This is the second article in my Quad 405 series, this time a review of the legendary Quad 405-2. Read on to find out why so many people love these great amplifiers.
What an incredible amplifier the Quad 405 is, even 40+ years later. Tellingly, Quad 405 and 405-2 power amplifiers are most loved by those for whom listening to music is more important than looking at power meters, fiddling with controls and discussing specifications.
Don’t get me wrong, I love power meters, but the 405 is a study in elegant, understated design and in leaving out what’s not truly needed. It’s a true champion of the ‘less is more’ philosophy and an amplifier that just gets out of the way and plays music. Surely this is what every true hi-fi lover wants in an amplifier.
Quad 405 vs 405-2
The Quad 405 and 405-2 are almost identical, compact, heavy, amplifiers. Both punch out 100 watts per channel, into 8 ohms. Where they differ is in their ability to deliver current into lower impedances.
Specifically, Quad modified the current limiting circuitry of 405-2 to be a little less aggressive. There’s a wider power envelope with the 405-2, allowing it to better drive lower impedance loads. If you own current-hungry speakers or those with a nominal 4 ohm impedance, a 405-2 is the better option.
Quad also implemented various iterative improvements over a long period. In my opinion, this makes the later amplifiers sound and perform slightly better. If you are interested in more technical detail, I recently overhauled a Quad 405-2 and you should also check out the ‘Further Reading’ section at the end of this article.
Quad’s legendary design engineer, Peter Walker, utilised a principle called ‘feedforward error correction’ in the 405 design. He wrote a paper about it, which you can read here.
As Stereophile pointed out in their review of the original 405:
“This is something we don’t see too often: an entirely new approach to power amplifier design. As Quad points out in its literature for the 405, class-A operation of transistors provides the lowest distortion, but drastically limits the amount of power an output transistor can deliver without overheating. (Most transistor amps use class-AB output operation, in which each of a pair of power transistors handles part of each signal cycle and shuts down during the other part.
Imperfect synchronism between the two halves causes the familiar “crossover distortion,” which accounts for most solid-state sound. In class-A operation, each output transistor draws current though the entirety of each signal cycle, eliminating the crossover transition but doubling the amount of time current is drawn, and thus tending to cause the transistor to heat up more.)
In “current dumping,” a low-powered, low-distortion class-A amplifier is used to control the amount of current passing through a pair of heavy-duty “dumping” transistors, and it is the latter which provide the driving power (100Wpc) for the speakers.”
Stereophile didn’t particularly like the original 405, but they reviewed the 405-2 much more favourably.
The Quad 405 and 405-2 are compact, measuring just 115 x 340.5 x 195mm. A mass of 9kg makes this a dense amplifier, heavy for its size. The 405 is able to deliver 100 watts per channel, more than enough to drive most speakers. I can’t think of a commercially available amplifier that’s more compact, with fewer parts or with a simpler layout than the Quad 405. Tell me if you know of one.
The driver stage consists of two TO-220 devices operating in class-A. The output stage utilises two identical N-channel TO-3 devices, per channel. The driver stage is of very high-quality, the output amp is the ‘current dumper’ and is a less precise, more robust unit operating in class-B.
Driver and output transistors are thermally coupled to the same block of aluminium, board-mounted, with no wiring. The assembly forms one amplifier module, there are two modules per chassis. This is evident in the images above and below. Thermal coupling helps lower distortion and the design as a whole minimises wiring and maximises serviceability.
The two modules make up virtually all the electronics in a 405 and 405-2. Other elements include a superb-quality potted transformer, two 10,000uF filter capacitors, diode bridge, some power supply wiring, a large front-mounted heatsink and some sockets. As a repairer, I can assure you that Quad 405 series amplifiers are the easiest to service.
405-2 Specifications (Courtesy Hi-Fi Engine)
- Power output: 100 watts per channel into 8Ω
- Frequency response: 20Hz to 20kHz
- Total harmonic distortion: 0.05%
- Input sensitivity: 0.5V
- Signal to noise ratio: 96dB
- Speaker load impedance: 4Ω to 16Ω
- Dimensions: 115 x 340.5 x 195mm
- Weight: 9kg
Quantifying sound quality isn’t easy but let’s get this out of the way – the Quad 405 and 405-2 are incredibly good sounding amplifiers. They aren’t the best at everything, but what they do well, they do very well and if you get a good one, you’ll know it. From the moment you switch on a 405 or 405-2, you’ll enjoy relaxed, effortless and fatigue-free listening.
Overall, the 405 errs on the warmer, mellower side of things, great for low-fatigue listening. The Quad 405 adds little in terms of distortion, something you’ll hear in the relaxed flow of sound from one. If you find a 405 that doesn’t sound super smooth, there’s something wrong with it.
