In a recent effort to upgrade the AA reference system and switch to a more flexible configuration of separates, I began the search for a suitable power amp. I wanted high current, high power, linear output and good cooling but in a compact form factor. Having chosen an M8s Preamplifier, going with a Musical Fidelity amp seemed a logical choice. I decided to take the M6sPRX for a test drive.
The M6sPRX (hereafter the PRX) is a dual mono power amplifier pumping out 230W per channel with a peak current output of 140 amps. At its heart are dual-mono power supplies with extensive regulation, and a unique dual bi-filar choke system more often seen in valve amplification but which Musical Fidelity has been pioneering for decades. A choke offers high resistance to AC and very low resistance to DC. It hugely reduces power supply noise and its harmonic spectrum. The normal power supply ‘saw tooth’ waveform is reduced by about 17 times, to a harmonically simple sine wave. Power supply noise is one of the big problems of amplifier design. Choke regulation offers a passive solution to this problem, reducing power supply noise and eliminating the effects of mains interference. The dual bi-filar choke system incorporates both the B+ and B– into the same choke, which cancel out each other’s noise and magnetic field.
The PRX is also designed with neutrality in mind. Loudspeakers can present horrendous problems to a power amplifier. Even when a loudspeaker is rated at an impedance of 4 ohms, it may have serious inductance and reactance and much lower real-world impedance than its spec implies. The PRX is a proper ‘voltage source’ and will deliver a constant voltage regardless of loading, for the vast majority of loudspeakers. The amp is fully balanced though also provides two switchable sets of single-ended inputs along with the balanced XLR jacks.
The aesthetic is typical musical fidelity, with a solid casing fronted by a slab of aluminium bevelled top and bottom. Large custom-extruded heatsinks run down either side with additional venting slots running symmetrically along both top and bottom panels. The finish is smart and clean and the amplifier feels like a quality product overall and certainly has a heft to it, made more noticeable given its relatively small size.
Inputs are switched by a button on the front panel, indicated by lights behind one of two display windows. Given that this amplifier will require some kind of preamplifier which will likely have a home theatre passthrough of its own, I see the switchable inputs as being redundant and would rather they were omitted in favour of a pair of XLR loop outputs which would be far more useful. As it is, there is no convenient way to add multiple PRXs into a system without losing the benefits of a fully balanced signal chain, as the only loop outputs are on single-ended RCA jacks.
Around back is a logical layout beginning with the central input section. Input A is on RCA jacks, and input B provides a set of balanced XLR jacks along with accompanying RCA jacks beneath. There’s an RCA Loop output, with an XLR loop out option lacking as above. A switch toggles between balanced and unbalanced. Musical Fidelity state that the balanced inputs can be used with the switch set to single-ended mode, at a loss of 6dB of gain and sacrificing the benefits of balanced operation. It’s also important to note that the XLR and RCA inputs on input B should not be used simultaneously, as they are connected in parallel and thus damage to the connected equipment could result. Using input A (RCA) and input B on either balanced or RCA is perfectly acceptable, however.
Speaker connections are on chunky terminals which can accept bare wire, spades or banana plugs. I used the latter, which necessitates removal of the end caps. This is easier than usual, as the terminal nut can be completely removed from these jacks, making it much easier to grab the cap with a small flat-bladed screwdriver. Power is via a standard C14 IEC jack. The amp has a hard power switch, so there are no trigger inputs or standby facilities.
As with all of Musical Fidelity products, the attention to detail in the packaging leaves a lasting impression. The amp is double-boxed, wrapped in a drawstring bag and sitting amidst thick layers of foam. Included are a spiral-bound instruction manual and a pair of cotton gloves to assist in installation. A standard-issue IEC power cable is also supplied.
Though not a large amp by some standards, the PRX does need room to breathe especially around the side-mounted heatsinks which do become quite warm in operation. Some heat exits from the top grill slots too, so this is an amp best kept on an open rack or table.
The amp powers up with a couple of clicks, after which it is ready for use. The clicks indicate soft-start and protection circuits. Some readers have expressed concerns that the M8 amplifiers lacked any kind of protection, pointing me to a review which suggests (incorrectly) that protection had been omitted from these units. The M8 500s and 700m do in-fact have ‘DC offset’ protection, monitored over time to detect any potential speaker damaging faults. The M6sPRX is a non-bridged amplifier and has more conventional ‘current limiting’ protection.
Sonically the PRX is neutral. What goes in is what comes out; just a heck of a lot louder. The PRX copes with dynamic swings with ease, with heaps of headroom, bags of power and endless drive. Raise the volume to unsociable levels and the tonality and effortlessness remain the same, the PRX maintaining a vicelike grip on the drivers, pumping the cones to and fro with increasing vigour. At no point does the amp appear strained or lacking in either power or control. And its character is consistent regardless of the speaker, from point-source Tannoys and Fynes to our diminutive full-range CHN-50s to larger, more traditional multi-driver boxes.
In an ideal world. An amplifier does nothing other than amplify a small signal without imparting any character of its own. It must do this not only with minimal colouration but with sufficient power to handle peaks in dynamics as brief as they may be without compromising low-level detail. It must also be load-independent; that is, to deliver the same performance under dynamic conditions and the variable conditions presented by any given speaker. Not every amplifier can achieve all of these things, but it has been long recognised that Musical Fidelity’s amplifiers are technically excellent, and thus sonically excellent too if your intent is a true-to-source sound.
And the PRX is no different. It’s a compact yet powerful stereo amplifier, sonically neutral, untroubled by challenging or variable loads and able to push just about any speaker to high levels with ease. It’s ideal as the centrepiece of a high-end system, especially in a medium-sized room with efficient speakers where too much power can be far worse than too little. Perfecting this amp would be a matter of refining the inputs and outputs, ditching the switchable inputs in favour of a single input configuration providing balanced loop outputs to chain multiple M6sPRX amps together. But I can’t fault it sonically. Highly recommended.
