Unas bocinas diferentes: Aurus PM desde Suecia


PM is a three-way loudspeaker system which I started to develop in the mid-seventies. The goal was to construct a loudspeaker that was able to reproduce the real acoustic environment from the recording. Discussions about time-delay in those years, which were confirmed by measurements, led me to a design with a complex baffle and exponential horns. Later on I've made fibreglass moulds for casting the baffle in concrete. The sound from this loudspeaker is something you must experience. It really is competitive with the "big" names on the hi-fi scene.
The PM loudspeaker is a three-way system with a 10" Peerless CC-line woofer, a 4.5" Peerless midrange, and a 1" Peerless soft-dome tweeter. The midrange and the tweeter are mounted at the back of the baffle in exponential horns with machine screws and nuts in the cast concrete. The woofer is mounted in the same way at the front of the baffle. The baffle is designed to produce low diffraction effects and to have a nice appearence, and care is taken to handle the time-delay between the midrange and the tweeter. The exponential horn, especially for the tweeter, works as an acoustic transformer, and the benefit of using it in the design is that, when you compensate for the amplification of the horn in the filter, you also lower the distortion. The use of the horn loading also allows you to place the treble in the correct time-delay plane. The effect of the midrange horn is not as good as for the treble in my design, because of the impossibility of integrating a horn of the full required size that would be needed in the PM's baffle. I had to compromize in that matter, as in the matter of the amount of time delay between the woofer and the midrange. I shifted the polarity of the woofer instead so that I came up with a time delay error corresponding to only a few centimeters in distance.
The concrete baffle is mounted in the enclosure with wooden sqrews so that you can change a driverif anything should happen. The design of the baffle was quite a drawing and calculating experiencein the mid-seventies, as in those days I only had a pocket-calculator at my disposal. The design involved both calculatingexponential curves and geometric drawings to get the right angles of the drivers to make themcoincide at a proper listening distance. But the design really was correct, which I found out later when I started measuring the PM's acoustic responce. The slope of the baffle also places the sound "image" at a good height even though the loudspeaker is not very tall.
The enclosure is made of veneered 16 mm MDF exept for the bottom and the back of the enclosure.These pieces are made of 22 mm veneered birch-plywood. The veneer used for the enclosure in the picture on the homepage is called pyramid-mahogany. I've also veneered in birds-eye maple, padoc, American walnut, ash, Nordic pine, cherry and some others. It all depends on each person's individual taste.
The bass vent is tuned to about 25 Hz. The midrange has a separate chamber that is also vented. The midrange vents come out in the front at both sides of the baffle. The bass vent is placed in the enclosure base where the crossover filter also resides.
Both the woofer and the midrange are acousticly damped; that is, you put the damping material at the back of the driver basket. This is a damping method that is not commonly used, but it is recommended by, for example, Doctor R.H. Small. Adding this type of damping allows you to adjust the driver Thiele-Small parameters. The PM's outside dimensions are: W = 37.5 cm, H = 71 cm (including Sonic Design damping feet which I use to minimize vibrations in the enclosure) and Depth = about 48 cm.
The crossover filter used in the PM's includes CFAC inductors in series with the woofer and midrange, and MIT capacitors in series with the treble. The other elements in the filter are polypropylen capacitors and non-inductive resistors. The crossover frequency between the mid and treble is roughly 4000 Hz with a 6 dB/octave slope on the treble and 6-12 dB lowpass slope on the midrange. The woofer-midrange crossover is at about 400 Hz and uses filters with 6-12 dB/octave slopes. In the 6-12 dB/octave filters you start with a 6 dB/octave filter and then you cut 6 dB/octave more at about one octave higher (or lower). The filter is hard-wired. The filter construction is computer optimized using the CALSOD software package and fine-tuned by listening. Special attention has been paid to the impedance responce to produce a "nice" load for tube amplifiers.
The PM loudspeakers was tested in a Swedish magazine about two years ago and the journalist found them really capable of reconstructing the "acoustic room". He also made a bass test where he used a 40 Hz and a 100 Hz tone and drove the loudspeaker to a distorsion level of 10 % and then checked the output level at 1 m. The PM's had exactly the same level at both 40 and 100 Hz, and it was comparable in level with another loudspeaker in the test with 18" woofer. The PM's have really got a tight and clean bass responce and with a capability of playing very loud. You get that subwoofer feeling withoutrumble. The "imaging" is almost perfect because of the clean mid-bass and the homogenity between the drivers, which makes you "see" the whole recording room. Someone called the reproduction quality "seemless". This person also said that he never before had that feeling of "having the whole symphony orchestra in his room".

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