Monday, December 18th, 2017

Quad 405 Stereo Monitor Amplifier

By editorJuly 19, 2014

CLASSIC GEAR REVIEW

In 1936, Peter J. Walker formed a company, in London, initially called S.P. Fidelity Sound Systems, then later, after a move to Huntington (near Cambridge UK), renamed to the Acoustical Manufacturing Co. Ltd., finally becoming best known for the Quad brand (from an acronym ‘Quality Unit Amplified Domestic’.)

Quad_405-2

After building a reputation for designing quality valve hi-fi amplifiers and electrostatic loudspeakers for the hi-end domestic market, Peter Walker applied a little known audio circuit technique called feed-forward error correction (first patented way back in 1929 by Harold Black) and designed the ‘current dumping’ Quad 405 power amplifier, an innovation still used in Quad power amplifiers to the present day.

From Peter’s December 1975 technical paper in the ‘Wireless World’ magazine,

‘An audio power amplifier is required to produce an output signal that differs from the input signal in magnitude only.  It must therefore have occurred to every circuit designer that it should be a simple matter to take a portion of the output, compare it with the input to derive an error signal. It is then only necessary to amplify the error signal and add it to the output in the correct amplitude and phase to cancel completely the distortion of the primary amplifier.’

Peter put this principle into practice using two amplifiers per channel instead of one.

The first stage ‘error’ amplifier is low powered but very high quality. The second amplifier is high powered, but of lesser audio high quality.  (It’s a lot more difficult to achieve very low distortion in high powered amplifier stages).  Peter designed a way to compare the high powered output with the original audio input and derive the required error correction signal which is then injected into the audio path, in such a way that the high power audio output achieves a very low distortion figure, even at very high power levels.

This innovative product earned Quad the Queen’s Award for Technological Achievement in 1978.

In the late ‘70’s and ‘80’s, if you were seriously into hi-fi and had the spending power, then a Quad 405 would be sitting in your listening room.  I had the desire, but sadly not the spending power!

Whilst the Quad 405 was most common in home hi-fi set ups, I came across many examples of them and their brethren in the BBC during the 1980’s.

Fast forward to 2014 and there I am scanning eBay on an almost nightly basis looking for a good condition Quad 405 with some ‘service’ history.  Quite a few with ‘seems to work okay but can’t be sure’ and ‘sorry about the scratches on the case’ but never just the right one.  Finally, one evening, up pops a Quad 405-2.  The -2 version had some design improvements and this one had been in the same ownership for a long time.  Lots of reassurance that it worked just fine.  So I bid, and bid, and bid and …. got it!

You don’t pop a classic (and very heavy) power amp in the post and I was not going to trust it to a carrier, so after the minor inconvenience of a 300 mile round trip, there I am staring at an icon from the late ‘70’s hi-fi world, all ready to couple it up to the PMC DB1+ passive speakers in my home studio.  Well not so fast!

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Today, you would typically reach for a pair of ¼” jack or XLR leads to connect to a monitor amp or powered speakers.  But we are back in 1978 and the ‘Philips’ 5-pin DIN connector was king.  Back on to eBay and I am relieved to find someone selling a five pin DIN to twin phono sockets (with the correct pinouts!) and I’m nearly in business.

Next little challenge is operating levels.  In those days, typical maximum input levels to hi-fi amps were very low.  In the case of the Quad 405, you only have to input 0.5V RMS (-3.8dBu) to get the full 100 watts output.  So feeding it directly from my converters which output +18dBu for 0dBFS was not going to work unless I wanted to blow both my speaker cones and probably the studio windows as well.  Oh, and did I forget to mention that the Quad has no input gain control?  Or input source selection?

Then we have the small matter of unbalanced inputs designed at a time when digital recording systems (read lots of digital noise from PCs into the mains) were not even a glimmer in the eye.

So I brought into play my trusty stereo mastering compressor and devised a cunning plan (with a few simple mods and some front panel button pushing) to use it as both a level converter and as a transformer isolated feed to the Quad.  Although the Quad has unbalanced inputs, transformer isolation of the feed audio from the D-A converters still cures any ground loops carrying digital noise.

Connection to my PMCs was easier but still required sourcing four 4mm banana plugs which were the loudspeaker connection ‘fruit of choice’ at the time.  Some fretting if they were going to provide a good connection (which they do) but finally, it was switch on time.

First impressions were awesome.  Hand on heart, my PMCs have never sounded so good.  I used to wonder why some folks used the adjective ‘punchy’ to describe quality audio reproduction and now I know why.  I also used to be convinced that it was the loudspeaker which would totally dominate the sound quality in a monitor setup.  I stand corrected.

This most amazing and innovative classic power amp design brings a quality of monitoring experience which still leaves me wondering, and slowly understanding, what I’ve been missing all these years.  Stereo imaging and instrument placement are outstanding.  The really hard to reproduce very dynamic instruments such as piano and brass simply come alive when amplified through the 405.

Even with my level matching mods, I still need to take care not to blow the speaker cones up!  You just want to turn this Quad 405 up and up.

Well of course nothing is perfect, so let’s delve into the dark side of this wonderful classic amp.

Quad_405-3

You’ll see from the image above that the inside is dominated by the mains transformer which is very large, and in this case, getting on for 35 years old.  The core has begun to gently hum as often happens with old mains transformers.  This is not ‘down the cable’ hum; this the actual transformer core physically vibrating (in sympathy with the 50Hz mains voltage which it is transforming from high to lower voltage) into the atmosphere.  With the reality that getting a replacement transformer would be impossible, the Quad 405 now sits discreetly out of sight below my monitor desk and the hum has ceased to be a practical issue.

I know there are plenty of quality monitor power amps available today and some of these are at quite reasonable prices.  But with a little care and attention, you can bring into practical use a wonderful icon of the monitor amplifier genre for a very modest budget, in my case £160 plus some door to door delivery mileage.

Useful Links

Quad Website

You can also find a whole bunch of useful information including User Manual, Test Report and Schematics at Audio Gear.  Click on the link below

Audio-Gear-Mini-15  Quad 405 Database Listing

Technical Specification

Power output: 100 watts per channel into 8Ω (stereo)

Frequency response:

-1db at 20Hz

-0.5db at 20kHz

-3dB at 50kHz

Total harmonic distortion: < 0.01%

Hum and Noise:

‘A’ weighted -96dB ref full power

Unweighted -93dB ref full power (15.7kHz measurement bandwidth)

Left/Right Channel Crosstalk:

80db @ 100Hz

70dB @ 1kHz

60dB @10kHz

Input sensitivity: 0.5V for 100 watts into 8 ohm load

Speaker load : 4 to 16 ohms (nominal)

Signal to noise ratio: 95dB

Speaker load impedance: 4Ω to 16Ω

Dimensions: 115 x 340.5 x 195mm

Weight: 9kg

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