Sound character
The 303
has very much a characteristic sound; there's a very
slight roll-off towards the top of the treble, not
unlike valves. Bass is more solid yet sounds quite well
damped and has better
transient response than some modern transistor amps. Where the
303 shines in my mind is in imaging and clarity of the
midrange when sitting down and listening to vocal music
at mid volume, coupled with a good pre-amp (not
necessarily the Quad 33) and when running from a good
source and driving good speakers, the transparency and
imaging might surprise you.
If your speakers can reach low enough into the
bass, you won't even need tone controls; paired
with the right speaker combination you just don't need them
and after you get used to the sound without them, they
just mud up the transients and slow everything down.
Every 303 I've owned or restored (30+ of them) has
exhibited a barely perceptible 'ringing' on the upper
midrange into the treble on some material - it's only
really there at higher volume levels and on modern
(digital) recordings on modern speakers (my vintage
hybrid ESLs don't show it up but my Celestion A3s do),
adjusting bias slightly upwards helps but nothing
I've ever done has resolved it completely; I concluded
it's just a side effect of a circuit design from the
days when component technology was far less perfect than
it is today.
Power was
rated at 45 watts RMS per channel. Now, this isn't a
great deal, admitted - as good as the 303
is, if you run larger speakers at high volume levels and you will inevitably
notice the bass harden up and become thinner at times as
compression creeps in. Even a good
pair of 303s running as bi-amped on a well matched pair of sensitive
speakers will tend to run out of steam on low bass, at high volume levels
there's no getting away from the fact that you start
eating into large amounts of power. BUT, for low/mid
volume listening, they can still shine.
A restored & refinished Quad 33 and 303 from my
collection. They now power some B&W monitors in my
workshop. The B&Ws actually go well with the sound of
the 33.
A close-up of the 33 shown above - the new colours
actually match the original very well, however the
original paint actually dulls a great deal and loses its
metallic lustre.
Back to contents
Noise levels
Noise
levels are quite good for an amplifier from that era,
but you're not going to see a noise floor to rival
modern higher end audio gear. White
(thermal) noise is the only real problem, this emanates
from the semiconductors and resistors - depending on your
loudspeakers, you may or may not be able to notice this.
I usually run 303s on either of two pairs of Celestions,
the
huge A3s or a pair of Ditton 4s. The
A3s are a much louder, much brighter, much more
projected sounding speaker, yet I can barely notice any
white noise when driving them with a 303, on the Dittons it's
quite noticeable if you listen for it.
Another type of noise may be entirely physical, an
audible hum emanating from the mains transformer inside
the amplifier itself. Mains transformers do sometimes
hum or buzz, this is normal (it's caused by a phenomena
called 'magneto striction'); essentially this noise is
caused where portions of the copper windings aren't
securely fixed in place and vibrate as the alternating
current passes through them. As stated before, this is
normal behaviour but good quality transformers like
those in audio gear are usually filled with a
varnish/glue material (shellac in the case of the 303)
which among other things fixes the windings in place and
prevents any noise.
I've heard of people vacuum filling the mains
transformers to quieten them, but in my experience I've
never had a 303 with a badly noisy mains transformer -
the main culprit tend to be the original rubber feet, which
after 35 years plus have turned to brittle plastic and
will sometimes transmit vibrations through whatever
surface the amp is stood on and cause an audible hum
when the amp itself is actually quiet. This is why I
always replace the rubber feet when restoring an amp.
Back to contents
Quirks
Many
higher end amplifiers have audio outputs which are relay
switched, both to protect the loudspeakers against DC
output voltage if anything goes wrong inside the
amplifier, also to prevent any clicks or thumps when the
amplifier is switched on or off.
The 303
doesn't need relays for loudspeaker protection as it uses output
capacitors (these keep the amplifier biased and keep the
DC voltage present inside and hence prevent the 'magic smoke' from escaping).
Hence there are no relays and any combination of thumps,
hissing sounds or clicks are to be expected, especially
when you turn off the amplifier. When you turn off the
power, it may take as much as 20 seconds for the
amplifier to deplete the remaining charge in the
capacitors and for the sound to finally fizzle out,
which it may do with merely a quiet thud or a loud
hissing sound, depending on the speakers you have
attached. Don't be alarmed by this - it's a quirk caused
by the design of the amplifier and does NOT indicate a
fault.
Another thing to mention is that if you switch on the
amplifier without speakers attached and then attach them
within a few seconds after turn-on, you will get an
audible pop through them, possibly quite loud. Again,
this isn't a fault but is also related to the fact that
the 303 is an output capacitor coupled type design; when
the amp is switched on, the positive terminal of the
output capacitors suddenly sees around 33V and as such
the negative terminal (to which the speakers are
attached) initially wants to follow this. Normally, the
capacitor will charge as the initial current flows
through it and this gives the characteristic turn-on
'thud' that a 303 normally exhibits; take the
loudspeaker away and the capacitor will take far longer
to charge, and as a result you will get a loud pop if
you connect the speaker as this is happening. Again,
this is harmless to the amp but could possibly damage
delicate speakers.
