UTC A12 microphone transformer as phono step-up transformer
Bit of a long gestation period on this one ... many years ago (over a decade, indeed), in the midst of my then quest for that mythical beast, the perfect phono step-up, Guy at Puresound was kind enough to loan me one which featured the UTC A12 transformer - a legend in the world of mixing desks and microphones, but also at the time occasionally put to use as a phono step-up.
I was impressed with this at the time (used with a Koetsu Rosewood I had at the time, if I recall correctly). I believe Guy's was hard wired to a 1:30 ratio, but in fact the A12 (and the A11, which is also sometimes used as a phono step-up) is very flexible in its configuration options, with a wide choice of primaries and secondaries and the option to use the secondaries in parallel or series. Anyway the A12 has to go back as it was a loan, but it was remembered fondly for its resolute bass, strong dynamic capability, and musical presentation.
Microphone/mixing desk transformers were quite popular as SUTs in those days, coming off the back of the vintage revival in Japan, in part because there were relatively few commercial options around at the time and many people either went vintage or used active head amps. (Not so nowadays with quite a few designed-for-phono step-ups available at all sorts of price points). I later came to own several such transformers, from Beyer, Newcombe, and Altec - in my explorations I have also had SUTs from Audio Note (fan AN-UK S2 , S4 and AN-J/Kondo era AN S6c), EAR (MC3), Hashimoto (HM7), Stevens and Billington (TX103), Ortofon (various), KAB, Bob's Devices, Denon, Fidelity Research and a few other random ones. Fairly recently I bought Miyajima's mono step-up, the ETR-Mono, which works beautifully with my mono Miyajima cartridges. And the notion of 'best' really is mythical - even the best one I've ever heard, the AN S9 (which, since you ask, was breathtakingly fabulous), would probably sound a bit rubbish with anything other than an Audio Note Io.
At the moment I'm fairly spoiled as my EAR 912 phono stage has some very high quality internal step-up transformers, similar to (but not quite the same in terms of its four available step-up ratios, and also higher quality) compared to the EAR MC4 external SUT. But old habits die hard and when Guy alerted me out of the blue to a pair at sensible money on ebay in the US, I decided this was an itch I still wanted to scratch.
The transformers duly arrived, and Andrew Rothwell of this parish volunteered to help me do something with them, in the form of creating a switchable SUT - something that required not just construction ability but also some proper maths, and a willingness to measure and experiment with the various switching options. At one point we were looking at a very complex switching arrangement that would allow some very low ratios in addition to the usual ones, but in the end we settled (out of the many combinations available which ranged from 1:67 down to about 1:6) at 1:33.5, 1:20 and 1:12.5.
Into a 47k phono stage such as the MM stage of the EAR 912, Andrew calculated that these ratios would give reflected impedances of 42, 117 and 300 ohms respectively - a nice fit for a decent range of cartridges including Ikeda, SPU and Miyajima stereo which are my staple at the minute.
Certain ratios/wiring combinations were rejected because of bandwith limitations - relative to transformers custom designed for phono use, the UTC transformers have a high number of windings - good for bass performance (tons of inductance) but not so good for headroom. In the end the ratios chosen were measured to have decent bandwidth (above 20 khz) when used with cartridges in the normal range for impedance and gain.
The result arrived earlier today - Andrew doing a good job of keeping the wiring complexity hidden from view with just a bypass toggle and three-position switch for each channel, while showing off the UTCs on the top. Took a little while to find a zero hum positioning (the internal SUTs in the EAR 912 are silent - tough competition) but then we were off.
The UTC was very much as I remember it. Very sold bass performance with fabulous articulation, lovely mids, impressive dynamics. Initial perception was a touch less detail than the EAR but a couple of back to backs suggested that might not really be the case - more of a different balance. Certainly very musical. It's staying in for a few weeks while I play with it - the real test will come when I take it out again and return to the EAR.
Anyway some pics - delighted with this and grateful to Andrew for his work in putting this together for me, also to Guy for remembering and sending me the link!
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Originally Posted by montesquieu
