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rongon 3rd February 2013 01:28 PM

Choose proper load resistance for MC step-up transformer
Hi there everybody,

I'm hoping for a pointer to where I can read up on the proper termination of MC step-up transformers.

I'm lucky enough to have a pair of Tamura TKS-27 input transformers. I also have a Denon DL103 cartridge.

According to Discussion on MC Cartridge Loading, the proper input impedance for a DL103 is 100 ohms. OK, the TKS-27 has a 150 ohm primary, which should be close enough.

So, my question is... If the TKS-27 transformer says its secondary impedance is 100k ohms, is it OK to terminate it into the typical 47k load resistor on the input of the phono preamp?

Or... Since this is a step-up *ratio* we're talking about here, am I then effectively turning that nominally 100k secondary into a 47k secondary, and I need to adjust which primary I use to compensate?

It would seem to me that terminating a 100k secondary with a 47k ohm resistor would be trouble. Am I doing this wrong?

My phono preamp probably has a pretty high input capacitance. The input tube is a triode-wired 6J52P, with a 9k plate resistor, IR LED for cathode bias, 15mA plate current. I have been using the preamp with a 1000 ohm input load resistor (grid leak resistor) and a Denon DL110 (high output MC cartridge, 160 ohm output Z).

Thanks for any help. As you can tell, I haven't done this SUT thing before...


rongon 3rd February 2013 01:29 PM

I did find this -- Sowter Type 8055

It seems a zobel network may be necessary across the SUT secondary. How to figure the correct one for a TKS-27?


merlin el mago 3rd February 2013 02:07 PM

Web link with calculator - Home

rongon 3rd February 2013 02:21 PM

Hey merlin, thanks.

I'm confused. My SUT has primaries labeled as 150R, 200R and 300R. Secondary is labeled as 100k. The turns ratio is not stated in its specs --

I need to figure the step up ratio. Using the impedances, I figure that for the 300R primary, it's 300:100,000.

100,000/300 = 333

1:333 <-- ???

That can't be right. How do I figure the 'step-up ratio' of this Tamura TKS-27 SUT?


merlin el mago 3rd February 2013 02:40 PM

Tamura pdf don't stated turns ratio. Yes you are calculating well the ratio.

150R - 100K = 1:666,66
200R - 100K = 1:500
300R - 100K = 1:333,33

rongon 3rd February 2013 03:04 PM

The problem is that those figures do not make any sense when plugged into the calculator on - Home.

I think the gain is the square root of that 1:333.33 impedance ratio (square root of 333), which equals approximately +18.4dB of gain.

So, the "natural impedance" calculated for the TKS-27 using the 300R pri and the 100k sec is about 680R.

That is a rather high load for a Denon DL103 with output Z of around 40R. The question is whether a zobel network will be necessary across the TKS-27 secondary windings.


merlin el mago 3rd February 2013 03:16 PM

The ratio seems to high for a MC Step-Up, are you sure is a step-up MC transfomrer 50,45dBs!!!!!.

I'm using Denon DL103R with a Partridge 977 1:6 ratio & is enough gain for my MM phono preamp.

rongon 3rd February 2013 03:24 PM

Tamura TKS-27 is 100% a MC cartridge step-up transformer. Supposedly used by Sakuma, et al with a Denon DL103 cartridge.

I think the step-up ratio is equal to the "x factor" in that calculator, so when using the 300R primary, the TKS-27's step-up ratio is actually about 1:8 or thereabouts.

My problem will not be getting enough gain. I've got plenty of gain available. My goal is to get the load impedance set just right for the DL103 so I don't get overshoot on square waves (odd order harmonic generation).

Thanks for helping me with this!

Zeta4 3rd February 2013 03:38 PM


You can calculate the step up ratio as follows:-

Primary impedance = Load Impedance/(step up ratio) squared.

So for a 100K load impedance and a primary impedance of 150R the step up will be
1:26. For 200 R the step up is 1:22 and for 300R the step up is 1:18.

If my maths is right of course

If you want the correct load for the Denon you will have to adjust the load impedance

Zobels across the secondary are used to damp the resonance caused by the transformers leakage inductance and winding capacitance. For most signal level transformers its at about 100Khz. The load resistance will not be small enough to damp this so its usual to add a zoblel (series resistance and capacitance) to provide the neccesary damping resistance (say 6.8K) at the resonant frequency without affecting the load impedance.

Hope this helps.

rongon 3rd February 2013 03:51 PM

Does a 47k grid resistor (the phono preamp "load resistor") across the SUT secondary change the secondary's impedance?


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