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SUT: The Moving Coil Step Up Transformer

SUT: The Moving Coil Step Up Transformer

This is a basic introduction to step-up transformers for those new to hi-fi, or those who’ve been enjoying their system for years and are considering a dive into moving-coil cartridges and the esoterica which accompanies them.

Black magic. Voodoo. These are replies I give when asked what’s inside one of the small silver boxes kept near my turntable. The answer doesn’t stray too far from the truth, because the passive step-up transformer, or SUT, offers such a marked increase in the level of performance wrested from an moving-coil cartridge as to be considered supernatural. This is especially true when used with a valve moving-magnet phono stage. A SUT, like a powered MC head amp, or MC preamp, sits between a moving-coil cartridge and an MM phono stage, because the tiny electrical signal from an MC cartridge requires further amplification to correctly mate with the phono stage’s input.

If you’ve got a moving-magnet cartridge none of this matters to you because an MM cart has a higher output for MM phono inputs, an MC cart has a much lower output due it’s design. (Understand MC vs. MM carts HERE). Hence the need for a way to increase the MC cart’s output to the level of an MM cart. But while both the active gain of an MC preamplifier and the passive transformer windings of a SUT achieve that same end – but because the function of each is radically opposed – they produce a significantly different type of sound. I find the sonic traits of a SUT to be far more musical, dynamic, present and textured than an MC preamp. As in all things hi-fi, YMMV, but over the years most MC cart diehards end up coveting the transformer. Many go deep and acquire various vintage transformers by Tamura, Western Electric, RCA, Altec, Thordarsen, etc. (some originally designed for microphones), and DIY their own housings and wiring. Keep in mind that running a SUT from your MC cart means you'll also need an extra set of RCA cables as the phono cables from your turntable will connect to the SUT, and the second set runs from the SUT to your MM phono stage.

The passive windings of a SUT (basically two closely spaced, but separate coils of magnet wire spun around a laminated iron core – the input winding from the cartridge is considered ‘primary’ the output ‘secondary’) not only offers a demonstrably lower noise floor than its active MC brethren, but due to the fact its input impedance is dependent upon frequency (the MC preamp instead implements constant resistive input impedance), the harmonic distortion inherent to a SUT’s design occurs at the lowest frequencies, whereas an mc amp’s distortion occurs as the frequency increases. Because of the SUT’s architecture an electrical transference of AC power occurs via induction (voodoo!).

These passive windings have ratios. These ratios operate as approximations to specify a SUT for matching with specific cartridge impedances. Standard ratios tend to be 1:20 or 1:10 (this is the “gain” usually translated to dB) but numerous ratios exist for differing cartridge specifications, and many SUTs feature multiple winding ratios to add flexibility for different carts (a 1:20 SUT may also include a setting for 1:40 windings). A general rule for MC carts is they prefer a load impedance of 3~6 times their output impedance (for example, a 20-Ohm output impedance cartridge would work well with 60~120 Ohms of input impedance). A 1:10 winding ratio means that the secondary winding has 10 times the number of turns of the primary winding, so the voltage for the secondary is roughly 10 times that of the primary. But, that’s not all the magic the transformer is performing, it is changing the resistive load the cartridge “sees” by squaring the turns ratio divided into 47,000 Ohms (this is the impedance). This translates to the cartridge “seeing” a load impedance of 470 Ohms instead of 47,000 Ohms, a number most MC carts are quite happy with. The math looks like this: 47000/10^2=470, and it, along with a lot of other math on the subject of SUTs, is available with a few quick searches online for anyone looking to deep dive on the subject.

Once you know your cartridge’s impedance output, you can use the equation above to figure out what windings ratio would be best suited to your cartridge. And while close matching load impedance with a cartridge will never hurt, it is not imperative. Experience has taught me that if you’re in the ballpark you're OK – the exact value matters less than you’d think, just keep it above the cart’s source impedance. You should use your ears above all else when auditioning a SUT and not get caught up in a numbers game. If you find the unseen forces at work inside these silver boxes aligning with your particular sonic aesthetic, you will be rewarded with a level of fidelity to the recorded event that is exceedingly musical and lifelike.

Article courtesy of Rafe Arnott - Resistormag


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