Naming a few of the clients that use products from the Professional Monitor Company will probably spark some interest in the AML1 monitors: Sony Mastering in New York, Digidesign UK, ORF in Vienna, Paramount Mastering in Los Angeles, Abbey Road Studios in London... And those are just some of the names that make up the company's impressive and extensive résumé. Since it was founded in 1990, PMC has offered high-quality products aimed at both audio professionals and audiophiles. In its drive to close the gap between the recording studio and the living room (its motto is "Hear it as it was intended, not as a poor interpretation"), every PMC product uses the same drivers and the same operating principle: acoustic transmission lines.

So, from the impressive active BB5-XD down to the smaller DB1S, including the model at hand, the AML1, every model is based on the Advanced Transmission Line.

Acoustic transmission lines
Over the years, loudspeaker cabinet design has kept evolving.

At first, closed (sealed) cabinets were used, with no opening through which air could escape. Since speakers radiate both forward and backward, all the energy radiated into the cabinet is wasted.

The next step was to open a hole in the cabinet (a bass reflex port), so that all the energy inside would add to the energy radiated outward. Because of the phase difference between the energy coming out of each side, these ports have to be designed precisely to control frequency cancellations.

Bass reflex models improve efficiency and extend the response in the lowest frequencies, but they have the drawback that the resonance of the port tube colors the midrange and the phase response is not entirely accurate.

The evolution of bass reflex is the acoustic transmission line, which consists of creating a path inside the cabinet that makes the speaker behave as if it were in a larger cavity than it actually is. The effect is that the low-frequency response is extended considerably, but in a very controlled way and with good phase response.

The problem, of course, is that building these cabinets is much more complex and expensive. It requires heavy investment in research and development, since many variables affect the behavior of the transmission line: the length of the line, its cross-section, the thickness of the wood it is made of, what material it is filled with, the density and placement of that material, and so on. For all these reasons, it is no surprise that monitors based on acoustic transmission lines are not the most common.

PMC's design is called the Advanced Transmission Line (ATL), and it is the result of three years of work. In this design the line is heavily damped with absorbent material to attenuate the upper bass frequencies, so that only the lowest frequencies emerge from the end opposite the woofer, which acts like a subwoofer.
Controlling the frequencies in the line reduces unwanted cone movement, and therefore distortion. The low-mid and mid frequencies are not masked and more SPL is obtained, allowing a flatter response at different listening levels.

The AML1
A quick look at the monitors is enough to realize they come from good stock. The first thing that catches your eye is the bass driver, which has a completely flat surface. Even though we all know we shouldn't, it's impossible not to reach out and gently feel the matte surface. That light touch is enough to push some air into the monitor and have it come back with a deep sigh. I'm really starting to look forward to seeing them in action.

The 32 mm tweeter (somewhat larger than usual) is protected by a metal grille, perforated to improve dispersion. Below the two drivers, the transmission line's output port balances out the front of the monitors.

The back is almost entirely covered by a heat sink, which has an internal chimney to improve ventilation. Below it we find the power socket and an XLR connector next to a small diagram showing the connector's wiring.
The level and EQ controls are located on top of the monitors, hidden behind a cover with magnetic catches to prevent vibrations. The placement of these controls is welcome, since it makes them much more accessible than if they were on the rear panel, as is usually the case.

Digging deeper
The PMC 6.5, as the 165 mm bass unit is called, is made from a blend of carbon fiber and Nomex® that makes it extremely light and rigid. These qualities are essential for use in transmission lines: because of the large mass of air the bass driver has to move for the ATL to be effective, the pressure behind the woofer is very high. Cone-shaped diaphragms don't handle that pressure well and they flex, causing distortion. A flat disc ¼" thick provides the solidity needed to withstand the pressures generated in the line.

However, the bass piston has one drawback: its response in the upper midrange is not entirely good. That is why a somewhat larger tweeter was chosen as the PMC 6.5's partner: the response of the high-frequency unit allows the crossover to be set at an unusually low frequency (1.4 kHz), so that the upper mids are handled by the tweeter. This way, the woofer isn't forced to reproduce frequencies it finds uncomfortable, improving its performance and reducing distortion.

