Equipment Profile: Hartley Model Zodiac ‘76 Speaker System (June 1977)

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Speakers: One 10-in. woofer; one 1-in. dome tweeter.

Frequency Response: 35 Hz to 25 kHz.

Crossover: 2 kHz.

Impedance: 8 ohms.

Minimum Power Requirements: 10 watts.

Cabinet Size: 30 in. (76.2 cm) H x 15 in. (38.1 cm) W x 12 in. (30.5 cm) D.

Shipping Weight: 50 lbs. (22.7 kg).

Price: $140.00.

The Hartley Zodiac 76 is a full -range loudspeaker system using two drivers. A 254 mm (10 in.) woofer is used in an infinite baffle to cover the frequency range from 35 Hz to the crossover at 2 kHz. A 25.4 mm (1 in.) dome tweeter is used to carry the frequency range from 2 kHz upward. (Editor's Note: Hartley informs us that systems produced after March 1 have a tweeter with greater power handling ability.) Housed in an attractive oiled walnut cabinet measuring 381 mm by 305 mm by 762 mm (15 X 12 X 30 in.), the system is not too large to be classed as a bookshelf speaker, but at 22.7 kg (50 lb.), I would suggest that this speaker principally be used as a floor-standing system.

One of the interesting features of the Zodiac series of speakers by Hartley is their deliberate design of systems in asymmetrically matched pairs for stereo. Thus, there is a left speaker and a right speaker, and their identity is clearly marked on the rear of the speaker. The intent of this asymmetry is to provide a greater sound dispersion, and Hartley recommends the normal left-right configuration when the speakers are 10 feet apart or less, and a reversed configuration for wider speaker spacing to improve the spread of sound.

Rated at 8 ohms, a minimum of 10 watts is required to produce normal listening level, according to Hartley. Connection is made to spring-loaded binding posts mounted in a recessed cavity at the rear of the enclosure.

I must give Hartley bad marks on a technicality that is a personal gripe of mine. Other than the suggestion concerning the effect of speaker spacing on which left-versus-right enclosure to set up to use, there is absolutely no information to help a new owner set up his Zodiac speakers for best performance. Nor is there any warning about the maximum safe amplifier power or other operating conditions that could get a user into trouble with the warranty, which is prominently displayed on the rear of one system. A purchaser does get a very attractive piece of printed material extolling the virtues of the speakers, but I assume a purchaser is already sold on the merits of the loudspeaker and now he should be told how to use it.

The warranty, my gripes to the contrary, is excellent and good for five years.

Fig. 1--Magnitude of impedance.

Fig. 2--Complex terminal impedance.

Fig. 3--Amplitude of one-meter anechoic sound pressure level with one-watt average drive.

Fig. 4--One-meter axial anechoic sound pressure level phase response corrected for acoustic position of the tweeter.

Fig. 5--Three-meter room response.

Technical Measurements

The measured terminal impedance of the Hartley Zodiac 76 is shown in Figs. 1 and 2. The bass resonance peak occurs at 55 Hz, with a minor subsidiary impedance peak at 850 Hz.

While rated at a nominal impedance of 8 ohms, the lowest measured value is approximately 5.5 ohms. This means that reasonably large hookup wire should be used when employing long runs of speaker lead.

The detail nature of the complex impedance plot indicates that there is a small amount of interaction between the two speakers and the enclosure in which they are housed. These glitches occur at 160 Hz, 1.16 kHz, and 1.3 kHz. Their nature is not severe but they do show some possibility of cabinet resonances.

It is an unusual, and fortunate, circumstance that the lowest value of impedance is slightly in the inductive reactive side. Thus, while the value is low and occurs at a frequency of 5 kHz where strong human presence vocal peaks can place strong demands on an amplifier, the amplifier is not under the strain of a capacitive reactive load. In short, most amplifiers should drive the Hartley with ease.

The Hartley Zodiac speakers are designed for stereo.

There is a left speaker and a right speaker. In order to perform our standard tests, we picked the left channel speaker for all measurements. This means that a one -meter axial an echoic sound pressure test might not look so great for a speaker intended for off -axis listening, and that was indeed the case. Figure 3 shows the amplitude of measured sound pressure for one watt average drive, and Fig. 4 is the measured phase response.

The on-axis amplitude response shows a pronounced dip in the 2- to 5-kHz voice presence range. Rotating this speaker in azimuth and elevation revealed that this response smooths out for a microphone position to the left and slightly above the geometric speaker axis, precisely where a listener should be for a left channel speaker placed on the floor. But the axial response is what we use as standard for this particular test.

The anechoic phase response has several unusual features. The Hartley Zodiac 76 is non-minimum-phase in the 2 to 5 kHz range for any of the orientations we tested. The on axis response has a full 360 degree phase lag at 1.3 kHz. The phase measurement indicates that the acoustic position of the woofer and tweeter are approximately aligned. But even more unusual is the 90 degree phase lead of the tweeter.

Thus the tweeter pressure is neither in phase nor out of phase with the terminal voltage, but leads by 90 degrees.

This unusual characteristic was also verified from observation of the sound pressure impulse response. I do not know what the general subjective effect of this might be, but I do know I liked the sound of this tweeter in the earlier listening test.

