A brief
revision history of the 405
The revision history of the 405 is quite a bit more
complicated than it was with the 303; depending on the
serial number of the amp, there were a total of eight
official circuit revisions and two different amplifier
PCB types. Most of these revisions contain minor changes
and improvements and the only notable change was the
introduction of the 405-2.
Clamp circuit: One feature which wasn't
present in the earliest examples is the 'clamp' circuit;
at first this was fitted to a separate PCB directly on
the output terminals, later on it was incorporated into
the amplifier PCB itself. In the event of an internal
malfunction causing a sizeable DC voltage to appear
across the speakers, the loudspeaker outputs are shorted
to ground which prevents damage to the speakers and
ideally blows the supply fuses. I'm not a huge fan of
this idea (I prefer the security of having loudspeaker
relays as the Japanese designs do), but in the small
chance of a malfunction the circuit does work quite
well.
The 405-2: This was a circuit revision
with improved current limiting introduced to answer
criticism about the original amplifier's power delivery
into low impedance loads; whereas the current limiting
in the original 405 will limit output into 4Ohm speakers
to around 70w, the 405-2 will allow the full swing of
100w. Other improvements included a pair of RCA phono
input sockets and binding posts in place of the original
'pushbutton' loudspeaker sockets (both of which can be
added quite easily to an earlier amp), but in all other
respects the amplifier is still very similar to the
original 405.
Let me be clear here; the 405's lower power output into
low impedance speakers is caused by the over-zealous
current limiting circuit (it's doing its job, which is
to protect the output transistors) and not because of
any deficiency of the amplifier itself.
I've been asked many times if it's possible to 'upgrade'
a 405 to 405-2 performance, but the circuit layout
around the current limiters is quite different and would
really need the revised PCBs (and if you went to all
this effort you might as well just buy a 405-2 in the
first place). I'm aware that there are mods which can
improve the 405's low impedance drive performance to
almost 405-2 levels, but any alterations which could
compromise the reliability of the amplifier are not
something I'd be willing to do for a customer.
In short, if you want to drive low impedance speakers
and the loss of 30 watts is a problem for you, then go
for the 405-2. Otherwise, the original version will be
fine.
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Sound character
If you've been used to the 303, the sound from the 405
(and later current dumping Quads) is a solid evolution.
The soundstage is deeper and more projected, yet the
overall sound is less 'bright' while still having more
in the way of sparkle. The 303 is capable of a very
detailed soundstage when restored, but the 405 is better
still; on a good pair of speakers they can image VERY
well.
Out of the box with original components, the low end
isn't the strongest you'll ever hear for a 100w/ch
amplifier, but the bass is very well controlled and
'fast' and if you have the speakers to match then you'll
notice how well it manages to reach down into the lower
bass registers.
Noise levels are quite good for the era; there's a bit
of white noise as is to be expected from a vintage
design, although noise levels are usually quite low (I
don't normally see any more than 10-15mV noise floor on
the bench). Strangely, the left channel always exhibits
worse (in the region of 20-30%) noise characteristics
than the right, this is caused mostly by the way the
original wiring loom is configured with both AC mains
primary and audio signal wires bound together. Of
course, this is easily corrected with a better wiring
loom - see the
restoring
page for details.
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Bridging
Much like Quad's earlier 303, the 405 can be bridged to
make it in effect a 180w/ch into 8Ohms mono amplifier.
The bridging procedure is more technical than it is with
the 303 (requires de-soldering and changing a few
wires), the procedures are described in the service
manual (which is quite widely available online).
My opinion is the same as with the 303, in that they
sound better as a normal stereo amplifier; if you need
more power, then why not just buy a Quad 606 and keep
the 405 for when you want a smaller (but sweet sounding)
system for vocal music? That said, if you have a spare
pair of 405s and can use a soldering iron, then bridging
is something to try to get some 'free' extra power.
Finally, if you do have a pair of similar 405s, why not
just bi-amp them instead? Technically you won't get any
extra power this way, but if you connect one on each
stereo channel (with one channel of each amp doing bass and the
other doing mids & treble), you'll net a sonic
improvement with better channel separation and from this
an apparent increase in power as each amp's power supply
is effectively only being loaded by one set of speaker
drivers rather than two.
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Driving the 405
Quad intended the 405 to be part of a
full Quad system and to be driven by their 44
preamplifier. The Quad 44 is still a very good
preamplifier; is contains a very good phono stage, tone
controls if you need them are of Quad's famous 'tilt'
design; especially when restored, the 44 makes a very
capable preamplifier even by today's standards.
