MM Pro By Michael Fidler Phono Stage Reviewed

Michael Fidler’s phono stages are well liked here. So much so that some months ago I received contact from a competing manufacturer offering substantial remuneration to alter the narrative, slating Fidler’s efforts to their benefit. Sleazy indeed, and all too common in this industry. I asked if they would consider providing their product for an honest assessment and, unsurprisingly, they refused stating that reviews were not necessary due the prevalence of real user feedback from their existing client base, and the reputation of the product and designer. In plane English, that translates to “We know our product is shit, and we haven’t got any confidence in it, so we aren’t prepared to provide you any measurement data to publish and we’d rather you didn’t listen to it in case you’re able to determine how terrible it really is”.

If the maker in question possessed any skill in analogue design their products would speak for themselves. Instead they rely on a reputation built over decades selling products to buyers who through no fault of their own don’t know any better. If their products could speak, they’d screech like a banshee. Luckily they can’t, and their only utterances are excess noise, horrible harmonics and an inaccurate shaping of the signal you feed them, despite the many claims to the contrary. Perhaps they should supply a pack of cotton wool earplugs with each product to hide some of the obvious deficiencies in their performance. Then again, most of the user base seems to be retirement age men with enough time on their hands to bicker and whinge whilst dipping their heads in reverence to their worshipful master on the manufacturer’s own and other online forums, so most of them probably can’t hear anything above 12kHz anyway.

The moral of the story is that if a manufacturer can’t back up their claims with measured data, those claims should be taken with a heaped tablespoon of salt – far beyond the recommended maximum of six grams per day. Regrettably I can’t name the maker due to ongoing legal processes, but I sleep better at night having refused their money. Don’t believe everything you read in the hi-fi press.

I mean no offence to the wider population of men of retirement age who do possess common sense and the ability to research, read, think and act for themselves, and who are not therefore patrons of the aforementioned forums or customers of the maker in question. You should not be unfairly tarnished by the actions of a select few.

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Back to Michael Fidler, who has made quite a splash in the industry with his phono stages. He started with the Spartan 10, a moving magnet model quieter than almost anything out there which also introduced the concept of a low-frequency crossfeed feature. The Spartan 5 brought most of the excellent performance of the Spartan 10 to a smaller, cheaper model, probably the best £150 you can spend if you’re looking for a cost-conscious phono stage. Then the Spartan 15 replaced the Spartan 10, taking its performance to new heights, and the Pro series allowed Michael to flex his design skill in a cost-no-object design that represents the best he currently has to offer.

The Pro Series

The Pro series includes both moving magnet and moving coil options. Available in silver and black, the two models are physically identical besides the lettering, so much so that a blind man can’t tell them apart. Build quality is excellent. The enclosure comprises two aluminium extrusions with a 6 mm thick front panel and an aluminium rear panel, with neat silkscreened lettering, precise control cutouts and neat countersunk hex head screws. The control layout is logical with a satisfying ‘clunk’ to the switches, a smooth chunky aluminium knob with clear position mark and central detent, and quality connectors around the back including a 4 mm binding post for the turntable ground.

Internally though they couldn’t be more different. Rather than simply adding a gain stage to a moving magnet phono stage to produce a moving coil model, like most manufacturers do, the typology in the Pro series is purpose-designed for each input. Moving magnet and moving coil cartridges have wildly different characteristics and requirements in an input stage, and if you want the best performance this is the only way to go about it.

There are some common features between the two. The output stage is the same, capable of a massive 21V RMS (in excess of 29V peak to peak into a 10K load) output via the balanced output or 10.5V RMS via the single-ended output from 20Hz to 28kHz. Distortion is a vanishingly low 0.0005% from 20Hz to 22kHz at the full rated output. Both have a 23Hz 3rd order subsonic filter giving -23dB attenuation at 10Hz. And both include the variable low-frequency crossfeed, adjustable between 65Hz and 600Hz, -4.3dB to -21.6dB attenuation at 50Hz. The boxes are smaller than half-size hi-fi at 172 (W) x 60 (H) x 155 (D) mm.

