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Introduction

The design of the PLUTO loudspeaker brings together several concepts and ideas about a small size loudspeaker that had been on my mind for some time. In preparation for a 2002 AES Convention paper I had looked at the sound output capability and distortion behavior of several 5" drivers, and the minimum requirements for accurate sound reproduction in a surround system setup. I also liked the ease with which I could build, place and hide 4" pipe constructed ambience loudspeakers in my living room. When I then saw a 2" driver that had flat response to 200 Hz and more displacement capability than typical tweeters, I had to build a prototype to hear first hand whether this unit would hold up to the selected 5" driver when crossed over at a very low frequency. My goal was to obtain an acoustically small source with wide and uniform sound dispersion over most of its frequency range. Next to dipole radiation, as with the ORION, I consider omni-directional behavior desirable, provided that either the room has neutral acoustics or that one listens to such speakers from a close distance.  The prototype promised that I could develop from it a low cost, yet highly satisfactory music system. A system that would not be an embarrassment when played right next to the ORION. Now that it is done I would not hesitate to recommend PLUTO even to ORION owners should they look for a smaller, second sound system or surround setup. But if you add the recently designed subwoofers, then the 3-way PLUTO+ becomes truly an alternative to the ORION. Their biggest difference is in their interaction with listening environment.

First, some after-thoughts:
Now that I have lived with PLUTO for some time, have compared it to the ORION often, had other people listen to it when they visited to hear the ORION and then noting their observations, I have come to the conclusion that uniformity of the polar response is even more important than I had thought previously. When comparing PLUTO to ORION the thought immediately arises, how can they sound so similar when one is essentially a closed box and the other an open baffle speaker, and both operate in a fairly live room? The drivers in PLUTO cost a fraction of those in the ORION, and though they are exceptionally well suited to this application, they are not equivalent. Both speakers share a uniform on-axis and off-axis frequency response from the very lowest frequency up into the low kilohertz range. One is figure-of-eight, the other is circular, both horizontally and to different degrees of uniformity also vertically. The typical box speaker has a transition region from circular (4p) radiation to increasingly forward radiation (2p and less), the so called "baffle step", which occurs in the 300 Hz to 800 Hz range depending on front baffle and driver size. The smaller the box, the higher in frequency this transition occurs. For PLUTO it is above 3 kHz. Now, if we listened in an anechoic chamber or outdoors, then none of this would matter, but since speakers are used in rooms we also hear the sound that is radiated off-axis. It merely arrives at our ears with slight delay, attenuated and possibly changed in timbre by objects and reflecting/diffusing surfaces in the room, and often reinforced at specific low frequencies. Listening to PLUTO or ORION highlights the importance of illuminating the room uniformly over a very wide frequency range. The strength of illumination from ORION is 1/3rd (-4.8 dB) of that of PLUTO, thus the room is less of a factor with ORION. This can be completely compensated for by listening to PLUTO from a closer distance. Regardless of listening distance PLUTO elicits a significantly more neutral response from the room than the vast majority of box speakers is capable of. 

A few days ago I had the opportunity to set up the PLUTO at a friend's house - electro-acoustic consultant Brian Elliott - and to compare it there to the most sophisticated and lowest distortion open baffle system that I am aware of. A day later I compared PLUTO to the Quad ESL 989 at a local high-end audio store. Using immediate A-B switching and also longer uninterrupted listening we were all amazed by the similarity of sound from such dissimilar sources. Actually more so when listening from a greater distance than I had suggested below and with the PLUTOs at least as far apart as the comparison speakers. The two listening sessions also confirmed what a wonderful job these little speakers can do with voice, classical and jazz. Even Mahler's 3rd Symphony was a delight to hear on them. (Chailly, Concertgebouw, Decca B0002336-02)
After these experiences I widened the subtended angle in my own setup as in the graph below. I had not tried this before, because I thought it would create a hole in the center, but the speakers resolve images even better this way and with a large sound stage that extends uniformly from left to right. 

How does it sound? My comparison to the ORION might give an indication. When I sit in the sweet spot for the ORION, then PLUTO is set up to be closer to me, because the reverberation distance of a monopole is only 60% of a dipole's distance and I want to have a similar ratio of direct to reverberant sound from both speakers. PLUTO should be listened to from closer distance and be placed at least 3 feet (1 m) away from large reflecting surfaces, otherwise the room masks sound stage detail and bass, though the tonal balance remains neutral. Switching back and forth between PLUTO and ORION can be confusing as to which one is playing, but depends on program material. When very deep bass is required then PLUTO only hints at its presence, but cannot deliver the rumble. The male and female voice range, though, whether single or in mass, is handled so well that I sometimes wonder what the higher cost of the ORION is buying me. I do not sense a lack of very high frequency extension nor of volume capability. Imaging of PLUTO is very precise, the speakers disappear, the sound stage is tall and wide, but with less depth than the ORION. The performers appear closer. Overall, PLUTO lacks at times the ease, authority and refined clarity of sound that ORION can deliver on good program material. Even so, PLUTO has that special quality that allows me to lose myself for hours completely in the music and to forget about these speakers, except to marvel occasionally.

