Available now for £650/€750/$850 direct from Classic Audio Ltd., the MC PRO promises an exceptionally revealing performance for low and very low-output moving coil cartridges. The ultra low noise input amplifier using discrete transistors, feeding a new gain structure, yields a lower noise floor than possible in a moving magnet system, despite the 10x smaller input level. Downstream, the low distortion triple-stage architecture, precision RIAA equalisation, variable low frequency crossfeed, and a robust balanced output complement the low noise input to realise what might just be the best dedicated MC phonostage on the market.
MC PRO front view, black finish style
The MC PRO continues to push the boundaries of practical phonostage performance by using standard components in new circuit architecture. As a result of the new dedicated MC optimised circuit structure, MC PRO delivers the following highly practical features..
- Hybrid discrete input amplifier, combining parallel PNP input transistors with IC op-amps for both very low noise and distortion
- Variable and fully defeatable low frequency crossfeed, cancelling low frequency noise to reveal bass detail without loss of low-end
- 63/73dB gain switch to match the lower levels of very low 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
- Superb RIAA accuracy within ±0.1dB, with no colouration exhibited by most other phonostages that struggle to meet ±0.5dB
- Exceptionally low distortion, less than 5ppm as a result of careful gain distribution throughout the three stage design
- A dead silent backdrop from the very low-noise input transistors, allowing even the dimmest of details to shine through
- Crystal clear high frequency reproduction through combined loop gain of both transistors and op-amps in the high-gain head
- 23Hz 3rd order subsonic filter, using hand-matched capacitors to eliminate low-end fraying in contemporary filter designs
- Fully balanced XLR outputs, permitting a high quality line connection over a long length of cable for extra flexibility
- Split linear ±17V power supply with precise start-up muting for hash-free listening with over 10V RMS output
Rear panel, showing connections
Although the MC PRO's relatively low cost compared to many 'high end' phonostages might not indicate its class-leading performance, the full feature list and overall technical performance will rigidly hold it at the top of any objective comparison list. An exhaustive search by one audio reviewer concluded that it may well be the best practical moving coil phonostage currently available for any price. If you're searching for the finest phonostage for your moving coil setup, then further frustration may perhaps be avoided by looking no further...
Moving coil transfer using the MC PRO
As can be heard in the above video, the MC PRO works wonders with high-performance moving coil cartridges, being used exclusively for many other MC transfers on the AudioPhil YouTube channel. The channel also features a comparison video with an outgoing phonostage, demonstrating just how much input lower noise it has compared to its contemporaries.
Side view, silver finish
Classic Audio Ltd. is currently manufacturing the MC PRO with units black and silver finish styles in stock and ready to go. Available to ship worldwide with UK and EU plug 240/220V PSUs, along with 120V NEMA PSUs for US and Canada customers.
The MC PRO sits inside a compact sandblasted aluminium enclosure that fits neatly into most setups, making close placement to the turntable possible. Thick aluminium panels add a stabilising 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, glass fibre, 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 MC PRO phonostage
The MC PRO is hand assembled from component level in Kent, England, using carefully sourced parts exhaustively selected as the best available for the intended application, with all units being tested against the specification for noise, distortion, and RIAA accuracy to ensure consistent quality. Like its sister product, the MM PRO, highly linear socketed NJM5532 op-amps are used throughout the signal path in the head, equaliser, subsonic filter and line output. Rather than selecting components for marketing appeal, the MC PRO's parts selection was made for the best overall performance and reliability in-situ, offering both great value and longevity.
PCB layout showing socketed IC's and careful component placement
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 MC 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!
In contrast to many other offerings currently on the market that focus on exotic components made with esoteric materials and hide the specification in the small print, the MC PRO was designed with its real-world practical performance in mind. There's no need to drone on about 'authority', 'resolution', 'expressiveness', 'insight', or any other meaningless flowery nonsense here! As such, instead of putting forward a wall of extremely vague and unfalsifiable subjective claims with the hope that nobody looks at the actual numbers, the measured performance is proudly presented below, along with how they make for great listening.
