MM Pro Moving Magnet Phono Stage by Michael Fidler

Features

Available now for £600/€700/$800 direct from Classic Audio Ltd., the MM PRO offers arguably the very best all-round performance for moving magnet and high-output moving coil cartridges on the market. Active loading from an original dedicated head amplifier reduces input noise below what's possible with conventional passive cartridge loads. This, combined with a triple-stage architecture, precision RIAA equalisation, variable low frequency crossfeed, and a robust balanced output, make for a highly versatile phonostage with stratospheric performance.

Front view, black finish style

The MM PRO sweeps away the critically acclaimed Spartan 10's previous innovation as a high-performance moving magnet phonostage. As a result of some very careful design choices, the MM PRO offers the following highly practical features, many not seen before on the former product...

• Active loading, for lower noise with an MM cartridge connected than any other phonostage using a passive 47kΩ load
• Variable and defeatable low frequency crossfeed, cancelling low frequency noise to taste, revealing bass detail without loss of low-end
• 42/52dB gain switch to match the levels from high-output moving coil cartridges to other line sources in the system
• Mono switching, revealing detail by reducing noise and distortion on mono discs, making worn discs more listenable
• Class-leading RIAA accuracy to ±0.1dB, with no colouration exhibited by most other phonostages that struggle to meet ±0.5dB
• Exceptionally low distortion, less than 5ppm from the use of careful gain distribution in an original triple-stage topology
• A dead silent background from low noise input amplifiers, matched to moving magnet cartridge impedance characteristics
• Crystal clear high frequency reproduction through the use of limited high frequency feedback in an uncompensated input amplifier
• 23Hz 3rd order subsonic filter, using hand-matched capacitors to eliminate low-end fraying in contemporary filter designs
• Fully balanced XLR outputs, allowing for a high quality line connection over a long length of cable for extra flexibility
• High quality ±17V split linear power supply with start-up muting for switching hash-free listening with over 10V RMS output

Rear panel, showing connections

Despite the relatively low cost of the MM PRO compared to other 'high-end' offerings offering only a fraction of these features, the rich feature list and future-proofing puts it firmly into the domain of 'cost-no-object' offerings. Objectively, it may well be the finest practical moving magnet phonostage currently on the market for any price. If you are looking for the very best for your moving magnet setup, then the MM PRO is well worth comparing against other offerings on your shortlist.

HOMC transfer from the AudioPhil YouTube channel

Thanks to its dedicated low-noise head amplifier and gain switch, the MM PRO works brilliantly with both moving magnet and high-output moving coil cartridges. Hear the some of the results with HOMC for yourself, featured in the video above from an independent YouTube channel, now exclusively using PRO series phonostages for vinyl rips.

Side view, silver finish style

Black and silver finish MM PROs are currently in stock to ship worldwide. Along with GB 240V PSUs, EU 220V and US/NEMA 120V types are also available - see order page for more details.

Build quality

The MM PRO sits inside a compact sandblasted aluminium enclosure that fits neatly into most setups, allowing for close placement to the turntable. Thick aluminium panels add a pleasing weight to the unit, affording ample shielding to the electronics contained within whilst downplaying their exceptional performance, derived firstly from the innovative circuit architecture and in part from high quality components...

• All hand-soldered through hole technology, no cheap surface mount components
• Double sided, fibreglass, plated through hole PCB, with ground planes on both sides for very low cross-talk
• Film capacitors in the equaliser network and subsonic filter, distortion-free NP0/C0G ceramic parts filter RFI on the input
• All electrolytic capacitors are long life Panasonic FR and FM series, lasting for decades to come
• 1% tolerance metal thin film resistors in the audio path for high gain accuracy and excellent linearity
• Professional quality amplifier ICs in turned IC sockets, easy to change if destroyed by faulty electronics downstream
• Gold plated toggle switches rated for 50,000 cycles instead of the usual 10,000 cycles of cheaper push switches
• Precise conductive plastic track potentiometer for variable LF XFEED, rated for endurance at 100,000 cycles
• Panasonic telecoms grade output muting relays for many decades of high-reliability operation
• Gold over nickel-plated RCA and XLR connectors on the rear panel
• Large nickel-plated ground binding post that can accept banana plugs for easier connection
• 6mm CNC milled aluminium front panel, anodised for scratch resistance
• Stainless steel panel screws, all hex heads

Inside the MM PRO phonostage

The MM PRO is hand assembled in Kent, England, using carefully sourced parts exhaustively selected through rigorous testing as the best available for the application. Like the now-discontinued Spartan 10 phonostage, the MM PRO utilises the legendary 5534 opamp as a front-end, followed by 5532s for the equaliser, subsonic filter, and balanced outputs. All the components contained within were selected with the end result in mind, as opposed to their individual marketability, leading to an extremely reliable product that offers great value for money.

Through hole PCB

Great care has been taken to lay out the PCB as efficiently and intuitively as possible, while at the same time minimising parasitics and crosstalk, so the MM PRO manages a triple stage design in a 172mm width enclosure. Unlike many other commercial products, the enclosure design was done after the electronics were perfected, leading to an understated design that eliminates as much dead space as possible, potentially reducing domestic friction!

