Harman-Kardon Citation "A" stereo preamplifier
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Vintage test report


Taken from High Fidelity Magazine, March 1963

AT A GLANCE: The Citation A by Harman-Kardon is a twin-channel preamplifier and control centre, to the best of our knowledge the first professional-calibre preamp to employ "solid-state" (transistors and other semiconductors) circuitry. As is characteristic of the Citation line, the A boasts some unusual features and is designed according to wide-band response philosophy. Measurements made at United States Testing Company, Inc., indicate that the Citation A literally has flat response to beyond one million cycles and distortion that is nonmeasurable by the usual methods. Its listening quality is superb. The Citation A thus is not only an unsurpassed audio component; it also marks a major break-through in the application of semiconductors to high fidelity sound. The model tested was factory-wired and is priced at $350. The kit version is priced at $250. Dimensions are 14 7/8 inches wide, 5 5/8 inches high, 7 inches deep. Manufacturer: Harman-Kardon, Inc., 55 Ames Ct., Plainview, L.I., N.Y.

IN DETAIL: The Citation A dramatically demonstrates that transistors not only can do as well as tubes, but when correctly applied and used - can do better.

To begin with, the unit has been beautifully styled and crafted, inside and outside. The front panel contains twenty-one controls and a stereo headphone jack, yet it does not look cluttered; and everything is logically placed and neatly arranged. Many of these controls are familiar, but some are in the professional class, and not usually found on home audio equipment. Across the top of the panel are separate bass and treble tone controls for each channel. These are step-type controls that provide fixed amounts of cut or boost. When in the "flat" position, they are electrically out of the circuit and the signal bypasses the associated networks with the resultant benefit: less distortion. To the right of the tone controls are: a gain (volume) control that controls both channels at once; a five-position mode selector (stereo, blend, A+B, monophonic A, monophonic B); and a six-position signal selector (auxiliary, tape amplifier, tuner, phono 1, phono 2, tape head). Below the tone controls are two power switches: one turns on the entire sound system; the other permits turning off the power amplifier only when listening with headphones. The headphone jack is located at the lower left of the panel. To its right is a row of four push-button switches for channel reverse, rumble filter, loudness control, and tape monitor. Next are: a blend control that doubles as a volume control for a derived centre channel; a channel balance control; and a complete set of tape and disc equalization controls. Disc equalization is handled by separate turnover and roll-off controls. The tape adjustments are most unusual in that they provide, separately for each channel, a means of matching the preamplifier's circuit to the input signals from an external tape deck by compensating for tape head characteristics as well as for the capacitance of the connecting cables. They may be used for tape speeds of 15, 7½ and 3¾ ips.

The rear of the Citation A contains input jacks for all program sources, as well as two power amplifier output jacks per channel, a derived centre-channel output jack, and tape recorder feed jacks. Also, there are three switched AC and one unswitched AC convenience outlets.

The unit contains a total of thirty-three transistors, plus four zener diodes and four silicon rectifiers. Its main circuits are contained in eleven printed circuit modules, each mounted on its own glass-epoxy board. The boards are fitted into a slotted rack along the rear of the chassis. Any module can be removed readily for testing or servicing, as shown in the photograph. Should the need arise; an entire module can be obtained as a replacement for any defective stage at nominal cost.

The unit's power supply is rather unusual in design, and is located at the rear of the chassis to the left. The supply contains two full-wave rectifier systems, one to obtain the positive voltages and the other to obtain the negative voltages required by the preamplifier. The output of each rectifier is fed into a transistor voltage regulator circuit for constant-value supply voltages regardless of supply voltage or load current. The zener diodes are used for added regulation and as voltage references.

The preamplifier contains seven individual amplifier modules, each having three transistors, and each acting as a stage of flat audio gain and as isolation for the various functional stages such as the equalization networks, tone controls, etc. Each amplification module, initially designed to provide 67dB of gain, utilizes 34dB of feedback for low distortion, and thus has a net, undistorted gain of 33dB. Two of these modules are used in each channel as low level preamplification stages with passive equalization networks between the modules. One module is used as a voltage amplifier in each channel. The seventh amplifier module is used as the output stage for the derived or A+B centre channel.

There are also two emitter-follower modules each containing two emitter-follower circuits. The emitter-follower circuit is used as a unity-gain impedance-matching device. One emitter-follower is located at the high level inputs to the preamplifier (just after the volume control) and a second emitter-follower is found at the end of the circuit where it feeds the amplifier and headphone output jacks.

The preamplifier also contains a two-transistor time-delay module, which provides a 20-second warm-up period during which time the preamplifier output jacks are shorted to ground. The warm-up assures that the preamplifier is disconnected from the basic or power amplifier until all the energy-storing devices (capacitors) in the circuit have reached a steady-state condition. This prevents overloading the power amplifier and driving the speakers too hard during the warm-up period.

The output impedance of the Citation A is very low, being on the order of 50 ohms. This value negates any frequency-limiting effects of cable capacitance to a power amplifier and also is suited for driving high quality headphones. Citation recommends using 400-ohm impedance headphones for an optimum match to this output. Lower impedance headphones may also be used but with a slight increase in distortion. We listened over a pair of 8-ohm headphones and detected no distortion. The A's input impedance at its high level inputs is also rather low, being on the order of 27K ohms. This value helps maintain the preamp's high-frequency bandwidth and is a suitable match for most high quality program sources such as FM tuners and tape deck preamplifiers which themselves have a low output source impedance, such as a cathode follower. A few high fidelity components may have high impedance outputs; such units can be used with the A but with a possible loss of bass response.

