Solid State Preamp Reviews

I performed a complete set of tests on the Quad 33 preamplifier with my Audio Precision SYS2722 system. Looking first at the 33's performance as a line preamplifier, all the inputs and outputs preserved absolute polarity, ie, were noninverting. The balanced input impedance was close to 20k ohms from 20Hz to 20kHz; the single-ended input impedance was 8.7k ohms at 20Hz and 1kHz, 8.2k ohms at 20kHz. The preamplifier's balanced output impedance was a low 111 ohms from 20Hz to 20kHz; the unbalanced impedance was 119 ohms from 20Hz to 20kHz; the headphone output impedance was a very low 3 ohms across the audioband.

The Quad's maximum gain was 0dB (unity gain) for the balanced input to the balanced and unbalanced outputs and the unbalanced inputs to the unbalanced output. For the unbalanced inputs to the balanced output, the gain was 6dB. The maximum gain at the headphone output was 3.1dB for the balanced input, 9.04dB for the single-ended inputs.

Fig.1 shows the 33's balanced frequency response into 100k ohms (blue and red traces) and 600 ohms (green and gray traces) with the tone controls bypassed and the volume control set to the maximum. The response is flat up to the 200kHz limit of this graph and was identical from the unbalanced and headphone outputs. At lower settings of the volume control, the level of the left channel was 0.3dB higher than that of the right channel. With the tone controls activated, the output was down by 3dB at 100kHz. The Bass control gave a boost or cut up to 3.5dB below 50Hz (fig.2), while Quad's classic Tilt control tilted the response up or down by up to 3dB (fig.3).

Channel separation was very good, at >90dB below 1kHz in both directions and still 82dB at 20kHz. With the volume control set to the maximum, the wideband, unweighted S/N ratio (ref. 2V output) was 70.9dB in both channels, which increased to 87.5dB when the measurement bandwidth was restricted to the audioband, and to 90.2dB when A-weighted. Spectral analysis of the Quad's low-frequency noisefloor with the preamplifier outputting a 1kHz tone at 2V with the volume control set to the maximum is shown in fig.4 (blue and red traces). Repeating this analysis with the volume control set to –12dB and the level of the input signal increased so that the output was the same 2V (green and gray traces) dropped the level of the noisefloor by 12dB.

Fig.5 plots the THD+noise percentage in the Quad preamp's balanced output against voltage into 100k ohms with the volume control set to the maximum. Actual distortion lies below the noisefloor up to 4V output. With clipping defined as when the THD+N reaches 1%, the 33's balanced outputs clipped at a high 9.5V into this load. The balanced clipping voltage into 600 ohms was 5.8V, while the unbalanced outputs clipped at 4.65V into 100k ohms and 3.6V into 600 ohms. The headphone output clipped at 4.9V into 100k ohms. Unusually, when I repeated these tests at lower settings of the volume control, the maximum voltage was reduced in proportion to the setting. For example, with the volume control set to –6dB, the balanced clipping voltage into 100k ohms was 4.75V, ie, 6dB lower than it had been at the maximum setting. With the control set to –12dB, the clipping voltage dropped by another 6dB. This behavior, which was consistent with both balanced and unbalanced input signals, suggests that the clipping voltage that I measured was actually due to the input stage being overdriven rather than the output stage. Given the preamplifier's low maximum gain, I don't believe this behavior will be an issue in use. However, it is unusual.

The balanced outputs' THD+N percentage at 5V into 100k ohms was very low across the audioband (fig.6, blue and red traces). While it rose at this voltage into 600 ohms (green and gray traces), it was still low in absolute terms. While the distortion harmonics lay below the random noisefloor at 4V into 100k ohms (not shown), the second harmonic lay at a very low –99dB at the same voltage into 600 ohms (0.001%, fig.7). Intermodulation distortion with an equal mix of 19 and 20kHz tones was similarly very low, even into 600 ohms (fig.8).

Turning to the Quad 33's phono input, I examined its measured performance in both MM and MC modes. To minimize noise, I connected a wire from one of the Audio Precision's ground terminals to the grounding post on the preamp's rear panel. The MM input impedance was 45k ohms at 20Hz and 1kHz, 40k ohms at 20kHz. The MC mode's input impedance was 101 ohms from 20Hz to 20kHz. Both input modes preserved absolute polarity at the balanced, unbalanced, and headphone outputs.

The phono input's RIAA correction (fig.9) was well matched between the channels and extremely accurate in the audioband, though the low-frequency response was down by 2dB at 10Hz. The MM mode's wideband, unweighted S/N ratio, ref. 1kHz at 5mV, assessed at the balanced output with the inputs shorted to ground and the volume control set to the maximum, was a good 66dB in both channels. Restricting the measurement bandwidth to 22Hz– 22kHz increased the ratio to 86.4dB, left channel, and 84.9dB, right, while inserting an A-weighting filter resulted in a ratio of 90dB in both channels. In MC mode, ref. 1kHz at 500μV, the ratios were respectively 60dB, 68dB, and 74dB.

To be sure I wasn't prematurely clipping the preamplifier's output, I examined the phono input's overload margins with the volume control set to –12dB. Both the MM input's and MC mode's margins were impressively high. The MM mode's margins ref. 1kHz at 5mV were 26dB at 20Hz and 1kHz, 19dB at 20kHz. In MC mode, the margins ref. 1kHz at 500μV were 29.5dB at 20Hz and 1kHz, 23dB at 20kHz. Even with an input 6dB below the 1kHz overload level, the only distortion harmonics present were the second and third at an inconsequential –100dB (0.001%, fig.10). The levels of the intermodulation products with an equal mix of 19 and 20kHz tones were similarly negligible (fig.11).

While I was puzzled by the relationship between the maximum output voltage and the volume control setting, the Quad 33's measured performance as a line preamplifier, phono preamplifier, and headphone amplifier was otherwise excellent.—John Atkinson