SUT Plans

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Hello. I'd like some help in choosing and implementing a SUT into the tube phono stage I'm building.
I have two LOMC carts I like very much and were dear to purchase so they are permanent residents for now.
An Ortofon "Per Windfield" 0.3 mv and >10 ohm recommended impedance. This sounds best to me at 800 ohms.
And a Soundsmith "Paua" 0.4 mv and 470 ohm as a strongly recommended load.

The problem seems with the reflected load and the 47K resistor I was planning on using. With a 10:1 ratio turns ratio the reflected load is 470 ohms - perfect for the Paua. Although some have said the 10:1 ratio may not be enough. The real problem comes in with the Ortofon. This cart may well need a 20:1 ratio step-up and that will leave me well shy of the desired load of 800 ohms.

I had a recommendation from a transformer winder to use a 20:1 ratio transformer and substitute a 400K ohm resistor for the 47K. Here is what he had to say: Your example of the per winfield (0.3mv 4Ω) and the Paua (04.mv 10Ω) are a perfect example. The 4Ω and 10Ω cart impedances of the two are at the edge of what I would consider close enough to work well with a single SUT. The design of the paua limits the step up ratio to about 7 which combined with the 0.4mv output will net 2.8mv of output which is at the low end of what I like for feeding a typical MM stage with ~40dB of gain. If you sub in the winfield the 1:7 ratio would cause your output to be 2.1MV which is at the edge. The winfield really wants a 1:20 to better integrate into a system and of course the 1:20 becomes a liability for the Paua.

Since you included a schematic for the phono I trust you could increase the 47K input Resistor to a value that will make a single SUT more suitable and a 400K would allow a 1:20 to work well for both. It is important to realize that the secondary of the SUT serves the same purpose as the 47K resistor so with a SUT installed or connected, the resistor is no longer needed at all. A grid tot ground reference (47K or larger) is only required in the event the SUT get disconnected while the system is on.


Thoughts on this plan?


I can easily swap the 47K resistor for the 400K ohm resistor but I thought I might run two sets of RCA inputs so I could have flexibility to run a HOMC or even a MM. Thoughts on this as well would be welcome?

Can I wire the inputs as I've drawn them below?

Thanks in advance!
 

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The output levels for both carts are close, so assuming the gain is enough with a 10:1 SUT, what if you used a switch to change the input impedance from 800 to 470?

If instead you use a 20:1 SUT, the 47K becomes 188K and the 33K becomes 132K.

The statement from the xfmr winder that the 47K is not needed is incorrect; while the secondary of the SUT does provide a ground reference for the grid, without a termination resistor it does not provide an impedance that is transformed to the input side of the SUT.
 

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Still working on this but I have a couple of Sowter 1990 SUTs on the way from the UK. They are switchable between 1:10 and 1:20 but cant figure out how to wire in a switch or switches to accomplish this. Attached is the PDF file with the pinouts and wiring schematic. These will be octal socket mounted within the phono-stage chassis. Sketches would be greatly appreciated!
TIA
Dave
 

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PRR

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Joined 2003
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The key clue is the funny numbering on the primary. This is a conventional way to lead-out dual windings for series/parallel connection. It is *also* used for 120/240V switching, and I stole a diagram for that.
 

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Still working on this but I have a couple of Sowter 1990 SUTs on the way from the UK. They are switchable between 1:10 and 1:20 but cant figure out how to wire in a switch or switches to accomplish this. Attached is the PDF file with the pinouts and wiring schematic. These will be octal socket mounted within the phono-stage chassis. Sketches would be greatly appreciated!
TIA
Dave

I have the 1990’s as well and they are my fav!
I lived with the Lundahl LL1971 for awhile and the Sowters sound sweet in my system, versus the Lundahl being analytica.
Nothing more than preference on my part.
 
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