Product Review -
AudioControl Phase Coupled Activator - February, 1996
By John E. Johnson, Jr.
AudioControl Phase Coupled Activator-Series Three; Digital subharmonic restoration system and electronic crossover; frequency response 20 Hz - 20 kHz plus or minus 1 dB; harmonic distortion 0.007%; S/N 110 dB; Crossover factory setting at 90 Hz, 24 dB/octave; size 2.5"Hx 17"W x 11"D; weight 10 pounds; $299; AudioControl, 22410 70th Avenue West, Mountlake Terrace, Washington 98043; Phone (206)-775-8461, Fax (206)-778-3166.
The Phase Coupled Activator technology is not new, but the Series Three is their latest version (call it the PCA3). The difference between this model and previous ones, is the ease with which signals can be passed first through a crossover network (two channel electronic crossover built into the PCA3, which is separate from the subharmonic generation circuitry) then into the subharmonic circuit. Secondly, the PCA3 has speaker level inputs. Third, the current unit has reduced the 1/8 phase shift that was a problem in older versions. The principle of operation is an assumption that very low frequencies in the original music have been lost or removed during recording and CD production. The PCA3 generates subharmonics at half the input frequency, with a sampling range from 50 - 100 Hz in the "Digital Restoration" mode. So, if the input signal were 80 Hz, you would have an output of 80 Hz and 40 Hz. By limiting the sampling range to 50 - 100 Hz, and only with stereo signals, this eliminates boominess that would occur if subharmonics were added from original frequencies at 100 - 500 Hz, and eliminates adding subharmonics to voices that are sent monophonically (the same signal in both channels, in phase) to the center channel speaker. In other words, any signal that is monophonic will not have subharmonics added to it. The separate crossover can be used to send <90 Hz frequencies (line level inputs and outputs) to a pair of powered subwoofers (one each for left and right channels), and >90 Hz (line level) to the main front left/right amplifiers (pre-outs and main-ins on a receiver can be used).
The front panel has, from left to right, the on/off power button, the subwoofer output level trim (for use with the separate crossover), a crossover/bypass button (disables the subwoofer outputs from the crossover and sends all frequencies to the main left/right), an external processor button (a tape loop), an enhancement mode/soundtrack button (filters out frequencies below 25 Hz at 18 dB/octave, i.e., "subsonic filter"), a range button (alters the sampling range to 30 -70 Hz in producing subharmonics), a digital restoration button (activates the normal range of subharmonic restoration to 50 - 100 Hz sampling), and a restoration level volume control (controls the amount of subharmonics added to the original signal, but does not affect the overall volume of the original signal passing through). Red LEDs indicate the various functions are activated.
The rear panel has, from left to right, two pairs of RCA jacks for main left/right - in/out (the subharmonics are generated in this circuit), two pairs of RCA jacks for the left/right - in/out tape loop, three pairs of RCA jacks for left/right in, left/right low pass out, and left/right high pass/full range out (these jacks are for the separate crossover network), and two sets of speaker level (maximum power 150 watts) input/ output connections (this signal is passed from the inputs to the outputs unchanged, but it is crossfed to the subharmonic generation circuits so that a pair of subwoofers can be driven from the line level outputs of the PCA3 as well).
On to the performance. We tested the PCA3 with four different powered subwoofers, two of which are servo-feedback (Velodyne F1500R and Mirage BPSS-210), the third is a passive radiator design (Velodyne VA-1012X), and the fourth is active push-pull (M&K MX-5000THX). The PCA3 worked with all four. Regardless of the sub, the PCA3 produced a noticeable addition to the VERY low end. For CDs, sometimes it was a nice effect, and at other times, irritating. It really depended on the music. I have a CD version of a Boston Symphony recording that was originally released on LP decades ago. The composition is Tchaikovsky's Romeo and Juliet Overture. The CD version (bought in 1988) definitely had some of the low frequencies removed, compared to the LP version, and I had been very disappointed with the CD. Using the PCA3 with the CD, the resulting low end was closer to what was on the LP. Keep in mind that the low frequencies from the PCA3 are generated with certain assumptions; there is no actual restoration, because the PCA3 does not really know what was in the original signal. So, the Romeo and Juliet CD with the PCA3 sounded better, but certainly distinguishable from the real thing (the actual low end on the LP). But, it was an improvement. On other CDs, such as Telarc discs, which are famous for their low frequencies, the PCA3 was too much. On one CD, we found that by adding subharmonics, the acoustic string bass began to sound like an electric bass guitar. Obviously, then, the PCA3 is not for all music, but that is why the volume control is on the unit; with it turned all the way down, there is no subharmonic addition. In any case, it is important to use good bass drivers with the PCA3. If used with less than adequate speakers, it can sound awful, and the automobile version has received some criticism because of this. Much better to use no restoration at all than to try and shove 20 Hz signals through a small inexpensive speaker. On movie sound tracks, the PCA3 fared much better. Just about every movie benefited from subharmonic addition. At 50 Hz, using the Mirage BPSS-210 sub and a 98 dB SPL, this setting on the PCA3 produced a 94 dB subharmonic at 25 Hz as measured on a real time spectrum analyzer. With the "range" setting activated, subharmonics are generated from frequencies in the 30 - 70 Hz range, which means that the factory 25 Hz subsonic filter will reduce the subharmonics of the 30 - 49 Hz originals going through (very little 15 - 24 Hz subharmonics will pass). If you have a sub which will reproduce signals in the 15 - 24 Hz range, then get a 15 Hz subsonic filter module with the unit when it is purchased. Otherwise, the stock filter module is fine.
The separate crossover network is very good. The factory setting of 90 Hz is OK for many situations, and it appears this frequency is a standard for some consumer electronic equipment that contains crossovers. We connected the crossover to subwoofers and a pair of Krix Esoterix I Mark II full range speakers, driven by a Rotel RB985 power amplifier. When the bypass switch was used, all frequencies were sent to the Krix, or alternately, the Krix received only >90 Hz signals, and <90 Hz were sent to the subs. The crossover frequency is programmable with modules obtainable from AudioControl, or you can make them yourself (instructions in the well written manual). However, if you want a different crossover frequency, we would suggest that you simply order the module with the unit when you purchase it. With the Krix full range speakers and subwoofers we used it with, a crossover frequency of about 60 Hz would have been more suitable. Since the crossover network is separate from the subharmonic generation circuitry, to use both, one would need to run jumper cables from the low pass out jacks of the PCA3 to the main-ins of the PCA3, and the main-outs of the PCA3 to the powered subwoofers.
In general, we feel that the subharmonic generation circuit is a nice "gadget", but the electronic programmable crossover network is much more useful and worth the three bills price just by itself. By connecting the outputs from the front left/right channel preamp to the PCA3 and then to the main amplifiers and speakers, using the crossover network, your left/right power amps will have to do less work (no power used at <90 Hz), and stereo subs (will become important with AC-3) can be utilized to drive the <90 Hz info. If you have good subs (capable of handling an intense 20 Hz), the subharmonic circuit can be revved up just for fun once in a while.
John E. Johnson, Jr.
© Copyright 1995, 1996, 1997 Secrets of Home Theater & High Fidelity
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