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Why no balanced output? “Adding the noise of a second amplifier input doesn’t make sense for a true floating source which only has a reference on one side and thus isn’t prone to the kind of ground loops that cause trouble in conventional unbalanced connections,” says Michael.

Like the MC Pro, the MM Pro includes adjustable gain. In the case of the MM Pro, 42.39dB, 132x, or 52.30dB, 412x (RCA) and 48.41dB, 264x, or 58.32dB, 824x (XLR) both at 1kHz. It will comfortably drive a 1kΩ load via the single-ended RCA output and a 2KΩ load via XLR, both to full specified output at full bandwidth. Most RCA inputs are specified somewhere between 30-5kΩ, where most XLR inputs are 10-20KΩ with a few notable exceptions. The Topping PRE90 and A90D preamplifiers for example have a 2kΩ input impedance to reduce input amplifier noise, to the detriment of the frequency response of some components which can’t drive such a low input impedance. Not so the Pro Series, which does a Stirling job driving my A90D as promised.

Pcb Front View

The gorgeous PCB layout isn’t just for aesthetics, though it is pleasingly symmetrical. The input section and front end is close to the input jacks on the right, while power regulation sits on the left and the input stage is mostly in the middle. All of the ICs are in the best quality sockets so they are easily replaced if something upstream or downstream misbehaves and causes damage. Op-amp rolling is strongly discouraged as the best components for the job are already on the board. The switches are made especially to Michael’s specifications. The soldering is impeccable, and the obvious effort in each hand-crafted board commendable.

Pcb Tilted

The Pro series use a linear power supply. A 9V plug-in transformer is supplied depending on the shipping country. The external transformer eliminates the potential for electro-magnetic interference injecting noise into the sensitive circuitry of the phono stage, while blocking potential ground loops due to double isolation and leakage capacitance. The 9V linear ‘wall wart’ feeds an internal ±17V split-rail power supply, regulated by a precise zener diode circuit allowing the amplifiers to output nearly 30V peak to peak. As opposed to the cheap switching units supplied with many phono stages at this price and greater, the Pro series power supply emits no spurious noise (audible or otherwise), does not negatively impact the phono stage in any way, is immune to mains noise, and will be reliable for decades to come.

I reviewed the MC Pro moving coil model previously. It was so good I kept it. For reasons of brevity I won’t go over old ground, but instead focus on the MM Pro and the differences between the two. The first of which is ‘Active Loading’.

Active Loading

A moving magnet cartridge is a high-impedance device with a high-inductance. The high inductance causes a sharp rise in impedance as the frequency increases, from a typical 600Ω at  C resistance (resistance measured essentially without a frequency load) to over 30kΩ at 10kHz. You can imagine what those numbers might look like at the top of the audioband, around 20kHz, and it gets even worse if you take into account that a good cartridge is quite capable of a frequency in excess of 50kHz as evidenced by the carrier frequency on a quadrophonic record. Generally the impedance drops from 25kHz onwards as the capacitance starts to let more current through, but 25kHz is still well above the audioband and it’s a far cry from the 47kΩ nominal load quoted for most phono stage inputs. An example of a conventional moving magnet input amplifier is below.

Passive Loading

The problem is that when an amplifier’s current noise flows through such a load, much more voltage noise is generated than is produced by the amplifier itself, particularly evident in the midband once RIAA equalisation is applied. The MM Pro uses an NE5534 as an input amplifier, spiking an optimal balance between voltage and current noise so neither can dominate or degrade the noise floor.

Not only does the cartridge add noise, but so too does the loading resistor which adds more current noise to the input than the amplifier produces. Like any resistor of any type the load resistor exhibits a thermal noise at any temperature above 270ºc or ‘absolute zero’. Only dropping the temperature substantially or increasing the value of the resistor can reduce the current noise, though the former is impractical and to increase the value sufficiently lessens its dampening of the cartridge inductance and capacitance, causing an undesirable peak in high-frequency  response.

