Pre/Power Amplifiers

The world’s oldest hi-fi magazine was, of course, on the trail of Quad’s ‘33 Control Unit’ and 303 power amplifier back in the day [HFN Apr ’68]. Moreover we also re-tested ‘mint’ originals as part of our Vintage Review series [HFN Jul ’11]... so how did they perform? The 33 preamp offered a useful +13.6dB gain, sufficient to raise a full 1V output from a contemporary tape or radio source. Also, with just 450mV required to generate the specified 45W/8ohm output from the 303 power amp, the 33 was well within its comfort zone even if its limited maximum 1.6V output and high 900ohm-1.8kohm source impedance is out-paced by today’s newcomer. Distortion was 0.014-0.075% (20Hz-20kHz) while its response had a gently downtilted presence/treble at –0.2dB/10kHz, –0.4dB/20kHz to –4.1dB/100kHz. Quad had also engineered a steep LF filter with a –3dB point at 20Hz, falling away to –18dB/10Hz.

Above: Original Quad ‘33 Control Unit’ and 303 power amplifier reviewed in HFN Apr ’68

The original 303 power amp had a milder LF rolloff (–3.6dB/10Hz), a moderate 0.16-0.76ohm output impedance and a progressively obvious treble loss with decreasing speaker loads (–0.55dB/20kHz into 8ohm to –1.8dB/20kHz into 2ohm). However, distortion was impressively uniform at ~0.02% through upper bass and midrange and although its power output was good to a full 2x50W/8ohm, its dynamic prowess – just 10W/1ohm – suggests it was rather less capable of driving low impedances than the reimagined 303 of 2024. PM