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Quad 405-2 Amplifier

The recently introduced Quad 405-2 is an uprated version of the original 405 amplifier, which first arrived in 1976, and like its predecessor, the Quad 303 is becoming something of an industry standard for monitoring purposes.

Current Dumping

In 1976, the original 405 broke new ground, being radically different to conventional class AB bi-polar power amplifiers. It was the first mass produced amplifier to employ 'feedforward' techniques (contrary to Japanese claims), although Quad prefer to use the term 'current dumping', because the feed forward loop bypasses the output devices, which become 'current dumpers'.

In effect, a highly accurate, low current, class A stage accurately controls the output devices, which can be regarded as 'switches' between the supply rails and the speaker. In theory, this arrangement can offer zero distortion, and the practical realisation certainly offers cleaner sound — especially at low levels. This was a significant achievement back in 1976, when competing class AB power amplifiers often suffered from audible crossover distortion.


In the past seven years, amplifiers in general have improved, so the Quad's intrinsic accuracy is less spectacular. Nevertheless, it retains at least three distinctive advantages over competing bi-polar topologies.

Firstly, the performance of conventional bi-polar amplifiers is dependent on the temperature of the output and driver devices. If the recent programme 'history' has involved high levels, or a predominance of frequencies which see a big impedance dip in the speaker, the resulting temperature rise upsets the ideal conditions for low distortion, and succeeding quiet passages in particular may be less than perfectly reproduced.

Because the Quad 405's output devices are tightly controlled by a low power stage (which stays cool), thermal dynamics have no effect on the sound, and in particular, the DC offset at the output remains stable, and very low. This is especially good news if you're using a pair of expensive headphones!

The other two advantages are inter-related. The current dumping topology spells a simple circuit, that avoids messy presets; it requires no setting up. This contributes to a low price, and also to reliability. As high power amplifiers aren't exactly renowned, as a breed, for longevity, despite the claims of enthusiastic advertising copywriters, the Quad 405's healthy track record in this area commands a lot of respect in the music business, especially as it's strictly intended for domestic use, as opposed to less than genteel Rock 'n' Roll (mis)applications!

However, it should be noted that Quad make it clear in their manual that this amplifier is not designed to withstand misuse to the extent of driving it into a short circuit for long periods. As, for our purposes, the amp is likely to be wired up on a fairly permanent basis, but this is no real shortcoming; it's simply advisable to adopt the good habit of switching off the amplifier before unplugging or swopping over the output leads.

The only significant shortcoming of the original 405 was its inability to drive at full power into low impedances — and the impedance dips of many nominally '8 ohm' speakers presented difficulties here. The new 405-2 features bigger, output devices, which can safely handle the increased power dissipation into lower impedance loads. As a result, the protection circuitry has been 'backed off', allowing a continuous power of 150 watts into a 5 ohm resistive load (cf. 6 ohms for the 405-1) and 100 watts into 3 ohms — as opposed to 25 watts into 3 ohms for the 405-1.


Of course, for short, but musically significant periods, full power is still available into these very low impedances. This uprating is particularly timely in view of a recent research paper, which demonstrates that under dynamic conditions, the impedance of real loudspeaker loads can fall to 25% of the nominal, viz. 2 ohms for an 8 ohm system, for short periods. The differences between power amplifiers are most audible when they're driven at high power, and the premature tripping of protection circuitry is a key factor here. In this respect, Quad's design has been overtaken by high technology devices, in that the latest MOSFET power amplifiers are even simpler, and don't require any protection circuitry, though to be fair, the difference as regards driving nominally 8 ohm speakers will be purely academic in most instances.

Quad internal layout.

In any case, it should be stressed that the 405-2 has a short term peak current capability of 8.5 amps, and the protection circuitry's current limiting is considerably more 'intelligent' and less obtrusive than that found in most bi-polar designs, being based on a thick-film custom IC — an 'analogue computer' of sorts.

From a glib audiophile perspective, it's easy to criticise the design in view of current practices in that a single pair of relatively small reservoir capacitors are shared by both channels, and whilst a token effort appears to have been made to prevent contact non-linearities at low levels by gold-plating the fuse-holder clips, the fuses themselves are not gold-plated! Also, the cheap DIN input connector and the 4mm banana sockets are not the highest quality connectors, although to be fair, you can easily upgrade them. At best, however, these criticisms are minor and at worst they're simply irrelevant. In practice, the Quad 405-2 measures well, and providing it's used within its capabilities, it comes very close, soundwise, to Quad's elegant euphemism for a perfect power amplifier, "A straight wire with gain".

Track Record

However, the real tour de force here appertains to a long service life, and reliability. For your £247, you're investing in equipment which has a proven track record, and if you do have problems after the guarantee expires, the amplifier boards are readily unplugged and replaced without ever using a soldering iron.

Quad have an unrivalled reputation for service — they still repair, recondition and stock spares for the valve amplifiers they manufactured three decades ago(!) and all of these services are available at a sensible price. So on this basis, you can expect your 405-2 to remain serviceable for many years to come. And that's not all. The 405-2 may not be the world's newest design, but it's most certainly a sound piece of engineering design, and accordingly, it's unlikely to be surpassed by a totally new Quad amplifier for some time to come. Neither is it an amplifier that will readily grow out of date, or out of fashion; rather like the Mini or the Russell-Hobbs kettle, it's something you can be proud to own for years to come.

Needless to say, the quality of assembly and finishing is immaculate. There's also a comprehensive manual, with twelve pages of good English prose and all the info, you're likely to need.


To sum up, the 405-2 scores very highly as a serious and long term investment at the 100+100 watts level — perhaps more highly in this respect than any other power amp.

At the same time, reservations are still in order as regards its suitability for driving high power into speakers presenting a 'difficult' load, especially if their nominal impedance is well below 8 ohms, which is perhaps more often the case with professional, as opposed to domestic speakers.

The real answer here is to obtain honest data from the speaker manufacturer, and also, if possible, audition the speakers at high levels, checking for any reduction in the anticipated SPL capability.

Recommended retail price for the 405-2 is £247 inc. VAT.

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X-15 Multitracker in Action

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Monitoring On Loudspeakers

Home & Studio Recording - Copyright: Music Maker Publications (UK), Future Publishing.


Home & Studio Recording - Sep 1983

Gear in this article:

Amplifier > Quad > 405-2

Review by Ben Duncan

Previous article in this issue:

> X-15 Multitracker in Action

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> Monitoring On Loudspeakers

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