Quad 405 Current Dumping Amplifier

Quad 405


The Quad 405 and the later much revised Quad 405-2 Current Dumping Amplifiers are one of the most unique and much loved classic british made power amplifiers of the last twenty five years. The skill of the design, the engineering and thought that have gone into this amplifier make it stand out as an oustanding product and it is no coincidence that the Quad 405 first won a Design Council Award 1976 and then later in 1978, The Queen's Award for Technological Achievement, the only Hi-Fi manufacturer to have won such awards.

The Quad 405 and 405-2 boasts a generous 80 watts per channel into 8 ohms, and has a very neutral open sound that is quite detailed in some areas but just slightly lacking in others. Part of the reason for this is the age and quality of some of components. Fortunately for those who are willing to invest a little time and money and get companies such as Net Audio to upgrade the op-amps and electrolytic capacitors, these amplifiers can compete with the best modern day hi-fi has to offer. There will be more articles to follow on upgrading the Quad 405 and 405-2 in later posts.

The other beauty of these amps is because they operate in pure class A mode as opposed to class AB which requires bias adjusting after time, the amplifier will never sound different or degrade overtime so long as the capacitors are replaced every 10-12 years. This means that because of their build quality, the Quad 405 and 405-2 should keep on going and going without issue.

Since this article was originally written, Net Audio have started manufacturing brand new "Mk 3" boards for the Quad 405 and 405-2. More information can be found here, but having listened to them for the last six months, these new boards for the Quad 405 and 405-2 offer significant improvement especially if used with Net Audio's Dual Mono Power Supply.

This entry was posted on Friday February 10th, 2006 at 1:56 PM and is filed under Amplifiers, Classic. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response.

13 Responses to Quad 405 Current Dumping Amplifier

Nigel from beds Says:
December 10th, 2007 at 12:12 PM

How many units were made of the 405 and mk2 please?

Net audio Says:
March 7th, 2008 at 11:26 PM

Quad have made around 110,000 405s since the mid 70's.



Adrian Says:
October 10th, 2008 at 12:19 PM

Net Audio upgraded the boards in my 405-2 a few years ago and I was amazed at the difference in sound afterwards. I did opt for the expensive OPA627 opamps too. The sound after the upgrades is detailed and very satisfying. Like many old 405's mine had been very well used in a studio environment and probably due servicing anyway.

Apart from a later power supply caps upgrade I did myself the NetAudio board was the only upgrade and I'm very happy with them.

Peter woodland Says:
March 6th, 2009 at 11:31 PM

what are the differences between a 405 and a 405/2.? can the original 405 be upgraded to the same standards\?

Max Says:
March 16th, 2009 at 11:09 AM

The only specific change was to the current limiters in the output stage. The 405 used classic 'load line' limiters which reduce the current limit as the voltage across the output transistor increases. The 405-2 limiters looked at the history of the current & voltage of each transistor to adjust the current limit according to the degree of recent abuse.
The effect is to allow, for brief periods, higher currents into low impedance loads, without risking damage to the output transistors. The change had to be made because manufacturers started labelling speakers as '8 ohms' when in fact they had much lower impedance at some frequencies, to boost the sound level.
You can upgrade a 405 to a 405-2 by swapping the two power amplifier circuit boards, if you can find them. If you don't drive the amplifier to the point where a 405 starts to current-limit (not clip) then there is no performance improvement

Shane Says:
August 21st, 2009 at 5:51 PM


Has someone ever compared the Quad 405 to the Quad 11 valve amps? If so, what is the difference between the two?

As I find the 405 to have a valve like sound.



Keith Says:
September 26th, 2009 at 6:51 PM

In the 1970's I had one of the original Quad II amps and also one of the first of the 405's; it was my impression back then that they did indeed have a very similar sound apart from the fact that the 405 had a lot less mains hum than the valve amp.

I bought the 405 after reading the article about current dumping by Peter Walker (and the ensuing debates about it) in Wireless World. Being an Electronic Engineer myself, the first thing I did was to open it up and look at the build quality and the circuit schematic. I was dismayed; the robust external construction was in complete contrast to the cheap and nasty components that they had used internally and the printed circuit board which was built on SRBP rather than the fibre-glass substrate looked like something built by a hobbyist in their kitchen. After one of the PA boards blew up a few months after purchase I re-engineered it using decent components and it sounded much better and worked reliably until the day twenty years later when my Missus nagged me into selling it because she wanted some crappy matt black box with loads of LED's on it in the living room instead.

Max Says:
October 5th, 2009 at 9:28 AM


In 1978 Quad commissioned James Moir & Associates to organise a blind (but not double-blind) A/B listening trial between the QUAD 405, 303 and II amplifiers. The panellists were 'experts' of one sort and another, and at the insistence of one panellist, the speakers used were Yamaha NS1000's (remember those?). These required a parallel pair of QUAD II's to drive each speaker at the same realistic level.

The test used a few mutually agreed musical excerpts. These were played first in position 'A', then in position 'B', and panellists recorded only if they heard a difference or not (no preference). The A & B positions were randomised among the three amplifiers, and could of course be the same on both. No statistically significant ability to distinguish any pair of amplifiers was found.

This test wouldn't quite meet today's standards (not double-blind, no listener control of A/B position) and the general preference these days is for A/B/X testing, but the result is a pretty good indication that they do all 'sound the same'.

I've got a copy of the test report if anyone is interested, (although I'm not clear about any copyright issues.)


Shane Says:
October 16th, 2009 at 11:21 AM

Cheers Max, very interesting.

Carolo giuseppe Says:
October 10th, 2010 at 8:25 PM

sono in possesso di 2 amplificatori quad 405,sono stati ascoltati per alcuni anni ,poi fermi fino a oggi,vorrei sistemarli e mettrli di nuovo in sala per l'ascolto.Io sono di Padova itaty,a chi mi potrei rivolgere per un buon lavoro di sostituzione dei componenti che il tempo ha degradato? Un grande ringraziamento per la eventuale risposta ,grazie.Giuseppe Carolo.

Carolo giuseppe Says:
June 17th, 2011 at 9:17 AM

A Carolo Giuseppe. sono Italiano tecnico elettronico appassionato Hi Fi e di Ampli Quad. Ne ho fatti molti e upgradati oltre 20. se vuoi contattami con email o telefono
al 388 1219118 mi chiamo Francesco Vona

Steve abbott Says:
April 29th, 2012 at 2:04 PM

i have a quad 405-2 power amp. i am a bass player. i use an Ampeg 115-300 combo. i have another pair of 12" and 15" EV speaker cabs. each rated at 200w. i would like to slave the Quad, off the line out of the ampeg, and therefore drive the additional 2 x EV cabs. Is this possible to do, and would i require any mod to thye quad, and/or a pre-amp. the line out sig from the ampeg is popst pre-amp btw.
anyone be able to help???

Ed davies Says:
February 7th, 2013 at 1:09 AM

Quad were good but prefer the sound and the state of the art technology of the NAD 2200PE. Far more advanced than the 405 and the 606

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