The main advantage of going for this modification is increased reliability as all the components are replaced with modern high performance parts. There is a secondary advantage, the high tolerance parts allow the circuitry to maintain optimum settings over a longer period. What we do to your 303: All three PCBs are stripped down to bare boards and new high performance components are fitted. If your boards are poor quality or the first generation type,we fit new FR4 glass fibre boards. The four smoothing/output caps are replaced using our capacitor PCB modification, which includes L & R rail fuses for added protection. The full array of transistors included those on the heat sink are replaced with modern devices. All small signal resistors used are high stability 1% types. The key resistors that can overheat under fault conditions are replaced with flame proof types. All semiconductors used in the high voltage stages have higher breakdown voltages and increased current capability. The output transistors in particular are 250W devices compared to the original 117W types. This improves the reliability and output power handling. Another improvement, the critical voltage amplifier stage (VAS) which has to handle very large signal swings with low distortion is fitted with a 300V transistor with low 'early effect' . again helping to reduce distortion. As an option, the output stage can be modified from 'quasi complimentary' format to a 'fully complimentary' format. This was not possible when the Quad 303 was first launched. Complimentary high power transistors (NPN/PNP) were not readily available. To change the power output transistors to complimentary MJ15003/MJ15004, involves a simple (reversible) component change and PCB link. The fully complimentary modification will improve the overall distortion figure of the 3O3 by a small amount for a given bias current. However, the distortion of the native 303 is already pretty low ( 0.03% max at 700Hz 5W) on a fully serviced unit. Whether you can hear the difference between your completely rebuilt 303 and one properly and conventionally serviced by us depends on the quality of your source material and other HI-FI equipment as well as your aural acuity. If you are looking for an improvement in sound quality and are convinced your existing 303 is as good as it can get, then you may prefer to go for our 303 new circuit option with our SUFI board upgrades. (Q303HPUG)

Full component replacement and new front panel