Specification

In contrast to the norm, the specification for the MM PRO is put forward before going further into its new features and their benefits. Rather than launching into a tirade of nebulous, and conveniently unprovable, subjectivist statements that might apply better to wine-tasting than to audio, cold hard numbers that will afford the reader easy comparison to other products are supplied below, with the reasoning behind them. This specification has been very carefully wrung out of the readily-available parts used in the design, and it would be a pity to obscure it behind a wall of waffle or vague claims about magic components; architecture is the name of the game here!

Parameter	Measurement
RIAA Accuracy	±0.1dB, 40Hz to 22kHz
Channel balance	±0.1dB, at 1kHz
Signal-to-noise ratio, ref 5mV MM	79dB, MM cartridge load, flat 220Hz-22kHz
Signal-to-noise ratio, ref 2mV HOMC	77.5dB, 200Ω input load, flat 220Hz-22kHz
Total harmonic distortion	<0.0005%, 20Hz to 22kHz, at 21V RMS
Maximum output (20Hz to 28kHz)	21V RMS (XLR), or 10.5V RMS (RCA)
Maximum input at 1kHz	82mV RMS at 42.4dB, 26mV RMS at 52.3dB
Maximum input at 10kHz	400mV RMS at 42.4dB, 125mV RMS at 52.3dB
Overload margin ref 5mV, at 1kHz	24.3dB at 42.4dB gain
Overload margin ref 2mV, at 1kHz	22.3dB at 52.3dB gain
Gain at 1kHz (RCA)	42.39dB, 132x, or 52.30dB, 412x
Gain at 1kHz (XLR)	48.41dB, 264x, or 58.32dB, 824x
Minimum load impedance	1kΩ (RCA), 2kΩ (XLR)
Output impedance	75Ω (RCA), 150Ω (XLR)
Input impedance	50kΩ//120pF
Subsonic filter	23Hz, 3rd order, for -23dB at 10Hz
Low frequency crossfeed	65Hz to 600Hz, -4.3dB to -21.6dB at 50Hz
Dimensions (W*H*D)	172*60*155mm
Power consumption	6W max powered on, 0.2W standby

The MM PRO offers an excellent set of numbers that will translate well into an uncoloured, undistorted, and unmitigated listening experience. A product of a 'performance from architecture' approach, the MM PRO manages this feat without resorting to any exotic or obscure components that might become unobtainable in the future. The all-round specification, measured with a physical cartridge loads on the inputs, soundly beats all of the big name brands by a substantial margin, especially in terms of RIAA accuracy and distortion. All of these targets have been assiduously balanced against each other so as to combine for the best final result in the listening room.

Carrying forward the excellent channel balance and RIAA accuracy of the Spartan 10, the MM PRO uses active loading to push the signal-to-noise ratio derived from a standard moving cartridge further to 79dB. Vinyl-to-digital transfers supplied by customers with pristine record collections over the past year have revealed that many vinyl pressings are quite capable of delivering an effective SNR against 5mV of over 70dB (3160 times) from 220Hz upwards. It's therefore highly important that the phonostage exceeds this margin by a considerable factor of at least 6dB to ensure that its noise contribution doesn't audibly increase the noise floor of excellent discs.

While many phonostages are marketed with 80dB or more SNR (usually A-weighted for an easy extra 2dB of apparent performance), they crumble when a real moving magnet cartridge is connected, usually dropping to below 76dB or much worse. This happens because amplifier current noise effects are shunted away from the input when connected to a low impedance distortion analyser, along with noise from the loading resistor. The MM PRO, however, is designed specifically for moving magnet cartridge loads and doesn't use a conventional loading resistor. It therefore happily exceeds the 72dB benchmark by a comfortable 7dB, ensuring that it never masks low-level detail that hovers just above the groove noise.

High-output moving coil cartridges, intended for moving magnet inputs in the region of 1-2.5mV, might also pose a challenge to moving magnet phonostages when the gain is increased. Voltage noise on the input can start to swamp the lower signal level. These devices are still capable of great noise performance when married to an input that considers their lower output impedance than moving magnet cartridges. By keeping the series impedances in the head amplifier as low as possible, to take advantage of the low voltage noise of the 5534 input amplifiers, the MM PRO retains an excellent SNR of 77.5dB against a 2mV HOMC cartridge, despite the extra 8dB of gain required to make up the difference in level. Even with the lower level, no detail is lost!

Low noise input amplifiers

Very low noise is great for low-level detail in quiet passages, but headroom and linearity to cope with louder passages and transient peaks is equally important at the other extreme. At least 20dB of overload margin will be required to stave off peak clipping and overload during loud passages. Some MM cartridges are quite capable of up to 50mV of output when subjected to heavy modulation levels, such as those found on 12 inch singles. Fortunately, the MM PRO realises a comfortable overload margin of 24.3dB against a 5mV cartridge input, and 22.3db against a 2mV input with the gain set to 52.3dB, cleanly handling any transient thrown at it by MM and HOMC cartridges.

Specifying the overload margin at 1kHz is all very well, but it's well-known that magnetic cartridges express a rising output as frequency increases, so any decent phonostage must be able to handle a hotter level at high frequency to avert transient distortion and accompanying intermodulation products that cloud the mid-range when equalised by the RIAA curve. Surface clicks in the region of 10-20kHz that would otherwise be benign after being attenuated by the RIAA curve will generate obtrusive distortion products in the mid-band if allowed to clip.

