Original Website © Amplabs 2009 This new site created April 2018

Quad 303 Full Left & Right PCB upgrade with high performance circuits (Q303HPUG)


The Quad 303 has a beautiful warm, valve like sound when correctly serviced. It is well know for its sound quality and robustness. Despite this, we receive lots of requests from enthusiasts and audiophiles alike, to ‘improve’ the audio quality. Electronic devices have advanced a lot over the years and today’s well designed amplifiers offer an improved performance in terms of slewing rate, power handling and distortion. With a well produced recording, these amplifiers  separate out the individual instruments and voices across the sound arena more readily than the 303 but the difference can be quite minimal unless you have a decent pre-amplifier and of course good quality source material and speakers.  If you  want your 303 to really bring out the qualities of your favourite titles and other HI-FI equipment, we have listened to your requests and have finally got around to designing and producing a new circuit and corresponding upgrade PCBs. The new PCBs contain new circuits, designed and configured by Rob Beales our engineer. He named the new boards ‘Sufi’. For the technically minded, each Sufi circuit is based on discrete high performance transistors, with a long-tail-pair front end using, current mirrors and constant current sources, featuring a slow turn-on circuit. These drive a fully complementary output stage.  On good quality recordings, the result is a noticeably uncluttered ‘sound stage’ with a good bass response and beautifully clean mids and highs. The sensitivity is the same as the original 303, so you won’t have to change your pre-amplifier settings. The output power capability of the new circuit is in excess of 50W r.m.s. per channel into 8 ohms. This is more than adequate for even the largest living room, and more than the standard 303.  We have purposely used ‘the old way,’ (through hole components), rather than modern SMD technology, to make parts procurement and replacement easier in the unlikely event of a component failure. What we do to your 303 to bring about this transformation: The L & R PCBs are removed from your Quad 303 and new high performance plated through hole, double sided FR4 boards are fitted.  The four smoothing/output caps are replaced and fitted using our L & R fused PCB capacitor board. The heatsink transistors are replaced with fully symmetrical MJ15003/4 matched pairs. (5 off)  We also fit our front panel upgrade which incorporates an on-off switch. gold plated ,RCA sockets and professional 4mm LS binding posts. This improves the overall interconnectivity and electrical safety. If you prefer we can leave your existing front panel in place, the price is the same. 
Full component replacement with new, Amplabs SUFI circuits, output transistors  and .capacitor  PCB
For   ure listening pleasure