But the anechoic test is a measure of a specific type of technical perfection. When it gets down to listening, the three -meter room test ought to be a better indication of how the speaker sounds. I was glad, therefore, to find that the response, shown in Fig. 5, was much closer to my earlier listening impressions than the anechoic response.

I tested the Hartley where I heard it. Twelve inches from a wall and placed on the floor. The measurement position was the listening position, three meters away and one meter above the carpeted floor. I listened 30 degrees off axis, the lower measured curve. The upper curve is what we get when we sit directly in front of the speaker, or rotate it so it points to our listening position. For clarity, these two measurements are displaced 10 dB on this plot.

In my listening, I had thought the top end was great, vocals needed a mild 3-kHz lift for most accuracy, and there was a slight mid -bass bump around 200 Hz. Oh well, two out of three isn't bad. Actually my spectrum analyzer showed a bump at 100 Hz, but because of the 13 millisecond time gate used in this test, I cannot honestly show measurements below 200 Hz. Sounds great, and measures good.

The measured dispersion of sound energy is shown in Fig. 6 for horizontal spread and in Fig. 7 for the vertical spread.

The Zodiac definitely has left-right asymmetry, just as Hartley claims. However, this left-channel speaker would be hotter as a right channel source, since more energy is launched away from nominal listening position. The Hartley also launches more energy upward, which is a proper design for such a floor-mounted system. These plots indicate that large objects should not be closely positioned either outboard or above the speaker location.

Fig. 6--Horizontal energy response, left channel speaker.

Fig. 7--Vertical polar energy response.

The Hartley woofer has problems in handling large amounts of super low bass, as there is a definite tendency to crunch on Low E (41.2 Hz) if driven at high levels. This is shown in the harmonic distortion measurement of Fig. 8 and the intermodulation distortion measurement in Fig. 9. In the case of intermodulation, the nature of the modulation of low E on A above middle C (41.2 Hz on 440 Hz) is that of amplitude modulation up to about 10 watts, then a progressive increase in phase modulation above that level. At 10 watts there is five degrees peak-to-peak phase modulation on A4.

The Hartley fared much better in the crescendo test. A reference tone of 440 Hz is reproduced at a test power level and its energy observed in a one -Hertz filter centered about instrumental blurring should not be experienced even when cymbal crashes and brass peaks are encountered.

In yet another type of distortion test, the Hartley showed a smooth general drop of acoustic transfer gain with an increase in drive level. At 10 watts average power, tones are reproduced at one half dB lower sound level than would be expected from the pressure-versus-drive volts transfer gain measured at 0.1 watt drive level. This is similar to a mild compression in dynamics as the sound level is raised. A good feature about the Hartley is that this compression is not dependent upon frequency. Thus the sound should have a slight softening in dynamics with increased drive, but suffer no change in timbre.

The measured energy -time curve is shown in Fig. 10. The first peak of energy at 3.15 milliseconds is due to the tweeter. The second peak at 3.3 milliseconds appears to be due to the first sound components of the woofer, which for this axial one meter microphone position means that they are closely aligned in time. The energy fall off is at a rate of about 50 dB per millisecond until small diffraction peaks are encountered at around 4 milliseconds. Further scattered energy arrives after 4.7 milliseconds.

This is actually a very good transient response and the only indication of technical difficulty is the fall off of energy following the first tweeter peak, which hints of slight super top end ringing on sharp percussive sounds.

Fig. 8--Harmonic distortion for tones E1 (41.2 Hz), A2 (110 Hz), and A4 (440 Hz)

Fig. 9--IM distortion on A4 (440 Hz) when El (41.2 Hz) and A4 are mixed one-to-one.

Fig. 10-Energy-time measurement at one meter on axis.

Listening Test

As pointed out earlier, the Hartleys are supplied as a left and right matched set. These are upright floor-mounted speakers. After some experimentation I found that I preferred these speakers about 12 inches away from a wall and pointed straight outward.

I am favorably impressed with the sound of these speak 440 Hz. Then random noise is added at a level 20 dB higher than the single tone when averaged over a 20 kHz spectrum and the change noted in the property of that tone. This process is repeated at increased test power levels, always at the same ratio of tone t3 noise. Any modulation of the tone by the superimposed random signal indicates that peak program bursts may distort lower level musical voices. The Hartley showed no measurable modulation on up to a peak-to-peak test level of 80 volts. Since this is well above the recommended peak power of this speaker, it can be inferred that ers. The super lows are down, but not out. I thought there was a slight mid -bass bump and felt that the range around 3 kHz needed a small lift. But, for the money, the rest of the spectrum is good. The tweeter in particular is a star performer and has a good bite on brass instruments.

The left-right and stage center stereo illusion is good, in my opinion. The illusion of depth, however, is somewhat lacking.

This speaker is quite efficient and puts out a lot of sound.

At high sound levels, approaching the 250 watt/chan. limits of our Marantz 510 amplifier, there is a mild crunch in midrange which sounds a bit like cone cry, but this occurs well above the levels of anything reasonable in home listening.

It also sounds pretty good at low levels. At $140, I think this speaker is a good sound bargain.

Richard C. Heyser

(Source: Audio magazine, June 1977)

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