That said, much like any other power
amplifier, you can drive the 405 with pretty much any
source you like as long as it has some way of adjusting
volume output and can provide enough current at around
half a volt. As with the 303, when listening to digital
music I mostly drive my Quad gear from a USB sound card
running through a good DAC and into a passive
preamplifier containing only a stepped attenuator as
volume control; and it doesn't need to be a DAC, it
could be a CD player, DVD or anything else, as long as
its audio front end can provide enough current to drive
the 405 and whatever you control volume with.
The 405 is quite sensitive compared to
most power amps, in so much as that it only takes around
500mV to drive it to full output power (most power amps
require around a volt to do this). Some people advocate
altering the input circuit to reduce the amp's
sensitivity by the required 6dB or so and this has
another positive effect in that it will also reduce the
noise floor of the amp too, but bear in mind that if you
reduce the sensitivity in this way that you'll no longer
be able to get full volume if you hook up to a Quad
preamp (or any other preamp) which abides by the old
500mV standard.
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Buying - condition and
reliability
Cosmetic condition: As with all vintage
Quads until the 606 era, everything got painted in one
of two shades of metallic gold/beige. Now, unless the
amplifier has spent its life in a sealed container away
from sunlight, the chances are that the original finish
has faded quite severely. Giving it a gentle clean may
help, but usually there's not much you can do beyond
this.
 |
|
An average un-restored 405 |
The black section on the front of the amp is a
functional heatsink (takes heat away from the power
devices inside) and this is aluminium with a black
anodised finish. Anodising is a very thin but very tough
finish for aluminium (harder wearing than even powder
coat) and cleans up very well, but obviously if there
are digs and scrapes down to the bare metal then there's
not much you can do about this besides repainting them.
Re-finishing the painted casework can be done, and is
discussed in more detail
here.
Reliability: In all honesty, I see more
reliability problems with later Quads than earlier ones,
and this carries through to the 405. I'm not saying that
the 405 is unreliable here, far from it; as with all
Quad gear the overall build and construction quality is
very high but as far as components go, I get far more
405s into my workshop with noise problems or outright
failures due to factors such as bulging or completely
expired electrolytic capacitors than I ever see on the
303.
 |
|
Bulging capacitors in an
original 405. The terminal end of the
component should be completely flat |
I mentioned in my Quad 303 articles that in my
experience the later blue ITT branded power smoothing
capacitors are more likely to leak and cause problems
than the earlier types, and this is the same on the 405;
the main 10,000uF 63V power reservoirs are almost always
showing signs of bulging and leakage and are almost
always at least 10% or more down on value and high on
ESR (Equivalent Series Resistance).
The Bakelite bodied PCB capacitors are a similar
problem, on the 303 I don't often see these bulging but
on the 405 it's often a different matter; they don't
expand or 'vent' so won't obviously look bad on
inspection, but as they degrade pressure builds up
inside and the plastic casing crazes, with the crazing
being a beige colour (this is dried electrolyte). These
capacitors are sometimes completely dead (open circuit
on both my LCR and ESR meters) and will cause all manner
of faults.
Quad later moved to using UK made BHC capacitors in
their power supplies (I've sworn by this brand for years
now and use them in all my restorations where possible).
I don't know when this switch happened - my early Quad
606 had last been back to them for service in the early
90's and came to me with a set of BHCs fitted, I've also
seen a late 405-2 with them - but these prove to be a
far more reliable component; the ones in my 606 were
date coded 1990 and still measured within spec for both
value and ESR, I'm yet to have a single issue with them
in anything I've restored.
Renewing the capacitors is actually a simpler and
slightly less expensive job than it is on the 303; if
you use premium quality parts and replace 'like for
like' (i.e. don't spend extra money increasing capacity
of the parts), the total parts cost is around £60 and
for a qualified tech like myself it shouldn't take any
longer than an hour or so to do.
I guess that what I'm saying here is that fundamentally
the 405 is a very reliable amplifier but considering its
age and expected component lifetime, unless it's been
back to Quad for service & parts replacement sometime in
the last 10 years or else been restored by someone such
as myself, no matter how well it works at present you
should budget in an extra £100 to have the capacitors
replaced, as they will probably need it in the near
future.
Upgrades and modification
Modifications, upgrades and
restoring are discussed in more detail
here.
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Last updated May 2014
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