[Review] Audiodinamica moving coil cartridge step-up transformer: SUT No.3

Product: moving coil cartridge step-up transformer SUT No.3
Manufacturer: Audiodinamica, Torino (Italy)
Price see below for options
Website: Audiodinamica
Reviewer: Richard Varey - TNT New Zealand
Reviewed: 2018

Introduction

No, not an Egyptian king, nor a new avant garde masterpiece. The box in question is a phono step-up transformer for matching a low-output / moving coil pickup cartridge to a (moving moving) phono preamplifier. The curious shape, the comments made by those who listened at hi-fi shows, the “any colour you like” fashion concept, the sonic function. I just had to try it. My audition coincided with several phono preamps arriving - like London buses - around the same time, so I had an interesting alternative to active preamplification with this product. And a chance to update my ears with several ideas about reproducing recordings from vinyl.

On first thought, the concept is simple. Instead of using active circuitry to match the delicate output of a moving coil to the input of an amplifier, a transformer is deployed. I discovered there's much more to it than that, in terms of sound quality from records, and the understanding of what's happening when a SUT is used. Some audiophiles seem to prefer the sound from their cartridge through a passive matching device rather than a solid-state preamplifier, and clearly so do the guys at Audiodinamica. Back in the day, few moving coil preamps were on offer, so the use of a SUT was necessary. Today that isn't any longer the case, with many standalone, dual, and integrated MC preamps available (indeed now the norm), so what is the interest in a SUT about? Given it's unusual form (size and shape), is it easier to use, and does it sound better than the more usual active preamp?

The product

The SUT No.3 is designed and made in Turin by Audiodinamica. It's an entry level flexible/adjustable step-up transformer that boosts the output of a moving coil cartridge to be suitable for input to a moving magnet phono stage preamplifier. The pair of high bandwidth 50% Nickel core permalloy core transformers (wound in the USA by CineMag) are housed in a mu-metal screening canister to minimise noise and interference. This makes for an usual form - the SUT No.3 is a simple Aluminium 150 mm cube sitting on a 5 mm thick no-scratch rubber pad. Think of it as a passive amplifier with adjustable output. Construction uses Grayhill gold contact rotary switches, 0.1% precision low-noise metal film resistors, gold plated RCA and XLR sockets, and a metal shielding case (available in a range of colours, finishes, and materials). The SUT No.3 was designed, tested, and refined in comparison with several acclaimed vintage SUTs (DENON AU1000, Uesugi U-Bros-5 Low, Ypsilon MC20), and paired with a wide range of pickup cartridges. As a result of careful tuning, it has an extremely linear frequency response between 10 Hz and 20 kHz, and enables adjustment of the interface with the moving coil cartridge - load and transformation ratio (gain) - easily and quickly - between three settings each to cater for a wide range of cartridges.

I listened first, then sought explanation. Why is it so different? Firstly, the Audiodinamica team want to see the home audio world differently, and have established a range of products that are different in design, emphasising emotion and personalisation. Secondly, the sound quality achievable from a transformer as feed for a phono pre-amplifier is very different from that of an active pre-preamp (or head amplifier). The latter colours the sound more than a passive device. The inherent distortion is different. From a transformer it is benign. SUTs and active phono pre-preamplifiers are fundamentally different in both function and sonic character. SUTs are often considered superior as they add no electrical noise to the tiny signal from the cartridge. They are passive and deliver exceptional noise performance and sound quality.

This is because the harmonic and intermodulation distortion is fundamentally different between moving coil pre-preamps and SUTs. While the former has a constant resistive input impedance, a SUT has an input impedance that is frequency dependent. The harmonic distortion produced by SUTs is highest at the lowest frequencies and decreases rapidly as frequency increases, whereas in most active devices the distortion increases with frequency. Intermodulation Distortion (IMD) is also significantly lower. Thus, the sound produced with an SUT is much more dynamic and open, with a spacious soundstage presentation, excellent instrument separation, and natural timbre. That is exactly my experience with the SUT No.3.

