Quad 33 Preamp and 303 Amp Reviewed

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Aaah, if only every manufacturer had this dilemma - too many classics in its c.v.! With Quad, do you think first of the original ESL or ESL63? The Quad 22/II? Or do you picture their biggest-selling pre-amp ever and their second-best-selling power amp, the 33/303 combination? For those of a certain age, who arrived on the hi-fi scene after 1967 but before the dawn of CD, that was the heart of a Quad system, and for many of them, it was the introduction to the UK's most venerable brand.

Additional Resources

• Check out our review of the mAMP at HomeTheaterReview.com
• Read audiophile preamp and amp reviews from the likes of Audio Research, Krell, Mark Levinson and many others at HomeTheaterReview.com.
• Read about classic tube companies on AudiophileReview.com's Tube Blog. 


Quad-303-Amp-Reviewed.gifBy the mid-1960s, the transistor had made such broad inroads into audio that even valve die-hards like Quad, McIntosh, Leak and Radford were having to produce both. Difficult though it may seem for younger readers to envision this, there was a time when major brands' catalogues included both technologies.

Gordon Hill recalled, in , 'Quad was one of the last audio manufacturers to introduce a transistor amplifier...Many famous names were early adopters and they were a commercial, if not an audiophile success. The original Leak Stereo 30 is one such example.

Competition and Comparison
You can compare the Quad 33 preamp and 303 amp against other products by reading our reviews for the Unison Research Mystery One preamp and the Beard BB 30-60 integrated amp.  You can find more information available in our Preamplifier review section and on our Quad brand page.

'In many ways Quad had built a rod for its own back. Prevailing transistor amplifiers had neither the power response nor the stability to drive the ESL-57 satisfactorily and the world at large would just have to wait if the remarkable qualities of this speaker were not to be thrown away by an unsuitable design.'

Unlike some, Quad did wait for a dependable device. It arrived in the form of the silicon epitaxial transistor, which Hill states, 'had virtually none of the disadvantages of its germanium cousin. There was a learning curve, but manufacturers did eventually produce high-gain, low-noise input devices and stable, wide bandwidth output transistors. The EF86 and KT66 were the devices of yesterday, the BC109 and the 2N3055 were the devices of tomorrow. Nearly 40 years on you can still find them, or some of their variants, in many modern amplifiers.'

Quad launched the 33 pre-amplifier and 303 stereo power amplifier in 1967 after a typically long gestation period. Many have noted that the Quad 33 is in many ways a solid-state Quad 22. Quad employee Roger Hill notes that, 'If you look at the Quad 22 and the Quad 33, it fits in the same furniture by just squaring off the corners.' The styling was brought up to date, allowing the unit to be free-standing or cabinet-mounted.

Quad provided mains power for the 33 to allow it to be used as a stand-alone unit and it had two switched sockets on the rear panel to supply mains to a tuner and a power amplifier. To maximise the available space, all the signal connections were DIN, at a time before the average audiophile grew to loathe them.

Its phono input is a conventional, two transistor amplifier with feedback equalisation, with a plug-in board to provide different cartridge sensitivities and impedances. (Moving coil was not a major concern in 1967.) Gordon Hill: 'The relatively low headroom of this stage requires that high output devices be attenuated, thus failing to maximise the noise performance of the preamplifier. This being 1967, all options for adjusting recording characteristics have been consigned to history and the response is within 0.5dB of the RIAA curve from 30Hz-20kHz. A built-in rumble filter cuts in steeply at 30Hz.'

A second plug-in board in the tape loop allowed the user to vary the output and input sensitivities, while the tape output could be adjusted to conform to the DIN standard. Gordon Hill remarked that, 'Those who were around at the time will recollect what a blessing that was. On just about every other British preamp of the period the presence of DIN sockets did not indicate conformity with the DIN standard!'

At the front, the 33 looked like no other pre-amp - other than a modernised 22. Customer loyalty was a huge part of the Quad client profile; to prevent culture shock for users moving to the 33 from the 22, Quad provided an extensive filtering and tone control system, with small rotaries for the tone controls, plus a row of press buttons for source select and filter settings. A primary rotary knob served the combined on/off and volume functions.