Active Loading1

The solution is to employ an active amplifier stage reference to the input to draw more current through the load resistor, correcting the response of a high resistance (560kΩ) is employed to reduce current noise below half that of the input amplifier. The technique was first discussed in the 1970s but never gained commercial traction, as when applied to a conventional input stage it brings with it significant amplifier noise, defeating the purpose.

A typical active loading circuit as shown above isn’t without its problems, however. The inverting amplifier adds its own voltage noise to the input amplifier, increasing the voltage noise at the input. Another problem with the conventional circuit is that the input amplifier has to perform RIAA equalisation. Driving the loading inverter from the equalised output will cause the active load’s impedance to fluctuate with the RIAA response, not the response of the input load.

Michael’s solution (below) is 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. Active loading, when done properly, reduces the effective noise contribution of the input load for a better overall signal-to-noise ratio than possible with a conventional passive cartridge load resistance.

Michael Active Loading

Input loading is a fixed 50kΩ at 120pF. As a fan of Audio-Technica cartridges the fixed capacitance is a tad higher than I’d like, given that they tend to perform best with a total capacitance of less than 200pF. That assumes however a confessional input stage with passive loading. This leaves 60pF to play with in the cabling. It’s do-able, and the active loading no doubt helps here too. AT cartridges tend to present a rather harsh distorted high frequency response of the capacitance is too high, but there is none of that here what-so-ever.

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The MM Pro has proven a fine match to the RigB LNR mounted on my Technics SL-1200G, which is fitted with an Audio-Technica VM700-series generator and MicroLine stylus. Nevertheless I’ve had a lot of questions regarding the fixed loading, so I put the question to Michael. “Variable load capacitance isn’t really necessary, as 120pF works well for almost all the MM cartridges you can think of. It also requires implementing the capacitance through some kind of switch network that will greatly degrade the RF shunting ability when integrated into an RF stop network on the input.”

RIAA Accuracy

Since 1955, the RIAA equalisation standard has been used in the cutting of records, attenuating the low frequencies to stop the grooves cutting into each other on highly modulated bass notes, and boosting high frequencies to overcome problems with surface noise. If a record were cut without equalisation it would be impossible for a stylus to track it – assuming the material was bass light and the grooves didn’t cross. The phono stage is responsible for reversing the RIAA equalisation, cutting high frequencies and boosting low frequencies to restore the intended frequency response. Any deviation from the RIAA standard will skew the tonal balance, and result in an inaccurate frequency response. See below the RIAA recording (red) and playback (green) curves.

Riaa Curve

Most phono stages quote a ±0.5dB RIAA accuracy. Better models quote a figure of ±0.3dB or so, but that’s still ±0.6dB of variation across the midrange where the human ear is most sensitive. Worse, most phono stages quote a figure reference only 1 channel, usually the best of the two stereo channels measured. A variance between channels can cause a stereo imbalance, more so than the cartridge itself. It is said that we have a maximum of about 20dB of stereo separation between our ears, so an imbalance of even 0.5dB is noticeable and significant.

The Pro series uses 1% tolerance polystyrene capacitors rated for telecoms applications where accuracy is paramount. These parts are not cheap, and nor is the time that goes into further hand-matching the parts for the best possible performance. The RIAA network also uses multiple 1% tolerance resistors in parallel to further reduce average tolerances. But the results speak for themselves. RIAA accuracy is a pessimistic ±0.1dB at its very worst, and often significantly better on the test bench. Even the pessimistic figure is well below the threshold of audibility, so a ruler flat frequency response with the best possible stereo balance is assured. Yes, you can get higher tolerance parts as modern SMT (surface-mount technology) devices. But to better the performance demonstrated here would cost a lot of money, for no practical gain.