As this is my first practical encounter with a SUT, I asked the Audiodinamica team why would someone use a SUT instead of the more usual active preamp, especially as there are now lots of options, including both MC and MM preamps in the same box? Here's the very helpfully detailed technical reply from product developer Gianluca Sperti, reproduced in full.

Manufacturer comment

«When only tubes were available, building an active MC headamp was really a challenge due to the unavoidable noise introduced by the circuitry and components so using a step-up transformer was the simplest and most usual solution; probably tubes were also much more expensive than today so, in terms of cost, transformers made sense as well. Today, a number of low noise active devices are available with performance somehow superior to tubes and active stages are a cheaper, popular and effective solution. But they do not sound all equals and performances varies. As we say in our paper: 'The step-up transformer is a simple and effective way to amplify the cartridge's signal adding very low noise and distortion'. But let me dig deeper into the details, reasons why we preferred a step-up will appear clearly.

Let's assume we fixed all the deterministic noise in the phono preamp (that is hum, buzz, mechanical noise, etc.) so we are only left with random noise generated by the resistive character of the components. Random noise is proportional to the square of the equivalent resistance of the components and it's there, it's the white or pink noise you might detect at very low level from the noisier phono preamps. As a reference, a 1K resistor has a 4.1 nV/sqrt(Hz) noise density and for the very same phenomenon, a 15 Ω MC cartridge with a typical 0.2 mV output signal has a signal-to-noise ratio of 69 dB: that is the limit the active stage or step-up needs to be over in order to preserve the tiny signal of the cartridge coils.

The random noise cannot be fixed, but can be minimized by clever engineering and good components. A low noise LSK170 jFET (we use a lot of them) has a noise density of 0.9 nV/sqrt(Hz), few op-amp get there, while the best tubes may hardly reach 1 nV/sqrt(Hz); so the 1 kΩ resistor is 4.5 times noisier than the jFET or +13 dB. In an active stage there are a few jFETs or opamps and a bunch of resistors that add noise to noise for an equivalent resistance of 1 kΩ or so. On the other hand, a step-up has typically a fraction of ohm of resistance for the primary winding and few tens at the secondary making its noise figure very competitive.

One more positive aspect of step-up transformers is the immunity to the common mode noise that pollutes the wirings from the cartridge to the step-up. It is mandatory to have the primary of the transformer floating (or its center tap grounded) to benefit from this immunity. Headamps may or may not reject common mode noise depending on their architecture and design: thus, step-ups are scoring one more point.

Unfortunately, the core of the transformers is sensitive to external electromagnetic fields and high permeability materials such as permalloy is more prone than the cheaper iron lamination to pick up noise, but it sounds better! The solution is to shield the step-up transformers in a mu-metal or soft iron can, and this adds cost to the step-up.

The active stages typically have a distortion pattern that increases with the frequency and may abruptly hit an edge during overloading or clipping if the stage is not properly designed. Transformers exhibit distortion at low frequency comparable to active stages or higher but it rapidly decreases as frequency increases: this is due to the magnetic behavior of the core and depends on the characteristics of the laminations. Moreover, clipping cannot happen in transformers and large signals (overloading) will only gradually increase the distortion. Finally, transformers have a negligible intermodulation distortion, while active stages do suffer from it depending on the arrangement of the schematic.

The very different distortion pattern, both in frequency domain and depending on the cartridge signal, makes the step-up transformer sound different from the active stage and the nature of the distortion in the transformer is always benign versus the sometime unpleasing distortion of active stages.

Transformers are expensive and good transformers with high permeability cores and properly wound primaries and secondaries (to minimize stray capacitance and inductance) are more expensive; silver wire brings the price to levels accessible only to the most passionate and wealthy customers. As a matter of fact, a nude step-up transformer may cost from $ 50 each up to well over $ 1,500. A headamp may cost down to a few $ in components and up to a hundred $ if specialized low noise components are used. When cost is important, active stages do have a clear advantage. Clearly, we assume expertise, knowledge, and prototyping are required for both the technologies.

