Manufacturer's Specifications:
Frequency Response: Phono, RIAA ±0.2 dB; high level, 20 Hz to 20 kHz, ±0.5 dB.
Input Sensitivity: MM phono, 1.0 mV; MC phono, 0.2 mV (high), 0.06 mV (low); high level, 80 mV.
S/N (A-Weighted): MM phono, 76.5 dB; MC phono (re: 0.5 mV input), 80 dB; high level, 96 dB.
Input Impedance: MM phono, 47 kilohms; MC phono, 100 ohms; high level, 50 kilohms.
Input Capacitance: MM phono, 50, 100, 150, or 250 pF, selectable; MC phono, 1,000 pF.
Distortion (THD, SMPTE IM, IHF IM, TIM): 0.006%.
Maximum Undistorted Output: Greater than 7.0 V rms.
Input Overload (High Level): 10 V rms.
Output Impedance: Main out, 560 ohms; tape out, 560 ohms; phones, 10 ohms.
Dimensions: 17 in. W x 2 1/4 in. H x 8 in. D (43.2 cm x 5.7 cm x 20.3 cm).
Weight: 9 lbs. (4.1 kg).
Price: $417.
Company Address: 176 Walker St., Lowell, Mass. 01854.
Tomlinson Holman is one of those young American engineers who brought to audio design a fresh, innovative approach that combined purity of sound reproduction with simple elegance of product appearance. Mr. Holman's company, Apt Corporation, like so many other small but dedicated audio equipment makers, fell upon hard times a few years ago, and Tom went on to other audio pursuits on the West Coast, where he now works for the Lucasfilms organization. His audio designs, however, have not been lost to the world. In the spring of 1984, all of the assets of Apt Corporation, including the trade name and patents, were sold to Wayne Friedrichs, a Boston-area businessman and engineer. Friedrichs has re-established Apt, rehired key production and technical personnel, and added extra management staff to properly service dealers and handle customers' needs.
The Apt Holman Preamplifier Two, which I tested recently in the lab, is a refined and slimmed-down version of the classic Apt Holman preamplifier I reviewed in 1980; it incorporates much of the same thoughtful engineering and excellent sonic qualities as did its predecessor. The "new" Apt Corporation has come up with this lower cost preamp by simplifying some of the circuitry and deleting controls that many audio enthusiasts do not regard as important, while managing to retain the most innovative features and sound reproduction accuracy of the older, more expensive model.
At the heart of the Preamplifier Two is a unique MM phono preamp circuit, the input of which employs a differential pairing of an FET input with a bipolar-transistor feedback circuit for low noise and r.f.i. immunity. The FET input ensures against response-altering interaction between the preamp and the high impedance of a moving-magnet cartridge. The high-level portion of the phono section uses an IC op-amp which provides the headroom needed for dynamic phono signals. This IC also provides both a low output impedance, for accurate RIAA equalization, and the high current needed to avoid transient distortion due to the falling impedance of the RIAA network at high frequencies.
A rear-panel selector switch provides several values of phono-loading capacitors and thus is suitable for the vast majority of MM cartridges. The rear panel also has an unusual, dedicated phono balance control. A simple calibration procedure allows the user to precisely "null out" any cartridge balance errors and restore correct stereo imaging without resorting to any test equipment.
The built-in MC pre-preamplifier circuit is a discrete bipolar design in which the base currents have been optimized for lowest noise at the low impedances of MC cartridges.
Gain of this stage can be set to either 15 or 25 dB, while its treble response can be adjusted, by means of plug-in capacitors, to correct for the rising high-frequency response of some MC pickups.
Another innovative feature is a full muting switch, which cuts off signal to the main outputs but allows you to listen to dynamic headphones plugged into the preamp's own low impedance headphone output. There's also a unique stereo imaging knob which gives total control over the proportions of L + R and L- R in any stereo signal source. It allows you to vary the apparent depth and breadth of the stereo spread, from mono to "beyond stereo." While not a totally new idea (I remember using the same principle in some products I designed way back in the early 1960s), its execution here is extremely well done.
Control Layout
The dark gray front panel, with its matching gray knobs and pushbuttons and its low profile, gives the Preamplifier Two an understated, elegant look. A "Power" button is at the left, and alongside it is the stereo "Phones" jack. The stereo "Image" control, with markings at the ends of its rotation for "L + R" and "L- R" and "Neutral" at its center, comes next, followed by rotary "Balance," "Bass" and "Treble" controls. Four oval-shaped pushbuttons select the desired program source ("Phono," "Tuner," "AUX," or "Tape"), and a large master level control is to the right of these buttons.
Finally, a single, oval pushbutton at the extreme right of the panel is used to mute the main outputs when headphones-only listening is desired.
Separate MM and MC phono inputs are found at the left of the rear panel, and near them is a screwdriver-slotted selector switch to choose the capacitive loading value for the MM jacks as well as the gain setting for the MC inputs. A screwdriver-adjustable phono balance control is accessible through a small hole on the rear panel. Tape in and out jacks, sets of high-level input jacks, two pairs of high-level output jacks, and three convenience a.c. outlets complete the rear-panel layout.
Measurements
My test setup measures RIAA phono accuracy directly. That is, I apply signals which vary in amplitude according to the inverse of the RIAA playback curve. Then I set a digital voltmeter to "0 dB" at 1 kHz and simply read any deviation.
When I tested the Apt Holman Preamplifier Two, I thought at first that my meter was hung-up. It kept reading "0 dB" (deviation) for every frequency I used-from 30 Hz to 20 kHz. Only after repeated checks did I realize that the meter was telling the truth. RIAA accuracy was absolute over the entire range of measurement-or at least as absolute as the resolution of my metering system, which is accurate to better than 0.1 dB! I don't know of too many other preamps that can claim this kind of equalization accuracy.
