Page 2: PSI Audio A23-M

Let It Flow

The A23‑M's low–frequency system is port–loaded, with the port itself located alongside (or, in portrait mode, beneath) the bass driver. The port is created by a shelf and slot incorporated within the enclosure in the same manner as the port in the A25‑M (and, it has to be said, quite a few other monitors), but the A23‑M incorporates an innovation that PSI call 'Flow Guide'. The concept of Flow Guide is to add a profiled wedge within the port that, effectively, results in the port's cross-sectional area varying with length. The aims are to maximise laminar airflow while at the same time suppressing organ pipe resonances. Diagram 1 shows the output captured by a measuring microphone inserted a couple of centimetres into the port, and it's clear that the latter aim has been achieved. The port output is very clean, with no evidence of organ-pipe effects. The port tuning frequency just above 40Hz is clearly apparent too.

Diagram 1: The A23–M's output, measured a few centimetres inside the port.On the more fundamental question of the pros and cons of ported monitors, PSI's stance is that if ports are designed appropriately and thoughtfully, both in terms of their dynamic behaviour (the low–frequency delay and overhang that they add) and of their potential distortion and compression effects, there's no reason not use them. I guess the proof is in the listening, but one thing that is apparent from the published specification, and my analysis with FuzzMeasure, is that, for a ported monitor, the A23‑M has relatively low levels of LF group delay (around 12ms at 50Hz). This, combined with the obvious care taken in the Flow Guide technique, augurs well for dynamic, accurate and tuneful bass.

Living In A Box

So I'm halfway through and, having been distracted by the A23‑M's drivers and its low–frequency characteristics, I've not really finished describing the monitor. While the A23‑M standard finish is PSI's corporate maroon (as pictured), my review pair were in a silver–flecked black (which I actually much prefer). The enclosure is of conventionally manufactured rectilinear MDF construction with slightly rounded front edges. The enclosure panels feel rigid and well damped.

The A23‑M follows the usual format in having an amplifier heatsink and connection panel around the back. Along with the connection sockets found there, input sensitivity and low–frequency level adjustment knobs are also present. The A23‑M amplification is rated at 140 Watts, 70 Watts and 50 Watts for the bass driver, mid–range driver and tweeter respectively. Interestingly, while the bass and mid–range amplifiers employ Class–G and Class–H variable–supply–rail technology, the tweeter amplifier is a traditional Class–A/B design. I came across a similar mix of amplifier technologies on the KEF LS50 Wireless, where the bass/mid driver used a Class–D amp and the tweeter a Class–A/B. A further interesting amplifier feature, one that's widely employed by PSI, is their Adaptive Output Impedance (AOI) error–correction feedback system. AOI is actually an enhanced version of the 'Current Drive' idea that I believe was first floated by Malcolm Hawksford and Paul Mills in an Audio Engineering Society paper of 1989. It's also employed on the Kii Three monitor. The idea is to sense the current flowing in the driver voice coils by measuring the voltage across low–value series resistors. Now, as driver output is actually proportional to the current flowing in the voice coil (rather than the voltage across its terminals), the sensed current signal can be fed back to the amplifier and employed for error correction. It's a neat idea and its use by PSI speaks of an appreciation that active speakers should be considered as integrated systems rather than, as all too often seems to be the case, drivers and amplifiers that just happen to share a common enclosure.

Finally, there's one more subtle feature that I really like: the electronics module is housed in its own enclosure, insulated from the acoustic battering it would otherwise receive from the rear of the drivers, and it's compliantly mounted too and so is also isolated from mechanical vibrations.

Electric Avenue

The electronics within the A23‑M are analogue all the way through and incorporate only a balanced input on an XLR socket. Monitors that remain resolutely in the analogue domain, in defiance of a trend towards internal D–A, DSP and A–D stages, are becoming less common. On this subject, Roger Roschnik, PSI's CEO, explained to me that the PSI philosophy is very much based on optimising the analogue electronics and electro-acoustics before considering any opportunities that DSP might offer. It's a philosophy I have some sympathy with: a better mid–range driver, for example, more fundamentally advances the art than DSP–based correction of the existing one. That's not to say that all applications of DSP within monitors are misguided, the Kii Three (again) being a prime example of where DSP brings significant benefits.