Passive RIAA network phonostages quickly run out of headroom at high frequency, as they throw it away into an attenuating network, greatly escalating the risk of overload when subjected to high frequency transients and surface clicks, making the latter much more noticeable. The MM PRO, on the other hand, uses a fully active shunt-feedback equaliser that allows the maximum input voltage to increase from 82mV RMS to 400mV RMS at 10kHz and an eye-watering 800mV RMS at 20kHz, faithfully reproducing the most challenging discs.

All of this input capacity still wouldn't be very useful if it isn't accompanied by excellent linearity. The uncompensated input amplifier manifests better linearity at high frequency, and thoughtful gain distribution spreads the burden of amplification through multiple stages so that no single stage has to work too hard with reduced linearity, so the MM PRO yields less than 0.0005% THD (5ppm) across the audio band. This conservative estimate sits on the current test setup's threshold of measurement, so it may indeed be lower than this! Accompanying intermodulation distortion that introduces 'harshness', 'grain', and 'veiling' to the sound and causes listener fatigue, especially during complex passages, is equally low.

With gains of 42 and 52dB, the output level is strong enough to easily contest the average level of digital sources, ramping 5mV and 2mV cartridge sources up to 660mV and 824mV output voltages on the RCA outputs respectively. This doubles on the balanced outputs to overcome the higher electronic noise of these more complex differential inputs, while matching conventional DACs that also double the balanced line output level. With the power supply rails increased from ±15V in the Spartan 10, to ±17V in the MC PRO, over 10V RMS is available from the RCA outputs, doubling to 21V RMS of differential output on the XLR outputs, making sure the stronger line output levels have space to breathe.

Balanced and unbalanced outputs

Typical unbalanced inputs downstream of the phonostage won't present a load of less than 10kΩ for the line output to drive, and the MM PROs RCA output easily exceeds this by a factor of 10, allowing multiple parallel RCA connections (to a digital recorder for example). A low output impedance of 75Ω keeps bridging loss below 0.1dB when driving a standard 10kΩ line load.

Balanced inputs can be far more demanding to drive, as many manufacturers have been recently reducing the input impedance to as low as 2kΩ so as to diminish resistor noise without buffering for more impressive measurements. In practice, though, this pulls down the level and overloads many contemporary outputs designed for conventional 10kΩ loading. The MM PRO takes note of this new practice and conquers the most demanding spec-chasing balanced inputs currently on the market. It can breezily drive over 20V RMS into a 2kΩ balanced line load without breaking a sweat, with less than 0.7dB of insertion loss from a differential output impedance of 150Ω.

Input impedance, at 50kΩ in parallel with 120pF, is raised slightly from the standard 47kΩ to allow for a slight extension in high frequency response for moving magnet cartridges when used with short cables. The capacitive loading is fixed at 120pF, so the cartridge sees a total capacitance of 200pF when 30pF of tonearm and 50pf of 0.5m cable capacitance are added, which extracts a flat response from virtually all MM HiFi cartridges currently on the market. Suitable RF filtering at the input also permits HOMC cartridges to be connected to this high load impedance, without the risk of interference from their ultrasonic electrical resonances, minimising insertion loss and eliminating the need for adjustment.

A 3rd order subsonic cutoff set to 23Hz attenuates low frequency disturbances in the 10Hz region by -23dB (14 times), blocking them from causing intermodulation distortion and over excursion of loudspeaker drivers further down the signal path. Despite the heavy pruning of subsonic frequencies, some very fine tuning of the filter has made certain that it doesn't affect the response at 40Hz by more than 0.1dB, so the baby isn't thrown out with the bathwater. Coupled with variable low frequency crossfeed, this dramatically reduces the effects of low frequency noise inherent to the vinyl format, improving stability and revealing bass detail otherwise buried in LF noise.

A maximum 6W power consumption when powered on, dropping to 200mW when left plugged, in keeps operating costs low, saving more money for records.

Active loading

Moving magnet cartridges are high-impedance devices, with an impedance that rises sharply as frequency increases. This caused by their high inductance, which can increase the impedance from typically 600Ω at DC (the DC resistance), to over 30kΩ at 10kHz. Amplifier noise flowing through this spiralling impedance is unhelpfully converted into voltage noise with great efficacy, often generating much more voltage noise than the amplifier itself produces, particularly in the mid-range when equalised with the RIAA curve. This is why so many phonostages are noise specified with their inputs shorted or at low-impedance, to hide this effect and demonstrate impressive looking numbers on the specification, despite being very noisy in use.

To mitigate current noise conversion in the cartridge, the MM PRO uses the NE5534 as an input amplifier; perfectly matched to strike an optimum balance of voltage and current noise, so neither dominates and degrades the noise floor when an MM cartridge is connected. Once this is done, the next noisiest component on the input isn't the cartridge on its own, but the loading resistor which adds more current noise to the input than the input amplifier.

The cartridge loading resistor, like all physical resistors made of metal, carbon, or cryogenically treated reconstituted vegetable peelings, exhibits a thermal noise current at any temperature above absolute zero (-273℃); rather outside the comfort zone for most listening rooms! This current noise can only be reduced by substantially dropping the temperature, or increasing the value of the resistor. Sadly, increasing the resistor value enough to meaningfully reduce its current noise in a passive cartridge load lessens its damping of the cartridge inductance and load capacitance resonance, causing an undesirable peak in the high frequency response.