Applying lower level test signals to the MC input, I found that results were not quite this accurate, but almost. While everything above 1 kHz was exactly correct, I did read a +0.3 dB deviation from the RIAA curve at 30 Hz. I don't consider this anything to get terribly upset about. Phono sensitivities were as stated by the manufacturer (1 mV for MM for 0.5 V out, 0.2 mV for the high setting of the MC inputs, and 0.06 mV for the low setting of the MC inputs).
Phono overload, not specified, was 135 mV for the high level inputs and 8.5 mV for the MC inputs (set to high level).
High-level frequency response was flat within 1 dB from 20 Hz to 75 kHz, and the-3 dB roll-off points occurred at 15 Hz and 160 kHz. The low-end subsonic roll-off is deliberate, as Apt has incorporated an infrasonic filter that begins to roll off response below 20 Hz. The filter is built in and cannot be turned off by the user.
Fig. 1--Tone control characteristics.
Fig. 2--Display of stereo test tones with "Image" control at "Neutral" setting.
Fig. 3--Same as Fig. 2, with "Image" control at extreme "L + R" setting.
Fig. 4--Same as Fig. 2, with "Image" control at extreme
A-weighted signal-to-noise ratio of the MM phono section, referred to a 5-mV input signal with the volume control set for 0.5 V output, measured 75 dB, a bit lower than claimed by Apt. With 0.5 mV applied to the MC phono inputs, I measured an A-weighted S/N of 67 dB for the high-gain setting of the pre-preamplifier stage and 69 dB for the low gain setting. Unweighted S/N for the high-level and tape inputs measured 91 dB, increasing to 97 dB when an A-weighting network was introduced.
Harmonic distortion, referenced to 1.0 V output, measured only 0.0035% at mid-frequencies and at the bass end of the spectrum, increasing almost linearly (and insignificantly) to 0.006% at 20 kHz. SMPTE IM measured an even lower 0.0025%, with IHF IM a still lower 0.0009%.
Figure 1 shows the action of the bass and treble controls when set to their maximum clockwise and counterclockwise positions. At 20 Hz and 20 kHz, boost and cut is at least-± 10 dB. Notice that the midrange, from around 300 Hz to around 3 kHz, is almost totally unaffected even when maxi mum bass and treble boost or cut are applied. The shelving action of these newly designed tone controls allows for better tonal compensation of frequency extremes if all you have available are bass and treble controls.
I was especially interested in the action of the stereo "Image" control. As I had guessed, it alters the proportions of L- R and L + R in any stereo program material being amplified by this unit. To put it another way, if you add some out-of-phase "L" signal to "R" and some out-of-phase "R" signal to "L," you can create the audible illusion of in creased stereo separation. Carry the trick to extremes, though, and you will end up with what essentially amounts to an all L- R signal coming from each speaker; that kind of out-of-phase mono reproduction will seem highly deficient in bass and will have other problems, too. So, the point I'd emphasize about the "Image" control is that if it is used at all, it should be used in moderation. If you keep the control at its mid-point, it does nothing (stereo reproduction is as it comes from the program source). To illustrate this, I applied a test tone to one channel while a near multiple of that tone was applied to the opposite channel. Using the horizontal and vertical inputs of an oscilloscope to display the left and right signals coming from the preamp, I obtained the display shown in Fig. 2. Turning the "Image" control fully counter clockwise to its "L + R" setting resulted in the in-phase diagonal display of Fig. 3; turning the control fully clockwise to the "L- R" extreme resulted in the typical out-of-phase diagonal display of Fig. 4.
Use and Listening Tests
With all tone controls set to their flat position and with the "Image" control at its mid-setting, the Apt Holman Preamplifier Two delivered the kind of neutral, uncolored sound that I find most pleasing. The accuracy of the RIAA equalization characteristic was especially evident with some of my favorite old LPs which I hauled out for these listening tests. Most of us tend to think that minor variations in frequency response caused by deviations of less than 1 dB in the RIAA curve will be inaudible. Nevertheless, when I come across a preamp that is as accurate in response as this one, I find I can appreciate its accuracy in very real and audible terms.
The tone controls, in real-world use, were significantly better in their action than the more familiar Baxandall types, which have a common bass and treble hinge point between 500 Hz and 1 kHz. I switched to a pair of smaller speaker systems expressly to try and compensate for their lower octave of missing bass, and I was more successful in doing so using a moderate amount of bass boost from this preamp's tone-control configuration than I had been with more conventional bass controls. I also liked the action of the step-type master volume control. This, I discovered, is not just a mechanically detented control, but is made up of a discrete, thick-film, precision resistor array, with channel levels matched over the entire range of the control. I should mention, too, that the power switch on the front panel activates only the three convenience outlets on the rear panel and can handle up to 1,000 watts of power. To avoid turn-on thumps, the Preamplifier Two's own circuitry remains on as long as the line cord is plugged in. Since power consumption is not more than would be needed by a low-wattage "night-light," there's really no problem with leaving the preamp plugged in (and active) all the time.
Summing up, I was pleased to see that the original Apt Corporation's penchant for audio purity and high quality has been maintained in its latest incarnation. The Preamplifier Two is not only a credit to its Apt Holman predecessor but is a welcome addition to that select group of audio components destined to become classics.
-Leonard Feldman
[ Orig. publ. in Audio magazine/Aug 1985]
Also see:
Apt/Holman Control Preamplifier (Feb. 1980)
Rotel RC-870 Preamp (Aug. 1985)
Amber Model 7 Tuner and Model 50b Amplifier (Aug. 1985)
Anthem Pre-1 Preamplifier (April 1997)
Arcam Delta 110 Digital Preamp (Jun. 1992)
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