Solid State Preamp Reviews

I ran a conventional set of measurements, not so much to find a cause behind any facet of the NAD's sound quality, but to see if there were any particular aspects which would lead to system incompatibility problems. At this price level, I would be surprised by any electronic component that didn't depart from neutrality, so careful system matching is even more essential with an affordable preamplifier than with expensive models.

The moving-magnet RIAA response is shown in fig.1. Sensibly curtailed in the bass with the infrasonic filter on, it starts to rise above the audio band, reaching a maximum of 1.5dB around 100kHz. This should have an innocuous effect on sound quality, given the claimed linearity of the circuit in the MHz region. The infrasonic filter cuts the response below 20Hz, measuring –3dB at 14.5Hz and –6dB at 11Hz. With it switched out (button pushed in), the response extends to well below that figure, reaching –1dB at 9Hz or so. The signals available from the two tape outputs and the EPL output are taken after the infrasonic filter so that turntable rumble and/or disc warp information that might lead to recorder problems can be removed. The line-level response is flat through the audio band, starting to roll off gently above 30kHz, being –0.3dB at 78kHz, which is higher than the specification would suggest.

Fig.2 shows the effect on the preamp's output response of switching in the LF EQ circuit. The measured response peaks by 6.8dB at 34Hz (compared with the spec's +6dB at 36Hz), which will effectively flatten the response of a small speaker to around 35Hz. Note the sharp infrasonic filtering, however. Such LF response restoration depends on the speaker's woofer having sufficient headroom that it will not be driven into overload on normal music program; filtering out very low-frequency garbage is essential when the boost is applied to speakers with small-diameter woofers if the sound is not to suffer from Doppler distortion (though all the published research indicates that such distortion has to be gross before it becomes audible).

Fig.3 shows the nearfield response of the Celestion SL700 used for the auditioning with (red trace) and without (blue) the NAD 1300's bass EQ. You can see that the speaker has acquired useful extension in the low bass, with its nearfield –6dB point dropping by almost an octave, from 60Hz to 36Hz. Measured in-room, boundary reinforcement will bring the –6dB point down to around 30Hz with the NAD, giving a musically flat response down to below the bottom of the double bass's range at 42Hz.

NAD's engineers have put a lot of thought into the design of the 1300's tone controls. Both bass and treble controls have three center frequencies, with the band boosted or cut covering one-and-a-half octaves. In this manner, if the controls are set to the lowest and highest frequencies respectively, the response at the frequency extremes can be varied without affecting the quality of the important midrange. Conversely, selecting the highest and lowest center frequencies, 250Hz and 3kHz, with both treble and bass controls set to full boost or cut, gives the curves shown in fig.4, with considerable overlap apparent in the midrange.

This "Semi-Parametric" action, as NAD terms it, in conjunction with the bass equalization, offers considerable flexibility combined with subtlety in getting the best tonal balance from a system. NAD offers useful advice in the instructions for getting, for example, the most musical sound from CD by judicious use of equalization. (I must say, however, that philosophically, I still prefer not to use tone controls, even when they are as well-designed as they are here. Am I perverse? Yep!)

Noise levels were very low, the noise floor on the MM input when short-circuited (unweighted) lying at –70dB unweighted and 76dB A-weighted ref. 0.5V RMS output (corresponding to S/N ratios of 82dB and 88dB respectively ref. 5mV input). These figures worsened to a still very good –48dB/63dB with the MC headamp switched in circuit (equivalent to 66dB/81dB ref. 500µV input). The large improvement with the A-weighting network switched in suggests that much of the noise on the MC input is low-frequency and thus relatively innocuous in nature. Separation on the line-level inputs with the appropriate input shorted was respectable, with 63dB, 71dB, and 62dB measured at 20Hz, 1kHz, and 20kHz, respectively.

The output impedances differed rather from the specification in that while I found the "Normal" sockets to deliver their signal from a 105 ohm source impedance, the "High" sockets had a 40 ohm impedance. A puzzle. One thing for sure, however: the NAD 1300 should both be able to drive long, highly capacitative cables without problem, even from the normal outputs, and should be relatively unaffected by different cables.—John Atkinson