The MM Pro’s RIAA implementation goes further. Conventional RIAA input amplifiers can require up to 60dB of gain below 50Hz, a challenge for most op-amps. This probably counts for the superior compared performance of conventional discrete input circuits. In the MM Pro however the RIAA equaliser is downstream of the input amplifier, unburdening the input amplifier, reducing distortion to negligible levels across the full frequency spectrum. The MM Pro RIAA equaliser network can accurately adhere to the RIAA curve well into ultrasonic frequencies and does so without any impact on the input stage.

All of this means an extremely high maximum input tolerance. 82mV RMS at 42.4dB, 26mV RMS at 52.3dB at 1kHz, and 400mV RMS at 42.4dB, 125mV RMS at 52.3dB at 10kHz. To put this into perspective, the hottest moving magnet cartridges are specified as having an output of around 8mV at 1kHz, measured at 5CM per second. Given that such a cartridge would be used at 42.4dB gain, the MM Pro’s input stage can handle more than 10 times the specified output. If we take a more typical 3mV cartridge output, again at 1kHz measured at 5cm per second, the MM Pro can handle more than 27 times the cartridge output at low gain, and more than eight times its output at high gain.

This is important as it means that in practical terms, it is impossible to overload the MM Pro regardless of the dynamic content on the record, or the output level of a cartridge. But more importantly than that, harsh transients such as pops, clicks, warps, crackles and scratches which cause sharp momentary peaks in the cartridge output will not exceed the headroom of the input stage, nor will they cause any anomalies in frequency response. As a consequence those unwanted noises are dissipated quickly, and are hardly noticeable.

Low-Frequency CrossFeed

The MM Pro includes the same variable low-frequency crossfeed feature as the MC Pro. A fixed version of this feature was first seen in the Spartan 10 and persists on the Spartan 15, but here the turnover frequency is adjustable between 65 and 600Hz. Set to 140Hz, the low-frequency crossfeed circuit maintains 18dB of stereo separation at 500Hz and 24dB at 1kHz. Increasing the turnover frequency narrows the stereo separation, through the midband, particularly evident when you max out the turnover frequency.

But when a record is mastered, most content below 200Hz is usually blended to mono. This isn’t a set in stone rule. If you have subtle low-frequency content panned to stereo, it is usually not a problem. Sudden bursts of highly dynamic LF modulation however can cause the stylus to jump right out of the groove if they’re not handled properly in the master and at the cutting stage.

Low-frequency crossfeed is a sloped filter that blends low-frequency content to mono. The purpose of this is to reduce vinyl roar caused by stylus friction, imperfections in the vinyl surface and mechanical noise from the turntable, accentuated by the low-frequency boost as part of RIAA equalisation. Its primary aim is to remove the distraction caused by surface noise and vinyl imperfection which directs the listeners attention to the extremities of the stereo field, especially when using headphones. The below image shows an unfiltered noise waveform in red and low-frequency crossfeed waveform in blue

Lfx Waveform

It also makes a surprising difference with some loudspeakers, particularly acoustic suspension designs which are known for their accurate, refined and highly revealing bass response.

I use the LF crossfeed when I’m listening with headphones, as it makes the listening experience a great deal more enjoyable. And once the turnover frequency is set to a sensible level, it has so little impact on the music that I usually forget to disable it. Where most records are concerned it doesn’t impact the music at all. Only in rare cases have I managed to actually detect the narrowing of the stereo image, and even then it’s subtle. I have never come close to maxing out the turnover frequency and can’t think of a situation where a 600Hz turnover would be required. Purists can switch the low-frequency crossfeed function completely out of the signal path using the toggle switch on the front panel.


One of my favourite features of Michael’s phono stages is the mono switch. Playing a mono record with a stereo cartridge, particularly through headphones, results in unwanted noise to either side of the stereo field that distracts from the music in the centre. Blending the channels to mono elevates the experience, giving monaural content a true sense of depth even without a dedicated mono cartridge. Even stereo pressings that contain mono content benefit from the mono switch, not just pre-stereo vinyl pressings that only have lateral modulations.


