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Vintage Amplifier mods and why we do them


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Vintage Amplifier modifications and why we do them!

Ok! You've seen our ads offering to upgrade your lovely vintage classic with modern circuitry and superior specifications. It has come to our notice on several forums that many of you are asking, why we do it?  Its simple really, “the all too human quality ... the desire to make money!” This is of course is not the right answer. It is more to do with another desire, call it a fetish if you wish, to improve and develop the things we love.

 Lets take a good example, the humble Quad 303. Even today it still looks the part. Built like a brick, and rendered in matching brown hues, it exudes quality. At launch and for many years during its success, most of us, including yours truly, could not afford one. Now you can buy this classic on ebay for a song.

The trouble is, electronics has moved on. Most of  the connectors are obsolete,and signal levels have changed. To cap it all, if it is going cheaply on ebay,  it  probably needs a good service. I bought one.  It was desperately in need of servicing. So after a considerable amount of de-soldering and component replacement, I did some intense listening on several loudspeaker set ups. I must admit, I was pretty impressed with the overall aural quality. Then as if this wonderfully warm and soothing  quality was just a start to something even more ethereal, I imagined how it  would sound using  modern, discrete, circuit techniques, without resorting to the use of ICs  It’s similar to the way a racing car mechanic tweaks an already potent engine to produce that extra ounce of performance. We do it because we get a great kick out of improving the things we love and of course, because we can!

First, I decided to get rid of the 303's voltage regulator circuit. I read somewhere that Peter Baxendall said they only included it because they were worried about the low Vceos (70V) of the output transistors. Modern high performance power transistors are readily available with Vceos of 180V or higher so this would not be an issue. The trouble is, changing the output transistors with modern epitaxial devices in a Quad 3O3, will inevitably cause instability problems. This is due to the excessively long wires used to connect the hinged PCBs to the transistors. These are about a foot long for the left channel. In the original design the transistors used were homotaxial types. These are fabricated using a large chip area and have a high inter-electrode capacitance. Epitaxial types have a smaller chip area as the fabrication extends deep into the wafer and a correspondingly small inter-electrode capacitance. Epitaxial types therefore exhibit a superior HF performance, but the down side is, they are more prone to instability, particularly if long interconnecting leads are used.  Extra capacitors could be used to stabilize  the HF response, (adding extra poles for the technically minded) but this is not good engineering practice.  

The solution would be to design new PCBs and place them near the heatsink and do away with the long wires. They would have to be mounted on the side panels or on an added mezzazine panel floating above the transformer.  Placing the PCBs near the heatsink would have the added advantage of allowing the LHS  and RHS power transitors to occupy the left  and right heat sink fins respectively.

The next step was to design a new and stable amplifier circuit based on modern epitaxial output transistors. A fast slewing rate, very low harmonic &  intermodulation distortion and the ability to drive awkward loads, were prime design objectives. 

Once the circuit design had been finalised a schematic was drawn up and captured to produce a prototype PCB layout.  The dimensions of which were obtained after constructing a stable mounting plate to house the two boards. 


pictures to be added here  when time permits


After stripping out the old PCBs, output transistors and large can electrolytics, we are left with the skeletal remains;  a well built chassis, transformer and two aluminium end castings, one of which forms the rear heatsink.


pictures to be added here  when time permits


The front panel was the next item on the agenda. The essential change immediately apparent due to current safety legislation is the replacement of the old 3 pin  Bulgin mains inlet. Many 3O3/33/FM3 owners are still using Bulgin plugs with the screw-on cap. It became apparent that the cap can develop a hair line crack, and come off in the hand during extraction, exposing the live terminals!  Not surprisingly, it is no longer approved for mains use. Unbelievably some suppliers are still selling these as mains connectors! 

Note:  If you still use this type of Bulgin mains plug please fit the approved type which uses small screws to hold the two plug halves together.


pictures to be added here  when time permits



The completed Quad make over: 

The result, a fresh looking Quad 3O3 externally, with its modern new sockets and on-off switch, but inside a new heart ready to take you into that strange ethereal world of pure sound.  It is something special, not just because it has been re-engineered  but because it has been honed with that other human  quality…love!


Please note:

At present, the completely modified Quad 3O3 upgrade (new heart) is not available as a kit or as a special service for your existing 303.

Depending on the interest we receive we may produce a limited number of boards as part of an upgrade service.

The modified 3O3 front panel with fused IEC socket and on-off switch, phono sockets and loudspeaker barrels is available now as an upgrade.

Please enquire for details on 01905 821737




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Last modified: Monday, 27. April 2015 12:57