The crossover is an active filter with a slope of 24 dB per octave that feeds two amplifiers (100 W RMS for the lows and 80 W RMS for the highs), capable of delivering a sound pressure level of 106 dB at one meter, with distortion below 0.009%. These amplifiers were developed by the prestigious company Bryston and use high-end OS-CON® capacitors in the key areas of their electronics.

The effective length of the transmission line (1.7 m) extends the bass response down to a surprising 33 Hz, while the upper limit is 25 kHz.

To adapt this response to our listening environment, we have three EQ controls. The first is a high-pass filter (LF Roll) with cutoff frequencies of 0, 50, 80 and 160 Hz. Frequencies up to 500 Hz can be adjusted by -9, -6, -3, 0, +1.5 and +3 decibels with the LF Tilt. Finally, the HF Tilt lets you apply gains of -5, -2.5, 0 and +2.5 dB to frequencies above 1 kHz.

One detail worth highlighting is the inclusion of a switch to turn the EQ on or off. This control lets us bypass this stage if we don't need it (as was my case), keeping the filter electronics out of our signal path. The switch is also very handy for quickly comparing the flat signal with the equalized one.

Continuing with the details, the drivers are matched by hand, and the manufacturer keeps calibration charts for each unit so that, should one ever need replacing, the substitute will fit perfectly into the working environment.

The front panel is carefully secured to prevent resonances and incorporates waveguides to improve dispersion. The rounded corners and large-radius curves of the surface also help with this.

Listening
As you'd expect, the first thing that stands out is the bass. Deep and very well defined. Powerful and extremely clear. If you get the chance to try these monitors, you'll probably catch yourself smiling at those frequencies that you feel more than hear.

Bass lines come across exactly as they are, and kick drums sound clean and transparent. Any small flaw jumps out at you right away, making the differences in quality between one record and another obvious.

The rest of the spectrum is also reproduced faithfully: there is no trace of masking in the midrange, and the highs are pleasant and precise, making for a wide listening window.

And in this case, the word window is more than apt, because you really can lean in and look through it. The stereo image is wide and deep, and absolutely precise. You can step into it and explore the different sonic layers in search of the tiniest details.

Even though the records I tested on the monitors were old acquaintances of mine, I was surprised to discover nuances I had never heard before: subtle reverb tails, small breaths in the vocals, slight shifts in the stereo image... Details that had always been there but that I hadn't heard until now. A real pleasure.

Conclusions
Good studio monitors should present audio as it is, without makeup. The AML1s are extremely accurate and present things as they are. Good mixes sound good. Bad ones don't.

Exceptional richness of detail. Solid, deep imaging. Powerful, well-defined bass. Balanced sound at different listening levels. Reliable and accurate...

Unfortunately, this quality comes at a price, and the AML1's will probably put them out of reach for many. Good tools are expensive, and these are no exception. In any case, even if your budget won't stretch that far, drop by the store and ask for a demo. It's worth it.

Specifications

Frequency response: 33 Hz - 25 kHz

High-pass: 0, 50, 80 and 160 Hz

Low-frequency EQ: -9, -6, -3, 0, +1.5 and +3 dB up to 500 Hz

High-frequency EQ: -5, -2.5, 0 and +2.5 dB above 1 kHz

Bass driver: 165 mm flat piston in carbon fiber and Nomex®

Tweeter: 34 mm silk dome

Sensitivity: 0.775 V for 106 dB SPL at one meter

Bass amp power: 100 W RMS

Treble amp power: 80 W RMS

Distortion: <0.009%

Signal-to-noise ratio: 90 dB (ref. 0 dBu)

Effective ATL length: 1.7 m

Input connectors: balanced XLR

Crossover: 1.4 kHz 24 dB/octave

Cabinet material: MDF (Medium Density Fiberboard)

Dimensions: 400 x 200 x 363 mm

Weight: 16 kg per cabinet