The output impedance of MC cartridges and its series inductance coupled with the loading parameters of the step-up may generate a loss at low frequency (insufficient primary inductance) and resonances at high frequency (typically above the 20 kHz threshold). Active stages have a more regular and extended bandwidth due to the lower parasitic of the circuit, this is clearly an advantage of headamps. But a properly designed step-up with multiple interfacing options (in terms of loading and amplification factor) can minimize the side effects of the cartridge/step-up interface even out of the audible band.

The super wide bandwidth of the active stages is actually not so desirable: records may be warped and/or the mechanical interaction between the cartridge suspension and the tonearm may generate low frequency resonances that will be amplified by the active stage and will excite the woofers: they happen at 5-15 Hz and are not audible, but cone excursion of the woofer may be visible with naked eyes under severe circumstances. A step-up transformer is a natural low-cut filter that will damp cartridge-tonearm resonances. Needless to say that the added bass rumble circuitry in the active stage is not required in the step-up with all the advantages of simplicity, noise, distortion, and reliability.

One added advantage of step-up transformers is, as mentioned, the rejection of common mode noise: so even a longish balanced connection from the cartridge to the step-up will offer all the benefits of noise immunity also in the case a single-ended preamplifier is used. We strongly encourage customers to use balanced cables (twisted and screened pair of wires) with XLR connectors as they shield the signal effectively from the tonearm to the step-up or preamp; RCA plugs can run twisted pairs of wire, but they are missing the third pin for the screen while ground is very often run with a separate wire; there is no effective screening possible in an RCA cable.

Finally, people love tube preamplifiers, and step-up transformers interface perfectly with tubes. So, Richard, in summary, we have a step-up in our catalog because from an engineering point of view it may provide:

The third above is also one reason that makes us prefer step-up active stages, from a listener's point of view. They cost more than an active stage but we didn't have low cost as the target, nevertheless we are pricing the SUT No.3 very competitively.

Let me add that we are introducing a fully differential MM preamplifier based on low noise jFETs, that works extraordinarily well with the SUT No.3 and shares its same stylish chassis; it offers different settings for input resistance, capacitance and gain to cover a really wide range of cartridges when teamed with the SUT No.3. Our objective is to give vinyl lovers high quality sound and engineering with the ease of use and flexibility that are always welcome. As a stand-alone piece of equipment, the SUT No.3 can be interfaced with any MM preamp and with almost any cartridge out there, it is not specialized for low signal or high signal MC; it basically matches ideally with all of them, thanks to the user selectable load/gain settings. We encourage users to experiment; I am using an EMT XSD15 cartridge on a EMT 938 turntable with a EMT 929K tonearm (the short version with international connection) and I prefer 24x/high load with a 47 kΩ preamplifier.

We will also offer an option for a hybrid jFET-tube differential MM preamplifier for our smart integrated amplifier, needless to say it matches very well to SUT No3.

So why do we offer the SUT No 3? It is the third level of the Audiodinamica sut range. The SUT No.3 has a 50% Nickel core. It's a very competitive transformer, better than some vintage transformers such as Denon AU320/340, Uesugi, Ortofon, and so on. In the Autumn we launch on the market the SUT No.2, our second level model. It has the same external construction of the SUT No.3, but with an 80% Nickel core transformer and more expensive winding technique. The SUT No.1 is our first level. We’ll build the SUT No.1 only on request. It's a cost-no-object transformer customized to the audio system of the buyer. We use a special core, special winding, special wire, and a special shield. More information will be available in January 2019.»

My own experience with the SUT No.3

My cartridge is a Hana EL, and I took advice from the designer on matching their SUT and my phono amplifiers. They suggested that I use the lower gain on the phono amplifier, then adjust gain with the 2-position switch (12x or 24x). The Hana EL has a 0.5 mV output and 30 Ω internal impedance. Thus, 12x (22 dB gain) with medium load (M) was suggested. They also emphasise that the best solution is to choose the most appropriate value according to your taste, while listening to your favorite records. This I did.

