Solid State Preamp Reviews

By far the most complicated of the three preamps i review in this issue in terms of facilities offered, NAD's "Monitor Series" 1300 ($398) provides two buffered tape loops, an external processor loop (which can also be used as a third tape-recorder loop), a headphone output, a "null" switch, switchable bass equalization to extend the low-frequency range of small loudspeakers, and treble and bass controls, each with a choice of three turnover frequencies: 3kHz, 6kHz, 12kHz, and 50Hz, 125Hz, 250Hz, respectively. An 18dB/octave infrasonic filter, cutting the response below 20Hz (–3dB at 14Hz) can be switched out by pressing a button; the default position of having the filter in-circuit seemed intuitively wrong to me.

The null circuit replaces the normal conventional stereo feed to the output sockets by the difference (L–R) between the two channels. It can therefore be used both to optimize cartridge alignment and to adjust an FM antenna to give the best rejection of multipath distortion and extraneous noise on the subcarrier. (You could also use this switch to eliminate a centrally placed Barry Manilow or John Denver from their recordings—but who would want to do that?)

Looking inside the 1300, one large printed circuit board carries everything apart from the mains transformer, headphone and bass EQ circuitry, and the volume control, this detent-less and similar in quality to those used in the other two preamps reviewed. Rather than use the pot as a conventional series voltage-divider to ground, NAD's engineers have configured the pot as part of the feedback loop around an operational amplifier. In this manner, the line-stage gain is varied according to the volume control setting, which, NAD feels, avoids clipping or slew-rate limiting.

The phono circuitry is based on discrete transistors, with an FET input stage. This is said to linearize the circuit well into the MegaHertz region, thus avoiding the possibility of demodulating any stray RF. A rear-panel slide switch selects a 26dB-gain moving-coil headamp, with a 100 ohm input impedance. This stage, which NAD claims takes full advantage of the inherent low noise of low-output MCs to yield lower-noise playback than with conventional MM pickups, is powered from voltage rails buffered from the rest of the circuit and regulated by Darlington capacitance multipliers. Another switch selects input shunt capacitance for MM cartridges.

The rest of the circuitry is based on JRC op-amps; the output circuitry is, again, discrete transistor. An additional pair of output sockets delivers the signal from a beefier output stage, in effect a baby power amplifier, the preamp then being able to swing its full output into a 600 ohm load. (This output, which also drives even low-impedance headphones from a front-panel socket, is set 14dB higher than that from the normal sockets.) The output is muted via a relay for five seconds after turn-on. The "normal" output is also muted when headphones are plugged in, though the "high" outputs remain operative.

Almost uniquely, in my recent experience, the 1300 comes fitted with shorting phono plugs fitted to its line-level inputs other than CD. These can be replaced by the interconnect from the appropriate sources, but if left plugged in, will cut down on inter-input crosstalk. There are four utility mains sockets on the rear panel, two unswitched and two switched, and attractive 19" rack handles, with red inserts, are available as a $30/pair accessory.

The Sound
The NAD 1300 was set for exactly unity gain, its tone controls, infrasonic filter, and bass EQ switched out of circuit, and inserted in the PS Audio 4.6's tape loop. Unlike the Parasound P/FET-900 that I also review in this issue, the signal polarity at the "Normal" outputs with the tone-control circuitry bypassed was still inverted by the line-level stage, the same proving true for the "High" outputs. This factor complicated the auditioning—both speaker cables had to be reversed every time I changed preamplifier. (The most useful test signals I have found for determining line-stage polarity are Track 51 on the Technics Test CD, which has a totally asymmetrical raised-cosine waveform centered around the waveform midpoint, and Track 88 on the Japan Audio Society Test CD, which also has a raised-cosine waveform, this offset to the positive side of the time axis.)

Cursing inwardly—limiting the variables under test to just one is always harder than it might appear from the outside, and I have found that reversing absolute signal polarity can often be more audible than the intrinsic differences between similar electronic components—I sat down to some serious listening. At first, the NAD 1300's line-level circuitry appeared not to be introducing any audible differences, but, over the long run, a number of aspects to its sound became apparent. First was that, like the Parasound preamplifier, the 1300 sounded more dynamic in its handling of the drums on Flim & the BB's "Tricycle" (DMP CD-443). Second was the bass. While not having quite the subjective extension of the PS Audio, the NAD's low frequencies had good midbass weight. Ultimately, however, I preferred the straightwire sound, the bass drum in "Tricycle" having a better-defined, less woolly pitch center, with a more realistic relationship between the sound of the beater hitting the head and the follow-on tone.