Multimedia speakers
Monacor SPH-60X dual-cone full-range loudspeaker in bass reflex cabinet
Used beside my workbench in the Attic
Associated equipment: Latest update: November 22, 2005
Zen Revisited Power Amplifier (Nelson Pass)
Bride of Zen Preamplifier (Nelson Pass)
Homemade FM Tuner (based on Ambit modules)
Homemade Phono Preamp (with pencil tubes)
Pioneer PL-12D Turntable with Audio Technica AT-66 MM cartridge (dating from 1972 !)
The Dutch magazine Audio & Techniek once published an article about one-way loudspeakers that were based on Audax units 1. Although speakers using just a single full-range unit per audio channel were disregarded in audiophile circles (high intermodulation distortion), they could also have important advantages (excellent imaging). The editors of Audio & Techniek were enthusiastic about the Audax sound; they claimed that the single driver flaws were minor and the advantages were huge. “We never heard so much soundstage for so little money” was their comment.
Audiophile convictions are subject to fashion like every other human opinion. In the 1960s, speaker builders considered full-range drivers from Philips (9710M, 9710AM) a “best buy”. Such speakers were efficient and could be used with valve amplifiers. In the 1970s, bextrene woofers, soft dome tweeters and complex filters became popular. Because of the low efficiency and heavy load of such speakers, powerful solid-state amplifiers were required to drive them. But history tends to repeat itself. In the 1990s, valve amplifiers and full-range speakers reappeared.
Since I wanted a pair of speakers for listening to FM stereo in my attic, I planned to copy the Audio & Techniek design. However, I could not acquire the Audax driver. Apparently it had been discontinued. Thus, I decided to design a speaker of my own. In the Monacor catalog, I found an interesting unit: the SPH-60X. A 5 inch dual-cone loudspeaker (paper cone with rubber surround, paper whizzer) with a strong, vented magnet:
The published frequency response is exceptionally smooth (100 Hz to 18 kHz within 3 dB) and free of major dips (see above). The price was only 30 Euros (for a single unit, 60 Dutch guilders in 1995).
Parameters of the SPH-60X are shown in the right column of the following table: free air resonance 60 Hz, power rating 30 W, Sensitivity 89 db (1 W / 1 m), Suspension compliance 1.19 mm/N, moving mass 5.9 grams, mechanical Q factor 2.17, electrical Q factor 0.53, total Q factor 0.42, Compliance Equivalent Volume 9 liters, DC Resistance of Voice Coil 6.2 Ohms, Inductance of Voice Coil 0.4 mH at 1 kHz, Effective Cone Area 75 square cm.
When I fed these data to the Boxcalc program, the software suggested the following enclosure: volume 13.2 liters, vent diameter 5 cm, vent length 14.25 cm, f3 48.9 Hz, SPL 87.3 dB (1W, 1m).
Based on the Boxcalc simulation, I built two speaker cabinets from MDF. Outside dimensions: height 400 x width 180 x depth 312 mm, wall thickness 18 mm, reflex port on the back (diameter 50 mm, length 150 mm, equal distances to cabinet sides, center of port 115 mm from cabinet bottom), drive unit on the front (equal distances to cabinet sides, center of cone 120 mm from top of baffle). One reinforcing lath (50 mm wide, 18 mm thick), running from front to back at the center of the front and back panels, to stiffen the cabinet. Enclosure stuffed with polyester wadding, amount as specified by the manufacturer (Monacor). I used only wood glue, no screws.
My wife smoothened and painted the cabinets (I wouldn't have patience to do this!) and I mounted the drivers. Since the flange of the units is quite thin, it is not necessary to sink them into the front panel; they can be mounted with four screws from the front. Wired with 5 m standard loudspeaker cable (2 x 2.5 square mm, red and black variety), soldered directly to the speaker terminals and fitted on the other end with good banana plugs (Hirschmann).
The sound surpassed my expectations. Smooth, natural, exact placement of instruments, very good rendering of the human voice. Only the lowest octave missing and not suitable in large rooms. Now I can understand why 9710M drive units from Philips were so immensely popular! With a bad amp, these Monacor speakers sound bad. With a good amp, they sound VERY good. One can listen to them for hours without getting fatigued.
References:
1 Peter Haase, PMR, The “Poor Man’s Reference”, Audio & Techniek, July issue, pp.35-41, 1989
Questions, Comments: aren.van.waarde@hetnet.nl
(Tip: Use only my initials if you want to reach me!)
Finally, two pictures of the speaker front and back. As you can see, it is quite messy in the attic!
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