Koetsu Urushi + MC step up transformer

I just did not want to be the dispenser of incorrect information. I've never used a SUT either, but I came close at one point and read some of the excellent white papers on the Jensen website at that time, to gain what little knowledge I have on the subject.

Spider, I hope it's clear in all this that the circuit should be as follows: cartridge to transformer to load resistor, so that the load resistance is on the other side of the transformer with respect to the cartridge and replaces the built-in load resistor in the preamp. To really make it simple, you could just leave the 47K resistor in place. With your transformers, that would result in a very acceptable load on the Urushi of about 200 ohms. (I'm too lazy to do the math.)

The maths works out as 47000/(13*13) = 278 ohms and I'll give that a try tonight.

Trying to replicate a 47K load through the transformer works out as 8M (47K*13*13) ohms as Lew points out. That will require modifying the pre-amp and seems quite a big change that is surely going to have an effect on the phono stage as a whole.

My thinking is that I probably don't need 47K load to get the sound I am used to and that I should take the SUT out of the equation and change the loading of the MM stage down until I find the lower bound. If that turned out to be only 3K for instance then that means I would only have to change the pre-amp to be 500K which seems more reasonable. Does that sound right?

I actually got the Lundahls from diyparadiso but Benny has been on holiday so I haven't been able to go through this with him. I'll also checkout the Jensen website and lookup Zobel networks.

Thanks for all the help so far.

Subbing the 47K load resistor with an 8M load resistor would only negatively affect the phono stage if the input tube or transistor cannot tolerate it. I'm not sure how it would work with a transistor input, but with a single-ended (as opposed to "balanced") tube preamp, the "load resistor" is actually the grid resistor as far as the input tube is concerned. (Actually, this is true for full balanced input as well, but it's a bit more of a complex topology.) The question would be whether that tube can tolerate such a high grid resistor. The typical tubes used for input in a tube phono stage (12AX7, 6922, etc) can tolerate a 1M resistor with no problem, depending upon the bias voltage. To determine whether an 8M resistor is problematic, one would have to consult a manual for the specs of the tube in question, and it would be prudent to know the bias voltge. But most likely - no problemo. As for the rest of the circuit, the value of that grid or load resistor would have no effect on RIAA equalization and should not effect the sonics except in that it affects the cartridge output, which is what we've been talking about. The only other thing that occurred to me was whether the behavior of the transformer will be unaffected when there's an 8M resistor (i.e., a very tiny load, almost no load) in it's secondary. But I think you're doing the right thing by leaving the 47K R in place, so the Urushi sees 278 ohms. You may be quite happy with that.

You're rationale for selecting the next highest load R below 47K sounds reasonable but tedious.

On the Jensen website you will find a white paper on use of Zobel networks with their transformers (which are excellent by the way). I don't see why the info is not also applicable to any other SUT, but it may be so if the design of the Zobel is based on the frequency response of their trafos in particular.

It is a transistor pre-amp, I don't have a schematic of the PF1, the instruction manual only states that it is the industry standard 47,000 ohms shunted by 100 pF. It describes how to lower the impedance with a parallel resistance but not how to increase it.

I cannot find a Zobel network diagram on the Jensen website but their MC transformer diagram adds a capacitor in series with the modifying resistor - that may be worth trying.

At the moment I'd rather not dig inside the pre-amp without some guidance, perhaps I should try contacting CJ for a schematic. Otherwise I'll wait and see if it burns with more use.

Have you yet tried connecting the trafo to the input with the standard 47K-ohm resistor in place, in other words with no other changes? Should at least work well. The networks etc are for touching up the response to make it as flat as possible; a Zobel or added capacitance is certainly not a requirement.