PLUTO, the small 2-way active loudspeaker system that was introduced in 2005 has undergone several changes to improve its acoustic performance and to make it easier to build. Most significantly the woofer/midrange driver was replaced by a special Seas unit to obtain lower non-linear distortion and increased bass output. This became Pluto-2 . It required component changes on the printed circuit board of the original PLUTO to update the equalization and crossover for the new driver. Also a new mounting scheme was introduced for the woofer/midrange driver to improve the looks and to provide an alternative in situations where the original rubber pipe coupler is difficult to find or not desired. The base, which houses the two power amplifiers, the electronic crossover and equalization, was increased in size to make room for larger heat sinks, for ventilation and for easier access to the circuit board.

No changes were made in the tweeter selection. The Aura driver is instrumental to the omni-directional PLUTO concept since this tweeter allows for a low 1 kHz crossover frequency and uniform radiation over large off-axis angles up to several kHz due to its small external dimension. I am not aware of another equally well suited driver for this application.

I have built, measured and listened to a number of Pluto-2 systems both at home and in other locations. The new Seas driver had required a change in equalization. That change indicated further potential for refinement and I have revisited the equalization process as if starting new. Equalization in my loudspeaker designs is usually a correction of trends in the frequency response. The measured frequency response often hides these trends in some fine grain structure. The fine grain structure is usually the result of reflections in the measurement environment and diffraction from the loudspeaker cabinet. Thus the chosen equalization for a seemingly flat on-axis response may not yield a flat response, if the artifacts could have been removed. Some educated guess work is involved in equalization. It yields more realistic results than the blind equalization of every detail often used in computer software. It is not surprising then that re-equalization can lead to somewhat different woofer and tweeter drive signal frequency responses, especially when the underlying physical phenomena are better understood.

The low frequency equalization was left unchanged but the midrange, crossover region and tweeter notch were modified. The tweeter level was reduced by 2.5 dB. The 220 Hz notch corrects for the remainder of the attenuated half-wavelength pipe resonance. The audible effect of this is a warmer presentation with an increase in low bass. There is no loss in neutrality or transparency. The all-important voice range is rendered naturally.

PLUTO-2.1 is a worthy alternative to ORION+ when cost, size or room placement do not allow for a large system at this time. If you think that you need another octave of bass, then you could add the PLUTO+ subwoofer, but you should wait with that decision until you have lived with PLUTO-2.1 for a while. This is such an enjoyable speaker system by itself that the doubling of expenses for the subs may not be necessary. You can drive these speakers directly from a universal player that has volume controls, such as the OPPO DV-980H, from an iPod, a laptop, from the outputs of your preamplifier or the loudspeaker output terminals of a receiver.

The most cost effective way for acquiring the PLUTO-2.1 loudspeakers is to build them yourself. The PLUTO documentation and its updates on the owner-support page provide detailed information and ongoing support.
The loudspeakers can also be purchased ready made from Wood Artistry. Of course, you now pay someone else for turning basic plumbing materials that you can find in a local hardware store into a unique looking product' You pay for fit and finish, for assembling and testing the electronics and getting it all safely to your place. This might be the most expedient way for you to become a PLUTO owner. Don Naples will thank you for it.

You can listen to PLUTO-2.1 in our Sea Pine Cottage at The Sea Ranch in northern California, or visit me in Corte Madera, 10 miles north of the Golden Gate Bridge in San Francisco.

Do-it-yourself and Wood Artistry versions of PLUTO-2.1 that show construction and finish alternatives.

The enclosure is made up of 4 inch and 1.5 inch or equivalent metric PVC or ABS plumbing pipes that must be cut to length and of associated end caps, knees and pipe couplers. The mounting section for the woofer/midrange driver can be a rubber plastic-to-clay pipe adapter or a wooden crown that is assembled around a standard pipe coupler.

The same electronics assembly works for either construction.

Power amplifiers, electronic crossover and equalization are built into the base.

Construction requires the ability to follow the labeling on a printed circuit board for placement of electronic components with leads into wire holes and then soldering them to circuit pads. The components are described in a material list and are ordered from suggested suppliers. The housing is a simple wooden box.

A review of the earlier Pluto-2 loudspeaker has been published by Peter Aczel of The Audio Critic.