Quad 405 Power Amplifier Review

This is the second article in my Quad 405/405-2 series. In this piece, I review the legendary Quad 405-2. Read on to  find out what all the fuss is about and for restoration details, check out my first article!

Tellingly,  the Quad 405 and Quad 405-2  amplifiers are most loved by those who enjoy listening to their music, rather than worrying about controls and meters on their amplifiers. I’m not out to bag metered or complex amps, but the 405 is a study in elegant, simplified design, a champion of the ‘less is more’ mantra. The amplifier does an amazing job of just getting out of the way and playing music, surely the goal when designing any good amplifier.

Quad 405 vs 405-2

The Quad 405 and 405-2 are small, heavy, ‘loaf of bread’-sized 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 of 405-2 to be a little less aggressive. This allows the 405-2 to better drive lower impedance loads, like the 4 ohm speakers commonly available now. If you own current-hungry speakers, or those with a nominal 4 ohm impedance, the 405-2 is a better option.

If you are interested in more technical detail, I recently overhauled my own Quad 405-2. You can read about that here. You can also skip to 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.

This is a schematic, representing a typical iteration of the 405 layout. Note that this is one entire channel, minus the power supply transformer. Not a lot there really…

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 here.

Simple, Elegant Design

I can’t think of a commercially available amplifier with fewer parts or a simpler layout than the Quad 405. Tell me if you know of one, I’d be keen to take a look. The driver stage consists of two TO-220 devices operating in class-A, the output stage two perfectly matched N-channel TO-3 devices, per channel. The driver stage is a very high-quality amplifier, the output amp is the ‘current dumper’ and is a less precise, more robust unit.

This is one entire amplifier channel!

The driver and output transistors are thermally coupled to the same block of aluminium and board-mounted. You’ll see this in the images above and below. This is great for uniform heating and minimising superfluous wiring, another example of clever design.

Notice how compact the amplifier modules are, and the low parts count.

Two boards make up almost the entire amplifier. Other parts include a high-quality transformer, two modestly-sized filter capacitors, some wiring, a large front-mounted heatsink, an LED and some sockets.

The 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 small size. The 405 is also powerful, able to deliver 100 watts per channel, more than enough to drive most speakers.

The entire amplifier consists of the chassis, power transformer, two filter capacitors and two amplifier boards. The whole thing comes completely apart in minutes.
405-2 Specifications (Courtesy of the Hi-Fi Engine)
  • Power output: 100 watts per channel into 8Ω (stereo)
  • 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
Relaxed, Effortless Sound

I’m specifically talking about the Quad 405-2 here and it’s easier to describe this amp’s sound by using an oft-used hi-fi euphemism – it doesn’t really have any. That’s not exactly true, but seriously, if you get a good one, you can immediately tell that these are special. From the moment you switch it on a good 405-2, you’ll enjoy relaxed, effortless and fatigue-free listening.

That’s not to say it has no sonic signature, of course it does. Overall, I would say the amp errs on the warmer, mellower side of things, which is always good for low-fatigue listening. The 405 also features very low distortion and you can hear it, or the lack of it. Evidence is the long listening times one can put in with the Quad and the lack of glare, listener fatigue really noticeable by its absence. The 405 is definitely a big smoothy.

This is going too far, but you probably get what I mean!

Some have described the amp as sounding rolled off. To me, it sounds creamy and smooth and lacking that edgy quality that many transistor amplifiers possess. Measurements don’t support this claim of a rolled off top-end, so I don’t believe it’s actually rolled off. Instead, I think what one is hearing are the results of low distortion and perhaps also, the payoff from signals passing through only a small number of parts.

Bass is clean, full and deep, but perhaps not the last word in authority. There’s no ‘bass bloom’ here to flatter small speakers, just clean, mostly accurate bass. Remember though that this amp has just one pair of output devices per channel. This is fewer than almost any other commercially available amplifier and yields other benefits.

I find the mid-range to be 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, the Quad 405-2 is is a very good sounding amp. My only criticisms are that sense of a slightly rolled off treble and the resulting slight lack of sparkle and air compared with the best. This, combined with slightly reduced energy in the bottom octave are things that separate this amplifier from the very best I’ve heard in my listening room, like the Krell KSA-150, Accuphase A-30 and BAT VK-500.


