In this second article in my Quad 405 series, I review the legendary Quad 405-2. Find out why so many love these amazing little amplifiers and why you should buy one – a real one.
The Quad 405 was and remains an incredible 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 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. Keep in mind that the Quad 405 is about as far away from typical US amplifiers of the time as one could get. It’s unlike any other power amplifier from that era.
The Quad 405 and 405-2 are compact, measuring just 115 x 340.5 x 195mm. At 9kg, this is 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 on 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. But, as might be expected, original units can sound tired, ‘hummy’ and in general need of TLC at this age.
Some describe the 405 as sounding rolled off, but I disagree. To me, these amplifiers sound smooth and lack 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. It lacks the last word in slam and authority. If anything, it’s the bass that’s rolled off to my ears and keep in mind that these amplifiers are capacitor coupled at the input, so they 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, so the most current hungry speakers are going to stretch these diminutive amps.
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. Treble is smooth, never etched or bright. What a welcome antidote to the bright and even painful amplifiers out there.
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 reduced however with a series of improvements that I implement in these amplifiers.
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 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 . I had a customer who owned two ‘405-3’ amplifiers contact me and we chatted for a while about how good these modules sounded! More on what they sound like here.
Clones & Knock-Offs
I’m often asked about the cheap boards and 405 clones found on eBay and elsewhere. Check the comments and you’ll see I’ve had long discussions with readers about this. Understandably, many will have questions, especially those with limited experience working on electronics, owning and listening to a range of good audio gear.
First off, I’ll never understand trying to save just almost nothing on something that’s already a bargain. Let me draw a parallel with replica watches. Just like a real Rolex or Omega, a real Quad 405 contains premium quality genuine parts rather than cheap fakes. All the original electronic and mechanical parts are of very high quality and all contribute to the sound. The clones use the lowest grade re-labelled and fake parts.
“But Mike, they are Quad 405 boards, it says so..!”
No, they definitely aren’t. Look, I can wear a badge that says I’m Brad Pitt, and I’m still not, unfortunately.
Did you know that the superb original potted transformer makes a big contribution to the sound? Clones often use cheap toroidal transformers wound with sorry that’s not real copper and will not sound anything like the original, for this reason alone. Don’t even get me started on semiconductors and how important they are.
China is the number one source of counterfeit semiconductors and most are simply rebadged, cheap parts made to look vintage that are clones or copies of other well-known parts. Real On Semi 15024 devices are $10 – $20 each, depending on how many you buy. Do the math and figure out how each board can sell for $40 with way more than $40 worth of ‘real’ parts on it. Anyone..? Bueller..?!
A real Quad 405 or 405 board contains the actual DNA and parts specified in the original design. Replicas have redrawn boards, circuit changes, and cheap board material, populated with the aforementioned fake parts. They have no performance guarantees and are built and sold by people cashing in on the desirability of the original product and on buyers looking to find a ‘bargain’.
If you’re looking to buy a clone, you are likely inexperienced, hunting for a bargain and probably haven’t heard a real 405 or 405-2, so the replica watch comparison is a good one.
But Mike, I’ve read the reviews of people who’ve bought these, they love them!
Look harder, definitely not all of them.
There’s something called confirmation bias which virtually ensures that you’ll read positive reviews from people who’ve convinced themselves that these are great value. These reviews are written early and can’t be changed later, when the amplifier fails, the reviewer hears a real Quad 405 and the ‘value drunkenness’ wears off.
A lack of experience means that, for many, a clone may actually be the best amplifier they owned. It may even be the only amplifier, further reducing the credibility of such reviews.
Would you wear a fake Rolex? Do you think a ‘clone’, with a crappy imitation case, heatsink and fake parts is good value when you can get a real one for around the same money? If you answered yes, buy a clone, but don’t call it a bargain, just call it what it is – a low-quality knock-off of one of the great amplifiers, for not much less than the real thing.
For everyone else, I assure you that clones, replicas and counterfeit parts are just a waste of money, for the same reason fake watches are. A real Quad 405 or 405-2, made in England, is a genuine hi-fi bargain, often selling for only a few hundred bucks. These amplifiers still represent the ultimate high-value hi-fi, despite people waking up to them.
Buy the real thing. You’ll be helping not fund the production of counterfeits and ripoffs and you’ll also have the real sound of the lovely Quad 405 / 405-2. Invest in having it professionally overhauled and you’ll have a low-cost, high-value friend for life.
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, though as a commenter pointed out, the ESL-57 can be too tough a load for many amps, so the 405 and 405-2 are definitely better with the ESL-63.
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
Unimproved 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. I have a series of improvements I can apply to your 405 or 405-2, developed, tested and implemented for various customers, and in my own 405-2 of course. The sonic benefits are not subtle, turning this already great-sounding amp into a serious heavy hitter.
A full restoration is an even better idea, 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, polystyrene and polypropylene capacitors, various premium electrolytic capacitors, new lab-grade power supply caps and more. Everything has been carefully chosen and tested for the best results.
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 decent but that probably won’t sound 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 no-brainers in hi-fi!
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! If in doubt, look at the 405 above.
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 encourage you to listen to a Quad 405 or 405-2. Better still, just grab yourself one to play with on a rainy day, you won’t regret it. For the price you’ll pay for the genuine article, you simply cannot go wrong.
If you already own a Quad 405 or 405-2 and you’d like me to service, overhaul or improve it for you, visit our Contact page to get in touch.
If you enjoyed this article or maybe it’s helped you choose a Quad 405, you’re very welcome to say thanks by buying me a drink via the donate button! 🙏
Quad 405-2 Power Amplifier$500 - $1500AUD
Chassis / Build-Quality9.5/10
- Superb build and parts quality
- Smooth, relaxed, refined sound
- Ease of service, high reliability
- Tiny for the level of performance
- Deep bass and air slightly lacking
- DIN connectors and no power switch
- Capacitor coupling non-DC design