Solid State Preamp Reviews

303 power amplifier: tech story
The transformer in the new 303's power supply is a low-noise, low-impedance toroidal, updated from its E/I laminated precursor. The rated power output from the new transformer is a modest 200VA. The new design builds on the original concept, though the new 303 has significant advantages. The new design can easily be configured for either stereo or monoblock use. This modern, cool-running, class-AB solid state amplifier provides a baseline 50Wpc into an 8 ohm–rated load in stereo, rising (not doubling) to 70Wpc into 4 ohms. In stereo at least, the 303 is not intended for very demanding loudspeaker loads.

Via masterful sleight of hand, the new 303 replaces not just the early 303 but also, with the addition of bridge-mode connectivity, the early 405 "current dumping" amplifier. Working in mono, the 8 ohm output power now increases to a useful but safely limited 140W, rising to 170W into 4 ohms. The modest 4 ohm value reflects the output current limit of 10A. While it depends on how loudly you play and how large your listening room is, even in bridge mode, relatively sensitive speakers of higher impedance will make the best match.

The original Quad 303 utilized the very smart and cleverly named "Quad Triple" transistor array in the output stage (above). The Quad Triple was a clever way of offsetting the fact that in the 1960s, no PNP output transistors were available that could result in sufficiently low crossover distortion in a complementary arrangement with an NPN transistor. So the 303 was configured in an equivalent array using only NPN transistors. (TR9 and TR10 are both NPN.) Today, well-matched NPN and PNP power transistors are readily available, so the new 303's output stage is fully complementary.

Another fundamental change: The original quasi-complementary output stage was coupled via a large electrolytic capacitor. The new 303 operates from a modern dual-rail power supply instead of the single 67V rail of old, allowing for direct coupling to the loudspeaker for better damping at low frequencies.

During my testing, I found all the power transformers in these three units—the 33 preamplifier and a pair of 303 amplifiers—to be silent mechanically. During lab testing, I misconnected an amplifier and powered it up. I expected a blown fuse at least, but the protection circuits proved effective.

The sound: The 33 preamplifier and the 303 amplifier
The sound quality of this new pairing inspires confidence, with an impressive neutrality—no fireworks but rather an easy refinement, everything in balance, almost (but not quite) self-effacing. After a week or so of conditioning, I substituted the Quad 33 (with its included phono stage) for my Naim Superline phono equalizer and Townshend Allegri Reference autoformer line control. The reference power amplifier remained in place: my Naim NAP 250 (2024). Initially, right after the substitution, the sound seemed a little reticent—softer, pleasant, with good resolution but not particularly involving. That impression soon passed. The more we—my listening panel and I—used the control unit (beyond 50 hours), the better it got, until the 33 no longer seemed in the way. Rather, it was now providing free-flowing, musically rewarding replay when fed by my reference-level ND 555 dual-supply streamer. The 33 did not perform like a reference-level design, but then it is not intended to be. It does classic, full-function preamplifier tasks without fuss and with substantial musical neutrality. Transparency rates Good+—very good at the price.

With the Quad 33 well run-in, we experimented with the facilities; eg, tone controls, on and off. We found "bypass"—no tone controls in the circuit—a little sweeter, more transparent and dynamic. There is some mild midrange bloom and loss of transparency with "tone" invoked. We also noted slightly better stereo image clarity when the display was muted. (We ran software version 1.7; updates are available online.) The line level via CD was clearly better in definition and soundstage clarity, though LP via the built-in phono preamplifier was certainly pleasant. With my Lyra Delos cartridge, I achieved a fortuitous gain match. Both MM and MC input options worked well, if perhaps not quite to maximum volume via MM.

I put on Vivaldi's The Four Seasons, with Sir Neville Marriner and the Academy of St. Martin-in-the-Fields (LP, Argo ZRG 654). The MC input was respectably quiet with no supply hum, just a little more, unobtrusive background ssh than with the MM setting. The timbre was well balanced: a little bland, reasonably spacious, with the solo violin honey toned, perhaps more decorative than involving.

Conveniently, MC/MM switching may be done remotely, allowing easy comparison of the low- and high-gain inputs after adjusting for the volume changes. A tough call, but I fancy that the MM input was a tad clearer, more open and more dynamic than MC, so a higher-output moving coil might be favored. It is worth trying the MC input nonetheless. The quality was well up to the standard required for a component at the price. I compared Bruce Springsteen's Born in the U.S.A., LP vs CD. The latter was better in definition, focus, and attack, but the LP (via the MM input) was easier on the ear and proved more rewarding.