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’ve had the MM Pro for months and in that time I’ve used it with a range of turntables and cartridges. My reference SL-1200G with RigB LNR integrated headshell and cartridge, fitted with VM700-series generator and VMN-40ML stylus. A RigB9 body fitted with a VM95 cartridge generator and microline stylus on an HS-6 headshell. A full refurbished Pioneer PL-12D with AT91 cartridge and both the original .6 mil conical stylus and an extended contact stylus upgrade. A Sony PS-5011, a Lenco B52 derivative with GL75 arm and its stock rebadged Shure B55. And various other Technics models, including the SL-QL15 linear tracker with P33 cartridge and elliptical stylus, SL-QD33 with a P30 and an elliptical stylus, SL-QL1 with Ortofon OMP20, and probably some others I’ve forgotten. As you can see, quite an extensive range and an extended period of testing. The system downstream is a Topping A90D with EXT90 input extender, Hypex Nilai500 monoblock amplifiers and primarily restored AR28LS loudspeakers with the desirable in-house tweeter, along with a couple of prototype loudspeakers of my own design that I can’t discuss in detail yet.

I’m used to phono stages that impart a healthy noise of their own, as most do. It’s on elf the reasons going from vinyl to digital is such a stark contrast, and gives a good digital recording the illusion of better dynamics. Yes, even 16-bit, 44.1kHz CD quality audio offers more dynamic headroom than vinyl. But vinyl is better dynamically than you’d think. When you effectively remove phono stage noise from the equation, the dynamics of a good pressing are quite startling. The double LP release of Eric Clapton’s MTV Unplugged is a beautiful recording and a top-notch pressing. His 1939 and 1966 Martin acoustic guitars have all the zing and shimmer you could want, with the subtle detail of his fingers on the frets fully unmasked. His vocal, at times delicate and at times with great emotive power, leap from the speakers slightly to the right of centre, while his backing singers are positioning in a wide stereo image that flows around the listener letting the speakers completely disappear.

Records with a lot of bass, like Rag’n’Bone Man’s ‘Human’ are delivered with far greater impact than can ever be so when the music immerses from a bed of noise. Acoustic suspension speakers portray bass like nothing else, controlled by the pressure of the air inside the sealed cabinet. They show up low frequency error with ease, in particular distortion. Other phono stages do a passable job with this record, which has a lot of LF extension and is quite a loud pressing with a lot of detail through the midband as well. But the MM Pro delivers it with near digital precision, so clean and with all its impact intact. Something infinite baffle loudspeakers cannot do is produce the copious amounts of false low end that a ported loudspeaker can. Most people initially find them bass light. But my god, this record through an MM Pro with a gutsy amp will knock you for six.


You can make up all the audiophile cliches you want, but like it or not measurements do tell us exactly how a piece of equipment will perform and the MM Pro is no exception. It is as good as the input source. So if your turntable is accurate, your cartridge is flat in its response, and the components downstream are of a similarly high specification, you will hear exactly what is on the record. Otherwise you will hear the colour present somewhere else in your system as the MM Pro has no colour, intentionally or unintentionally, audibly or measurably.

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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. I will stand by that statement until someone shows me verifiable scientific data, not just audiophile hyperbole, to prove otherwise. Until then, you can order one from

By Ashley

I founded Audio Appraisal a few years ago and continue to regularly update it with fresh content. An avid vinyl collector and coffee addict, I can often be found at a workbench tinkering with a faulty electronic device, tweaking a turntable to extract the last bit of detail from those tiny grooves in the plastic stuff, or relaxing in front of the hi-fi with a good album. A musician, occasional producer and sound engineer, other hobbies include software programming, web development, long walks and occasional DIY. Follow @ashleycox2


