SUT Advice - Which Ones Are The Best?

Thank you rauliruegas.
i will bear that in mind should I ever upgrade my phono-stage in the future..

@lewm , @mijostyn was undoubtedly trying to be helpful by linking to a page where a transformer winder (Sowter in this case) provides primary inductance data for one of their designs.

In addition to the Sowter Type 1990 webpage https://www.sowter.co.uk/specs/1990.php , I recommend also studying Sowter's Type 1480 page https://www.sowter.co.uk/specs/1480.php , as well as their Type 9570 page https://www.sowter.co.uk/specs/9570.php .

Starting from the Type 1480 page, it is specified for cartridges with coil impedances of 1~15 ohm, and its primary inductance is a fairly low 180mH. The accompanying frequency response curve is applicable when the source (cartridge coil) impedance is 3 ohm; and the frequency range is stated as "Essentially flat from 10Hz to 120kHz".

Next the Type 9570 page. Intended for cartridges with coil impedances of 1~100 ohm, its primary inductance is a far higher 1.9H (1900mH). The accompanying frequency response curve is applicable when the source (cartridge coil) impedance is 8 ohm; while the frequency range is stated as "+/- 1.0 dB 5Hz to 100kHz", instead of "Essentially flat from 10Hz to 120kHz" for the Type 1480.

Lastly we can look at the Type 1990 page, which has two primary coils to be connected either in series for 1:10 gain and 2000mH inductance, or in parallel with 1:20 gain and 250mH inductance. This is targeted at cartridges with coil impedances of 1~40 ohm (series coil connection, 1:10 gain, 2000mH inductance) or 1~5 ohm (parallel coil connection, 1:20 gain, 250mH inductance).

The accompanying frequency response curve is applicable when the source (cartridge coil) impedance is 40 ohm; while the frequency range is stated as "- 3 dB @ 5Hz to 100kHz". Once again, we see a transformer structure that the manufacturer recommends be set up for considerably more inductance when the cartridge impedance is expected to be higher, and commensurately less inductance when the cartridge impedance is expected to be lower. And on the same page Sowter declares "bandwidth improves with lower cartridge resistances".

You should be able to obtain primary inductance data from any company that winds its own transformers; Sowter, Ypsilon, Lundahl, Jensen, Cinemag, Intact Audio, Hashimoto etc. And even when the manufacturer does not wind the transformer units itself, they should be able to obtain the primary inductance data from their transformer supplier, or measure the value directly by placing an inductance meter across the au naturale transformer unit.

@jcarr , thank you Johnathan, exactly. I like Sowter transformers because I am not paying for a fancy box and I have been very happy with their customer service. I believe transformers should be internal in the phono stage. I have not personally seen a phono stage without enough internal real-estate for fitting a transformer. You can use the hardware already in the phono stage but I prefer to add another set of input jacks and a gold plated toggle switch so one can easily switch back and forth from MC to MM. When you internal them you avoid another set of interconnects saving even more money. 

@jcarr , I forgot to ask! What do you think of transimpedance or current mode phono stages? I love the Atlas and it's impedance is low enough that it can be used with one.

JCarr, thanks again for yet another great help to SUT users. The only SUT-maker that I have dealt with personally are Jensen transformers in California, and yes those guys really know what they’re doing and what they are talking about, and they offer extensive information on how to use their products. I am sure the same must be true of the other companies that you have named specifically.