Diagram 2: The on– and off–axis (20 degrees vertically) response of the A23–M.Despite being entirely analogue, however, the A23‑M does incorporate some signal processing beyond the expected crossover filtering: it's phase compensated from 200Hz upwards so that the various time–domain errors introduced by the drivers and filters are equalised. Although, as with any multi-driver speaker, such time–domain compensation begins to become less effective off axis as relative driver path lengths to ears change, it is still, I feel, a very worthwhile element of signal processing.

Along with measuring the A23‑M's port characteristic (Diagram 1), I used FuzzMeasure to analyse a couple of further characteristics. Firstly, Diagram 2 shows the A23‑M's frequency response from 200Hz upwards on axis and, simply for interest's sake, 20 degrees vertically below axis (in landscape mode). There are no surprises: the axial response is impressively flat to within just over ±1dB, and the off-axis curves shows the expected dip around the mid–range/tweeter crossover region followed by reduced tweeter level further up the band. The off-axis curve is very tidy in the grand scheme of things and suggests good driver integration.

Diagram 3: The A23–M's frequency response, in its default setting (purple) and with the LF response set to -10dB (blue).Diagram 3 shows the A23‑M's low–frequency character, measured using a microphone very close to the monitor and equidistant from the bass driver and the port. The A23‑M is unusual for port–loaded speakers in that it is possible, thanks to the port and driver being adjacent, to use a close-mic technique to get a reasonably accurate picture of low–frequency performance. I measured the nominal low–frequency cutoff (-6dB) at 36Hz, which isn't too far from the published 34Hz. The low–frequency roll-off slope appears to be around 36dB/octave, which suggests that the A23‑M includes a 12dB/octave high-pass filter (the natural roll-off of a ported monitor is around 24dB/octave). The blue curve in Diagram 3 shows the effect of the A23‑M's rear–panel low–frequency level control set at minimum (-10dB). For what it's worth, because all rooms and installations are different, I preferred the LF level set to -4dB.

Listening In

Which brings me finally to what I found on listening to some old favourite CDs and Pro Tools sessions. Some monitors have quirks that require a little learning, but not the A23‑M. I felt completely comfortable with it almost immediately, but more than that, the A23‑M displays an air of quiet competence. It's not at all showy in its tonal balance; there's no emphasised mid–range or overcooked tweeter, it seems completely trustworthy. And behind the reliability, there's a serious ability to resolve detail and offer clarity over a very wide bandwidth. Elements of a mix, from low frequencies right through to the limits of audibility, are clearly revealed, with explicit clarity, at any volume level from sensible to really very loud. But it's in the mid–range that the A23‑M really excels. The natural details of female voices and higher–pitched acoustic instruments, if they have survived the tracking and mixing processes, are reproduced spectacularly. And if they haven't survived the tracking and mix process, the problems are obviously revealed. The new dome mid–range driver is clearly a success.

And the A-23M's subjective bass performance? Well, as a dyed-in-the-wool fan of closed–box monitors, I'd love to hear a non-ported version of the A23‑M, but with my hopes for its performance raised by the low group‑delay specification and careful port design, I was not disappointed. The A23‑M's bass seemed subjectively beyond reproach — tuneful, dynamic and very extended.

It's not just in terms of tonal characteristics that the A23‑M excels, however: stereo images are tremendously well focused with great portrayal of depth and space, and not just centrally but right across the sound-stage. To my mind, strong stereo image performance such as this requires a monitor to display both a tight time–domain performance and low 'noise' — so no spurious resonant output from cabinet panels, for example. The A23‑M is extremely good in both those respects and I think it shows. It's actually extremely good in pretty much all respects.