Passive cartridge loading

A typical MM input amplifier, shown above, demonstrates this effect. In this case, the standard 47k load resistance generates 83pA of noise in the audio band at 20℃ - 48% more than the optimised input amplifier. To knock the load resistance's current noise down below half that of the input amplifier, a higher value resistor of 560kΩ must be deployed. To realise the correct response, however, it must be made to behave like a lower value resistor. Pulling on the bottom end of the resistor, with an active amplifier stage referenced to the input, draws more current through the resistor and does just this, creating an active loading input.

Active loading, sometimes referred to as 'load synthesis', 'synthetic cartridge loading', or 'electronic cooling', has been known of since at least the 1970s and has been described in several articles that can be found online. Although the theory itself is sound, active loading has never gained significant traction in commercial products since it always comes with significant amplifier noise limitations when tacked onto conventional input circuitry, somewhat defeating the purpose...

Active loading derived from the feedback tail

In the conventional active loading circuit with RIAA equalisation, shown above, the 47kΩ passive loading resistor is replaced with a 560kΩ one, now contributing just 24pA of current noise to the input; a very welcome reduction. The input amplifier applies RIAA equalisation and feeds the signal at the top of Rload into the inverter amplifier through its feedback tail via R3. The inverter then pulls on the bottom end of the 560kΩ Rload 10 times more than the cartridge input pulls on the top, dividing it's effective resistance by 11 to a usable 51kΩ so it can effectively damp the cartridge response.

Things may seem peachy, as the loading resistor current noise has now been cut to less than a third, but adding the inverter into the feedback tail also adds its voltage noise to the input amplifiers reference, bolstering the effective voltage noise on the input from 550nV to 950nV at audio frequency. This is hardly a win-win situation, making for little overall improvement in noise performance with too much input voltage noise for HOMC cartridges.

Buffered active loading adds current noise

In an attempt to fix the issue of voltage noise addition, the 'Self circuit' above was proposed over a decade ago. The load-driving inverter is now taken out of the feedback reference of the input amplifier, being driven by its own buffer stage, also connected to the cartridge input. This appears to be a sensible idea at a brief glance, but now the buffer adds its input current noise of 98pA onto the input; more than the original 83pA of the original 47kΩ passive load resistance added to begin with!

The problem with both conventional circuits is that the input amplifier has to perform RIAA equalisation. An RIAA input amplifier can't drive the loading inverter from its output, as it will vary with frequency and cause the active load's impedance to fluctuate with the RIAA response. The MM PRO's solution is to forget about applying the RIAA equalisation on the input, and make an input amplifier with a flat gain of 19.4dB (9.33 times) to bring the signal level up to a point where it can't be contaminated with noise. The loading inverter can then be driven from the noise-immune output of the input amplifier with the RIAA equalisation relegated to a later amplifier stage.

Dedicated head amplifier with superior noise performance

The MM PRO takes the dedicated approach above, elegantly abolishing the limitations that have crippled conventional circuits and finally bringing home the theoretical benefit of active loading into reality. The uncompensated input amplifier drives the inverter from its output, keeping its current and voltage noise away from the sensitive input and feedback tail respectively. The feedback resistor values are almost quartered compared to their previous values, halving their voltage noise contribution so as not to contaminate the lower output levels of HOMC cartridges when the gain is adjusted.

Bringing the gain up before feeding it into the inverter also increases its effective signal-to-noise ratio, which curtails the voltage noise present at the bottom of the active loading resistor, further upgrading the circuit's performance. The end result? Seriously reduced mid and high frequency noise, with a total un-weighted signal to noise ratio of 79dB against a 5mV cartridge; probably the lowest measured noise performance with a real moving magnet cartridge connected on the market! Get ready for true blacker-than-black backgrounds on your best LPs...

Too long; didn't read: a passive cartridge load resistor injects noise into a moving magnet input. The MM PRO's original active loading circuit increases the effective signal power across the cartridge loading resistance by actively driving the bottom end. This reduces its effective noise contribution for a better overall signal-to-noise ratio than possible with a conventional passive cartridge load resistance.

Precision RIAA

So as to match the power bandwidth of music to the physical restrictions of vinyl recording, all records are cut with some form of equalisation that shapes the frequency response to fit into the limitations of the medium. This involves attenuating the low frequencies to stop the grooves cutting into each other on heavy bass notes, while boosting the high frequencies to overcome rising surface noise. The phonostage must perform the exact opposite process on playback, boosting low frequencies and cutting high frequencies to restore the original balance - hence the term 'equalisation'.

RIAA recording (red) and playback (green) curves

Since 1955, the RIAA standard has been used, which amplifies treble frequencies above 2122Hz and attenuates bass frequencies below 500Hz when the record is cut. The phonostage must precisely invert the curve at these points, as any error will result in unwanted boost or cut around these turnover points. Contemporary phonostages usually only manage an accuracy of ±0.5dB, which might initially look benign when we compare them against the sometimes drastic response variations of typical MM cartridges at the extremes of the frequency spectrum. Unfortunately, human hearing is much more senstive in the mid-range - right around the turnover points where most cartridges have a very flat response.

