Audio Pro TPA-150 and Audio Pro A4-14 Tuner/Preamplifier and Speaker System Review

The A4-14 is a compact bookshelf-size speaker that can be installed almost anywhere in the room with equal effectiveness. Its bass section consists of two 5-inch long-throw woofers operating in a vented enclosure whose volume is only half a cubic foot. The woofers are driven by a specially designed amplifier that permits them to develop high undistorted sound levels at the lowest audio frequencies (the “ACE-Bass” nomenclature stands for Amplifier Controlled Euphonic Bass). There is an electronic crossover at 300 Hz to a second amplifier that drives a more conventional 4-1/2-inch midrange cone speaker, and there is a second crossover at about 2,500 Hz to a 1-inch soft-dome tweeter.

The two woofers face downward, are located near the rear of the cabinet, and radiate through openings in the metal grille around the base of the enclosure. When the system is placed near a wall, the woofers are so close to the reflecting boundary surfaces that there can be no response cancellations in their operating range. The front-facing midrange driver is so far from any reflecting surfaces that similar cancellations cannot affect its response. The result is an exceptionally smooth acoustic output through the entire bass and mid-frequency range.

The black metal base of the speaker, perforated on four sides for ventilation and sound radiation, houses its electronic circuits, including the two power amplifiers and their power supply and the crossovernetwork components. The input connectors and a number of controls are on the rear of the base and include DIN sockets for input and output cables, pushbutton switches to connect the speaker to the left or right program channel, and a bass blend switch that sums the left and right channels below 100 Hz. This reduces power-wasting, out-of-phase response to turntable rumble or record warps without loss of audible bass response. In the auto position of the auto/ on button, a signal-sensing circuit turns on the amplifiers when an audio signal reaches the speaker. Five minutes after all signals have ceased, the speaker amplifiers automatically shut off. In the ON position, the speaker amplifiers are powered whenever the system is plugged into an a.c. socket. Small pilot lights visible from the front of each speaker show its operating status; green shows that it is plugged into the a.c. line, and red indicates that it is fully powered and operational.

Each A4-14 speaker is supplied with a 20-foot cable, fitted with DIN connectors at both ends, and two 9-inch adaptor cables. When the speakers are driven from a receiver or preamplifier with DIN outputs (such as the Audio Pro TPA-150), the cable is simply plugged into the DIN socket on the receiver and into the IN socket of either of the speakers. The cable from the other speaker plugs into the out socket of the first speaker, and the channel-assignment buttons of the two speakers are set for the desired left-right orientation. If the speakers are to be used with receivers or preamplifiers lacking DIN outputs, the adaptor cables permit connection to any signal source through phono jacks or terminals designed to accept wire ends (such as most speaker outputs).

There are three knobs on the rear of the A4-14 speakers. The sensitivity control, with a calibrated range from 0.1 to 50 volts, sets the input voltage required to give a sound-pressure level of 96 dB measured at a 1-meter distance from the speaker. The more sensitive settings are used with low-output preamplifiers; the less sensitive settings make it possible to drive the speakers from the output of any power amplifier (the 20,000-ohm input impedance of each speaker amplifier allows it to “bridge” the normal speaker output of any amplifier without loading or matching problems).

The other knobs are room matching controls. The bass control adjusts the woofer output relative to the other drivers to provide uniform overall frequency response with different speaker locations. The calibrations refer to the solid angle into which the speaker radiates: the 2π setting is used for wall mounting, which is the preferred placement, whereas 4π is used in a freestanding placement and π/2 when the speaker is in a corner. The treble control tilts the frequency response at frequencies above 2,500 Hz to suit room conditions or personal taste. It provides a maximum variation of ± 6 dB at 20,000 Hz referred to the “0-dB” or nominal flat setting.

Audio Pro does not publish power specifications for the speaker amplifiers since they would convey no useful information. More to the point, the company states that the A4-14 is capable of generating sound-pressure levels as high as 110 dB (above 100 Hz) at a 1-meter distance. Many full-size speakers driven by the most powerful amplifiers cannot surpass that output rating. The drivers and amplifiers are protected by circuits that sense the energy in the high-and mid-frequency ranges over a period of time and shut down the high-frequency amplifier for 10 seconds if a potentially unsafe condition is detected. Operation is restored automatically after that time. The bass drivers are protected by fuses (inside the base of the speaker), and the amplifier heat sinks have thermal sensors that shut down the system if the heat sink becomes too hot.

The mid- and high-frequency drivers of the Audio Pro A4-14 are mounted flush with the front board to minimize the effects of diffraction on the polar response of the system. A black foam-plastic grille, retained by plastic fasteners, covers the drivers and extends around the sides of the front board. The top, sides, and rear of the wooden enclosure are walnut finished, and, except for a single horizontal piece of wooden trim, the front of the speaker is black. The Audio Pro A4-14 speaker is 20-1/4 inches high, 12-1/8 inches wide, and 10-1/2 inches deep. It weighs 35 pounds.

