This is the second article in my Quad 405 series, a review of the legendary Quad 405-2. Find out why so many people love these great amplifiers and why you should buy a real one.
The Quad 405 was and remains an incredible little amplifier, even now, some 40+ years after it was introduced to the world. This article you are reading right now is one of the most popular articles on my entire site. That should tell you something about the Quad 405’s enduring qualities and how people are drawn to these lovely amplifiers.
Tellingly, the Quad 405 and 405-2 are most loved by those for whom listening to music is more important than looking at power meters, fiddling with controls and discussing technical specifications. It’s curious, but this little industrially styled amplifier punches well above its weight, all these years later.
Don’t get me wrong, I love power meters, but the 405 is a study in elegant, stripped back, understated design. 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, power 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, 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. At 9kg, this a small, 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.
The driver and output transistors are thermally coupled to one block of aluminium. Everything is board-mounted, there’s almost 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. It really is genius in terms of packaging and miniaturisation at a time before SMD manufacturing.
The two amplifier modules make up virtually all the electronics in a 405 / 405-2. Other elements include a superb-quality potted transformer, two 10,000uF filter capacitors, a diode bridge, some power supply wiring, a large front-mounted heatsink and some sockets. As a repairer, the Quad 405 series amplifiers are just about the easiest to service if you know what you are doing.
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 beautiful sounding amplifiers and amongst my favourite solid-state designs. They aren’t the best at any one thing, but what they do well, they do very well. From the moment you hear one you’ll enjoy relaxed, effortless and fatigue-free listening.
Overall, the Quad 405 errs on the warmer, mellower side. A 405 adds little in terms of distortion, something you’ll hear in the relaxed flow of music and long periods of fatigue-free listening possible with one. If you find a 405 that doesn’t sound smooth, there’s something wrong with it or with another part of your system.
I work plenty of 405s and 405-2s and whenever I test one, I’m taken by how incredibly sweet they sound. The lack of glare is noticeable and unusual with solid-state gear.
Some describe the 405 as sounding rolled off, but I disagree. To me, these amplifiers sound smooth and lacking that edgy quality of many transistor amplifiers. Measurements don’t support the claim of a rolled off top-end, but it’s not a fast, wide-bandwidth amplifier by any means. It’s also unlikely to blow up like one either!
Bass is clean and rich but lacks the last word in slam and authority. If anything, its the bass that’s rolled off to my ears and keep in mind that these amplifiers do have an input filter. There’s no ‘bass bloom’ here to flatter small speakers though, just clean, accurate bass. Remember, this amp has just one pair of output devices per channel.
The midrange is warm, resolved 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 amplifier. My only criticisms are a slight lack of sparkle, air and deep bass compared with the very best. These audible traits 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.
These limitations are dramatically addressed however with a series of improvements that I now implement in these amplifiers, customer budget allowing of course.
Quad 405 Serviceability
The Quad 405 is an easy amplifier to work on compared to most. The whole unit can be completely apart and broken down to board level in minutes. 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 original factory parts quality is very 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.
Clones & Cheap Knock-Offs
On this point, I’m often asked about the cheap boards and 405 clones found on eBay. Check the comments and you’ll see I’ve had long discussions with readers about this. Understandably, those without experience working on electronics and comparing replicas to the real thing will have questions like this.
Let me draw a parallel with replica watches. Just like a real Rolex or Omega, the real 405 contains genuine parts rather than cheap fakes made to look real. The original parts are of very high quality, especially the transformer, chassis, heatsinks etc. The fakes are often the lowest grade, made to look genuine.
Did you know that the superb original transformer makes a huge contribution to the sound? Don’t even get me started on semiconductors and how important these are. China is the number one source of counterfeit semiconductors and most are simply rebadged versions of super-cheap parts, made to look vintage.
A real 405 or 405 board contains the real design DNA from the original design. Replicas often have redrawn boards and circuit changes. They have absolutely no performance guarantees and are built and sold by people simply cashing in on the desirability of the original product and on the number of buyers looking to find a ‘bargain’.
Would you wear a fake Rolex, lacking all the quality elements of the real thing? Do you believe that knock-off parts made to look like the real thing are electrically the same as the originals? Do you think a clone at $500 – $1000 AUD, with a crappy case and heatsinks, filled with fake parts is good value?
If so, maybe a clone is for you. For everyone else, I can assure you that replicas and counterfeit parts are no bargain, they are just a waste of money. A real Quad 405-2, made in England, with original parts, for just a few hundred bucks is one of the few genuine bargains in hi-fi. Save up, buy the real thing and you’ll be helping not fund the production of counterfeits and replicas.
People also 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, it just won’t work. The Quad is better suited to small to medium-sized speakers of moderate sensitivity.
Overhaul & Upgrade
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 have your 405 or 405-2 overhauled 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 do quite a bit as part of the complete overhaul and upgrade process I’ve developed for these amplifiers. This includes output matched devices, thermal pads, op-amps, special MIL-spec silver mica capacitors, various premium electrolytic capacitors, new lab-grade power supply caps and more. Everything has been carefully chosen and assessed.
I’ve had customers ask if it’s worth upgrading a 405 or 405-2. 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 improve your Quad is one of the great audio no-brainers.
Have a go if you know what you are doing, but if you don’t, spend the money and have the work expertly done. I’ve seen too many of these amps where people have fiddled and caused problems. Consider what could you be doing to earn money while an expert works on your amplifiers instead!
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. I’ve written a lot more about the Quad 405, including several pieces about restoration, modifications and upgrades, and servicing. Positive Feedback has a great piece, with lots of mods and other technical details. Yet another page lists more modifications and upgrades.
One of my favourite resources is Keith Snook’s fantastic Quad repository. Keith goes into detail about his own journey with the 405 and breaks things down over two large and very detailed pages. The first covers the original 405 and its various iterations, the second covers the 405-2.
The Bottom Line
If you are looking for an amp to soothe the soul, I urge you to listen to a 405 or 405-2. Better still, just grab yourself one to play with on a rainy day, you won’t regret it. If you already own a 405 or 405-2 and you’d like me to service or overhaul it for you, visit my Contact page to get in touch.