... please don't phase me in ...

ForcePanel ForceSubBlock

Small subwoofers try to squeeze 10 pounds of performance into a 1 pound box. Thus, they need to 'adjust' the amp response to provide reasonable performance from a driver and volume smaller than could be considered optimum. The Force Sub uses a 250W OEM amp to power a custom 10" driver and achieves remarkably good performance of 20-250Hz ±3db. The Force is no longer manufactured, but its principles are the same for small sealed subs.

The Force Amp

Input Eq Phase Control

The amp has a large 15db @10hz low end gain starting @ 40Hz to linearize the driver and box.Whenever an equalization is applied, Phase is affected.

The amp has an All-Pass filter to adjust phase, but the Input Gain phase shift negates the phase control markings.

Like all such All-Pass phase controls, it operates at only a single frequency with varying degrees of shift across the passband. In the right graph, the horizontal bars on the 22.5° to 135° lines cross at about the frequency at which the marked phase occurs for the All-Pass filter alone. The 0° point is 2341Hz and the 180° point is 4.67Hz. Input Gain phase is added to these values. More than likely, many small subs suffer similar problems.

FilterResponse OutputPhaseTimeDelay

The filter section consists of two 12db/oct 35-250Hz filters. 2nd Order filter phase is about 180° at 1 octave below the corner frequency. Both Level and Phase are cumulative, thus by careful adjustment of the two filters one can approximate a desired level and phase within the limits of the filters. The left graph shows the combined response of A Filter 130Hz and B Filter 180Hz.

The right graph shows the cumulative input, phase and filter for the overall amp response. As can be seen, the phase, and hence time, response @ 180° is much worse than at 0°. The dotted lines show tΦ. Spica's Servo subwoofers used an 18db/oct filter and were inverted polarity. The advantage of this arrangement is, with the mains rolled off an additional 6db/oct, the sub can be placed in the same plane as the mains, summing will be in phase through the crossover region, and the total phase shift from adding the sub will be the minimum possible. At John Bau's suggestion, I added a hardware Phase Invert for an absolute 180° flip to use in concert with the Force low pass filters and passive XO1.

Bottom line is that integrating a sub can be an exercise in futility when the expected response is not possible because the controls do not reflect actual performance. It is possible to get excellent performance if one works slowly with small adjustments of a single control and knowledge of what is actually happening. Manufacturers would do well to provide accurately calibrated controls, level and phase information to make integration easier.