Its sister, the 303, was rated at 45W/ch into 8ohms, producing 28W into 16ohms and was believed to be unconditionally stable into any load. Gordon Hill felt that it was, 'Outstanding in partnership with the 16ohm ESL-57; the amplifier's performance features low distortion and a controlled bandwidth of 20Hz-35kHz, -1dB. As transistor amplifiers go, the output impedance is a relatively high 0.3 ohms, fine for 16ohm loads, less good into lower impedances. At very low impedances, performance falls off.'

Various iterations exit of the 303, including models adapted for pro use, but the basic model featured a special DIN-type connector to take the signal feed from the 33, with mains in via a 3-pin connector. Earlier versions (S/N 80,500 and below) used a miniature 3-pin Bulgin socket, while later versions use a 3-pin IEC connector.

Gordon Hill remains impressed with the 303. 'In complete contrast to today's design philosophy, the 303 uses a fully regulated power supply. The genius of the circuit lies in the innovative use of "output triples", which renders the current in the output stage virtually immune from temperature changes and ensures stable performance under widely varying conditions.' Additionally, Quad fitted the 303 with automatic current-limiting to render it virtually indestructible under nearly any combination of input and output, including an open circuit or dead short across the output terminals.

As for the sound, well, let's just say that a mint 33/303 combination will upset audiophiles who refuse to believe than vintage solid-state gear can produce satisfactory sounds. I lived with my 33/303 on a daily basis, using it 40 hours per week for four years, driving the LS3/5As on my desk. I found it so easy on the ears that, most of the time, I was simply unaware of its presence - high praise, I assure you. (While the ESL 57 is an obvious match, you simply hear it with LS3/5As.) Clean, sweet, devoid of the nastiness of most early tranny amps - it stood out amongst its contemporaries as a harbinger of doom for the commercial dominance of the valve.

Gordon Hill felt that, 'Certainly with 16 ohm loads, the amplifier behaves impeccably. On loads where the impedance plunges heavily at low frequencies, the amplifier can run out of steam and its 4 ohm performance is just about adequate. That said, there are thousands in current use all over the world and, in its day, the 303 was extensively employed in domestic, broadcasting and professional applications, satisfied users including (improbably) Pink Floyd.'

Inevitably, what goes around, comes around, and, as of 2005, Quad - as do McIntosh, Audio Research and others - produces both tube and solid-state ranges. This year, Quad relaunched a facsimile of the Quad II valve amp. But will they ever reissue the 33/303? Unlikely, and for two reasons. I was once told - emphatically - that both pieces would be too expensive to produce today, using the methods and technology of their day. Change the innards to surface-mount technology, ICs, etc, well, it wouldn't be a 33/303, then, would it?

And the other reason? The survival rates of both the 33 and the 303 are so high that, at any given time, the classifieds and the audio fairs are full of them, at bargain prices. And, yes, Quad will still repair them.

The 33/303 combination ranks amongst Quad's all-time best-sellers. As for the disproportionate number of 33 vs 303 sales, Quad accounts for that by reminding us that the arrival of the 405 power amplifier preceded the arrival of a matching pre-amp - the 44 - by four years, so a number of 405s were sold with 33s.

Quad 33 Control Unit: 120,000 produced, 1967-1982
Quad 303 Power Amplifier: 94,000 produced, 1967-1985

'My' Peter and Peter Bax [Baxandall] worked together on the 33 and 303 and did their triples [a way of making output devices so that the biasing wouldn't shift with changes of temperature], which then came out in 1967. That worked very well, so the 33/303 really started to motor, although we managed to build into the 33 a catastrophic failure.

They used to have these bloody little plug-in circuit boards that were frightfully dinky and frightfully clever and we thought we could change those for servicing, etc. It was done for all the right reasons. But the original edge connectors had tinned contacts and the boards were silver-copper, and of course with subtle vibrations they went through the tinning and oxidised, so you got resistance building up in them.

Read more about the crazy Quad story on Page 2 . . .

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