The ±0.5dB figure quoted so often - a result of using low-cost 5% tolerance capacitors in the equaliser circuit - can introduce a full 1dB of variation across the mid-range, audibly colouring the sound with its error signature. The effect is as if conventional tone controls are left askew at 1 and 11 o'clock position, considerably more harmful than the tone controls now shunned by many audiophiles ever were. Worse still, this can vary between the two channels, affecting stereo balance more than the cartridge itself while also shifting mid-range balance randomly around the sound-stage with frequency. Clearly, much better accuracy is needed for transparent processing...

Polystyrene telecoms capacitors in the equaliser amplifier

High-precision equaliser capacitors are not cheap, but the MM PRO seeks the very best, investing in 1% telecoms parts to reap an exceptional RIAA accuracy of ±0.1dB that beats the current standard by a factor of 5. Parallel 1% tolerance resistors are also used in the equaliser network for further tolerance averaging, making the specified ±0.1dB quite a pessimistic figure. The tiny RIAA error is well below the threshold of audibility in even the most imaginative estimates, so you can be assured of perfect transparency from a beautifully uncoloured frequency response with the best possible channel balance, delivering the original sound that the engineers intended when the record was cut.

Conventional RIAA input amplifiers can require up to 60dB of gain below 50Hz; quite a challenge for most op-amps, and certainly not optimum for top-notch low-frequency distortion performance. By using a separate equaliser stage downstream of the input amplifier, the MM PRO takes this burden off the input amplifier. Doing so keeps distortion very low throughout the frequency range, compared to many other phonostages that see a rise in low-frequency distortion as the input amplifier runs out of corrective open-loop gain.

As the equalistion isn't performed by the input amplifier, it isn't constrained by the need for a high input impedance for the cartridge. The equaliser exploits this by using a current input shunt-feedback topology in a dedicated equaliser amplifier stage, instead of a conventional series feedback arrangement. The series feedback circuit can't track the RIAA curve at high frequencies below unity gain and requires an extra 'compensation pole' to take up the slack which can introduce its own error. The MM PRO's equaliser amplifier has no such problem, making for a very elegant circuit that tracks the curve accurately all the way into ultrasonic frequencies without any extra components in the signal path.

Too long; didn't read: ultra-precise replay equalisation five times more accurate than most competitors produces perfect channel balance and zero colouration!

Variable LF X-feed

Most of the noise on vinyl records is at low-frequency, which is unsurprising given that the RIAA curve boosts bass frequencies and most surface imperfections such as warps are rarely smaller than a centimetre in size. This 'vinyl-roar' has been known for a long time to almost disappear when mono mode is selected on the replay amplifier. The majority of low-frequency noise exists in the vertical up-and-down plane of the stylus's movement and is therefore cancelled when mono mode, only sensitive to lateral side-to-side motions, is enabled. Stereo low-frequency noise below 100Hz can be particularly distracting when listening through headphones with their near-perfect separation, as it unnaturally straddles the stereo field, causing listener fatigue.

Stereo (red), and mono (green) groove noise waveforms

Amplified waveforms of real groove noise, after 24Hz subsonic filtering, neatly illustrate the equal-and-opposite nature of low-frequency noise on stereo records. As shown above in the red waveform, the worst peaks of noise neatly move in the opposite direction, while the green mono sum reveals substantially diminished overall levels; a threefold measured improvement well worth having, to be precise. Regrettably, stereo sound is rather desirable in a HiFi system, so switching to mono isn't an option if stereo pressings are to be fully enjoyed unattended by this 'road noise'.

When records are mastered, all low-frequency information before 200Hz has to be carefully panned to mono. This is to prevent the stylus from jumping up out of the groove upon playback when excited by heavy bass notes, and also as a consequence of vertical range of motion being severely restricted. If the grooves are spaced well apart, they can move a good deal side-to-side with mono lateral modulation, but the groove can't move vertically up and out of its tiny cutting depth and into the thin-air above the record's surface with stereo information. Digital analysis of vinyl-rips support this, showing that all bass energy below 200Hz is mono.

Unfiltered (red), and LF X-feed (blue) groove noise waveforms

The MM PRO's low frequency crossfeed function gently blends the two channels together as frequency falls, cancelling much of the stereo noise, while preserving stereo separation in the more critical mid-range where the stereo image is generated. Because bass frequencies have to be panned to mono on mastering, no useful information is lost by blending to mono below 200Hz or lower. As shown in the above waveform of the same stereo groove noise, LF X-feed cuts back surface noise almost as much as switching to mono does, annihilating the worst peaks and easily halving the average level.

Although not a new concept, previous attempts at making an LFC circuit for vinyl playback featured in electronics magazines over the years, that used complicated summing and differential amplifier arrangements, all pass filters, and sometimes didn't even work in the first place! The MM PRO instead implements an elegant bridging arrangement that excludes any additional series components from the signal path. It can therefore be completely switched out of the signal path by a toggle switch on the front panel, satisfying purist sensibilities.