This is an easy amplifier to work on. I’m not kidding, you can have the whole thing completely apart and broken down to board level in minutes. Its equally quick to reassemble. I can’t think of an amplifier that’s easier to work on. You can even remove a side panel and work on a board whilst still attached to the chassis.

All parts are replaceable with modern easy to find equivalents. The only caveat on that is the current limiting sub-boards are no longer available, but even this is no real problem. You can remove them altogether, if you like to fly by the seat of your pants 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. Others look amazing however, NET Audio used to offer 405-3 ‘mark 3’ boards, though it appears they no longer do, which is interesting.

NET Audio 405-3 board
Bang for Your Buck

Quad 405’s and 405-2’s currently sell for around $500AUD. Seriously, this would be a great deal even at $1500, so for half a ton, it’s a great deal. Seriously, what else can you get that sounds good for 500 bucks? And the best thing is how understated they are, like a fine piece of 1950’s English sculpture.

System Matching

People often ask me very good questions about “what would go best with this” and so on. I run 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 exuberant, clarity of the wonderful NS-1000M. 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, a no-brainer if you own 57’s or 63’s.

The Bottom Line

If you are looking for an amp to sooth and reinforce your faith in transistors, 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 🙂

Further Reading

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, however, 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 really is a wealth of information, so I encourage you to visit and spend some time reading and looking at the schematics, which Keith has painstakingly redrawn into one document showing the changes at each version of the board. You’ll also find the service manual along with all revisions and parts lists, very handy indeed.

Quad 405-2

Quad 405-2

Chassis / Build Quality




Sound Quality







  • Superb build quality
  • Relaxed, effortless sound
  • Ease of servicing
  • Plenty of power
  • Awesome bang-per-buck


  • Slight lack of slam and sparkle
  • No power switch!

6 thoughts on “Quad 405 Power Amplifier Review”

    1. Hi, any high-quality, transparent preamp will work with a high-quality power amp like the 405. Obviously, synergy with the rest of your system and room is very important here, so you’d need to consider this. The 405 can be bridged and run as a mono amp, but realistically, there’s plenty of power there with the 100 Watts a side available with a single amp. It would certainly be interesting to try though!

  1. Interesting article. Why do you think that the Chinese boards are rubbish? Looking at the circuit diagram for the 405, it’s pretty basic and the component cost won’t be a lot. With modern cad I would expect a better pcb layout to be achieved than the original along with better modern components than the original. Have you actually tried the copies to justify the comment? I’m asking as I’m looking at makin a modern ‘copy’ but adding a streaming server to it.

    1. Hi Richard, thanks for leaving a comment and I’ll elaborate on a couple of points here that hopefully will help.

      Have a good look around my site and you’ll see that it’s a little different. I’m a working specialist, interested in the science, engineering and sound of hi-fi gear, and in helping people make smarter, more informed choices! My comments are based on many years experience with electronics/boards/parts/ etc. If I don’t have first-hand, factual information about something, I generally won’t comment on it 🙂

      With that in mind, yes I’ve seen some of the Chinese Quad boards and they were of poor quality, hence my comment. Some I’ve seen are not copies, hence you are not getting a 405, and board quality was poor. I haven’t seen all of them of course, there may be some good copies. Sadly, most Chinese hi-fi and audio boards are of low quality.

      Regarding parts, no, modern parts are not automatically better. Some modern parts are better, some are worse and the commonly available Chinese parts are usually terrible, especially capacitors and semiconductors. With Chinese kits, there is no QA re what you’re getting, no way of knowing if the CAD layout is better, it could be worse. There are so many terrible fake semiconductors and caps out there now from China that many professionals won’t buy them at all, without quality assurance from a large vendor, myself included.

      So, with these things in mind, I can’t see the point when you can buy a real Quad 405, with superb boards, transformer, case, parts and pedigree, for a few hundred bucks. There are of course the Net Audio ‘mk 3’ boards, they look good, havent heard them though.

      In my opinion, there is only one path to follow with classic audio gear that’s to get the original! It’s all about the history, the pedigree, the fact that it sounds the way it does because of the layout and the parts.

      Anyway, hope this helped!

Thanks for reading, leave a comment and let me know what you think!