Soundcraftsmen SP-4001 Preamplifier-Equalizer
One chassis that is a preamp and an equalizer, with a phono stage for each of two turntables.
Overview
The front panel is black with white lettering you can read from across the room. Each channel has ten vertical sliders, numbered at the top, with center frequencies printed below - 30 Hz, then doubling band by band up to 15,360 Hz. That is the SP-4001. Soundcraftsmen's own literature calls it a "signal-processor preamplifier-equalizer." The company's argument for combining the two was simple: one chassis, internal interconnections instead of patch cables, no need for separate tone controls, and a price advantage over buying the pieces individually. Audio (November 1980, p.46) had Howard A. Roberson review it, and he concluded the combination worked - "an excellent value for anyone needing both a preamp and an equalizer."
The phono section is rated at ±0.5 dB RIAA response from 20 Hz to 20 kHz, harmonic distortion under 0.01 percent, and a signal-to-noise ratio of 97 dBA. The line amplifier tightens the frequency response to ±0.25 dB across the same band, holds both harmonic and IM distortion below 0.01 percent, and reaches 105 dBA signal-to-noise referenced to a 10 V output. Those are quiet, clean figures for a component of this class. The review called the performance "excellent across the entire audio band with very low noise and superbly low distortion," even with aggressive boost and cut dialed in.
The equalizer has ten octave bands per channel, with ±12 dB of boost or cut on the sliders. The octave control range reaches ±16 dB when every control is at maximum. Roberson measured the filter shapes and found them fairly consistent at a Q of about 2.0 at full boost. They narrowed to a one-octave bandwidth (Q = 1.4) with the 960 Hz band pushed 11.5 dB, and Q = 1.0 at 9.6 dB of boost. He also noted that the center frequencies were "not dead accurate," with some unequal spacing between bands. That matters more on a lab bench than in a listening room. Detent repeatability was excellent: ±0.1 dB with a little care.
Between the two banks of sliders sit the equalized-signal zero-gain controls. This is the feature that separates the SP-4001 from a generic graphic EQ. Each channel gets its own control with top and bottom LEDs. Light them equally and the gain with EQ engaged matches the gain with it bypassed. Audio (November 1980, p.48) called the scheme "very worthwhile," noting it prevents the level jumps that make A/B comparisons of equalization so misleading, and, less delicately, prevents damage to amplifiers and speakers. The measured range of those controls ran from +6.7 to −13.6 dB around the detent. With pink noise and the LEDs for guidance, gain matching stayed within 0.5 dB. Matching at an 80 mV input was possible but fussy. 150 mV at the AUX input made it easy.
The volume control looks and feels nearly continuous, and it can be set between detents. It is actually a stepped attenuator with steps from 0.75 to 1.5 dB, larger toward the bottom of rotation. The two sections tracked within a dB from full output down more than 60 dB - superb, in the reviewer's word. The balance control is center-detented, and both knobs have fine knurling that makes small adjustments easy to feel as well as see.
Around the back, the SP-4001 is unusually well connected for its price. There are two phono inputs, each with its own separate preamp section. Selection happens at line level, which Roberson pointed out avoids the contact-resistance problems that plague low-level switching, and means two turntables can coexist without an outboard box. Add a tuner input and an auxiliary source, two tape recorders in and out with cross-dubbing in both directions, two external processing loops, and two sets of line-out jacks with low enough impedance to drive headphones directly. There are three switched and one unswitched AC outlet with a combined 1200 W capacity, far beyond the 300 W or so typical of the era. Both the outlets and the power cord are three-prong, a detail Audio (November 1980, p.48) flagged as uncommon in this kind of equipment.
The switching is all push-button. Four interlocked buttons handle source selection. Six non-interlocked buttons cover the subsonic filter, the two external loops, EQ in/out, mono A+B, and Route to Tape Record. That last one shifts the processing section out of the main signal path and into the tape-out feed, so dubbing can proceed while the main system plays something else. Four more buttons manage tape monitoring and dubbing direction. The external loops let an owner insert anything, a dbx II processor or a bandpass filter, and the review used exactly that path to demonstrate the EQ's flexibility. It pulled down an excessive presence hump in a recorder's playback without dragging the top end down the way a tone control would.
Inside, the build is tidy: two large circuit boards for most of the electronics and two small ones for the phono preamps. The main boards have socketed ICs, and the phono stages use discrete components. Soldering was judged excellent, and intercard connections are wirewrapped. The cards carry no parts identification, which is a small annoyance for anyone planning to service one decades later. The unit ships with wood end pieces and stick-on feet for shelf or table duty. Pull the end pieces and it drops straight into a rack, occupying 5¼ inches of height. Overall dimensions are 19 inches wide, 5¼ inches high, and 11 inches deep, at 23 lbs.
Treat that as marketing, but the independent review reached much the same place on its own. The original price was $549.00, and the review's verdict was that it delivered excellent value for the money. The documentation that came with it was a four-page stiff-paper manual, a test record, and a dozen full-size "Computone" charts imprinted with scales for each control, so favorite EQ curves could be stored on paper and reset by eye. The charts and test disc go missing. The unit works without them, but completeness does affect what a careful buyer will pay.
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