good, neatly bent to follow
the components, but are inadequate by
today’s standards. So I replaced them
with 0.7mm diameter (0.5mm2) multicore cable.
Photo 2 (right): on the rear panel of the QUAD 33
preamplifier, you can see the Disc Adaptor
board that plugs in at lower right, the
Tape Adaptor card to its left, plus
the various inputs and output
connectors.
Photo 3 (above): the Disc Adaptor can be
plugged in on any of its four sides, setting the
preamp up for one of four different input signal types.
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Australia's electronics magazine
95
Fig.4: the tone control, output level
control and top-cut circuitry
plus the final amplification
stage of the preamplifier; another
extract from the QUAD 33 Control
Unit Service Data manual.
The power-on indicator (Photo 5) is
a neon bulb connected to the incoming
mains supply through a 100kW resistor. This may be faulty; the recommended replacement is a square LED
with a 12kW series resistor across the
DC power supply; still, a new neon
indicator could be used.
Along with the indicator, there are
other exposed mains conductors on
the fuse and voltage selector on the
rear panel, near the input connector,
so care must be taken to avoid contact
with them.
I unclipped the PCBs but did not
disconnect them from the cabling. I
replaced the components following
Photo 4: the
QUAD 33
preamplifier
internals are very
neatly organised
and laid out, with
highly organised
cable routing.
96
Silicon Chip
standard procedures, carefully observing the polarity of diodes and electrolytic capacitors. The PCBs are old phenolic types; care must be taken when
desoldering and soldering to avoid
lifting tracks.
For testing, the boards must be kept
away from the chassis to avoid short
circuits as the chassis is Earthed. As
the history of this unit was unknown,
I disconnected the positive rail from
both amplifier boards before power-on
and added a 3.3kW 5W resistor as a
load. I then used a variac to gradually
apply AC voltage while monitoring
the DC output volts.
The power supply’s regulated DC
output voltage increased slowly and
stabilised at around 70V DC. Adjusting the trimmer (RV200) reduced it to
the desired 67V. However, the voltage
decreased further; at least 10 minutes
passed before it was stable.
Power amplifier testing
I switched the power off and connected the first amplifier board to the
supply through a 100W ½W resistor
as a fuse. I also connected 10W highpower resistors across the amplifier
outputs as loads.
I re-applied power and monitored
the amplifier DC output voltage (5 on
the PCB).
Fig.5: the 1kHz square wave response
of the QUAD 303 amplifier is very
clean.
Figs.6(a) & 6(b): the leading edge (left) of the QUAD 303 amp output with a 1kHz
square wave fed in. The rise time allows us to calculate the time constant of the
high-pass filter formed by the coupling stages throughout the amp. The trailing
edge (right) of the same waveform indicates that the response is symmetrical.
It steadily increased to 29V and
nothing was getting hot. I then
adjusted trimmer RV100 until the output was at 33.5V. The output current
can be monitored by checking the voltage across both 0.3W emitter resistors
(4-6 on the PCB). I adjusted VR101 to
get 8mV (allowed range 6-9mV), corresponding to 13mA.
This time, both the supply voltage
and the amplifier settings were drifting, so I repeated the adjustments after
20 minutes.
I removed power and connected the
amplifier directly to the 67V supply,
then wired up the second board via
the 100W resistor. After verifying that
it worked, I removed that resistor and
I repeated the adjustment procedure
for the second board, aiming to have
the standby current in both channels
the same.
The upgrade instructions state, “use
for some hours at normal volume and
repeat the calibration”, so I followed
that recommendation.
It is interesting to compare the procedure with the setup for the Silicon
Chip Class-A 20W Amplifier that was
initially designed 25 years ago (in
1998, as a 15W version) and improved
to 20W in the May-August 2007 issues
(siliconchip.au/Series/58).
While the Class-A amplifier power
supply is unregulated, the separate positive and negative supplies
with the input referenced to ground
ensure the output voltage is close
to zero regardless of voltage fluctuations. Also, the quiescent current (1A)
remained stable after setup.
After monitoring voltages for a
while, I ran a few simple tests. The frequency response (-3dB) was from 10Hz
to over 60kHz, measured with a digital
oscilloscope. It achieved 19.9V RMS
output just before clipping, and with
a magnified trace, crossover distortion
could not be seen on an oscilloscope.
A 1kHz square wave output looked
good with a reasonably flat response,
no overshoot and rise and fall times
around 10μs (see Figs.5 & 6).
A 32Hz square wave showed significant low-frequency roll-off. However,
a good frequency response does not
necessarily translate to a good square
wave response. I measured the time
constant (time for the waveform to
drop to 63% of the original value) on
the CRO as around 15ms.
The most likely cause of this time
constant is the 0.68μF capacitor 22kW
resistor in series at the amplifier input.
The calculated time constant is T = RC
= 68μF × 22kW = 15ms, in agreement
with the measurement.
I connected a 22μF electrolytic
capacitor across the 0.68μF capacitor,
and the response improved significantly, but it still was not flat. Rather
than attempt to redesign the amplifier, I left the input coupling capacitor at 0.68μF.
Restoring the preamplifier
Photo 5: there are exposed mains connections on the front panel, including for
the neon indicator. That indicator can be replaced with a modern LED powered
by the DC supply.