|RIAA Accuracy||±0.1dB, 40Hz to 22kHz|
|Channel balance||±0.1dB, at 1kHz|
|Signal to noise ratio (220Hz to 22kHz)||81.5dB, ref 500µV, 10Ω cartridge|
|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||7.3mV RMS at 63dB gain, 2.3mV RMS at 73dB gain|
|Maximum input at 10kHz||35mV RMS at 63dB gain, 11.3mV RMS at 73dB gain|
|Overload margin, ref 500µV, at 1kHz||23.3dB at 63dB gain|
|Overload margin, ref 150µV at 1kHz||23.8dB at 73dB gain|
|Gain at 1kHz (RCA)||63.17dB, 1440x, or 73.1dB, 4520x|
|Gain at 1kHz (XLR)||69.19dB, 2880x, or 79.12dB, 9040x|
|Minimum load impedance||1kΩ (RCA), 2kΩ (XLR)|
|Output impedance||75Ω (RCA), 150Ω (XLR)|
|Subsonic filter||23Hz, 3rd order, for -23dB at 10Hz|
|Low frequency crossfeed||65Hz to 600Hz, -4.3dB to -21.6dB at 50Hz|
|Power consumption||6W max powered on, 0.2W standby|
The MC PRO realises an audibly superior set of numbers translate into a much more immersive listening experience, with greater combined freedom from noise, colouration, and distortion than any other MC phonostage currently available. This might seem like a very bold claim indeed, especially with most of the components used being rather ordinary types, but with stronger architecture it simply becomes an inevitability. Every advantage the improved architecture brings to the table at each stage of the signal path adds to create a structure far greater than the sum of its parts.
Continuing with the excellent RIAA and channel balance of the SPARTAN series the MC PRO focuses on the critical input amplifier, using discrete input transistors to realise half the input noise of conventional IC head amplifiers used at this extremely sensitive point in most other designs. Many transfers from high quality vinyl records have shown a signal-to-noise ratio relative to 5cm/s that reaches 72dB, so it's very important the the phonostage beats this figure by a further 6dB. This makes sure that when the noise of the phonostage sums with the surface noise of the record, the overall SNR doesn't drop by more than a decibel, the smallest perceptible change.
With most MC phonostages holding a mediocre SNR around the 70dB mark, as a consequence of using IC input amplifiers or even just an MM input amplifier with extra gain, it's a given that the phonostage will hide more detail under a veil of electronic hiss than the surface noise of the record itself! The measurement is usually made with the inputs shorted, removing the thermal noise generated by the 10Ω cartridge resistance too, also using A-weighting which modifies the frequency response to hide even more noise in the measurement. In a real setup it's therefore significantly worse than the already compromised figure, throwing away the benefit of using a moving coil cartridge in the first place.
Low noise input and equaliser circuitry (right)
By using a new gain structure with discrete input transistors at the input which exhibit half the noise voltage of the best IC amplifiers on the market, the MC PRO achieves a practical signal to noise ratio of 81dB against a 500µV input level. This guarantees that it can never perceptibly degrade the signal-to-noise ratio of even the very best pressings, easily exceeding the 78dB minimum requirement; lower than a moving magnet input can manage with a cartridge connected! The flat measurement from 220Hz, connected to a real moving coil cartridge load, proves that great dynamics will be delivered off the test bench. Tiny details that were previously lost in the roar of amplifier noise can now shine through, bringing an even closer connection to the music and performers.
The very low noise of the input amplifier wouldn't be very useful if it didn't also have a great amount of headroom to deal with the loudest dynamics along with the quiet ones. Overload margin is particularly important for undistorted loud sections, as many discs capable of produce transients ten times greater than the nominal level. This is certain to cause gross distortion during loud sections and ruin the impact of hard transients in many typical phonostages. The MC PRO has this covered with plenty to spare, with its overload point over 14 times greater than the nominal level of 500µV, making sure that even the most diabolically mastered records remain impactful and transparent as the music heats up.
Premature overload can also make high frequency clicks on worn or dusty records much worse by generating far more audible distortion products in the critical mid-range. Passive RIAA designs wastefully dissipate the available headroom in a primitive attenuator network, but the MC PRO's active RIAA equaliser causes the overload margin to increase as frequency rises. Unlike moving magnet cartridges, moving coil cartridge frequency response extends well into into ultrasonic frequencies, so the MC PRO is designed handle an input voltage of over 100mV from 28kHz to 100kHz; a full 200 times the nominal level. This means that otherwise inconspicuous little clicks on the on the edge or even outside of the frequency spectrum can never create artefacts that take centre stage.
The combination of vanishing noise and ample headroom in the MC PRO would not be very practical for dynamics without corresponding very low distortion. Thanks to the gain distribution of transistors and op-amps working in harmony together in the head amplifier, with further balancing between the equaliser and subsonic filtering amplifiers, no single amplifying element has to work too hard. Consequently, everything stays beautifully linear and total harmonic distortion remains below 0.0005% from 40Hz right up to the top of the audio band. THD produces intermodulation distortion in music; a form of dynamic noise that leads to a grainy and veiled sound. With so little of this, the MC PRO stays transparent whatever the source material.
