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  PMC AML1 Powered Monitor Loudspeaker

      Date posted: October 24, 2001

PMC AML-1

Sugg. Retail: $5200 US pr
Size: 15 3/4″H x 7 7/8″W x 12 1/2″D
Distributor: Bryston Limited,
677 Neal Drive, PO Box 633, Peterborough, ON K9J 7Y4
www.bryston.ca

(Reprinted from the Fall 2001 Audio Ideas Guide)

      There’s a lot more to this compact speaker than meets the eye, especially if you look at it only from the front. The AML1 is not only a powered loudspeaker, it is bi-amped, with a sophisticated electronic crossover built in. In fact, it’s a truly hybrid Bryston/PMC product, with the all the circuitry of a 3B ST driving the woofer/midrange (140 watts rms), a 2B ST driving the tweeter (70 watts rms), and a modified 10B providing the driver integration after the balanced XLR input (pin 2 hot). The amp/crossover module takes up most of the rear panel, and is custom made in England to Bryston specifications.

     And that’s about $5000CA worth of electronics before you get to the drivers, a flat 6 1/2″ carbon fibre/Nomex sandwich woofer/midrange and 1 1/4″ Audax soft dome tweeter. At top rear of the enclosure is a flip-up panel that hides a level control at left, with a button to its right that engages controls for low-frequency rolloff, low-frequency tilt, and high-frequency tilt, these to the right of the button. Thus, you can engage EQ to fine tune the response, or run the speaker at a predetermined flat EQ bypass setting.

      Given all that’s in these compact boxes (available in slate grey, as thge review pair were, or iridescent blue, as shown), it could be argued that the AML1 is something of a bargain at close to $8000 Canadian. All you need is a preamplifier, preferably one with balanced output. The speaker is more amenable to room tuning than most, and provides freedom from the possible phase and amplitude distortions of passive crossovers. It is also designed to play at very high levels with very low distortion, essentially a studio monitor built to audiophile specifications.

     Acoustically, a couple of interesting things are done in the AML1. The flat woofer/midrange, crossed over at 1.4 kHz minimizes lobing with the tweeter by virtue of its pistonic motion (not to forget the phase-accurate electronic crossover), while the tweeter is lensed to virtually eliminate diffraction; with a curved lens around it, the soft dome also has a perforated phase plug cover that smoothes and controls dispersion.

     Bass loading is a transmission line, which vents at the bottom of the enclosure’s front. Unlike reflex loading, transmission lines do not have resonances, but push a column of air out the port, which extends down to a frequency dependent on the length of the transmission line folded inside the cabinet. They are notable for faster and smoother bass than can be achieved by reflex porting, and do not roll off quite as steeply in the deepest bass. Perhaps this explains why the AML1’s perceived deep bass seems more powerful than the measured curves would suggest.

PMC AML-1 Frequency Response

     And comments like this pretty much lead us into the measurements. At top are the closely interlocked Pink Noise Sweep (PNS) and Summed Axial Response (SAR) curves. Their close correlation is already an indication of excellent engineering, in particular good driver design and integration. The AML1 does have a slight peak at 3 kHz, and another one, largely on axis, at 6 kHz, but these are the only departures from almost perfect neutrality. Rolloffs are smooth at both frequency extremes, the tweeter within 3 dB at 20 kHz, and the bass rolling off quite gradually below 60 Hz. But, as suggested, there’s more to the bass performance that I’ll explore in the listening comments.

     At bottom, PNS measurements show the range of the tonal adjustments, the top one the highest settings of all of them, the middle, the lowest settings of the low- and high-frequency tilt, and the bottom curve in the bass showing the lowest setting of the LF rolloff. Looking at this latter curve, it occurred to me that such an adjustment might appeal to studio engineers who were once partial to the dreaded Auratone small monitor, once used to mix 45 singles to sound good on AM radio. God forbid! But the good thing is that bass can be extended a little, as can upper treble to deal with a large room, though at some slight loss of transparency relative to the bypass mode.

     By the way, the other measurements were made with the tone controls bypassed, as were our listening tests. During this enjoyable process I was drawn in by the transparency and dynamic excitement of the AML1. Most active speakers I’ve heard before have been underpowered, and when their low distortion encouraged one to crank the volume, they started to clip. Here that was not the case: over 200 watts is just enough, the AML1 capable of 106 dB/metre with .775 volt input.

I just kept cranking these speakers, and they just got louder. There was no compression or distress heard from either driver, and the capabilities of the woofer in particular were prodigious. I couldn’t make it bottom or double, its harmonic and Doppler distortion totally inaudible.

     This meant that at higher levels bass was very clean and seemed to extend deeper than the measurements suggest. Perhaps it’s a Fletcher/Munson perceived loudness thing, but these babies really boogied at higher levels, but that is not to suggest that they didn’t have delicacy and ability to reveal nuances.

PMC AML-1 Rear

     The tweeter also held up well at high levels, and was outstanding in its microdynamics, the overall effect a very realistic and punchy sound that was very much like live music, rather like that heard from high sensitivity speakers like some recent Kliipsch models. Tonally, I heard a little extra midrange that gave female voice a slightly papery quality, but sibilance was well controlled, and treble in general very sweet and accurate.

     This contributed to the outstanding imaging, both behind and outside the speakers in the soundstage. The AML1s really do have that “listen through” quality that distinguishes true high end loudspeakers…and, speaking from my pro experience, identifies great studio and location monitors.

I was so drawn into the sound of these miniature PMCs (and the company also makes some very large monitors) that I listened to the whole movement from the Mahler 4th from our speaker evaluation compilation CD. The sound of orchestral music was both relaxed and powerful, and, as they used to say at Linn, you could “follow the tune”.

     As well, our Debussy piano and Bach cello recordings sounded utterly natural and powerfully present, as did other recordings. This is a speaker that passes Robert Deutsch’s LIAR test…LIAR meaning even when Listening In Another Room they sound like real music.

     I’m not sure I’d like the AML1 in electric blue, and hopefully a wood finish might become an option, but I did find the grey blended in well with my system, and, of course, the speakers literally disappeared when they were playing. The PMC AML1 may not be for everyone, powered speakers a special niche in North America, but for those who comprehend the virtues of active loudspeakers, professional or audiophile, this system is a must listen.

Andrew Marshall

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