Quad Revives Its Iconic 1960’s 33 Preamp And 303 Power Amp

Probably one of the most celebrated amplifiers ever made—the 33/303 two-box pre/power amp system from Quad—is back. The British high-end hi-fi brand is reviving the design 57 years after it first made transistor-based amplifiers respectable.

Teased in prototype form at May’s High End audio show in Munich, the reimagined Quad 33 preamplifier and Quad 303 stereo power amplifiers take the best of the most sought-after ‘vintage’ amplifiers on the second-hand market.

When they were launched back in 1967, the innovative circuit design established transistors as a viable form of amplification. The new 33/303 contain the same signature technologies, in an industrial design that pays homage to the past while embracing the future.

Transistor-based amplifier technology was relatively new in the mid-1960s. Valve-based amplifiers dominate the high-end from a sound quality perspective. The 33 preamp and 303 stereo power amp were Quad’s first transistor designs and were highly unusual inside and out. They were also well built and many remain in use and Quad offers a refurbishment service for those who still love their 33/303 combos.

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Most transistor amps in the 1960s were integrated models with the preamp and power amp stages—and often a radio tuner—in a single box. Quad’s design put the preamplifier and power amplifier in separate cases and sold a separate FM tuner.

The 303 power amp, with its vertical orientation and larghe heatsink fins, was designed to be tucked out of view, leaving just the 33 preamp on show. The 33 with it grey metal casework and orange and white buttons, contrasted with the flush-mounted rotary controls. The design won a Council of Industrial Design Award in 1969 and the combo gained iconic status, confirmed by its presence at the Museum of Modern Art in New York.

Quad’s founder, Peter Walker, introduced several innovations that set the 33/303 apart. The 303’s ‘triples’ transistor output configuration solved the thermal instability issues that plagued early transistor amplifier designs. This circuit layout produced 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 special. It is sometimes described as ‘warm,’ especially in the context of other solid-state amplifiers at the time, which tended to sound rather hard and harsh. The richer sound made it less tiring to listen too, bu there was plenty of detail and punch on offer. It had the sound of a valve amplifier with a cool-running transistor design. The 33 and 303 amps were so good, they are still sought out by made in the 1960s and ’70s are still sought by enthusiasts today.

Quad’s decision to revive the 33 and 303 was due to the continuing demand for these amps on the second-hand market, four decades after manufacturing was discontinued. Taking such an iconic design and updating it provided the opportunity to take onboard advances in design, engineering and manufacturing.

The project was undertaken by an in-house team at Quad’s Huntingdon HQ in Cambridgeshire. The team consisted of Jan Ertner, lead electronics designer for Quad and its sister-brand Audiolab. Rob Flain and Paul McConville are Quad’s most experienced service engineers and their knowledge was key to breathing new life into the new 33/303. David McNeill, Quad’s industrial design chief for the last two decades took charge of the overall look of the two new units.

McNeill opted for an evolutionary design of the external design instead of a radical revamp. The new 33 preamplifier’s small size and shape reflects the original, but with more detailing. The color of the casing is grey with less of a taupe accent and more matt silver.

The control layout is similar, with a volume knob to the left and a trio of flush-mounted rotary controls on the right, although the functionality of the controls is slightly different. Importantly, the original 33’s orange accents remain, although they have been cleverly updated. The buttons beneath the rotary controls are now all orange with the addition of LEDs to illuminate them. The orange strip with the labelling for the three flush-mounted rotaries now includes an LCD display with orange backlighting. These advances weren’t possible back in the 1960s, but the essence of the new 33 is still there and completely in keeping with the original aesthetic.

The backlit buttons and LCD display—which can be dimmed or switched off—are not the only modern twists. Between the 33’s two rows of orange buttons are a 6.35mm headphone socket and an IR receiver for the remote control.

Like the 33, the new 303 stereo power amplifier looks like the original but slightly updated. The vertical orientation remains, together with the classic heatsink fins at the front. There are now ten fins rather than eight which have been neatly squared off. The addition of an illuminated orange standby button ties the new 303’s aesthetic with the 33. Finally, the Quad’s musical note symbol is etched into the top surface.

The original 33 and 303 were admired for their reliability and performance. Even some recording and broadcast studios used them. The new editions have been engineered to offer the same kind of longevity, supported by Quad’s legendary technical service team led by Flain and McConville who helped design them. In half a century, these new amps could still be providing music pleasure, just as the originals do now.

The original 33 preamplifier was launched when vinyl was the main source material and CDs were still 15 years away. The 33 preamp offered several vinyl-oriented facilities for users to sculp the sound, including 5k, 7k and 10k filters, accessed via the buttons under the rotary controls for bass, treble and slope adjustment. These filters were designed to remove the sound of vinyl surface noise and high frequency distortion when used in concert with the slope dial.

The new 33 preamp is decisively analog without a built-in DAC for digital sources although it has been adapted to better suit contemporary needs. 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 on the original 33, it’s now a classic Quad feature and so has been added to the new 33.

