The 33/303 two-box pre/power amp system from QUAD is making a triumphant return 57 years after it first recalibrated expectations for transistor-based sound.
Teased in prototype form at the High End audio show in Munich in May, the reimagined QUAD 33 preamplifier and QUAD 303 stereo power amplifier reflect not only the appeal of modern-retro design in audio, but also the high esteem in which the original amps are held.
Upon their launch in 1967, their circuit design elevated transistor-based amplification to new heights. The new 33/303 contains highly evolved versions of the same signature technologies, wrapped in an industrial design that echoes the past and embraces the future.
Transistor-based amp technology was still relatively new in the mid-1960s, with valve-based circuits continuing to dominate the most highly prized hi-fi amplifiers from a sound quality perspective. The 33 preamp and 303 stereo power amp were QUAD’s first transistor designs, following several revered valve amplifiers, and were highly unusual inside and out.
For start, most transistor amps of the time were integrated models with the preamp and power amp stages – and often a radio tuner – combined in a single box. QUAD’s design housed the preamplifier and power amplifier in separate compact-width units (there was a separate FM tuner too).
The 303 power amp, with its distinctive vertical orientation and prominent heatsink fins, was designed so it could be tucked away from view if the user desired, leaving only the 33 preamp in view.
The latter sported grey metal casework accented by orange and white buttons and flush-mounted rotary controls, earning it and its partnering power amp a Council of Industrial Design Award in 1969. Today, the 33’s design has gained iconic status, confirmed by its presence at the Museum of Modern Art in New York.
Under the lid, QUAD’s founder Peter Walker introduced several innovations that set the 33/303 combination apart. Chief among these was the 303’s ‘triples’ transistor output configuration, which solved the thermal instability problems that plagued early transistor amp designs. This circuit topology produced impressively low distortion, prompting QUAD to promote the 303 as “the world’s first low-distortion transistor amplifier”.
The sound of the original QUAD 33/303 pre/power amp combination was considered something special. It is sometimes thought of as ‘warm’, especially in the context of typical solid-state amplifiers of the time, which by comparison sounded rather hard and sometimes harsh. Its richer style of sound made it less fatiguing to listen to, yet there was no shortage of detail and engaging ‘punch’.
QUAD’s decision to revisit the 33 and 303 was in no small part due to the continuing demand for these amps on the second-hand market some four decades after they were discontinued in the 1980s.
To achieve this vision, QUAD assembled an in-house team at its HQ in Huntingdon, Cambridgeshire, led by Jan Ertner, Lead Electronics designer for QUAD and sister-brand Audiolab, Rob Flain and Paul McConville, QUAD’s most experienced service engineers, and David McNeill, QUAD’s Industrial Design Chief for the last two decades.
In deference to the 33 and 303’s industrial design, David has delivered a continuation of the external form rather than a radical overhaul. The new 33 preamplifier’s compact size and shape is faithful to the original, but the detailing has been enhanced and the casework’s grey colour is a little less taupe and more matte silver.
The distinctive control layout is similar, with a volume knob to the left and a trio of flush-mounted rotary controls to the right (although the functionality of these controls is now somewhat different).
Crucially, the original 33’s orange accents remain, albeit y updated. The buttons beneath the rotary controls are now all orange (no white), with the addition of LEDs to illuminate them. And the orange strip containing labelling for the three flush-mounted rotaries now includes an LCD display with orange backlighting. Such things were not possible in the 1960s, yet the way these visual updates have been applied to the new 33 is in keeping with the classic aesthetic.
The backlit buttons and LCD display – which can be dimmed or switched off entirely – are not the only nods to modern expectations. Between the 33’s two rows of orange buttons are a 6.35mm headphone socket and an IR receiver for the supplied remote control – more ‘mod-cons’ added to the original’s specification.
Like the 33, the new 303 stereo power amplifier is instantly recognisable yet smartly updated. The original’s vertical orientation remains, together with the distinctive heatsink fins at the front – albeit there are now 10 fins rather than eight, and these have been neatly squared off.
The addition of an illuminated orange standby button ties the new 303’s aesthetic with the 33, topped off with QUAD’s musical note symbol etched into the top surface.
The original 33 and 303 were so admired for their reliability and performance that they were not only used in homes; some recording and broadcast studios adopted them too. The new editions have been engineered to offer the same longevity, supported by QUAD’s technical service team.
The original 33 preamplifier was launched at a time when vinyl was king and digital audio was still some 15 years away from entering the domestic scene in the form of CD. It offered several vinyl-oriented facilities for users to tailor the sound, including 5k, 7k and 10k filters, accessed via the buttons beneath the rotary controls for bass, treble and slope adjustment. These filters were intended to remove record surface noise and high frequency distortion when used in concert with the slope dial.
The new 33 remains a staunchly ‘analogue’ preamplifier, with no built-in DAC for digital sources, but its facilities have been adapted to better suit contemporary requirements. For a start, the filters for vinyl have gone and the bass, treble and slope dials have been replaced by bass, tilt and balance.