Rear connections and gold-plated selector switches
Through the selectable gain settings of 63 and 73dB, the output is hot enough to match the levels of digital sources with ease regardless of the cartridge output voltage, boosting tiny 500µV and 150µV inputs all the way up to 700mV on the RCA outputs and 1.4V on the XLR outputs. The doubling of level on the XLR outputs comes at no cost to headroom and the stronger signal offsets the higher electronic noise of differential line inputs that most manufacturers conveniently forget to reference in their marketing literature. The ±17V internal power supply rails allow the line output amplifiers to drive the unbalanced and balanced outputs to 10.5 and 21V RMS respectively, allowing the strong nominal output level plenty of breathing room.
The majority of unbalanced line inputs have an input impedance of 10kΩ or more, so with a minimum unbalanced load impedance ten times less than this at 1kΩ the MC PRO can easily drive several such connections in parallel from its RCA outputs without breaking a sweat. 75Ω of single-ended output impedance keeps insertion loss into a 10kΩ line load below 0.1dB, with strong immunity to electrostatic interference from poorly shielded cables or current noise injection from parallel line inputs.
While most unbalanced inputs have an undemanding input impedance of 10kΩ or more for the line output circuitry to drive, balanced inputs can be far more demanding, especially on many new line preamps that drop the input impedance to as low as 2kΩ. This is done to remove the noise contribution of buffering conventionally applied ahead of the low impedance balanced input amplifier. Many modern line outputs designed for conventional 10kΩ loading will therefore generate heavy distortion driving such a challenging load, however the MC PRO's 5532 output amplifiers will happily deliver 20V RMS into a heavy 2kΩ line load without complaint and the low differential output impedance of 150Ω prevents insertion loss of more than 0.8dB.
Linear power supply and subsonic filter networks
While many MC phonostages offer a range of input impedances, the MC PRO's fixed input loading is based on real filter tuning principles rather than blind subjectivism. The 120Ω load resistance critically damps radio frequency interference in the otherwise resonant circuit made up of the cartridge inductance and input capacitance. Insertion loss from loading which degrades signal-to-noise ratio at the input by pulling the tiny input voltage down even further, is also held below 10% for the vast majority of MC cartridges. Likewise, the 800pF of input capacitance lowers the natural roll-off point to 1.3MHz; well clear of the wanted audio while shunting the worst sources of RF interference away from the sensitive input amplifier.
To prevent loudspeaker driver over-excursion which causes severe distortion and amplifier power waste further down the signal path, a 3rd order subsonic filter with a cutoff point of 23Hz attenuates the inevitable low frequency disturbances inherent to the vinyl format by a factor of 14 times (23dB) at 10Hz; where typical cartridge and tonearm resonances make them most prevalent. Although the heavy attenuation stops these unwanted additions dead in their tracks, careful tuning and hand-matching of filter components ensure that the frequency response at 40Hz doesn't droop by more than a tenth of a decibel, so no bass potency is thrown out with the rubbish. Low frequency crossfeed also reduces vinyl roar, for a pristine, deep and detailed bass response.
Power consumption is a maximum of 6W driving maximum output into the lowest specified load impedances; much more than it will typically be in normal use. It then falls all the way down to below 200mW in the external power supply when powered off, so there's no need to worry about it racking up extra costs while you're not listening.
In order to realise the groove-tracking benefits that come from a lighter moving mass, the moving coils in MC cartridges are made out of only a few turns of very fine wire. This makes their output voltage 10 times lower than moving magnet cartridges, but luckily their 600 times lower coil resistance means that the cartridge resistance itself generates almost 25 times less thermal voltage noise, so in theory it's quite possible for a MC system to realise a better signal-to-noise ratio than an MM one. There is, however, one very important prerequisite for this little-known advantage to be brought about in practice; the input amplifier must also have at least 10 times less noise than its optimised moving magnet counterpart.
Conventional IC op-amps found in most modern moving coil phonostages might be great for moving magnet inputs, but the most exceptional types exhibit barely four times less voltage noise (0.9nV√Hz) than the kind optimised for MM inputs (4nV√Hz). An op-amp input's signal-to-noise ratio with low-voltage MC cartridges is only going to be mediocre at best and probably worse than the noise of the record itself, even with the most expensive chips. This is especially true in the ubiquitous 'dual purpose' input circuits that simply switch up the gain of an otherwise normal MM input by a factor of 10, resulting in an SNR of 65dB or worse when used with a 500µV MC cartridge...