Run at full gain, the Citation A provided a maximum output signal level of 5.8 volts rms per channel. At this level, frequency response from 20 cps to 20 kc was perfectly flat, within ±0.1dB. USTC then checked its response well below and above the normally accepted 20-cps to 20-kc audio range. The results were amazingly good: there was a 1.7dB peak at 1 megacycle, and response continued beyond this point to 1.5 mc (1,500,000 cps) where it was down only 3dB. At the low end, response was maintained perfectly flat to below 5 cps (this frequency is the lowest at which USTC can make measurements using sine waves). At full gain, then, the over-all response of the Citation A was found to be +1.7, -3dB from below 5 cps to 1.5 mc.

Since a preamplifier, however, is seldom, if ever, operated at full gain, USTC repeated the measurements in the more rigorous mode of "unity gain" in which the signal leaving the preamplifier is the same voltage level as that entering it. Under these conditions, frequency response from below 5 cps was flat within +0, -1dB to 400 kc, rolling off to -3dB at 620 kc, and to -7.5dB at 1 mc. This superb response characteristic is not matched by any other known preamplifier.

The A's square-wave response was equally outstanding, indicating excellent transient characteristics and virtually no phase shift throughout its extremely wide range. At the usual test frequencies of 50 cps and 10 kc, the A reproduced almost perfect square waves that were hard to distinguish from the input test signals. Because they were so excellent, USTC went to lower and higher test frequencies to see just what the limits of this preamplifier were. At 5 cps, the square-wave response was still nearly perfect, showing no more than a 5% tilt in the flat portion of the waveform. Going up in frequency USTC found that a 200-kc square wave—an unusually high and demanding test signal—still had a very recognizable and relatively undistorted square-wave shape, with a remarkable rise-time of only 0.6 microseconds. These measurements were made at unity gain.

The response of the Citation A through its magnetic phono input was outstanding over the range from 20 cps to 20 kc, showing no more than a negligible 0.5dB error at any frequency when compared to the standard RIAA equalization curve. The NAB tape head playback characteristic was excellent, being flat within +0, -2dB from 20 cps to 20 kc.

Harmonic distortion in the Citation A was actually less than the residual distortion of the measuring equipment and thus could not be measured directly. However, using the more complicated technique of direct harmonic analysis, USTC calculated the THD of the Citation to be 0.026% at 1 kc and 2 volts output; and 0.065% at 1 kc and 5.8 volts output on the left channel. The right channel had even lower distortion. IM distortion was measurable, but was very low in value, being 0.36% at 5.8 volts, and 0.08% at 2 volts output on the left channel; and 0.26% at 5.8 volts, and 0.05% at 2 volts on the right channel.

The sensitivity of the Citation A was 270 millivolts on the high-level inputs, 69 mV on the crystal phono input, 2.2 mV on the magnetic phono input, and 2.4 mV on the tape head input. The signal-to-noise ratio was 77dB on the high-level inputs; 58dB on the magnetic phono inputs; and 55dB on the tape head inputs. The low-level input figures are obviously fine. The high-level measurement is "unweighted"—that is, it includes a signal component that actually is in the supersonic region due to the unit's extremely wide response, and thus is inaudible. If the high-level signal-to-noise figure were weighted, or made comparable to what would be derived from an amplifier whose response rolled off in the 50 kc to 100 kc region instead of in the megacycle region, the figure would be even more impressive, about 85dB. The Citation A thus is a truly quiet instrument.

Because the Citation A has step-type, rather than continuously variable, tone controls, USTC measured the response of each step in each control separately and then plotted the results as the curves shown here. As can be seen, these controls can provide ample amounts of low or high frequency tone cut, but relatively small amounts of bass or treble boost. This design is deliberate; Citation "thinking" holds that the wide-band response—so flat and undistorted—obviates the need for, or even the desirability of, introducing excessive tone boost. Such boost can overload power amplifiers and damage a speaker; what's more, it is felt that the quality of the speakers logically used in a system that also included the Citation A would require a minimum tinkering with tone controls. Similar thinking with regard to the choice of a turntable in such a system is seen in the design of the A's rumble filter which attenuates relatively little of the very low frequencies - 4.3dB at 30 cps. The loudness contour introduces no treble boost, but does supply an ample amount of bass boost at low listening levels.

The relative absence of extreme variations in the tone control action was found, in listening and use tests, to be a completely correct and valid design approach. The extremely wide response of the A, combined with its virtual lack of any form of distortion, actually improved the sound of familiar program sources and loudspeaker systems. When substituted in a system that had been using a Citation I preamp, the most obvious improvement (over what already had been considered to be an excellent-sounding response) was in the transient, characteristics of music. Percussion instruments, for instance, were lent a more live and realistic quality, with excellent, sharp attack. The sound in general took on a more "opened up" and lifelike quality. This enhanced clarity and transparency was bestowed on discs, tapes, and broadcasts. The over-all effect of using the A is, simply, more music and less listening fatigue. The A was no less impressive driving high quality headphones. It is, in sum, a unit that should meet the demands of the most critical listener and audio perfectionist. It suggests that, by combining this unit with a transistor basic amplifier of comparable quality, a sound path could be set up that approaches the classic goal of amplifier design—that is, a "straight wire with gain."