Continuously adjustable LF XFEED

As an upgrade over the original LF X-feed function on the Spartan 10 phonostage, MM PRO adds a continuous variable turnover, allowing users to decide from themselves the optimum balance of stereo separation in the lower mid-range and rejection of vertical low frequency noise. Increasing the turnover frequency increases noise cancellation, but narrows the stereo image slightly below 1kHz. When set to 140Hz, LF X-feed maintains 18dB of channel separation at 500Hz and 24dB of channel separation at 1kHz, better than most cartridges, and continues to extend to 30dB and beyond past 2kHz where the direction-finding ability of human hearing is most sensitive.

Too long; didn't read: LF X-feed cancels low-frequency 'vinyl-roar' while retaining the same level of stereo separation on the record at mastering, unveiling bass detail previously hidden in the noise. No more fatiguing 'road-noise' when listening with headphones, with a wide range of adjustment to suit all tastes!

Subsonic filter

Even a perfectly set up turntable will produce low frequency disturbances below 20Hz. Other sources of subsonic energy include the turntable bearing, the original cutting lathe, and potentially a resonance mis-match between the cartridge and tonearm. The greatest source of subsonic energy is the record itself, with small surface imperfections and warps being amplified 1000 times or so by the phonostage. This low-frequency noise usually intensifies where cartridge-tone-arm resonance occurs around 10Hz, and while not audible on its own, it creates a lot of trouble in any sound system if allowed out of the phonostage.

At normal listening levels, subsonic disturbances can easily push the unloaded drivers of bass-reflex loudspeakers well past their precious few millimeters of linear excursion, guaranteeing severe distortion while dissipating excess energy in the voice coil. The same is true for sealed loudspeakers and headphone drivers, delivering an ill-defined stereo image as the acoustic drivers wobble, with an unstable sound-stage and a 'shimmering' or 'tremolo' distortion effect as they amplitude modulate the wanted audio. Despite all this, many manufacturers still insist that filtering isn't necessary for high-quality listening...

Raw (red) and subsonic filtered (blue) vinyl waveforms

To prevent intermodulation artefacts, a truly high-performance phonostage should reduce subsonic disturbances by at least an order of magnitude in the troublesome 10Hz region, as shown in the waveforms above, taken from a typical LP. The MM PRO does just this, using a 3rd order high-pass filter with a cut-off frequency of 23Hz; enough to attenuate signals at 10Hz by a factor of 14, making certain that they can't cause any trouble further down the signal path. Fastidious tuning ensures that while subsonic energy is heavily reduced, the desired response doesn't change more than 0.1dB at 40Hz, preserving the ultra-accurate replay response of the precision RIAA network that precedes it.

Conventional subsonic filters have been lambasted by audiophiles for their in-band response variations that fray and distort the low frequency response. These are usually caused by picking the nearest component values to those calculated for the job, in tandem with the usual effects of 5% capacitor tolerances deviating the filter's tuning. This inevitably causes either undesirable peaking around and above the cutoff frequency with high group delay that muddies the sound, or premature roll-off that leads to thin bass. Channel balance can also suffer from tolerance effects, helping these essential filters to earn a bad reputation in some audiophile circles.

Hand-matched red epoxy-dipped polypropylene filter capacitors

The MM PRO solves the problem of component tolerance effects in the subsonic filter by using distortion-free polypropylene film capacitors, hand-matched to within 1% of each other, completely preventing tolerance effects from altering the response by more than 0.1dB from the ideal. By using one matched group for both channels, superb channel balance is maintained all the way down to 20Hz and beyond, while designing the filter iteratively for real-world component values keeps the response as close to ideal as possible right off the bat.

Unlike other subsonic filters, based around a unity-gain buffer with component values taken out of a textbook, the MM PRO implements the final 6dB of gain after the filter network. Applying the final gain after the subsonic filter stops subsonic disturbances from eating into the final headroom of the phonostage, as it does in conventional designs that apply all the signal gain ahead of the filter network. The distribution of gain away from the input and equaliser amplifiers also helpfully enhances the overall linearity of the design by increasing the overall feedback factor and therefore distortion correction of the first two stages. Common-mode input voltage on the filter amplifier is also halved, which further reduces its distortion.

Too long; didn't read: the strong subsonic filter attenuates subsonic disturbances by 14x, stopping them from destabilising the sound-stage, generating fluttering distortion in speakers and headphones, and wasting amplifier headroom. Unlike contemporary designs, hand-matched capacitors keep the response of the filter ultra-flat in the lower bass without any colouration, for deep, transparent, perfectly balanced bass with no thinness or muddying.

Mono switch

For the best results with mono pressings, a mono function that sums both stereo channels together and puts the phonostage into mono mode is highly necessary. When enabled on mono discs, all stereo noise and distortion that sits irritatingly across the stereo field and draws the listener's attention away from the mono centre is cancelled. This yields a less fatiguing and more revealing listening experience. The mono function can also be used to improve heavily worn stereo discs by reducing distortion and surface noise enough to make them listenable again.

A-weighted stereo (red) and mono (green) groove noise waveforms

Many obtrusive clicks and pops exist as stereo information, and can be at least partially cancelled in mono mode, as demonstrated in the red and green waveforms of groove noise in a lightly used LP above. A-weighting removes the effects of low-frequency vertical noise to show a further improvement to the constant noise floor in the critical mid-range. The first click is almost eliminated completely, whilst the others are at least halved in amplitude - very useful for old mono discs that have been through the wars.

