Workshop #6
Active Loudspeakers - An Example

Active loudspeakers tend to have an unfair advantage over their conventional cousins. In these speakers the amplifiers, crossovers and loudspeaker drivers are mated together to form a complete synergistic system. Such speaker systems have been very popular in the pro-sound arena for quite some time, but have yet to make a significant impact in the audiophile community. This may begin to change as more compact and efficient power amplifier technologies capable of high sonic quality emerge.

A set of DIY active loudspeakers developed by Bob Cordell and called the Athena was described and auditioned in this presentation (not to be confused with products from the company called Athena Technologies). This loudspeaker integrates conventional Class-AB MOSFET power amplifiers (4 X 125W per channel) into its cabinet, and demonstrates what can be accomplished in an audiophile active speaker system. The active configuration allows the loudspeaker to use a special form of vented box active equalization, called EQSS™, to achieve good bass extension and SPL in a small footprint. The 3-1/2-way mini-tower is only 39 inches high, 9 inches wide and 16 inches deep at its base.

The loudspeaker consists of a 1-inch silk dome tweeter, a 3-inch cone midrange and four 5.25-inch woofers in a TMWWWW arrangement. The tweeter and midrange are each driven by a dedicated 125-Watt amplifier fed from a 3-way active crossover. They are crossed at 4 kHz with a first-order slope. The top two woofers are fed by a dedicated 125-Watt power amplifier, as are the bottom two woofers. The top two woofers are active from the lowest frequencies up to the midrange cross at 750 Hz. This crossover is also accomplished with a first-order slope.

The bottom two woofers become active below 200 Hz. They are crossed with the upper two woofers with a constant-voltage half-way cross. This means that as frequency decreases and the bottom two woofers become active, the energy is re-distributed from the top two woofers to all four woofers without any change in frequency response. This is done so as to take advantage of the closer proximity of the upper two woofers to the midrange for purposes of the 750 Hz crossover. At the same time it provides the full benefit of the cone area of four woofers at frequencies below 200 Hz, where it is most needed. The proximity of the two lower woofers to the floor also mitigates floor-bounce effects.

The baffle is sloped back by 11 degrees to provide time alignment of the drivers. This, in combination with the first-order crossovers, results in a transient accurate loudspeaker. The speaker also includes adjustable electronic baffle step compensation.

The main objective of the speaker system was to achieve big sound from a small box. A related objective was to achieve this in a narrow box for improved imaging. This necessitated the use of small 5.25-inch woofers. The Equalized Quasi-Sealed System (EQSS™) technology made this possible. The 5.25-inch woofers have a free-air resonance of 53 Hz and are each allocated 9 liters of box volume. The box is tuned to 37 Hz with a 3-inch rear port. The system achieves a -3 dB response down to 35 Hz with a rolloff that is approximately second-order (Q = 0.7) down to below 25 Hz. Most importantly, excellent low-frequency SPL is achieved with this approach.