The MC input was well matched for the Lyra Delos, with very low noise and no audible hum. The Quad 33 pre will easily drive the Naim NAP 250 power amp, but unsurprisingly the latter reveals some limitation of image depth and dynamic expression. Nevertheless, this combination remained musical and entertaining, a win in this context. There was some loss of immediacy with tone invoked. However, the tone controls worked as advertised and could with practice be used to correct more difficult recordings. Out-of-kilter timbre can be leveled up, while for high-quality, well-balanced material the tone control section was best left on bypass.

In its class, it is neutral, almost nostalgic, with good clarity, a wide soundstage but moderate image depth despite a natural perspective, ie, not too much presence. The phono input is very pleasant, with natural tonality and a relaxed quality.

While the MC phono input was well balanced over frequency and respectably quiet, the sound in MM mode was superior, with enough gain for my Lyra Delos medium-output moving coil. A higher output cartridge, like the Hana EH, would likely be a good choice for use with the MM input—and a more appropriate price match than the Lyra Delos. The 33 via the moving coil input is quiet, and via the line input, it is silent. The line stage is neutral and detailed and drove other amps well, including the NAP 250, despite the price differential.

303 bridged
In time, it became apparent that the 303 power amplifier had superior potential. More listening, with higher-definition sources, proved rewarding in terms of transparency, dynamics, and image precision. Wishing to stretch this further, two 303s were bridged as monoblocks, one channel per amplifier.

The 303 comes to life in bridged configuration. The bridged-mono set had a firmer grip on my 8 ohm Fink Team KIM loudspeakers: tighter, more tuneful bass with better rhythmic patterning. The dynamic headroom was obviously superior—indeed, I heard improvement in dynamics, musical expression, transparency, and focus. The sound overall was more upbeat and bolder, as evinced by my The Best of the Art of Noise (CD, China Records WOLCD 1010), which displayed crisp, neutral, well-defined sound with good timing, satisfying clarity, and bold image focus. This last quality was particularly evident when I substituted the Townshend Reference control preamplifier for the Quad 33 preamplifier. Images gained additional focus, fatigue remained low, the soundstage had more depth, resolution and dynamic range increased, and pace and rhythm noticeably improved. Timbres remained natural. This system was now seriously entertaining.

Randomly, I substituted a Kimber Carbon power cable for the supplied generic cable; everything improved a notch, confirmed in a single-blind comparison. This is a clear demonstration that some potential remains unrealized.

Without a doubt more open and dynamic, the bridged-mono mode sounded well timed and refreshing, displaying fine microdynamics but also a correspondingly impressive dynamic range. Playing the third-movement Allegro from Scarlatti's K.115 Sonata in C Minor on Continuum (CD, Alpha 399), Justin Taylor was spellbinding: six minutes of densely packed notation played with verve and presence—all seriously arresting via the 303 monoblocks. This amplifier illustrated the huge difference in recorded soundstage when compared to the alternative, classically laid-back, almost polite rendition from Igor Kipnis.

Perennial CD review favorites including Rickie Lee Jones's The Magazine flowed well and were also rendered fresh and lively. My much-played reference from 1989, "Monkey's Paw," from Laurie Anderson's complex and varied Strange Angels album, provided the requisite attack and surprise. Also from Laurie, "Slip Away," from Life on a String, was exceptionally spacious and atmospheric—so much so that I played the album all the way through. The superclear and superdynamic "Dreaming in Colour" from the Art of Noise's The Seduction of Claude Debussy was convincing on these monoblocks, now well–run in, demonstrating ample dynamic range and rock'n'roll loudness with my nominal 8 ohm Fink Team KIM loudspeakers.

Conclusions
This is classic, timeless audio engineering at its best, beautifully functional, versatile, elegant, essentially self-effacing, and possessed of a calm, almost relaxed musicality respectful of its legendary roots in Quad audio history. Evidently, much thought has gone into the reinvention of these classic Quad solid state amplifier separates. The new 33 full-function preamplifier and 303 power amplifier complement each other in sound, style, and price. I am won over by the high-quality engineering, the sensitive interpretation of the classic modern style and finish, and not least the fine performances I experienced through all the range of inputs. The monoblock 303 in particular has potential beyond this pairing; bridged, as I auditioned them, these monoblocks should pair well with more costly units, though they do prefer more efficient, higher-impedance speakers. Quad has not overplayed its hand.

This versatile new 33/303 pre-power pairing is just right.