SAE P101 Preamplifier Review

The SAE P101 Computer Direct-Line preamplifier is a high-performance device with several advanced control and circuit features. As its name suggests, the P101 uses digital circuits to perform almost all the functions normally handled by physical switches and potentiometers. The knob-free front panel contains only momentary-contact flat pushbuttons with adjacent lights to show their status. The “Direct-Line” part of the preamplifier’s name refers to the fact that all physical signal paths are at or immediately adjacent to the rear panel where the signal-input and -output connectors are located. All signal switching is performed (separately for each channel) by tiny relays with gold-plated contacts; these are activated by digital logic circuits that respond to a light touch on a front-panel button.

The P101 has no tone controls, and a low filter is its only conventional signal-modification function. It has inputs for both moving-magnet (MM) and moving-coil (MC) phono cartridges through separate preamplifier sections optimized for their functions. The gain of the MC preamplifier is automatically adjusted by the impedance of the cartridge connected to it, adapting the circuit for the probable output voltage of the cartridge. The capacitance and resistance of the MM phono input are separately switchable to match the cartridge’s loading requirements. The two high-level inputs are marked tuner and video. There is a single set of tape connections with a monitoring (but not dubbing) facility, and an external-processor loop that can switch devices such as equalizers and other signal modifiers into the line or tape signal paths. It can also be used for a second tape deck. The preamplifier’s operating mode can be set to stereo, reversed stereo, or mono.

A unique and useful feature of the SAE P101 is its switchable phase (polarity). Each channel can be switched independently to an inverting or a non-inverting mode, making it easy to correct for an occasional recording with out-of-phase channels or an inadvertent speaker-line polarity reversal. There are two sets of main preamp outputs in the rear which are always in opposite phase. This makes it possible to drive a stereo power amplifier in a bridged mode for high-power mono output. (When a stereo amplifier’s two inputs are driven in opposite phase and a single 8-ohm speaker is connected across the two “hot” speaker outputs, it becomes a mono amplifier of roughly twice the normal stereo 4-ohm power-output rating. Also, by operating both front-panel phase switches, the absolute phase of the program can be reversed easily (there are those who claim that absolute polarity, from the original sound source to the loudspeaker cone motion, is important for fully natural sound).

There is provision for an optional speaker-switching device that plugs into a socket in the rear of the P101, making it possible to switch the power-amplifier outputs to either or both of two pairs of speakers. There is a headphone jack on the front panel. Whenever the preamplifier is plugged into an energized a.c. outlet, a small “on-standby” power supply keeps the computer memories active and illuminates a red LED next to the standby button. A momentary touch of standby turns on the preamplifier, extinguishes the light, and turns on red LEDs next to the various last-used selector buttons (input source, tape monitoring, etc.). Touching the gain button reduces the preamplifier gain by 15 dB.

The variable functions of the P101 (volume and balance) are performed by microprocessor-controlled resistor-ladder attenuators. The volume and balance can be changed in either direction in steps of 1.5 dB by a momentary touch on the corresponding button. Holding in a button causes the stepping to continue at a rate of about 6 dB per second. Although the changes are in discrete steps, the audible effect is of a continuous level variation (the total volume-control range is 94.5 dB). A pair of numerical LED displays in the upper center of the panel shows the gains of the left and right channels, including the effect of any balance offset between the channels (which is maintained during overall volume changes), but does not register the effect of the gain circuit.

The two pairs of phono-input jacks (for MM and MC cartridges) are gold plated for low contact resistance. The input termination of the MM input can be switched to 22,000, 47,000, or 100,000 ohms by a slide switch near the jacks, and the shunt capacitance can be set to 100, 200, or 400 picofarads by another similar switch. The SAE PI01 employs low-impedance circuits, including its RIAA phono-equalization feedback network, which is driven by a small power amplifier from the output of the phono-preamplifier stages. The combination of low-impedance circuits and the separate relays used for switching each channel gives the P101 exceptional isolation between inputs as well as between channels (typically 90 dB or better).

Laboratory Measurements

All tests of the SAE P101 were made using the IHF standard output termination of 10,000 ohms in parallel with 1,000 picofarads and the standard gain settings specified in EIA Standard RS-490. The distortion of the P101 is rated at less than 0.008 per cent; in our measurements it did not exceed 0.003 per cent for any output voltage up to 10 volts at 1,000 Hz. At the rated output of 2.5 volts, the distortion was no more than 0.004 per cent from 20 to 20,000 Hz, except that at 20 Hz the signal harmonics could not be resolved from power-line harmonics.

The frequency response through a high-level input was flat within about ±0.25 dB (the approximate resolution of the test equipment) from 20 to 20,000 Hz. The RIAA phono equalization was accurate within ±0.3 dB from 200 to 20,000 Hz rising to +1.5 dB at 20 Hz. It was virtually unaffected by the inductance of a typical phono cartridge in the input circuit. The phono (MM) input overloaded between 260 and 270 millivolts (mV), and the overload limit for the MC input was about 14 mV. The low filter began to roll off the response below about 100 Hz; it was — 3 dB at 30 Hz and continued to drop at an apparent 6-dB-per-octave rate below that frequency.

The impedance of the MM phono input was as specified, including both its resistive and capacitive components. The 1,000-Hz impedance of the MC input was about 350 ohms. The preamplifier input sensitivity for a reference output of 0.5 volt was 52 mV for tuner, 0.87 mV for phono (MM), and 0.057 mV for phono (MC). The A-weighted noise in the preamplifier output at reference gain settings was below our measurement limits (100 microvolts, or —74 dB referred to 0.5 volt) for all inputs. The gain button reduced the gain by 14.7 dB. The output clipped at just above 10 volts, although it was perfectly distortion free only a few millivolts below that level.

Comment

From a human-engineering standpoint, we found the SAE P101 very easy to operate and totally without such common vices as switching transients or any other noises (it was completely silent). The signal isolation afforded by its relay switching was truly extraordinary; the largest available signal fed to the tuner inputs produced absolutely no audible feedthrough to any other input at maximum gain and with open-circuited inputs. This is one preamplifier that does not need shorting plugs in its unused input jacks to prevent unwanted noises while switching; not only is there no noise to begin with, but the selection system permits changing from any input to any other directly without passing through the others. Although the test meters clearly showed the 1.5-dB steps, the digital volume and balance controls worked so smoothly that acoustically they seemed to be continuous in their action.

The SAE P101 is a very well constructed unit, built for the most part on a single large fiberglass circuit board with virtually no hand wiring or internal cabling. SAE claims that direct control of the signals leads to sonic benefits. Although we were unable to confirm any special benefits, the P101 was easily the equal of any preamplifier we have used and better than many in the important respects of freedom from noise and switching transients, lack of crosstalk, and overall ease of operation.

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