Walker believed traditional bass and treble controls were a clumsy way of tweaking tonal balance. The tilt control differs because it adjusts 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 tilts the audible frequency range on a 700Hz axis, adjusting the overall sound with 'warm' or 'cool' hints without impacting volume or coloring the sound.

The tilt control works alongside the independent bass adjustment, accurately applied from -3dB to +3dB. These subtle, precise and consistent methods of tonal tailoring are useful given the varying quality of the music sources people listen to today. They also help tune the sound to suit different acoustic environments and tastes.

The original 33 used DIN sockets for its inputs and outputs. These have been switched for a mix of single-ended RCA and balanced XLR sockets, providing a 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 (3 x RCA and 1 x XLR) and a phono input. 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 USB input shows that the new 33’s user interface is microprocessor controlled, the audio signal path is totally analog. A high-quality motorized Alps potentiometer adjusts the volume, while the three rotary encoders for bass, tilt and balance provide the precision of digital control but act entirely in the analog domain.

The new 33’s audio circuitry is faithful to the spirit of the original but acknowledges the quality of today’s electronic components, which is far superior to those available in the 1960s. The design team come up with 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 big advance on the original, with adjustable gain for MM and MC cartridges, precise RIAA equalization, precision input filtering and an upgraded power supply, delivering all the detail in a vinyl record’s grooves. The dedicated headphone amp is a completely new addition with a current-feedback design and high slew rate to ensure it works with all types of headphones.

The original 303 stereo power amplifier is still admired by hi-fi enthusiasts and studio professionals because of its exceptional reliability, the flexible way it can be set up and the compact form. It’s smooth and richly detailed sound also won it many fans. All these attributes are in the new 303, which evolves the core circuit design of Quad’s innovative original but with up-to-date techniques and technologies.

Peter Walker’s wonderful circuit innovation for the original 303 remains at the new amplifier’s heart. A breakthrough in transistor amp design required less negative feedback as well as exemplary thermal stability with lower distortion.

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 very low levels without sacrificing stability under all dynamic conditions.

The original amplifier’s Class AB design has been thoroughly revised with a new 200VA low-noise toroidal transformer 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 superior linearity and exceptional thermal stability. The result is a compact, cool-running power amplifier that has the capacity to deliver dynamic power and keep a firm hold on the music with rich sonic detail.

The original 303 delivered 45W per channel into 8Ω, reducing significantly into 4Ω with current limitations which might prove problematic with some of today’s loudspeaker loads. Maintaining design continuity, the new edition does not try 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 8Ω, rising to 70W into 4Ω. Although the 8Ω figure might seem a modest increase, a much higher maximum current delivery of 10A ensures it is adept at driving demanding loads. Quad says 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 might 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 a single speaker. This increases the power output to 140W per channel into an 8Ω load, providing more sonic muscle and control.

At the back of the new 303 are gold-plated binding posts to connect speakers, a choice of RCA and balanced XLR inputs for the preamp, plus a 12V trigger input for the convenience of automatic power up/down when connected to the 33 preamp’s trigger output.

The new 33 preamp and 303 power amp offer a compact and cool-running analog pre/power pairing faithful to the originals but updated inside and out. The fact that vintage 33 and 303 units made in the 1960s and ’70s remain in use today, and their sound still admired, is testament to the ingenuity of the original designs.

The new editions celebrate Quad’s heritage. They capture the silky sonic delivery of the originals that set them apart from other transistor-based amps of the time, adding more clarity, detail, bandwidth and dynamic impact.

Pricing & Availability: The 2024-edition 33 preamplifier and 303 stereo power amplifier are available from authorized Quad retailers from mid-November, priced at £1,199 each.

Tech Specs:

Quad 33 Preamplifier (2024 Edition)

• Inputs: 1 x stereo XLR; 3 x stereo RCA line level; 1 x stereo RCA phono (MM/MC)
• Outputs: 1 x stereo XLR; 1 x stereo RCA; 1 x stereo RCA AUX; 1 x headphone; 2 x 12V trigger.
• Other facilities: Bass and Tilt tone controls; balance control; remote control.
• Dimensions (WxHxD): 258 x 82.9 x 165mm.

QUAD 303 Stereo Power Amplifier (2024 Edition)

• Amplifier type: Class AB with ‘triples’ output stage; stereo and bridged modes.
• Power output – stereo: 2 x 50W (8Ω, THD<1%); 2 x 70W (4Ω, THD<1%).
• Power output – bridged” 2 x 140W (8Ω, THD<1%); 2 x 170W (4Ω, THD<1%).
• Distortion (THD): <0.003% (1kHz 8Ω @ 2/3 max. power).
• Maximum current delivery: 2 x 10A.
• Inputs: 1 x stereo XLR; 1 x stereo RCA.
• Outputs: 1 x stereo speaker binding posts; 1 x 12V trigger + link.
• Dimensions (WxHxD): 120 x 176 x 325mm.