Step up Transformer Mathematics

[Ammonite Audio]

Recent experience with Montesquieu's Newcomb SUT has shown me that I can do better than my Bob's Devices SUT, particularly in terms of optimum loading and overall gain. I see that Ortofon (http://www.ortofon.com/index.php?option=com_content&view=article&id=313&Itemid=347) have produced a new SUT called the ST-80 SE which is, unsurprisingly, optimised for cartridges like my Kontrapunkt B, and has a sensible amount of overall gain. Interestingly, it is made in Japan, which I take to be a Good Thing.

http://www.ortofon.com/images/stories/accessories/transformers/ST80-SE_contentImage.jpg

http://www.ortofon.com/images/stories/accessories/transformers/ST-80%20SE%20back%20white%20background.jpg

The ST-80 specs are listed as:

Recommended cartridge impedance < 10 Ohm
Gain 27 dB at 1kHz
Freqency response 10 Hz - 100 kHz (-2,5dB +0,5dB)
Trafo output loading 47 kOhm // 200 pF
Dimensions 95 X 153 X 100 mm

No cartridge loading impedance is quoted, but presumably this can be calculated from the data shown above. Can anyone explain to me how to do the maths?

Any other comments about this SUT would be useful too.

Thanks!

[Marco]

Hi Hugo,

If Nick (lurcher) doesn't spot this, PM him, as he'll have the answers you need :)

Marco.

[lurcher]

27dB is a gain of 1:22.3. Impedance ratio is square of turns ratio, so 1:500, so with 47k on secondary, the primary will be showing 95 ohm to the cartridge.

[pure sound]

The only issue being that 95 ohms presented by one transformer can sound very different to 95 ohms presented by another transformer if the winding arrangement, core material and shielding are different.

[Ammonite Audio]

Many thanks for the maths! So, it looks like Ortofon have designed it for a bog-standard 100 ohm cartridge loading, whereas the Bob's SUT loads the cartridge at 52 ohms. Of course, the proof of the pudding is in the listening, so there is only one way to work out if it's good, or not.

[MartinT]

It looks nice, Hugo, but as you say the proof is in the listening. I'm sure Ortofon will have conducted extended listening tests with their own cartridges, so it's very promising. I wonder who in Japan builds them? AT?

I'm eagerly awaiting my Choir Audio/Hashimoto step-up transformer to arrive and hope to boost the sound I'm getting from my Bob's CineMags too. There's no denying that the Bob's are great SUTs for the money, though.

[Ammonite Audio]

There's no denying that the Bob's are great SUTs for the money, though.

That's very true. On his site, Bob does state that loading is not the most important criterion and I have been attempting to understand what he's saying, with limited success. I can't afford the Ortofon anyway , so it's a bit academic for the moment.

[DSJR]

Ortofon will have priced this for their collective pension funds, so don't get too excited ;)