Hybrid input amplifier showing parallel input transistors with drive and servo ICs
To fulfil the potential of moving coil cartridges, the MC PRO uses discrete audio transistors that are a much better match for lower input voltage and impedance. Originally designed for low power audio outputs, BC327 transistors make up the front end of a current feedback amplifier, with half the noise power of the very best IC op-amps available. The lower input noise, due to the physically larger size of discrete transistors compared to what has to be crammed into a much smaller space on an integrated circuit, is further reduced by using two of these devices in parallel for each input channel which halves their noise power again, finally allowing a signal-to-noise ratio of 81dB with a cartridge connected - better than any moving magnet system!
Discrete transistors have very low voltage noise, but aren't quite as linear as IC op-amps when used on their own, generating excess distortion that can mask musical detail. The MC PRO's hybrid input amplifier brings the best of both to the table. Transistors provide low noise at the input of the head amplifier while 5532 op-amps supply the necessary gain and current drive to the low-impedance feedback loop to keep distortion below the noise floor. A 2068 servo amplifier stabilises the drive amplifier so no DC decoupling or biasing components are needed in the sensitive current feedback loop.
Too long; didn't read: moving coil cartridges can have lower noise than moving magnet ones, but need lower noise inputs than IC op-amps alone can offer. Discrete parallel transistors in the input amplifier realise a better signal-to-noise ratio in the MC PRO than a moving magnet system for an exceptionally revealing background.
Conventional HiFi wisdom states that purpose-built MC head amplifiers should simply boost the tiny cartridge output to moving magnet level, then present the signal into a standard moving magnet input at this still very weak voltage. This is very convenient for phonostages that combine MM and MC inputs into a single product, as it just requires a switch and an extra pair of RCA connectors, but is this staggered technique really optimal for the best performance?
The MC PRO's low noise input amplifier manages an equivalent input noise voltage of just 75nV from 220Hz to the top of the audio band. If this was amplified to moving magnet level the effective noise voltage would be 750nV, so the moving magnet input downstream would need to contribute less than half as much as this to avoid degrading the combined noise floor by more than a decibel. Unfortunately, moving magnet voltage noise has to be balanced with current noise for these high-impedance devices, yielding about 700nV of input noise. For a noisier MC head amplifier this approach might not compromise the noise floor, but it would certainly diminish the excellent signal-to-noise ratio of the MC PRO's input amplifier.
A typical combination input system
The limitations of the combination input architecture are laid bare in the simplified circuit above. The MC head alone manages a great noise performance, but even after being boosted by a factor of 11 the 700nV of input noise at the moving magnet input is enough so that the 825nV noise output of the head is degraded by 2.3dB to 1080nV, losing the theoretical noise advantage of MC cartridges. Looking back down the signal path from the output of the MM amplifier, which also doubles as an equaliser by incorporating an RIAA network into its feedback path, the effective input noise voltage at the MC input has been degraded from 75nV to 98nV; quite a significant loss of dynamic range. It's apparent that a better gain structure is needed to prevent noise contamination...
The MC PRO's architecture prevents noise injection downstream of the head
By taking the dedicated MC approach simplified above, the MC PRO drops moving magnet compatibility in favour of amplifying the moving coil input almost 10 times more than the combination circuit, so the stronger signal is now far less prone to noise contamination from the next stage. Even with the noisier low gain shunt feedback equaliser amplifier the relative degradation is now insignificant as its 1500nV of uncorrelated noise sums with the 7600nV of output noise from the head to just 7750nV. This means that from the output of the equaliser the practical input noise voltage has dropped from 98nV in the first example down to 77nV, far closer to the head amplifier's 75nV capability.
Full amplifier architecture showing head (red), equaliser (green), subsonic filter (blue), and line output (indigo)
Thanks to the combined gain of both the input transistors and the driver op-amp, the hybrid head seen above in red easily manages the very high 40.1dB of closed loop gain required for this architecture without generating any measureable distortion. The output current capacity of the head amplifier also makes possible a low impedance shunt feedback equaliser, for better high frequency tracking of the RIAA curve. The equaliser also allows the gain to be easily increased by SW4A by simply switching in more signal current without any attenuation networks for the lower gain setting, ensuring both headroom and gain are never wasted.