Noise and distortion cancelling mono switch

One of the most useful functions of the mono switch is its ability to instantly eliminate the jarring effects of fake 'reprocessed stereo', 'simulated stereo', and 'duophonic' processing on stereo re-releases of mono recordings. These apparently mangled pressings can be obtained for very low prices, as they're rightly shunned by most audiophiles as unpleasant to listen to. As such discs had to be mono compatible when released, the processing occurs only in the stereo difference and can therefore by removed in mono mode, stripping the reverb and phase effects away to reveal the original mono recording in the vast majority of cases.

Great reductions in distortion are also very possible on original mono discs, as well as later 78s that can be played back using the RIAA curve and a suitable stylus, that usually feature heavy vertical wear from the almost non-existent vertical compliance and heavy tracking force of mono cartridges of the era. This vertical wear distortion appears mainly during loud passages, generating stereo tearing artefacts that glaringly contrast against the mono source material, destroying dynamics and clarity.

Mono sum (green) and cancelled distortion (red) waveforms

By subtracting the stereo channels from each other to generate the unamplified red distortion waveform, while summing them to produce the green waveform in the above image, we see just how strong vertical distortion can be in practice on a visually pristine 1960s mono disc. Only a modest uptick in recorded level causes this highly unwelcome addition to jump savagely out of the noise floor and ruin the enjoyment and impact, with the distortion reaching a third of the level of the wanted audio. Luckily, the green mono waveform axes all of this rubbish, for distortion-free listening that conveys all of the detail in the loud passages otherwise masked by the distortion.

Too long; didn't read: if you have mono LPs, singles, 78s, or heavily worn stereo discs, the mono switch will halve surface noise and massively reduce distortion, unclouding the sound and allowing you to rediscover new details on these discs.

Balanced output

For extra versatility, the MM PRO features an inherently balanced output, permitting the total cancellation of hum and common mode interference over a long length of cable between its output and the line input of the next component in the signal path. This, along with its compact size, allows it to be placed as close to the turntable as possible without having to worry about the length of the line output cable. The fully differential output also produces a doubling of both signal level and headroom on the line connection, further increasing immunity to interference and improving the signal-to-noise ratio of the balanced input on the other end.

Chassis grounded balanced XLR outputs

Complementing the single-ended RCA outputs, the gold-plated XLR connections on the rear panel feature a chassis-grounded shield for extra immunity from radio frequency interference. With an output impedance of 75Ω on the hot and cold sides of the connection, the total differential output impedance comes to just 150Ω; low enough to minimise insertion loss into a standard impedance balanced input to less than 0.1dB. While the standard differential input impedance for balanced line inputs used to be an easy-to-drive 10-50kΩ, many products released over the past few years have reduced this all the way down to as low as 2kΩ.

Reducing line input impedance by a factor of five from the previous standard may be great for reducing resistor voltage noise in the differential input amplifier and creating a tidy input noise figure for the spec sheet, but it comes with a heavy price in practice. New low-impedance 2kΩ balanced line inputs can pose a serious problem to many balanced outputs on the market designed for the conventional 10kΩ, overloading their outputs by drawing much more current than they were designed for, reducing the available headroom, and increasing insertion loss to over 2dB when connected to a legacy 600Ω line output circuit.

Back-to-back output capacitors and muting relay

The trend toward low-impedance balanced line inputs is duly accounted for in the MM PRO's line output. Output impedance is kept low enough so that insertion loss stays below 0.7dB when driving a 2kΩ input. The ample drive capability of the 5532 line output amplifiers ensure up to 20V RMS of differential output without any increase in distortion while driving this challenging load. Regardless the equipment downstream, the MM PRO will always deliver a clean, distortion free performance.

A high-quality, high-reliability Panasonic telecoms relay, rated for medical applications, prevents turn on transients from escaping onto the line output when the MM PRO is switched on and off. The line output is DC decoupled using back-to-back output capacitors for low frequency distortion cancellation when driving potentially heavy loads and affords full protection (in both directions!) also from 48V phantom power, should it be erroneously applied to the line output.

Too long; didn't read: fully balanced XLR output connectors double the signal level, and afford total common mode noise and hum rejection on the line connection. 5532 line drivers easily drive challenging line inputs as low as 2kΩ to full level, for undistorted sound into the most challenging modern inputs.

Linear power supply

Many small phonostages today use an external switching power supply that can inject nasty audio frequency currents straight from the mains into the audio ground path between the phonostage and power amplifier, resulting in unpleasant buzzes, hums, and whines. In most cases this adds a most unwelcome accompaniment to the music. Switchers also have a very undesirable habit of allowing the 0V output 'ground' to 'float' at 80V RMS or so; more than enough to damage sensitive line inputs when hot swapping equipment. Switching supplies are also notoriously unreliable as they combine electronics and heat together into a small form factor...

External linear AC transformer

By using an external linear transformer to interface with the mains, the MM PRO affords far better isolation from the mains than switchers, without any high frequency switching noise or ground path current. The transformer's double insulation and leakage capacitance also block off a potential route for ground loops. As the transformer is external its troublesome magnetic field stays outside of the enclosure and well away from the sensitive electronics contained within, where they might induce hum. External transformer based power supplies, while not as cheap or convenient as switching ones for a low parts count and corresponding low cost, have an excellent track record for longevity and are easily replaced in the highly unlikely event of failure.

