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Audio Research's Classic Tubed Preamp
Audio Research SP-6B preamplifier, in rack-mountable metal case. Dimensions: 19 by 51/4 inches (front panel), 101/4 inches deep plus clearance for controls and connections. AC convenience outlets: three switched, one unswitched (1,600 watts max. total). Price: $1,500.
Warranty: "limited," three years parts and labor, except 90 days on tubes.
Manufacturer: Audio Research Corp., 6801 Shingle Creek Parkway, Minneapolis, Minn. 55430.
It's difficult to discuss tubed circuitry without becoming partisan, so beset with opinion, both pro and con, is the subject. If HF has traditionally taken the view that the results, rather than the means, are what matter, it's more a question of sanity than of impartiality. Suffice it to say that the SP-6B, which employs vacuum tubes in the signal stages, is a superb preamp-even a classic of its type.
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OUTPUT AT CLIPPING 10.5 volts
HARMONIC DISTORTION, THD; 20 Hz to 20 kHz
aux input </= 0.026%
phono input </= 0.028%
FREOUENCY RESPONSE
+0,-1 dB, 10 Hz to 52 kHz;
-3 dB at 181 kHz
RIAA EQUALIZATION +V. dB. 20 Hz to 20 kHz:
-V. dB at 5 Hz
INPUT CHARACTERISTICS (IHF loading; A-weighting)
sensitivity S/N ratio aux 30.5 mV 78 d8 phono 0.48 mV 73'h 08
PHONO OVERLOAD (1-kHz clipping) 550 mV
PHONO IMPEDANCE 50.6k ohms; ca. 60 pF
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To the audiophile used to solid-state gear, in particular, a number of features may stand out as unusual. When you turn on the preamplifier, the muting is designed to prevent any output for a full two-minute warmup period. That may seem interminable if you're used to a few seconds at most, but even much transistor equipment doesn't really settle down to fully stabilized operation for, say, a halfhour or so. The manual muting feature, a switch that kills all output, is nice for preventing nasty transients when you're changing pickups but startling if you're used to the usual 20-dB attenuation. (Perhaps the latter should be called "muffling" instead of "muting.") There's also a front-panel switch for the three switched convenience outlets. The back-panel outlets themselves are the heavy-duty grounded (three-prong) variety; Audio Research suggests that you plug your amp into one and switch it on only after the preamp has fully warmed up. (Automatic output muting occurs when the preamp senses DC conditions, which would represent an abnormal drive signal to the amplifier.) And note that this is one of the few preamps designed to accommodate the switching of behemoth amps with grounding plugs.
If you're one of those listeners who simply won't read manuals, you may wonder what the little back-panel switch marked "60 dB" is for. (You'll find it just after you finish exclaiming about the nonstandard signal jack strip, on which the right channel rather than the left is at the top.) If you remove the plate that prevents inadvertent switching, you'll find that the alternate position is marked "50 dB," and you may figure out that it reduces overall gain to keep the volume control (actually marked GAIN) operating in the normal range even with an amplifier that adds excessive gain for your room and speaker efficiency. (The plate is there because, as the manual warns, you should not flick this switch with the juice on.) The specs work out this way: The phono-section gain is rated at 34 dB; this level, plus the direct feed from the high-level inputs, is delivered to the tape-recording output; from that stage via the VOLUME to the main output is rated for another 26 dB (or 60 dB from the phono input) with the VOLUME wide open and this switch in its normal position. Switching it reduces the gain of this final section by 10 dB. One other rather uncommon feature harks back to earlier equipment: the dual tape inputs with one set evidently intended for playback only. One pair, marked TAPE, feeds directly to the corresponding selector position. You could use them for the playback side of a deck connected to the recording outputs, but then you would have no way of monitoring signal quality, even with a "three-head" deck, because switching to TAPE would disconnect your previous source feed. The logical hookup for this use is what Audio Research calls MONITOR-the tape side of a classic tape/source switch. According to the manual, it is there partly for switching in and out any ancillary equipment, like an equalizer. Unless the outboard unit has its own monitor switching (most do), you would lose that function in your system.
We don't want to belabor that point, however; the tape switching options are adequate, though they are markedly less elaborate than the sort of independent two-way dubbing often provided for these days in separate preamps and even some integrateds and receivers.
The listening tests were delightful. That is no subject for surprise; the data disclose no reason for any other reaction. Distortion never exceeds 0.01% (the limit to which we will chase today's vanishingly small steady-state distortion) by much; when it exceeds it at all, it is at frequencies of 10 kHz or above, where the harmonic distortion products are all at 20 kHz and above. Not incidentally, Diversified Science Laboratories could discern no harmonic other than the "soft" second (the least objectionable of all, given equal percentages). If you have a fussy phono pickup that alters sound depending on capacitive loading, the SP-6B allows circuit-board modification to suit. The approximately 60 picofarads that DSL measured is negligible; it's essentially a question of adding a capacitor whose value matches the pickup manufacturer's spec.
But good as it is, we are not so naive as to suppose that audible quality alone will make or break this preamp in the marketplace. The feelings about tubes vs. transistors harbored by owners, would-be owners, uncommitted shoppers, and the pontificators they meet along the sidelines have a powerful influence. To urge (as we often do) that you shop for the features--switching, and so on--that will best integrate with your system, your needs, and your habits would be idle here.
The SP-6B is a classic: superb in sound, unconventionally yet well conceived, solidly put together. Those are not statements we would be willing to make of all tubed preamps, high though their reputation as a class may be in some circles. But, as always, the ultimate decision is yours.
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[Preparation supervised by Robert Long, Peter Dobbin, and Edward J. Foster. Laboratory data (unless otherwise noted) supplied by CBS Technology Center or Diversified Science Laboratories.]
(High Fidelity, May 1981)
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