Solid State Preamp Reviews

I used my Audio Precision SYS2722 system to measure the Quad 303. I performed a complete set of tests in two-channel mode, then repeated some of the tests in bridged-mono mode. I preconditioned the amplifier at 1/8 the maximum power into 8 ohms for 30 minutes before starting the testing. At the end of that time, the heatsinks were hot, at 111.1°F/43.9°C. The top panel's temperature was 85.3°F/29.6°C.

The Quad 303 preserved absolute polarity with both the balanced and single-ended inputs in both two-channel and bridged-mono modes. The balanced input is wired with pin 2 hot, the AES standard. The balanced input's voltage gain at 1kHz into 8 ohms was 22.6dB; unusually, the single-ended input's gain was higher, at 29.2dB. The gains in bridged-mono mode were respectively 29.6dB and 34dB. The balanced input impedance was 18k ohms from 20Hz to 20kHz; the unbalanced input impedance was 5.8k ohms at 20Hz and 1kHz, 5.65k ohms at 20kHz.

The output impedance was low, at 0.05 ohms at 20Hz and 1kHz, rising slightly to 0.12 ohms at 20kHz. As a result, the variation in the amplifier's frequency response with our standard simulated loudspeaker (fig.1, gray trace) was negligible. The response into resistive loads was flat up to 40kHz, rolling off by 3dB at 200kHz into 8 ohms (blue, red traces), at 125kHz into 4 ohms (cyan, magenta traces), and at 61kHz into 2 ohms (green trace). The unbalanced input's frequency response was identical to that shown in fig.1; the response in bridged-mono mode into 8 ohms was down by 3dB at 150kHz. Even though there are now two output stages in series, the output impedance in bridged-mono mode was still low, at 0.065 ohms in the bass and midrange, rising to 0.165 ohms at the top of the audioband. The Quad 303's reproduction of a 10kHz squarewave into 8 ohms in two-channel mode (fig.2) featured short risetimes without any overshoot or ringing.

The Quad 303's channel separation was very good, at >80dB in both directions below 3kHz and still 80dB, L–R, and 69dB, R–L, at 20kHz. The unweighted, wideband signal/noise ratio taken with the single-ended inputs shorted to ground was a very good 79dB ref. 1W into 8 ohms (average of both channels). This ratio improved to 93.65dB when the measurement bandwidth was restricted to the audioband, and to 96.5dB when A-weighted. Spectral analysis of the low-frequency noisefloor while the Quad 303 drove a 1kHz tone at 1Wpc into 8 ohms revealed that the random noisefloor was very low in level, at –120dB (fig.3). While some power supply–related spuriae were higher in level than the random noisefloor, given the very small size of the 303's chassis, the low levels of these spuriae suggest exemplary circuit board layout.

Quad specifies the 303's maximum power with both channels driven as 50Wpc into 8 ohms (17dBW) and 70Wpc into 4 ohms (15.44dBW). With clipping defined as when the THD+noise reaches 1%, the Quad amplifier slightly exceeded the specified powers with a 1kHz signal, clipping at 52W into 8 ohms (17.16dBW, fig.4) and 77W into 4 ohms (15.85dBW, fig.5). The FTC's updated "Amplifier Rule" states that maximum power should also be assessed at frequencies other than 1kHz. I therefore repeated the clipping test with a 20kHz signal. At this frequency, the Quad 303 clipped at the specified power of 50Wpc into 8 ohms. (The amplifier went into protection mode just above this power, putting it into standby. Turning it on again via the front-panel button restored normal operation.) In bridged-mono mode, the Quad 303's specified powers are 140W into 8 ohms (21.46dBW) and 170W into 4 ohms (19.3dBW). I measured clipping powers of 152W into 8 ohms (21.8dBW, fig.6) and 180W into 4 ohms (19.54dBW). I didn't test the maximum powers in either mode into 2 ohms, as these aren't specified.

The downward slope of the traces in figs.4–6 suggests that the actual distortion lies beneath the noisefloor at powers below 20W. I therefore examined how the Quad 303's THD+N percentage varied with frequency at 12.67V, equivalent to 20W into 8 ohms and 40W into 4 ohms (fig.7). The THD+N was very low in the bass and midrange into both impedances, and while it rose at higher frequencies, the level was still low in absolute terms.

The distortion waveform with a 1kHz tone at 20W into 8 ohms was primarily the second harmonic (fig.8). Spectral analysis (fig.9) revealed that the second harmonic lay at a very low –97dB (0.0014%), and while higher-order harmonics were present, these were all lower in level. The spectrum at the same voltage into 4 ohms was similar, though the second harmonic had risen to –90dB (0.003%). Repeating the spectral analysis at 50W into 8 ohms in bridged-mono mode gave a very similar spectrum to that shown in fig.9. Intermodulation distortion with an equal mix of 19kHz and 20kHz tones at 40W peak into 4 ohms was impressively low in level (fig.10).

The Quad 303 offered excellent measured performance, with very low levels of noise and distortion. Notably, it met its specified power into 8 ohms with a 20kHz signal as well as with the usual 1kHz.—John Atkinson