PLUTO is a fully active loudspeaker. It houses two power amplifiers, active equalization and an active crossover in its base. Merely connect PLUTO to your preamplifier output, plug in the power cord, and you will have sound. If your equipment does not have a preamplifier output connection and only a power amplifier output terminal, then use a thin 2-conductor cable to connect it to PLUTO's binding posts as if PLUTO were a passive loudspeaker. The output signal from your power amplifier will be attenuated 20 dB inside PLUTO and then drive its electronics just like a signal at its low level RCA input connector. PLUTO can serve as a high performance speaker for any portable music device, PC or laptop. An active speaker design approach was also chosen because it allows to optimize each driver's performance with the least amount of overall power required and with a maximum of control over each driver's behavior throughout its whole frequency range.   

PLUTO has considerable volume capability. Its limit is set at low frequencies by  the volume displacement of the 5" driver and an intentional low frequency roll-off at 12 dB/octave rate, with -3 dB at 60 Hz and critical damping of Q = 0.5. The two built-in power amplifiers have a combined peak capability as if driving a corresponding passive 2-way speaker from a single 500 W amplifier. PLUTO was designed to perform without the need for a subwoofer and will do so in many applications, especially in small spaces and in close-up listening. The low frequency response could, of course, be extended down to 20 Hz by adding a subwoofer like THOR. For crossing over to PLUTO without any design change, the subwoofer would need to utilize a 2nd order Linkwitz-Riley lowpass filter, which is 6 dB down at 50 Hz. But, in order to also increase PLUTO's overall output volume capability, the crossover frequency must probably be raised to 120 Hz, which requires major changes in its low frequency equalization circuitry. The spatial integration between subwoofers and modified PLUTO now becomes a much more difficult design task. The addition of a subwoofer falls outside the boundaries that I set myself for PLUTO and I leave it up to anyone interested to work this out on their own. Chances are that the interaction of a subwoofer with the room decreases the tightness and definition of bass that PLUTO delivers without it. In general I would say, if your preference of program materials and volume levels is such that a subwoofer is required to satisfy you, then the recently designed PLUTO+ would the better loudspeaker system for you or you might consider the ORION. 

PLUTO has a small footprint and weighs only 15 pound (7 kg). With just two thin wires connected to each speaker it can be easily moved to its optimum location out in the room and close to your seat for full listening enjoyment and then be put  back and out of the way when not in use. 

PLUTO was primarily developed for do-it-yourself construction, but it is now available completely assembled from Wood Artistry. On the following pages I provide sufficient information so that someone with experience in building speakers and analyzing electronic circuitry can duplicate what I have built. It is important to match the terminal voltages in magnitude and phase, to properly absorb the rear wave of the drivers, and to adhere to the relative positioning of the two drivers. The closest equivalent metric pipe dimensions can be used, provided that they do not add air flow restriction on the backside of the drivers. I sell construction plans and blank printed circuit boards to make the building process easy for everyone. A pair of PLUTO loudspeakers would cost you around $680 and could take 30 hours of your time. If you purchase the assembled and tested electronics modules, the total cost will increase to about $900, but you save at least 8 hours of time.

This could be a fun loudspeaker project even if this is your first one. There is much that can be learned about loudspeaker and electronic design and construction from building and studying PLUTO in action. You will have given yourself speakers for a wide range of applications and flexibility in placement. Most importantly, PLUTO will provide you with sound reproduction of a very high quality level, which is likely to lead to many hours of music enjoyment. 

Finally, you may have different ideas about the enclosure design, the electronics or the driver selection. Please do not ask me for advice beyond what is provided on this site and in the construction plans. I have not done a parameter sensitivity study other than to analyze that the design with the specified components and dimensions is robust.  I recommend to first build PLUTO exactly as it has been described, to evaluate its actual performance, and then to apply your changes. That way we all might learn something worthwhile. Keep in mind that the PLUTO design was not an accident and that it has benefited from the strengths and weaknesses that I observed in other loudspeakers.

PLUTO is like any speaker the result of trade-offs between various parameters. In this case I have minimized size, and cost, and given up some output volume capability below 120 Hz, but in practice hardly any frequency coverage. I have maximized the ease with which PLUTO can be placed in the acoustically best location for any given room, so that its very high degree of sonic accuracy, neutrality, and spatial resolution allows maximum recognition and enjoyment of what has been stored in the recording that is being played. 
After all, it's about the music!

PLUTO can be auditioned at Sea Pine Cottage in Sea Ranch, California.

In the PLUTO Users Group forum you can find useful information for building PLUTO and possibly make contact with PLUTO owners for a listening session.

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Letter from a PLUTO builder

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