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Which SUT to match Koetsu Rosewood Standard in my system?

Howdy everyone,

I’m about to move to to my first MC cartridge, and I’d be grateful for your advice in choosing a step-up transformer. First, here’s my current system:

  • Table: Thorens TD-124 (mk I, with a few upgrades from Hanze HiFi)
  • Arm: Ortofon TA-110
  • Cartridge: AT-540ML
  • Amp: Audio Note Soro Phono SE Signature
  • Phono stage: Built-in MC phono preamp in the Audio Note Soro
  • Speakers: Klipsch Forte IV
  • Interconnects: Ortofon 6NX-TSW-1010L (came with the tone arm)
  • Speaker cables: 14 gauge Audio Note bulk wire (planning to upgrade this at some point, but not until I finalize the analog front end)
  • Room: I live in a city townhouse, so my listening room isn’t huge—about 15 feet long by 12 feet wide. It has been sound treated a bit: absorption at the first reflection points, front corners, and on the ceiling slightly in front of the listening position, and diffusion + absorption along the back wall.

It seems to me that the clear mismatch now is the cartridge, and after auditioning several contenders, I’ve settled on the Koetsu Rosewood Standard. At least in my system and to me ears, it gives me the most of what I want: a rich, organic, natural sound. When I auditioned it, the dealer lent me an Audio Note AN-S8 SUT. A fantastic piece of gear, but well above my budget at this stage.

So I need some help choosing an SUT. Here are a few priorities:

  • Maximizing the natural, organic sound of the Koetsu and amplifier, which seems to balance so well with my speakers—retaining the dynamism and drive of high-efficiency horns while taming stridency that sometimes also comes along, especially in my relatively small space.
  • Favor emotional impact over technical accuracy: “good” distortion can be fine with me.
  • That said, minimize noise. I rent a city townhouse, so there’s old wiring that I can do only so much to fix, and a healthy bit of RF noise. My turntable, while about as quiet as an idler wheel can be, isn’t as quiet as the best designs of today. My amp is great, but also not silent. And at 99 db sensitivity, those horns pick up everything. So it’s important that the SUT be well shielded from RF noise and have an effective grounding scheme.
  • Favor an ideal match with cartridge, amp, speakers, etc. rather than future flexibility. I don’t mind swapping the SUT out for something else if I decide to change cartridges sometime in the future. I prefer optimizing it to what I have now, not what I might have
  • A 1:8 step up ratio, if possible. The cartridge is 0.4mv; the amp’s phono input is 3mv. 
  • Budget: $3k or less, ideally 

I really look forward to your thoughts. Please don’t hesitate to ask for any other info that might be helpful. Thank you!

zazu22
zazu22 OP
9 posts
 

I don’t think that Audio Note publishes that spec. I could be mistaken, but here’s the manual: https://www.audionote.co.uk/_files/ugd/89c24f_e3f648926d024a13b9846ae2269296f9.pdf. See page 9. Is there a way to test for that? Or calculate it from the specs they do provide? Based on specs for some of their comparable standalone phono stages, it appears that it could be anywhere from about 36 db to 56 db.

As for the cartridge, output voltage is 0.4mv. Koetsu is also a bit stingy with specs, and doesn’t publish internal impedance (so far as I can tell). But other sources say it’s 5 Ohms. 

duckmanst3
52 posts
 

For about half what your budgeting you could get a Soundsmith MCP2 phono preamp which sounds great and features easily adjusted cartridge impedance loading. Koetsu recommends a range of 75-500 ohms. That’s a huge range and has an equally huge effect on the sound. It’s fantastic to dial in small changes while listening to find the best setting for your taste. I just recently bought the Rosewood Standard and went this route over using a step up transformer.

 

elliottbnewcombjr
5,574 posts
 

IMO: for that cartridge: you need a unit that has separate control of GAIN (x factor) and separate control of impedance shown to the amp's phono input.

.................................

Your intended Cartridge (KR) is 0.4mv signal strength; 5 ohm resistance.

1:8 gain ratio (x factor 8) is not right for that cartridge, it is too low for two reasons. 

Your Amp's phono specification: phono input sensitivity 3.0mv. Not Ideal, not preferred: That is the MINIMUM signal strength it needs IN to boost the signal up to line level. These amp's phono inputs also have MAXIMUM _____ mv, (before overload occurs) I cannot find what your amp's max phono signal strength  is. You can write the maker and ask. 

X factor directly effects the resulting IMPEDANCE SHOWN to your amp's phono input's impedance, (presumably 47k).

There is a 'guidance formula' for any cartridge's Impedance Shown: cartridge coil impedance x 10 (KR 5 ohms x 10 = 50 ohms). Thus guidance for IMPEDANCE SHOWN is 50 ohms, or a bit more. 

Impedance Shown Calculation: x-factor. x-factor squared. Divide phono input's impedance (presumably typical 47,000) by x-factor squared = impedance shown to phono input.

x factor 8 = x factor squared 64. 47,000 divided by 64 is: 734 ohms impedance shown to amps phono input. (over 14 times guidance of 50 ohms)

It is inverse: the higher the x factor, the lower the Impedance Shown.

x factor 16 = x factor squared 256. 47,000 divided by 256 is 183 ohms shown to the amp's phono input. (over 3 times guidance of 50 ohms)

Meanwhile, signal strength is getting higher, 0.4mv x 16 = 6.4, allow some loss, say 6.2 mv. More may be too high signal strength. i.e. KR 0.4mv x 20 x factor = 8.0. allow some loss, say 7.7 mv signal. Close to overload?.

IF you increase x factor, to lower impedance shown, the signal strength will be too high.

That is why this KR cartridge would be best with a unit with independent gain and independent impedance controls. 

Note: the other reason 1:8 x factor is not ideal: you want a phono signal strength that does NOT require a lot of gain from your preamp, BECAUSE, some (not all) preamps get noisy when you have to raise the gain control a lot. KR 0.4mv x 8 = 3.umv, allow some loss, say 3.0 mv. 

lewm
13,256 posts
 

To make it a bit simpler, you’re probably OK with a SUT that enhances the signal voltage by 10X. In other words, a turns ratio of 1:10. It is unpardonable that the manual doesn’t stipulate phono gain but we can hope it’s at least 40db. We do know the fixed phono load resistance is 47K ohms. For a 1:10 SUT this means your cartridge will see a load of 470 ohms. (Load seen by cartridge = 47K/(turns ratio)-squared = 47K/100 = 470.) over the decades, I have seen output voltage for the Rosewood quoted between 0.4 and 0.6mV. Other important variables and unknowns include linestage gain and amplifier input sensitivity and speaker efficiency. 

zazu22 OP
9 posts
 

Thanks for all of the advice so far. I’ll reach out to Audio Note to see if they’ll provide that info.

One other piece of potentially relevant info: I do have the Hegel V10. Would it be useful to play around with gain and capacitance settings on that first, and see what works best? I just wasn’t sure if the combination that might sound best on the Hegel would necessarily translate to what would sound best on an SUT. 

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