Manufacturer's Specifications:
Input Impedance: 8 ohms.
Power Rating: 25 Watts.
Frequency Range: 25 to 19,000 Hz.
Crossover Frequency: 3000 Hz.
Controls: High Frequency Balance.
Nominal Woofer Size: 10 inch.
Dimensions: 14" h x 22 1/2" w x 8 5/8" d.
Suggested Retail Price: $89.50.
An attractively styled two-way bookshelf speaker system in the modest price category, the Jensen TF25 could well be the answer for those seeking a reasonably good sounding "second system" or for others who require moderately high efficiency in a small enclosure. We were surprised to find that about five watts of clean amplifier power was all we needed in a medium-sized listening area to recreate desired dynamic sound levels.
By way of explanation, it should be noted that the enclosure design lies midway between the popular, low efficiency "air suspension" designs and the older, more conventional sealed enclosure more commonly referred to as an "infinite baffle." The woofer element of this two-way design comes from the "Flexair" series of loudspeakers which Jensen makes available separately as their C 10PF loudspeaker mechanism.
While these speaker elements have a fairly long "throw" or excursion and a soft rim suspension, they should not be thought of as true air-suspension or soft-suspension types. It is this "in-between" design, however, that accounts for the higher efficiency and surprisingly good bass response within the confines of an enclosure that might ordinarily be thought of as suitable for "air suspension" speaker elements only.
The 10-in. element is used all the way up to 3000 Hz, the crossover frequency, at which point a capacitive inductive crossover network, associated with a wirewound level control, routes the signal to the horn-loaded, compression-driver tweeter. The open end of the horn measures two inches by five inches.
Enclosure construction is solid, with sides and top and bottom made of 3/4-in. lumber and the back constructed of ½-in. material. The back is removable by means of ten screws. Attempt is made at sealing the enclosure by means of gasket material placed under the back cover. Styling of the front includes two vertical bars, as can be seen in the photo of Fig. 1. These enclosures may be oriented either vertically or horizontally, although we found that better angular dispersion of highs resulted when the enclosures were oriented with the long dimension running vertically.
We performed two types of electrical measurements in connection with the TF25 units. The first of these involves impedance. Although this system (and many, many others) is said to have a nominal impedance of 8 ohms, it is well known that this figure applies at perhaps one frequency or a specific range in the audio spectrum.
In this age of solid-state amplifiers, it becomes very important that a speaker exhibit an impedance characteristic which never falls below the safe limit of 4 ohms, regardless of frequency applied. Figure 2 is a plot of impedance versus frequency. Note, that the nominal impedance of 8 ohms applies at a frequency of 600 Hz. Happily, however, at no frequency did the measured impedance decrease below the safe limit of 4 ohms. This is particularly important at the extreme low end of the spectrum where inaudible but high-powered rumble might damage the associated amplifier if this limit were not maintained. Judging by the high impedance at 80 Hz, we would assume that this is the resonant point of the entire system, despite manufacturer's claim of 25 Hz resonance for the woofer element. It is, by the way, not unusual to have a steep rise in impedance at or about the resonant frequency of the system, as shown in Fig. 2.
Fig. 2 and Fig. 3
The second measurement performed was an open air frequency response plot, using a calibrated microphone mounted on-axis at a distance of three feet from the surface of the loudspeaker system. A word is in order in connection with this frequency-response plot, which is shown in Fig. 3. Most speaker systems exhibit small rises and dips over their entire usable range. In conventional graphic recorder plots of speaker response curves, there are many more dips and rises than are shown in Fig. 3. To present a more meaningful plot, however, we have averaged out the smaller dips and rises, since from an auditory point of view they are relatively unimportant.
We have included such variations from "flat" response that represent more than a tenth of an octave in spectral width, since these are believed to contribute to the overall coloration of the particular speaker system. Three runs were made at the high-frequency end of the spectrum, corresponding to various settings of the high frequency balance control. Later listening tests confirmed that the most "balanced" and pleasing sound occurred with this control set to its midpoint.
After testing so many "low efficiency" systems, the most startling thing about the Jensen TF25 is its loudness level when fed with just a little bit of power. The control on our receiver, which normally needs to be set at about 2 o'clock for low efficiency speakers, was set at about 10 o'clock for the same loud level at which we usually do our subjective listening.
Highs were quite clean and well defined, as was the upper midrange sound. The system seemed a bit deficient in the lower mid-range, through this may well have been caused by a pronounced peak at about 100 Hz, which tended to give the spoken voice just a bit of a "barrel" or hollow sounding effect. Admittedly, many listeners prefer this kind of sound, and that is probably why Jensen tailored the response of this system to meet that preference. We find the effect a bit unnatural. One thing is certain, however. Before this system could possibly be overloaded by excessive power, you would long since have achieved more than enough loudness for any listening you might want to do--even in relatively large listening areas.
(Audio magazine, Feb. 1970)
Also see:
Jensen Model 530 Three-Way Loudspeaker System (Jul. 1977)
Acoustic Research (AR) Model AR-5 Loudspeaker System (Equip. Profile, Jan. 1970)
Acoustic Research AR-6 Loudspeaker System (Equip. Profile, Jan. 1972)
AR-7 loudspeaker (Equip. Profile, Feb. 1973)
AR (Acoustic Research) AR-7 loudspeaker (ad, Feb. 1973)
AR M1 Speaker (Equip. Profile, Jan. 1992)
Acoustic Research AR98LS Speaker (Jan. 1985)
AR (Acoustic Research) MGC-1 loudspeaker (ad, Nov. 1986)
Acoustic Research LST Speaker System (Dec. 1972)
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