Split linear ±17V power supply

The low frequency 9V AC transformer output is internally rectified and converted to ±17V DC, using a precise Zener diode referenced voltage regulator circuit that allows the amplifier stages it powers to swing almost 30V of output peak-to-peak; their maximum safe possible output and over 50% more than a common 24V DC switcher can provide on a good day. Splitting the power rails between +17 and -17V keeps the non-linear amplifier power currents between these opposing points and out of the sensitive 0V audio reference where they could induce distortion. Another benefit of splitting the power supply into a pair of separate power rails is that there is no need to generate a DC 'bias' voltage from them that gets injected into the audio path, removing another potential source of trouble.

Reviews

After reviewing the MC PRO in March 2023, independent HiFi reviewer Ashley Cox decided to find out whether the its MM sister would be just as good. Fortunately, it was reviewed just as highly as the MC unit he tried a few months earlier...

So what does it sound like? Clean, accurate, and as good as the rest of your system. You can’t characterise the sound of the MM pro because it doesn’t have a ‘sound’. It is ruler flat from subsonic into ultrasonic frequencies (besides the intentional subsonic filter), is impossible to overload with any magnetic cartridge or record, has more than enough output to drive any line stage into clipping with a sufficient input signal, is so quiet as to be inaudible beneath the quietest pressing, and has almost immeasurable distortion.

I kept the MM Pro. I challenge you to find me a phono stage at any price with a specification that equals or exceeds that of the MM Pro. Multi-box esoterica, hand-built by virgins in desolate forests, infused with energy in spiritual ceremony and drizzled with cryogenically treated audiophile fairy wee don’t come close. Find me an example of esoterica that even lists a measured spec for a start. Then find one that is as good as this. Believe me, I’ve looked, and I’ve been doing this a long time. As far as I can determine, £600 is enough to buy you the finest moving magnet phono stage currently in production or that has ever been produced. And that phono stage is the MM Pro by Michael Fidler.

Ashley Cox, Audio Appraisal

The MM Pro was also positively reviewed alongside the SPARTAN 5 in early 2023 on Sound Perfection Reviews.

First off the sound difference between the MM Pro and the Spartan 5 is not as big as it was from the Phono box to the Spartan 5, but this is not to say that the MM Pro isn’t worthy of it’s price tag. The MM Pro seems to me to be a little more refined than the Spartan 5, it has a more expansive soundstage and clearer separation that allows to you pick up on even more detail than the Spartan 5.

Whilst not night and day, listening to them side by side you can hear that it sounds just a little more open with subtleties being even easier to pick up on. The midrange is more defined and cleaner, it’s more lifelike and realistic in it’s tonality and there is a bit more air between the different components in the mix. The sound is just so precise, so clean and so effortless that changing to another pre-amp really does highlight what you’ve been missing out on.

Oscar Stewart, Sound Perfection Reviews

Customer reviews

Customer feedback has also been very positive...

Received the preamp in perfect condition this past Monday. I began listening after it came to room temperature and have not stopped since.

Andy's MM Pro

Your preamp is extraordinary. It far exceeds my expectations. I have always listened first to woodwinds and piano for my assessment. Natural and realistic. Voices are expressive and textured. Bass is very deep and powerful. As detailed as any preamp I have owned regardless of price.

Andy's high-end setup

Thanks again for the great service and producing such an exceptional product. It is one of the best values I have encountered in 50 years with this hobby (obsession).

Andy Rocker, USA

All I can say is 'wow'. It took me a while to hear the difference between my built-in phono stage and the S10, but not with the pro. The bar has been raised and the game changed.

From the first track it was clear there was more sound being produced. An album, Avatar's Hunter Gatherer, I listened to only a few hours before it sounded better. More notes, more sound, more clarity, and the instruments sounded more individualised. How this is so much better than the S10, I do not know, but it is. I had to then see how it did with female vocals, so another album went on before dinner and wow, goosebumps. Evanescence's Fallen, specifically My Immortal was sublime, and in all honesty never sounded better. The CD was horrid, as it was right in the middle of the loudness war of the 2000s.

MM PRO in Michael's setup

If you're on the fence, buy one. In no way shape or form would you regret it. Yes, £600 is a significant amount of money, but I feel it’s excellent value. Somehow it sounds like it should cost more. I’ve been chasing a sound for a few years now, using the Audeze LCD 2s, and to me nothing else has ever sounded as good as it does with the MM Pro. Yes, I’ve spent a lot getting there: a £2000 amp, £1200 turntable and £1500 speakers, so that £600 isn’t seeming so much anymore. A change in speakers, as I find the Dali Rubicon 2 a bit fatiguing, and I’m probably done with upgrades.

What Michael Fidler/Classic Audio Ltd has achieved is truly great. When I ordered the S10 I did wonder what a Rega Aria or the iFi Black Label 3 would be like. I’m not wondering anymore. This for me is as good as it gets, and I didn’t have to break the bank. I’ve read this back and sound like a fan boy, but when something is this good it’s hard not to.

Michael Gordon, Manchester