We used the speakers with the Audio Pro TPA-150, another innovative product from the same design team. We will not describe it here in detail other than to point out that it is the first—and so far the only—receiver designed fundamentally around a microprocessor. Almost every function, including tone control, filter and equalization responses, and signal switching, is controlled by the central microprocessor. As a result, this receiver has only a single large knob, switchable by pushbuttons, to perform the functions of volume, balance, tuning, and bass, midrange, and treble tone control.

The various input sources, including five preset FM channels and two preset AM channels, two tape decks, two magnetic phono inputs, and an aux input, are selected by light-pressure, positive-action pushbuttons, and pressing any of the input buttons also turns on the power to the receiver (there is a separate off button). The tuner frequency is shown on a large red digital display, which becomes a clock readout when the receiver is switched off (or if the tuning button is pressed twice). The volume-contro! setting is shown by a horizontal row of red LEDs. The settings of the balance and tone controls are indicated by the positions of a pair of lights relative to the center of the volume display. FM tuning is aided by two red lights that glow with equal brightness when tuning is correct (an AFC system sets the tuner to the station frequency a few seconds after one comes close to the correct point by hand). A green LED is the stereo indicator for FM reception.

The large knob of the TPA-150 operates an optical wheel that sends pulses to the computer to signal the direction and amount of change of the selected control function, but it has no direct mechanical connection to circuitry in the receiver. The all-black TPA-150 (a wooden cabinet is available as an option) is 19-1/2 inches wide, 10-1/4 inches deep, and 4-1/2 inches high. It weighs 25 pounds. The price of the Audio Pro A4-14 speakers (sold in pairs) is $1,750 per pair. The Audio Pro TPA-150 tuner-preamplifier is $1,045.

Laboratory Measurements

The audio performance of the TPA-150 was comparable in most areas to that of the TA-150 we measured last year. The RIAA phono equalization was accurate within ±1 dB from 20 to 20,000 Hz, and it was not affected measurably by the inductance of a phono cartridge. The tone-control curves were somewhat unconventional. They tended to affect a large part of the audiofrequency range even when used in moderation, a characteristic that may or may not appeal to some users. The loudness contours boosted only the low frequencies to a moderate degree, and the two filters had 6-dB-per-octave slopes with — 3-dB frequencies of 50 and 9,000 Hz.

The audio distortion through the aux input at 1,000 and 20,000 Hz rose linearly from about 0.0015 per cent at a 0.5-volt output to 0.02 per cent at 4 volts, and the output waveform clipped at 5.4 volts. At 20 Hz the distortion was slightly higher, reading between 0.008 and 0.02 per cent from 0.1 to 5 volts output. The input sensitivity for a 0.5-volt output was 55 millivolts (mV) through the aux input and 1.8 mV through the phono input. The corresponding overload limits were 5.5 volts and 180 mV. The A-weighted noise level was 72 dB below 0.5 volt through either input. The phono-input impedance was 46,000 ohms in parallel with 80 picofarads.

The FM-tuner section had an IHF usable sensitivity (mono) of 11 dBf or 2 microvolts (μV). The stereo threshold was a rather high 35 dBf (30 μV), which also corresponded to the 50-dB stereo quieting sensitivity. In mono, the 50-dB quieting sensitivity was 14.3 dBf (2.8 μV). The tuner signal-to-noise ratio at 65 dBf (1,000 μV) was 71.5 dB in mono and 68 dB in stereo, and in both modes the distortion at that level was 0.068 per cent.

The stereo frequency response of the FM-tuner section was +1.6, —0 dB relative to the midrange output level from 30 to 15,000 Hz. The channel separation was better than 36 dB over that full range and was typically 42 to 46 dB over most of it. The 19-kHz pilot-carrier leakage measured — 35 dB, indicating relatively ineffective filtering of this frequency from the tuner outputs. The power-line hum was a good — 70 dB. Since the tuner’s AFC could not be defeated, we could not measure the capture ratio or selectivity. The muting threshold was between 9.8 and 16 dBf (1.6 to 3.8 μV), with the audio level changing smoothly through that range instead of cutting on or off abruptly.

The smoothed and integrated room response of the A4-14 speakers was impressively smooth and flat, with excellent dispersion of the highest frequencies (the response curves from on-axis and 30-degree-off-axis measurements did not begin to diverge until above 10,000 Hz). The speaker’s treble control produced roughly the indicated change in output (+5 to —6 dB at 20.000 Hz), with its effect becoming measurable at about 2,500 Hz.

To measure the woofer response, we had to place the microphone at the rear grille opening in the base of the speaker, as close to the cones as we could get without removing the base. The result was an amazingly flat response curve, within ±0.5 dB from 45 to 225 Hz. There was no sign of the usual woofer resonance peak, and the output dropped off rapidly (apparently at 36 dB per octave) beyond those limits.

To splice the bass curve to the mid- and high-frequency curve, we had to resort to subjective judgments based on the sound of the speaker since the woofer response rolled off so rapidly beyond the crossover frequency that we could not use an overlap of several octaves as a guide to the correct transition between the curves. The final result was a frequency response that was flat within ±1.5 dB from 36 to 2,000 Hz. When the treble control was turned down to —6 dB, the total response variation was only ± 1.5 dB from 36 to 20,000 Hz, something of a record in our experience with measuring speakers in this manner. However, the measurement method does not really warrant such a fine distinction, and we also tended to prefer the sound with the “0 dB” treble setting in our listening room (the manufacturer says this is the anechoically flat condition). This produced a smoothly rising high-end response, increasing about 5 dB from 2,000 to 20,000 Hz.