Back to contents
Bridging
Bridging
is the term given to connecting the outputs of both
channels in a stereo amplifier together to form a single
mono amplifier; the idea of this is that as long as both channels
are in phase they will work together and in theory will then give twice
the voltage swing (and hence twice the power output).
The
reason for bridging the 303 was to get more power into 4
ohm or 16 ohm loads; 45 watts isn't a huge amount of
power, especially if you are trying to drive less
sensitive speakers.
Many
people (including a well known Hi-Fi magazine) said that
the 303 sounded better when bridged; my own personal opinion is that they sound better when running as
standard and that bridging them degrades their tonality.
Bridging certainly does get you a lot more power and you
can certainly feel a more solid low end to the sound,
but it does make the overall sound brighter and glassier
and makes the soundstage much shallower, every 303 which
I've owned has behaved in this way.
One thing to mention is that if you want to
bridge a 303 which has been restored or modified, the
main loudspeaker capacitors must be 100v rated as per
the originals - some companies sell 63v capacitors for
the loudspeaker coupling, these are fine normally but
bridging the amplifier changes the voltage swing which
these see and this could cause reliability issues.
The information below on bridging was obtained
from an official Quad paper which someone sent me years
ago - you may be able to find a PDF copy of on the net
somewhere. I've just
presented it here in a clearer format and take no
responsibility for any damage you may cause if you try
bridging your Quads.
Bridging for 4
ohms
To
bridge a 303 to run on 4 ohm speakers, both channels
need to be connected in parallel. Ohms law states
that to drive half the impedance and hence get twice
the power output from the same voltage swing, we
need to be able to provide twice the current, hence
the reason for connecting both channels in parallel.
 |
|
Bridging a 303 for 4 ohm speakers |
Both
line inputs need to be connected together via a
potentiometer - ideally 5K ohms according to Quad's
literature although a lower value should be fine as
long as both channels are reasonably well matched to
begin with.
This
acts as a balance control allowing you to adjust the
output levels so that both channels match as close
as possible (better matching = more power and less
distortion). The ends of the potentiometer are
connected one each to the stereo line inputs, the wiper is
connected to the audio input from your pre-amp or CD
player etc. You should then end up with 3 wires -
ground wires for these three connections are
connected together as usual.
The
next step is to connect a speaker across the
positive loudspeaker terminals (that's one wire to
each red terminal) and then play some
sound (ideally a 1kHz test tone, although regular
music is fine), then adjust the pot until you get
minimum sound output, this is crucial to get good
results.
Once everything is adjusted,
the loudspeaker outputs for both channels can be
connected together in parallel with some jumper
wires (both reds connected together and both blacks
connected together), leaving the potentiometer in
place - this will give a mono amplifier
which can deliver a solid 90 watts into 4 ohms.
Bridging for 16 ohms
To drive a 16 ohm load,
you need twice the voltage swing but only half
the current that you would to drive an 8 ohm
load. In this case both
channels are connected together, end to end, one as
a mirror image of the other -
now one channel will need to be fed an
inverted version of the input signal.
 |
|
Bridging a 303 for 16 ohm speakers |
The reason for this
inverted input is that both channels are now
connected end to end and one will need to be
pushing in the opposite direction to the other
for both outputs to be the same, just like the
push-pull output stage in a normal audio
amplifier.
Otherwise one channel
will be working against the other as such the
signals will subtract, meaning very little or no
sound output.
Quad recommended a special
transformer to supply this phase inverted input,
though running the signal through an inverting
op-amp circuit to supply this inverting input should work fine if you
have some electronics knowledge.
Back to contents
Driving the 303 -
the Quad 33
Quad originally intended the 303 to be partnered
with their 33 pre-amplifier, the 33 never enjoyed quite
the same fame as the 303. To be fair, the 33 contains a
lot of electrolytic capacitors and after nearly 40 years these just aren't
going perform at their best, that's without even
considering all of the carbon pots, switches and PCB connectors which the 33 used
which have probably badly oxidised after 35+ years - it's
not fair to judge the 33 unless you've heard one which has been restored with
new
components.
That said, my honest opinion of the 33 is that
it's inferior compared to the 303 - the quality of
construction (soldering and wiring) tends to be low, by
modern standards the circuits are compromised because
they make use of ferrite core inductors (read distortion
and poor HF performance) and are forced to work on low
voltage rails (this increases output impedances and
destroys damping factor to name but two aspects). Also,
the quality of the volume and balance control pots are
dubious and the sockets aren't very good. In addition,
the nature of having five plug-in PCBs makes them ripe
for oddball intermittent faults.
Run a 303 through a 33 and then run it directly
and the difference may be like night and day; the output
from the 33 is glassier with a compressed soundstage,
the dynamics are overall more compressed and bass tends
to be boomy and thuddy (this is poor damping factor in
action). All this said though, you need to understand
that this is still a pre-amp which was designed in a
different era, when people's expectations (and also the
quality of most audio sources) just weren't anywhere
near what they are today.