Too long; didn't read: conventional MM/MC combination phonostages feed the moving coil head amplifier into the moving magnet input, combining the noise of both stages. The dedicated MC PRO boosts the moving coil input to a much stronger level, resulting in a very quiet performance immune to noise downstream of the head amplifier.
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...
Equaliser amplifier with polystyrene telecoms capacitors and parallel resistors
High-precision equaliser capacitors are not cheap, but the MC 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 MC 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 MC 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 MC 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 MC 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, MC 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!
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 MC 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.
PCB with hand-matched red epoxy-dipped polypropylene filter capacitors
The MC 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 MC 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.
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.
For extra versatility, the MC 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 MC 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 MC 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 MC PRO is switched on and off. A precise analogue comparator circuit senses the power rails and gives the high gain head amplifier 5 seconds to stabilise before driving the output relays closed. 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 reference to 'float' at 80V RMS or so if not connected to grounded equipment; 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 US, EU, and UK linear AC transformers
By using an external linear transformer to interface with the mains, the MC 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 and startup controller
The low frequency 9V AC transfomer 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.
Independent audio equipment reviewer, Ashley Cox of Audio Appraisal recently gave the MC PRO a glowing review, confirming the practical benefits of its features after 3 months of continuous listening...
I could delve into my box of audiophile cliches and tell you how the MC Pro has air, rhythm, timing, detail etc in spades. Or I could triple the length of this article listing the records I’ve played through it and give you a play-by-play commentary on how the MC Pro lifted each to new heights. Or I could simply tell you that the MC Pro has been a key component in realising the best sounds I have ever heard from a vinyl record. It’s that simple.
I have spent the past three months trawling through every phono stage on the market, trying to find something with a specification that comes close to that of the MC Pro. In that time I have played numerous albums with several cartridges, and not once has this little box provided anything less than stellar performance. My search of the market left me empty-handed. The MC Pro currently offers the finest technical performance of any phono stage I can find on the market at any price. It demonstrates that real-world performance is not wizardry or pseudoscience, but competence and ability.
If you’re stepping onto the MC ladder and considering a step-up transformer to use with your existing phono stage, buy the MC Pro instead. If you’re at the top of the MC ladder and you’re eyeing up some multi-box, five-figure esoterica that needs a rack of its own, consider its technical performance and buy an MC Pro instead. The MC Pro by Michael Fidler earns my whole-hearted, entirely unreserved recommendation.
Ashley Cox, Audio Appraisal
To showcase the excellent technical performance and really divide the MC PRO from the competition a test sample was submitted to Amir Majidimehr at Audio Science Review in May 2023. As expected, the confirmation of the exceptional performance resulted in the product being awarded a Golfing Panther, with full measurements being published on a forum thread.
Nice looking package, I'm fond of this white sort of colour in aluminium cases. The switches feel good so overall it's very nice. It's got some interesting features like this low frequency crossfeed where it turns the very low frequencies into mono to cancel out low frequency noise.
On the back panel it's nice to have upside-down fonts so when you're leaning over you can read what the labels are which is nice, but unusually it also has balanced output which I really like. As it is we have a lot of noise and hum issues with phonostages. Let's eliminate one stage of it which is the interface between this and your pre or power amplifier.
The inside is absolutely gorgeous, almost like a work of art. If you're an engineer, you look at this picture and it's like: 'Holy cow somebody really went to town to try to build a beautful looking device', even though you don't see it when the cover's on so quite a nice job.
Amir discussing the MC PRO on YouTube (quoted here)
What is extremely nice is flatness of the equalisation. The variation is just stunningly small, so very very very nicely done. This is one of the most important things where tonality is concerned. It also has a steep high pass filter so it gets rid of low frequency rumble.
One test we do is overload which is at what input level will the phonostage clip. It's clipping at about 8 millivolts, so it's able to tolerate around 15 times the normal level. Typical values I get before clipping are about 5 to 6 millivolts in other phonostages. Here we're going to 8 which is nice, so a pretty serious amount of voltage coming out of this thing.
I sweep it at different frequencies and basically it doesn't care which is very very nice and unusual. Usually, as frequencies go up your headroom reduces which is a bad thing, because ticks and pops on an LP tend to be high frequency so very nicely done. If I throw out noise and measure distortion, you see distortion levels are extremely good, we're talking digital audio levels of distortion.
So bottom line: this is just a fantastic phonostage. It looks good, it's got good innovation in the feature set and I highly, highly recommend it.
Amir Majidimehr, Audio Science Review
Both products were awarded a 'best buy' rating.