These figures were obtained with the 2π bass-control setting, since the speakers were mounted against a wall about 30 inches above the floor. When we measured the woofer response with the other settings of the control, we found that the π/2 (corner) setting reduced the bass output by 5.5 dB and the 4π (free-standing) setting increased it by about 3 dB relative to the 2π setting. There were only minor changes in the shape of the bass-response curve near its lower cutoff frequency.

The woofer’s distortion measurements also produced some surprises. According to the manufacturer, the ACE-Bass system tends to replace the physical properties of the woofer (its mass, compliance, damping, and voice-coil resistance) with effective values determined by the amplifier design, and to the extent that the basic properties of the speaker have been altered to the synthesized values, its distortion is also reduced.

We measured the bass distortion with a drive level of 0.5 volt at a 1-volt sensitivity setting. The result corresponded to a nominal acoustic output of 90 dB sound-pressure level, one that would require an input of about 2 watts with more conventional speakers. The distortion was not unusually low in the upper part of the bass range, but it remained nearly constant as we lowered the frequency. It measured between 1.4 and 2.8 per cent from 100 Hz down to 25 Hz, increasing to 7.9 per cent at 20 Hz. Of course, the acoustic output at the very lowest frequencies was far below the levels reached at 45 Hz or higher frequencies, but, in contrast to almost every other speaker we have tested, it remained basically undistorted. The speaker sensitivity was approximately as rated, with a 1-volt input giving a 95-dB sound-pressure level at 1 meter.

To verify the claims for the maximum acoustic-output capability of the speaker, we played music through the A4-14s, increasing the drive level and monitoring the sound-pressure level at a point in the room about 10 to 12 feet from the speakers. We obtained readings of 104 to 105 dB, more or less sustained during loud passages, without significant audio distortion. The peaks, which were too brief to register on the movement of the sound-level meter, were at least 10 dB higher than the average readings. Except for its deafening volume, the music was unstrained and perfectly listen-able. After a minute or so of this operation, the thermal cutouts of both speakers shut them down, but in a couple of minutes they had cooled sufficiently to resume normal functioning.

Comment

We felt much the same about the TPA-150 as we did about the TA-150 when we tested it last year. Although its FM tuner performance is, for the most part, not exceptional, and the tone controls and filters leave something to be desired, this is (in our opinion) one of the most intelligently designed hi-fi receivers on the market.

The Audio Pro TPA-150 is an exception to the prevalent pattern of control illegibility or functional obscurity. Its one large, easily grasped knob leaves the user in no doubt as to which control to turn. The buttons and lights next to the knob similarly leave no doubt as to the function being performed by the knob, and the lights on the panel display give positive indications of the actual settings of each control function even when viewed at a considerable distance. Since the unit steps automatically through the full control sequence each time the receiver is turned on, the operating status of the TPA-150 is clearly displayed whenever it is put into service. The only operating aid we missed was some form of FM signal-strength indication.

The Audio Pro A4-14 speakers were new to us, although we had previously read much about the ACE-Bass design. This is not a motional-feedback speaker, nor is it simply a conventional speaker with built-in amplifiers and electrical equalization. The design of the bass amplifier enables it, rather than the physical characteristics of the drivers, to determine the effective mass and compliance (and hence the resonance frequency) of the system. And because of the amplifier's control of the effective speaker resistance (using a sythesized negative resistance), the usual resonant rise in output is completely eliminated. The use of a ported enclosure (the design follows Thiele’s criteria) makes the ACE-Bass design much more efficient than the typical acoustic-suspension configuration normally used in powered speakers, and this helps give it a low-bass output capability that one has to experience in order to appreciate fully.

The sound of the A4-14 is about as uncolored as that of any speaker we know of. The speaker does not emphasize or diminish any part of the audio band relative to any other part. The sound is obviously “all there” from the first moment one hears it, but only with continued exposure does one begin to appreciate how “invisible" these speakers are in an acoustic sense as well as being relatively unobtrusive visually.

We put the A4-14’s protective system through its paces during our tests by playing the speakers as loud as we dared, and then a bit louder. As we mentioned earlier, before any of the drivers or amplifiers can be damaged they shut off for a while to let things cool down. Barring their use by a hard-of-hearing rock-music enthusiast, we cannot imagine that these speakers would protectively shut down in a normal home installation. Our ears were ready to give up long before the speakers showed any signs of distress.

These Audio Pro components are expensive indeed, but in our view they provide solid value both in sound quality and in ease of use. Only time will tell if a completely electronic, nonmechanical receiver will prove to be more reliable than conventional designs. We took a close look at the electronic sections of the speakers, and their quality and workmanship were manifest; in addition, the speakers themselves are nothing less than magnificent in their sound. The overall combination is hard to beat, even though it will leave little change from $3,000.

Read all