Circuit
upgrades and modifications
Before we start, I need to say that my opinion with
regards to upgrades is that I stay away from anything
which tries to re-engineer the original circuit topology; I
believe that Quad's engineers made many conscious design
decisions even over minute aspects of the design - some
of these relating to available component technology at
the time - these decisions help make the 303 sound like
it does and I don't see the reason to second-guess them.
To draw a parallel, there's a saying that goes if you
want to make your car go faster, just buy a faster car.
I've been there and found that to be true, the same goes
for audio. I'm not saying that modifications are a bad
thing, but by the same token I believe that if you get
to the point where you feel the need to modify a vintage
design such as the 303 to change its character more
towards a modern amplifier, you might as well just buy a
modern amplifier instead.
My own opinion based on years of experience is that a vintage Quad, restored
with sensible high grade modern components and with some small
tweaks to certain areas without changing the original
design in any way can vastly outshine an un-restored
example, but still retain all of its original sound character -
basically, you get the best of both worlds.
Schottky diodes:
In the past I've heard some talk on audio forums about
replacing the rectifier diodes with Schottky types,
presumably to reduce the voltage dropped across the
rectifier pack; I don't see the reasoning behind this
because although small Schottky diodes do tend to
exhibit less forward voltage drop (Vf) than
silicon, if you buy higher voltage rectifier types this
advantage decreases greatly (taken across the rectifier
pack, minimum of 1.0-1.2V for a Schottky bridge opposed
to 1.2-1.4V for a silicon one). In addition, even in
totally stock form the power supply in the 303 is
regulated to output around 13V less than the voltage it
gets fed from the rectifier so unless you adjust the
regulator output and DC balance out of spec, there won't
be any increase of voltage (and hence power output), no
matter what you do with the power supply.
Another advantage of Schottky diodes is that they can
operate at far higher speed than an equivalent power
diode, but again this isn't exactly an advantage when
rectifying 50 or 60Hz AC; even the most sluggish power
diode will work OK at these low frequencies and as a
matter of fact most manufacturers actually purposely
slow down rectifier diodes by paralleling them with
capacitors as this enables more stable low speed
operation with less switching noise.
Frequency response:
Over time I've done a lot of SPICE analysis,
mostly aimed at improving the HF response of the stock
303, something which always bugs me when I run a
frequency sweep on one. Most 303s have a
response which begins to roll off around 16KHz; it's
never more than just over half a dB or so by 20KHz but
it niggles at me every time I see it. In SPICE,
decreasing the time constant of the shunt RC filter
(formed by R106/C102 if you have a copy of the
schematic) near the input makes great improvements,
though in reality it's not anywhere near as much. Much
more improvement can be had by decreasing the value of
the capacitor in the feedback loop (C103 on the
schematic), but the major problem is that by decreasing
feedback at high frequencies, you may increase the top
end response but you risk making the amplifier unstable.
With regards to low end response, the LF response on the
303 is mostly limited by the input and output
capacitors; upgrading the loudspeaker output capacitors
from the original 2,000µF to 4,700µF helps move the
lower roll-off -3dB frequency to under 15Hz, and also
helps improve low frequency damping because of the
larger capacitor's lower impedance. The reason the
original components are smaller is purely because
component technology in the 1970's was light years
behind what it is now and as a result components were
far larger in size back then, probably 2,000µF
was realistically the largest size the designers could
fit into the chassis back in 1970.
Increasing the output capacitors is only
worthwhile if you increase the value of the input
coupling capacitor to around 1µF; beyond this point,
increasing the value of the loudspeaker capacitors any
further leads to diminishing returns even if you run
4Ohm speakers. The main bottleneck after this point is going
to be the input capacitor value as this has the most
effect on low end response, so doubling this or more is
probably the way to go if you're obsessed with lower
bass response although I find 1µF to work fine even on
large speakers.
Wiring: The gauge of wiring inside the 303 - it
has to be said - is feeble, and that goes for a lot of
Quad's other products of the era, including all guises
of the 405. Don't get me wrong, the wiring itself is
beautifully neat - in most of the Quads I restore it's
formed into neat angles and tied neatly into looms, but
the quality and gauge of the wiring itself is questionable
considering the amount of current some of it
is expected to pass - I've always been baffled why this
was a design choice for a company producing quite high
end audio equipment.
The problem with any length of wire is that it will have
an internal resistance between its ends, and the greater
the cross section of the conductor (its diameter), the
higher its conductance and hence the
lower this resistance will be. In accordance with Ohm's
law, as you pass more current along this wire the
voltage drop between its ends will increase (V=IR);
with regards to wire gauge, it's the current that
matters and not the voltage.
The driver amp stages on the amplifier PCBs have
decoupling capacitors fitted to them to help negate any
voltage fluctuation (and these sections don't pull
significant amounts of current anyway), but my concern
is the output stages of the amplifier; run at high power
levels you can see minor voltage drops for instance
between the reservoir capacitors and collector terminals
of the output transistors on the main heatsink, voltage drops which
shouldn't be present in an ideal design.
Of course, the portions of wiring which carry higher
currents can easily be upgraded, so why not fit upgraded
wiring when restoring an amp? The reason I stop short
of doing this as it becomes difficult very quickly to
install runs of heavy gauge wire into a tightly packed
chassis and keep things even slightly neat.
On the input side, the screened audio wiring is pretty
much standard fare for the era - it's a bit thin but it
does the job and besides, the wiring runs are only a few
inches; I highly doubt whether you'd ever hear any
difference if you changed it and I always leave it alone
because it does the job just fine.
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Building an 'improved' version of the 303
A couple of years ago I built a 4 channel custom
amplifier (intended for bi-amping) loosely based on the
303 design; a lot of the redesign work I did involved
modern regulated split rail power supplies so that I could lose
the loudspeaker coupling capacitors (not that difficult
actually) and also modifying the output stage
to use complimentary output devices in place of the
venerable 2N3055 - I don't remember exactly, but I used
some high frequency ST brand outputs with a TO-3P case -
linearity and high frequency are my main considerations
for audio transistors as long as I can keep them stable.
The resulting amplifier still sounded quite like a
303 in many ways; the bass was 'faster', probably
because of the better damping factor through losing the
output capacitors and increasing the wire gauge
throughout, the response towards the extreme top end was
perceivably brighter (again, probably because I used
modern transistors which are much 'faster' than the
originals). Imaging and channel separation didn't really
seem noticeably improved (the 303 always performs well
in this respect, especially if you bi-amp a pair of
them).
Overall I liked the amplifier, still have it in my
workshop and one day may write an article about it, but
it reminded me of the important lesson that I've
mentioned above; design choices in a piece of audio gear go quite
a way towards how the end result sounds, and the very
reason I value Quad equipment so highly is because of
how they add their own character to the sound.
Restoring an
original amplifier
The best thing to do by far if you have an original 303
and you want to make it sound better is to restore it.
Read about what I do when I restore an original amp
here.
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Last updated April 2014
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