I work plenty of 405s and 405-2s and whenever I test one, I’m taken by how incredibly sweet they sound. Evidence comes in the long listening times one can put in with the Quad. The lack of glare and listener fatigue are noticeable and unusual with solid-state gear.
Some describe the 405 as sounding rolled off, but I disagree. To me, they sound smooth and lacking that edgy quality that many transistor amplifiers possess. Measurements don’t support the claim of a rolled off top-end, but it’s not a wide-bandwidth amplifier by any means. Nor is it likely to blow up like one though either!
Bass is clean and full, but perhaps not the last word in slam or authority. There’s no ‘bass bloom’ here to flatter small speakers though, just clean, accurate bass. Remember though that this amp has just one pair of output devices per channel.
The midrange is warm, detailed and three-dimensional with good source material. This is definitely a strength of the 405-2, like a good valve amplifier perhaps. Sound staging works to enhance this feeling of three-dimensional realism and pinpoint imaging.
Overall, a good, fresh Quad 405 or 405-2 is a beautiful sounding amp. My only criticisms are a slight lack of sparkle and air compared with the very best. This dramatically improves with a series of improvements that I now implement in these amplifiers when they come in.
This slight lack of air, combined with slightly reduced energy in the bottom octave are things that separate the 405 and 405-2 from the very best I’ve heard in my listening room, like the Perreaux PMF-5150B, Krell KSA-150 and BAT VK-500.
This is an easy amplifier to work on. You can have the whole unit completely apart and broken down to board level in minutes. It’s equally quick to reassemble. I can’t think of an amplifier that’s easier to work on. You can even remove the side panels and work on a module whilst it’s still bolted to the chassis. Check out some of my Quad 405 work here and here.
All electronic parts are replaceable with modern equivalents, but factory parts quality is high. The only issue that is the 405-2 current limiting sub-boards are no longer available, but even this is no real problem. You can remove them altogether if you like to operate with no protection, the amp will work fine without them. Better still, they’ve been reverse engineered and are available from Dada Electronics for example.
Furthermore, the popularity of the Quad 405 and 405-2 has spurred on the development of a range of modern replacement modules. Some are rubbish, like these cheap Chinese boards found on eBay for example. But others look amazing, like the . More on what they sound like here.
People often ask me questions about “what would go best with this” and so on. For a long time, I ran my Quad 405-2 with a pair of Yamaha NS-1000M monitors. This is an excellent match, the slightly mellower sound of the Quad 405-2 is a great partner to the airy clarity of the wonderful NS-1000.
Many will also be aware of the legendary Quad electrostatic speakers like the ESL-57 and later ESL-63. Quad designed the 405 and 405-2 to run these speakers, so this is also an excellent match. As a general rule, the 405 and 405/2 lack really deep, tight bass though, so don’t expect one to drive your Infinity Kappas perfectly, for example, it just won’t work. The Quad is better suited to small to medium-sized speakers of moderate sensitivity.
Bang for the Buck
Quad 405s and 405-2s currently sell for around $500 – 750 AUD. They would be a deal even at $1500, so they are a fabulous buy for less. Seriously, what else can you get that sounds this good for 500 bucks? The best thing is the understatement of these lovely amplifiers, like fine pieces of 1950’s English industrial art.
It’s a very good idea to take your 405 or 405-2 in for overhaul if it hasn’t been. Many are running original capacitors and parts, a testament to their durability, but they can be dramatically improved. This is something I’ve been doing now for a while now, with great results. A full restoration is an even better idea.
I’ve had customers ask if it’s worth getting these units professionally upgraded. Consider that to replace a 405 or 405-2 with something probably not as good, will cost $5000 AUD or more. With this in mind, spending a few hundred dollars to refresh and seriously upgrade your Quad becomes a no-brainer.
The Stereophile reviews of the 405 and 405-2 are good places to start to get your head around what people were thinking when these amplifiers were released. There is a great 405 review and some background info, here.
Positive Feedback has a great piece, with lots of mods and other technical details here. Yet another page lists more modifications and upgrades – these amps are so great to work on that people are literally encouraged to play with them!
One of my favourite resources is Keith Snook’s fantastic Quad repository. Keith goes into great detail about his own journey with the 405 and breaks things down into two large and very detailed 405 pages. The first covers the original 405 and its various iterations, the second covers the 405-2. Keith’s site contains a wealth of information. You’ll also find the service manual along with all revisions and parts lists.
The Bottom Line
If you are looking for an amp to soothe and reinforce your faith in solid-state gear, I urge you to listen to a Quad 405-2. Better still, grab yourself one to play with on a rainy day, you won’t regret it.