The ‘Tilt’ control was created by QUAD’s founder Peter Walker as a more sophisticated form of tone control. It was introduced in 1982 on the QUAD 34 preamp, which succeeded the 33; although not contemporary to the original 33, it is considered a classic QUAD feature that remains highly useful and so has been added to the new 33.
The tilt control differs by adjusting both ends of the frequency spectrum together, either attenuating the bass and lifting the treble or lifting the bass and attenuating the treble in 1dB steps. Essentially, it rotates – or tilts – the audible frequency range on a 700 Hz axis, thereby adjusting the overall sound balance with ‘warm’ or ‘cool’ hints without altering volume or adding colour to the sound. The tilt control works in tandem with independent bass adjustment, accurately applied from -3dB to +3dB.
Connectivity has also evolved. The original 33 used DIN sockets for its inputs and outputs; these have been swapped for a mix of single-ended RCA and balanced XLR sockets, opening up a much wider range of cable options. The original’s four source inputs – labelled ‘disc’ (for vinyl), ‘radio 1’, ‘radio 2’ and ‘tape’ – have been replaced by four line-level inputs (3x RCA and 1x XLR) and a phono input, and there is also a choice of RCA and XLR outputs to connect the 303. Other modern conveniences include two 12V trigger outputs and a USB data input for firmware updates.
While the latter input shows that the new 33’s user interface is microprocessor controlled, the audio signal path is entirely analogue. A high-quality motorised Alps potentiometer adjusts volume, while the three rotary encoders for bass, tilt and balance provide the precision of digital control but act entirely in the analogue domain – classic QUAD features updated for 2024 and beyond.
The new 33’s audio circuitry is faithful to the intention of the original, whilst acknowledging that the quality of today’s electronic components is vastly better than those available in the 1960s. The design team has created an entirely new circuit design, including a low-noise, custom-specified toroidal transformer, numerous reservoir/smoothing capacitors and five regulated supply rails.
The new 33’s low-noise phono stage is also a significant advance on the original’s, with adjustable gain for MM and MC cartridges, precise RIAA equalisation, precision input filtering and an upgraded power supply, delivering every drop of detail dug from a vinyl record’s grooves. As for the dedicated headphone amp – an entirely new addition – its current-feedback design and high slew rate ensures a dynamic and detailed performance with all manner of headphones.
The original 303 stereo power amplifier was (and still is) admired by hi-fi enthusiasts and studio professionals alike for three reasons: its reliability, the positional flexibility permitted by its compact form and its smooth and detailed sound. All these attributes are equally applicable in a modern context to the new 303, which evolves the core circuit design of QUAD’s original using today’s techniques and technologies.
Peter’s circuit innovation for the 303 – the ‘triples’ output stage – remains at the new amplifier’s heart. It effectively forms a feedback amplifier-within-an-amplifier, requiring far less negative feedback, possessing thermal stability (meaning the current in the output stage is practically immune to temperature fluctuations) and producing lower distortion.
Whilst updated with the finest modern circuit components, the new 303’s ‘symmetrical triples’ design is faithful to the original, providing a completely symmetrical output stage and rendering the resting current independent of output transistor temperature. Distortion is reduced to emphatically low levels without sacrificing stability, maintaining this performance under all dynamic conditions.
The original’s Class AB design has been thoroughly revised, including a new 200VA low-noise toroidal transformer specially developed for the new 303. QUAD’s new generation ‘triples’ design provides an advanced feedback topology for the discrete power amplifier’s output stage, delivering linearity and thermal stability. The result is a compact, cool-running power amplifier that possesses the capacity to deliver dynamic power and maintain a firm hold on the music while providing rich sonic detail.
The original 303 delivered 45W per channel into 8 ohms, reducing significantly into 4 ohms with current limitations which might prove problematic with some of today’s loudspeaker loads. Maintaining design continuity, the new edition does not attempt to massively ramp up the wattage in line with modern trends – doing so, especially within its classic compact form factor, would change its circuit and characteristics to such a degree that it could no longer be considered a ‘303’.
The new amp is rated at 50W per channel into 8 ohms, rising to 70W into 4 ohms – the 8-ohm figure might seem a modest increase, but a much higher maximum current delivery of 10A ensures it is highly adept at driving demanding loads. Too much attention is sometimes paid to published power outputs without considering other factors – in reality, the new 303 will drive a wide range of speakers with as much ease as a typical amp with a 100W rating, but many such amps would lack the 303’s finesse.
The new-edition 303 offers an upgrade path through the ability to connect two 303s in bridged mode, combining each amp’s two stereo channels into a single mono channel so each unit drives one speaker. This increases the power output to 140W per channel into an 8-ohm load.
Around the back, the new 303 is equipped with gold-plated binding posts to connect speakers, a choice of RCA and balanced XLR inputs for the preamp and a 12V trigger input for the convenience of automatic power up/down when connected to the 33 preamp’s trigger output.
The 2024-edition 33 preamplifier and 303 stereo power amplifier are available from authorised QUAD retailers from mid-November, priced at £1,199 each.