Soundcraftsmen Pro-Control Four Preamplifier Review

The Soundcraftsmen Pro-Control Four preamplifier is a highly versatile stereo control center in which all signal routing is carried out by solid-state (C-mos) electronic switching circuits. In addition to eliminating unreliable mechanical switches, the solid-state switching circuitry allows the signal paths to be located near the input and output jacks on the rear printed-circuit board, close to the actual switching elements. All of the signal inputs are buffered, or electrically isolated from each other, which, in combination with the switching system, minimizes the noise and crosstalk that can be introduced when several signal sources are brought to common switching points.

Signal switching in the Pro-Control Four is done through DC voltages controlled by front-panel soft-touch pushbuttons. The preamp’s only knobs are for volume, balance, and bass and treble tone controls. The large volume knob carries a green led “pointer,” and the smaller tone and balance knobs are center-detented.

The input source is selected by pressing the LISTEN button, which steps the switches in sequence to the next available input: CD, tuner, phono, TAPE 1, TAPE 2, VID/AUD 1, VID/AUD 2, and VID/AUD 3 (the last three are high-level inputs that are electrically equivalent to the CD and tuner inputs). The selected source is indicated by a green led on the front panel above its name. Since input switching is possible only in the indicated sequence, a number of button operations may be necessary to make a change—for example, the button must be pressed seven times to get from tuner to CD.

Below the listen button is an identical button marked record. Used like the listen button, it selects the program source to be made available at the tape-recording outputs. The selection is indicated by a red led below its name. The total independence of listening and recording program selection is one of the novel features of the Pro-Control Four. It is possible to record any source on any or all of the five VID/AUD or tape circuits while listening to it or any of the others.

Additional pushbutton selectors, marked loop l and loop 2, control two independent external-processing circuits. Signal-processing devices such as equalizers or ambience enhancers can be connected to these circuits and switched either into the listening signal path or into the signal path going to the selected tape recorder. Red and green led’s show the status of these circuits in the same fashion as the main signal-source indicators. Another button toggles a subsonic filter into and out of the listening or recording signal path, or into both at the same time. Finally, the DIRECT button bypasses all extraneous circuits, including the external loops, subsonic filter, and tone controls, leaving only the volume and balance controls in the signal path.

Power is switched on and off by another light-touch button; since at least one signal-input source is always connected when the preamplifier is turned on, its led serves as a power pilot light. There is a stereo headphone jack on the front panel. The rear apron is well populated by RCA-type input and output connectors (the phono inputs are gold-plated). There are also two sets of main output jacks, one for normal and the other for auto-bridge operation. The AUTO-BRIDGE Outputs shift the signals into a reversed-phase configuration relative to the normal outputs, enabling the preamplifier to drive a pair of standard stereo power amplifiers in a bridged mode, which typically more than doubles their normal output power. There are also two AC outlets, a switched one rated for 600 watts and an unswitched one rated for 550 watts.

The Soundcraftsmen Pro-Control Four is finished in charcoal gray with white markings. It measures 19 inches wide, 10-1/2 inches deep, and 3-1/2 inches high, and it weighs 10 pounds. Price: $699.

Lab Tests

No instructions were provided with the Pro-Control Four preamplifier, but the specifications available to us indicated that it was a sophisticated, high-quality product, as one would expect from Soundcraftsmen. In general, our measurements supported that expectation. We were unable to verify a couple of minor items, however, because we had no information on their specific test conditions.

Although the preamplifier’s rated maximum output is 10 volts, our sample clipped at 8.1 volts output into an EIA standard load. Since no power amplifier we know of requires an input signal of more than 2 volts or so to reach its full output, that is hardly a serious problem. The headphone volume sounded very good to us.

The only measurement that surprised us was the phono-overload limit, which is rated at 150 millivolts. We measured it as 69 to 97 millivolts from 20 to 20,000 Hz (converted to the equivalent 1,000-Hz level). This limit is not likely to cause any signal-overload problems with today’s cartridges, but it is considerably less than that of most preamplifiers we have tested in recent years.

The harmonic distortion at a 1-volt output was about 0.006 percent from 20 to 7,000 Hz and rose to just under 0.01 percent at 20,000 Hz. At 2 volts the characteristic was similar, reading about 0.012 percent up to 10,000 Hz and rising to 0.0175 percent at 20,000 Hz. These readings were made with the direct switch disengaged. In the direct mode, the distortion at 2 volts output was about the same as the original 1-volt measurements, but above 3.000 Hz it rose to 0.0235 percent at 20.000 Hz—still entirely negligible under any reasonable condition of use. The 1,000-Hz distortion, typically about 0.01 percent up to 2 volts output, rose rapidly to 0.06 percent at 5 volts.

The frequency response was very flat, ± 0.05 dB from 20 to 20,000 Hz through the CD input. The subsonic filter reduced the output only 0.5 dB at 25 Hz, and it dropped steeply below that frequency. The RIAA phono-equalization error was +0.3, — 0.5 dB from 20 to 20,000 Hz. The phono input, designed for a moving-magnet cartridge, has a termination of 46,000 ohms in parallel with a 55-picofarad capacitance. The tone controls had good, conventional characteristics, with the bass turnover frequency shifting between about 50 and 200 Hz and the treble curves hinged at 2,500 Hz.

The high-level input required for a reference output of 0.5 volt was 120 millivolts (mv), and the phono sensitivity was 1.73 mv. The A-weighted noise level referred to a 0.5-volt output was —91 dB (high level) and —81 dB (phono); in the direct mode both figures were improved by about 1.3 dB.

We also made measurements to see if the solid-state switching system had the claimed effect on crosstalk. From a high-level input to the phono preamplifier, the crosstalk was down 80 dB at 1,000 Hz and 55 dB at 20,000 Hz. These measurements were made with the unused input terminated in 1,000 ohms to simulate the connection of a phono cartridge. The stereo channel separation through the CD input was 82 dB at 1,000 Hz and 56 dB at 20,000 Hz. While these readings did not equal those of a good CD player, they indicate that crosstalk problems are exceedingly unlikely to be audible with this preamplifier.

Comments

Our measurements confirmed that the Soundcraftsmen Pro-Control Four is an excellent control amplifier, which was not unexpected, given its heritage. Soundcraftsmen, known for its high-quality home and professional amplifiers, is one of the relatively few truly American manufacturers to have demonstrated consistently that it is not necessary to go overseas to find affordable top-quality audio electronics.

We expected the novel control features of this preamplifier to be among its more significant qualities, so we put them to the test. We found that a muting delay of almost a second accompanied any switching operation. The previous program disappeared for that interval and was then replaced by the new one with no trace of a switching transient. While it might be more convenient to have a bidirectional input selector to minimize the time it takes to select an earlier input in the sequence, the clear and unambiguous led indicators make missteps unlikely. We also appreciated the bright indicator on the volume knob.

Not many people are going to need all the capabilities of the Pro-Control Four, but it is comforting to know that they are there and available for use. We tried several combinations of dynamic expanders, equalizers, and similar accessories connected through the two loop circuits and found the switching of these components to be equally flawless. I suspect that audiophiles who have accumulated a number of accessory devices over the years would be encouraged to re-install them if they had this control amplifier in their systems. I know that I was! However you look at it, the Soundcraftsmen Pro-Control Four is a beautiful piece of equipment.

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