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Australia's electronics magazine
It’s important that the preamplifier has decent performance since the
power amplifier will amplify any noise
and distortion it introduces.
In the Dada procedure, the modifications are more extensive than for the
power amplifier. The following steps
are recommended:
1. Replace all electrolytic capacitors.
2. Replace all BC109 transistors
with lower noise BC550 types.
3. Replace some resistors with metal
film types for lower noise.
4. Change some resistors to alter the
gain so CD players do not overdrive it.
5. Increase the supply voltage from
12V to 16V for more headroom and
lower distortion.
6. Remove the secondary power
supply as it is not used.
January 2024 97
Fig.7: the 1kHz square wave response
of the QUAD 33 preamp (bottom) is
not as good as the QUAD 303 amp,
with a noticeable shift in the level
during what should be flat portions.
Fig.8: the 100Hz square waves
response of the QUAD 33 preamplifier
is noticeably triangular.
Fig.9: the preamp’s 32Hz square wave
response degenerates into something
barely recognisable as a square wave.
I changed the components as recommended and rebuilt the secondary 8V power supply that powers the
indicator.
The indicator bulb showed signs of
heat damage, so I replaced it with a
high-intensity LED soldered to the old
bulb metalwork. After changing the
lamp type, I moved the power supply’s
yellow (indicator) lead from position
2 (AC) to 5 (8V DC). I also changed
R502 to 330W to limit the LED current to 18mA.
I powered the preamp on without
the modules connected and measured the DC supply voltage as 15.9V.
As with the power amplifier, there is
exposed mains wiring that must be
covered while the lid is off.
I reconnected the modules and commenced testing with 1kHz sinewaves
into the Radio 2 input. As the output
reached around 0.5V RMS, it dropped
to almost zero. The output resistances
of both channels had dropped from
4.7kW to around 80W and stayed there.
After some checking, I determined
that the low resistance was from the
metal frame of the filter switches to
the output.
Powering it off and pushing switches
cleared the problem, but it returned as
soon as the output reached the critical
value. Several articles mentioned that
these switches can cause problems, so
I cleaned them without removing them
from the PCB. Removing them would
be challenging, as the spring-loaded
contacts are tiny and would pop out
much more easily than they would go
back in (see Photo 6).
Unfortunately, the problem returned
after cleaning, but only in one channel
this time. As the low resistance path
was to the switch frame, I sprayed the
gap under the switches well with contact cleaner and then washed them out
with isopropyl alcohol. After that, both
channels worked correctly.
This would likely have cleared the
faults in both channels if applied earlier. What the substance was and why
it was triggered into a low resistance
state depending on the signal level, I
do not know.
I checked the signals through the
preamp from the Radio input with the
filter switch in Cancel (no filter and no
tone controls). The sinewave response
was -3dB from 15Hz to 220kHz.
The square wave response at 1kHz
shows signs of poor low-frequency
response (Fig.7). At 100Hz, it is obvious (Fig.8), and at 32Hz, the response
is horrible (Fig.9). It is the same on
both channels, so presumably the original release had similar performance
as I did not reduce any capacitor values in the preamp.
Examining the amplifier more
closely, the signal is losing shape as
it arrives at the base of the first transistor, TR400. With the preamplifier in
“Cancel” mode, the output from TR400
emitter is coupled via a 2.2μF capacitor and 5.6kW resistor in series (time
constant = 6ms) to the base of TR401.
TR401 and TR402 constitute a
high-gain amplifier with the output
returned to the inverting input, making
the base of TR401 a ‘virtual earth’.
Investigating further would involve
breaking the feedback loop. The output has another 2.2μF coupling capacitor connected to a 4.7kW load resistor.
Increasing the value of capacitors
in this circuit would help the square
wave response, but they are not the
only factor. I was reluctant to change
any of those capacitor values as such
changes could have a flow-on effect
elsewhere. After all, this is a refit to
make the best of the existing unit with
minimal changes, not a redesign.
Note that the measurements and
comments above apply to these modified units alone.
Photo 6: one of the switches that
caused so many problems by
intermittently shorting the signal
to the case. Presumably, some kind
of conductive gunk had built up; a
thorough cleaning finally sorted it out.
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Silicon Chip
Australia's electronics magazine
Listening tests
Any comments on the sound must
acknowledge that my 70+ year old ears
are not in great shape. The ‘test’ was
listening to Fleetwood Mac’s Rumours
on a Thorens/Ortofon Blue combination played through home-built threeway Vifa Speakers.
For comparison, I used the 20W
Class-A amplifier I mentioned earlier, the Magnetic Cartridge Preamplifier (August 2006; siliconchip.au/
Article/2740) and a two-linear-IC tone
control network. This combination is
at least 30 years younger, so the comparison is unfair, but it is my reference.
I found the QUAD system acceptable
but not as clear as my existing system.
References (www.dadaelectronics.
eu/downloads):
QUAD 303 Power Amplifier Service
Data and Instruction Book
QUAD 33 Control Unit Service Data
and Instruction Book
QUAD 33-303 Service Supplement
QUAD 303 all versions illustrated
upgrade guidelines V2.0
QUAD 33 Revision – Illustrated
Guidelines V2.7
SC
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