Running the 303
direct
I hate to admit it, but I can gradually see
myself becoming a purist and audiophile. The best sound
I've had from a 303 is driving a pair of them as a dual
mono bi-amp setup, straight from a CD player or DAC via a passive
pre-amp containing nothing but a stepped attenuator (an
exotic name for a volume control). To get away with
this, you need a source with a good audio front end,
and one which can supply enough current to drive the
~25k Ohm load of a pair of 303s plus the volume control - otherwise bass and
dynamics will badly suffer. Shunt resistance in the
signal lines helps reduce the current demand at the expense of volume level,
really it's all down to the actual source.
 |
|
Home brew passive pre-amp - things don't get
much simpler than this. |
It
doesn't even need to be a CD player, any source can be
used as long as the output is high enough and it can
deliver enough current - DVD player, cassette deck,
tuner, even the output of a phono stage. You could even
use the headphone output on a laptop or MP3 player -
although I can't guarantee how good the resulting sound would be,
it should work fine.
All a
pre-amp needs to do is to let you control the volume of
the signal going in to the amplifier, anything else
(input switching, tone controls etc) is a luxury that
you don't actually need to get good sound.
The
pre-amp to the left was built one morning at 3am from
the remains of a set top TV box, added to that an Alps
100k
Ohm stepped attenuator, 4 phono sockets, 4 wires and 2
shunt resistors. A ground wire connects the chassis of
the box to the signal earth to prevent any hum pickup.
That's it, that's all that's needed.
Back to contents
Buying - condition and
reliability
The 303 is rugged, the major components such as power
transformer and output transistors can easily cope with
the demands placed on them.
If you're buying a 303, the main concern is, of course,
whether it actually works. A good seller who knows their
stuff should be willing to test the amp and state
honestly whether it works, if you're buying from
somebody who looks like an audio dealer and they say
they can't test the amp because they don't have the
cables or don't know how to connect it, be very wary of
them, especially if the price is high.
The main electrolytics are the major concern. If you're
not into electronics jargon, they're the four large can
shaped components on the chassis - often coloured blue
or yellow. as these degrade they will often leak, if it's
an earlier model then they will be mounted with
connections downwards and the fluid they leak might
damage the driver boards beneath. Later models had them
mounted with the connections upwards, in this case you
might even be able to see signs of leakage through the
vents in the case (often crystal growth or bulging). Leaking electrolytics are a terminal condition, there's no way
to repair them - replacing with new components is the only
fix, upwards of £80 of parts plus the cost of finding someone
competent to fit them for you.
 |
|
Inspecting a pair of Quad 303 power amps,
the electrolytics on the one on the left
have leaked |
Leaking or degraded electrolytics will cause any
combination of thin bass graduating towards hums and buzzes (especially
at switch-on), as they degrade further they will cause
instability as the supply voltage fluctuates wildly, in
extreme cases they might even vent or explode (a loud pop
or a plume of foul smelling smoke).
Strangely, I find the blue ITT capacitors fitted to
later 303s to be a bigger problem than the early yellow
Daly branded ones - the capacitors really need replacing
no matter how they look, but the ITT ones almost always
seem to be showing signs of bulging or leakage.
Solder joints tend to be quite good, although I have
come across one or two 303s with dry looking solder in
places (mostly on hard wired connections to the input
panel) - most times I'll just remove all the old stuff and re-solder
the whole panel.
Another problem is sometimes the regulator board, these
invariably run hot and this
can cause some issues (the adjustment pot sometimes goes
open circuit among other things). Signs here are burn
marks from the power resistors and/or regulator voltage
which can't be adjusted or is way out from what it
should be, if I find anything like this I'll usually replace all
resistors on the board.
There seems to be an increasing amount of people on the
internet (especially on a certain auction website) who
list 303s as 'serviced' to justify
huge prices. The word 'serviced' means nothing on a 303,
because there's so little to do - setting them to spec
means adjusting one pot to set the regulator voltage,
then setting the DC balance and bias on both channels
(the latter involves
de-soldering one wire on each channel and inserting a
ammeter, many don't bother to do this). This 'service'
will take an experienced tech like myself a maximum of
30 minutes, not something to justify doubling the price.
I wouldn't be happy to sell a 303 in this way because of
the age related reliability issues with the original components -
any people on the internet who genuinely do
sell these as restored will replace all of the electrolytics, clean and test everything and will be
willing to give a guarantee.
Upgrades and modification
If you have the money and are that way inclined, there
are people on the internet who offer upgrades for the
303. I haven't tried any of these, all I can say is that
they look nicely done. Modifications, upgrades and
restoring are discussed in more detail
here.
Back to contents
Back to Quad 303 main page
Last updated April 2014
All photographs and text on this website
unless otherwise stated are © 2008-2014
Briarsfield Hi-Fi -
www.briarsfieldhifi.co.uk
