Servo controlled active open baffle speaker
There are many different speaker concepts and we tried most of these. The main problems is coloration (distortion), this is usually caused by multiple standing waves of the same frequency within the speaker enclosure. Another issue is compression of air and the given slip between fixed magnet and moving voice coil. In other words when we apply a certain voltage, the voice coil moves to a certain position. But if we apply slightest mechanical force on the voice coil we can change the position of it, regardless of the applied voltage. So even when we manage to design a perfect speaker cabinet, we can't eliminate the asymmetrical load (compression) on the voice coil. This will always cause some distortion. With open baffle speakers we don't have these problems. The main problem with open baffle speakers is bass. This problem can be fixed using multiple bass units that compensate for the baffle step (loss of lower frequencies with finite baffle size). We ended up using one 8" full-range speaker and two 15" bass units. The baffle could be kept relatively small so it's not too intrusive.
Next problem is the power amplifier, when applying constant voltage steering (semiconductor power amplifiers) or quasi constant current steering (most tube amplifiers) there is no guarantee that the voice coil and attached membrane will always track the signal. So even when using a perfect power amplifier, the speaker will still distort significantly, typically 1% or more. In order to resolve full 16 bits we need much lower distortion, theoretically 0.002%. This basically means that given amplifier-speaker distortion makes high-res playback (16 bits and up) theoretically impossible.
In order to minimise this effect we need a servo system, something similar to the MFB (Motional FeedBack) servo bass system used decades ago by Philips, but now for the entire audio spectrum. Such servo system won't work very well with existing speakers as we have multiple electrical feedback signals from multiple voice coils plus the speaker crossover. So we decided to go for an active open baffle speaker. The end result is greatly reduced distortion, regardless of playback level, no audible coloration and very high resolution. The servo amplifier uses the ac voltage generated by the voice coil. The unique properties of this servo power amplifier eliminate crossover distortion. The servo amplifier soft clips like a tube amplifier, so it can be over-driven without problems. Because the servo amp works in synergy with the speaker (it doesn't try to force the speaker) less energy is wasted, this results in higher SPL for the same power level compared to conventional power amplifiers that fight with the speaker and thus waste power and related efficiency. Because of increased servo amplifier-speaker efficiency and the already high efficiency of the speaker units, the servo only needs a 7.5W rms output power rating. The servo amplifier also compensates for voice coil temperature changes, so related distortion is prevented too. The power consumption from the mains is constant at 15 watts. So these speakers are also energy efficient.
Each speaker has a RCA input and offers full output (no soft clipping) at approx. 1Vrms. These speakers not only greatly improve music reproduction quality of digital audio, but can also be used with analogue audio sources.