  1. I was asked to comment on this ‘review’ by a potential customer for a phono stage. The level of technical data led me to have some doubt as to the ability of the reviewer to carry out an objective test and the record playing equipment used gave rise to some doubt as to the subjective findings. Further, designing MC phono stages employing the 5532/5534 chips was something the industry left behind some forty years ago and utilising a ‘wall wart’ and what appears to a couple of LM317/337 regulators represents the most rudimentary solution to power supply design. A more serious aspect of this article was the inference that a ‘competitor’ for the maker of this product had approached you for some ‘favour’ in respect of this review and your refusal could be regarded as an attempt to bolster your standing as a serious player in this industry. To put the situation quite simply: I’m not able to endorse your findings or your account regarding the mythical competitor and as such, your allegiance to this manufacturer has to be brought into question. Having been a practitioner in this industry for over 43 years, I support and endorse the efforts of entrepreneurs almost without qualification however, in this case, I regret that I was unable to recommend either the manufacturer or the review of this product to my customer.

    1. I freely admit I do not have an audio analyser. Were I in the business of selling snakeoil to the audiophile industry, I could probably afford one but I am happy to be poor of an analyser in preference to strong morals. With regards to your comments on the record playing equipment used, please do feel free to suggest a turntable with better objective performance than a Technics SL-1200G (besides an SL-1000R perhaps), and moving magnet cartridges with better specifications than the Audio-Technica models listed? Perhaps you’d prefer I owned something British, driven by a rubber band and lubricated with snakeoil?

      Regarding your comments on the NE5532 and NE5534. 40 years ago they were state of the art devices, and too expensive to be used in a lot of equipment. These days they still offer excellent objective performance if correctly implemented in the circuit, and a lot of high-end audio equipment is full of them. I’d go as far as to say that a circuit based on an op-amp is preferable to a poorly designed discrete circuit, as found in so many audiophile products.

      You are incorrect in your assumption that the MM Pro (and MC Pro) use the LM317 regulators, and there is no disadvantage to using a ‘wall wart’ where such a device contains a simple linear transformer that would otherwise be contained within the product enclosure. Moving the transformer to a wall wart simply means that it cannot impart any electromagnetic effect on the performance of the circuit, and it means that the power supply can be purchased from a manufacturer of power supplies and therefore comes with safety certifications, unlike many audiophile products that so many people blindly plug into their sockets.

      Frankly, I couldn’t care less how you perceive my ‘standing’ in the industry. I note that your products are primarily geared towards offering ‘upgrades’ for old Naim equipment, which was poorly designed to begin with, and I have found little in the way of technical parameters or measurement data to assess the efficacy of those upgrades. It seems rather dubious to me. I did find many of the typical audiophile buzzwords – pace, rhythm etc – which came as no surprise. My standing in an industry that is primarily fuelled by dishonest, disreputable salesmen preying on the technical ineptitude or gullibility of “audiophiles” means little to me. I would prefer to stand in an industry that produces honest products, backed up by fact and science, and that doesn’t fleece people who are chasing a nirvana they will never achieve, because no matter what they buy, or what they spend, someone will always be there to convince them there is something better to be had. I would prefer to stand in an industry that isn’t deservedly mocked by anybody with even a rudimentary background in electronics, computer science, pro audio, music production or audiology.

      I do hope your customer chooses not to blindly follow your word, especially given that you have no first-hand knowledge of the product in question and therefore your opinion is of limited (or no) value. I hope they choose to make up their own mind, and perhaps hear one for themselves as I am confident it will not disappoint. After all, the measured results speak for themselves, not just those published by the manufacturer or in reviews like mine but also in the review of the moving coil model, shown over at the Audio Science Review forum. I would respectfully suggest to your customer that they make their own choices in their purchases based on their own research, and to disregard your opinion of a product you have neither measured, heard or even handled in person, especially as your opinion was formed on the basis of some incorrect technical assumptions.

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