A cartridge manufacturer's recommendation for loading their cartridge model is based on their listening experience and preferences, with their test system, in their test listening environment. The value given is only a guide, so most pleasing setting may be different in other systems. Usually the value given is for a solid state phono stage. It likely will be different when using a valve/tube phono stage with a step-up transformer. Some manufacturers will give values for loading for solid state phono stages and for valve/tube phono stages using an SUT.

The folks at Audiodinamica also emphasised the need to check the best combination to annul any ground loop. Since the SUT is a passive component, any hum from my system doesn't originate from our SUT, so I should pay attention during the connection of the ground cables from my turntable and to preamplifier. I found it necessary to connect both the turntable ground and the cartridge ground to the pre-preamp ground post to entirely eliminate hum. The SUT No 3 needs to be placed relative to the cartridge and pre-amplifier with careful regard to possible noise interference from power supplies, mains cables, and other electrical/magnetic field sources.

The several preamps I could use the SUT No.3 with for this audition were tried in turn, and I made the following notes. Three units also have moving coil inputs, so a comparison with the transformer was possible. Paired with the M2Tech Nash (mm input), to my ears (I'm 63 years old), the sound is bright and detailed, and very dynamic and revealing, including surface noise. I've also trialled the SUT No.3 with a QHW The Vinyl (mm input), a Black Ice Audio Fusion F159 tube (mm input), a Lounge Audio LCR MkIII (mm only), and an Etalon Acoustics supraFono (mm only). In each case, the SUT No.3 provided a more exciting sound image with my Hana EL cartridge than did the MC input on these phono stages.

The designers strongly recommend connecting the SUT to the cartridge via the XLR sockets due to the superior noise immunity of balanced connections. Since I have only RCA connections on my cartridge lead and preamplifier, I wasn't able to do this during my audition. However, I have Supra Sword ISL single-ended RCA interconnects. In each polarity conductor there are twelve individually insulated strands forming two layers, the inner six spun around an inner core, with high pitch in one direction, the outer six in the opposite direction separated from the inner layer by an extruded PE insulation layer. This strand orientation efficiently counteracts all unwanted electro-physical properties, giving all audible frequencies uniform conditions internally, and no possibility for malicious noise to enter.

“Let's change the tune!” is Audiodinamica's company rally cry. After extensive audition, I have concluded that in form they are doing that, yet in function the SUT No.3 enables perhaps the least coloured sound I have heard from my Hana cartridge.

Dynamic, yes, with nuance and subtlety, as well as heft and extreme detail resolution. There is a downside to this, though. Interference from record surface imperfections and dirt is very evident. This is no surprise, as transformers have lower masking effects and higher resolution and dynamic, so they clearly amplify micro details. This can largely be mitigated with thorough cleaning, though. This is the price for digging so much more detail out of your record grooves. It may not be to everyone's taste. Among the feast of record listening I've been enjoying since the SUT No.3 arrived in my system, I played The Style Council's 1984 album Café Bleu on original issue vinyl. One song resonated as so apt. So, to the SUT No.3, I say, you're the best thing that ever happened to my record playback. It seems that a SUT No.1 and SUT No.2 are also in the catalogue. If the SUT No.3 is the entry level performance, boy would I like to hear the more advanced models!

Prices

Audiodinamica are currently taking pre-orders, and sell their range of very different products directly from their website. For early bird buyers (only for a limited period) the price of SUT No.3 is 999 euro, in silver or in black anodized version. The price for the painted version (in any RAL) is 1,249 euro. After this initial phase, the SUT No.3 the price will be 1,199 euro, in silver or in black anodized version. The price for painted version (in any RAL) will be 1,449 euro. These prices include Italian VAT (22%). The price of the SUT No.2 will be 500 euro more than SUT No.3.

A complementary companion to the SUT, the BeCube jFET preamp, will soon be on sale.

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