Choose proper load resistance for MC step-up transformer

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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...

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?

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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.

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!

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.
Yes it does. By my calculation with the 1:26 step up (for maximum gain) and
to provide 100R for the Denon the secondary load impedance needs to be 67K.
This includes the grid leak resistor. So to provide the correct load for the Denon
the grid leak needs to be increased to say 68K with no other load resistor.

47K on its own provides a load of 69R for the Denon which may be near enough.
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Let's start again, you have a cart Denon DL103

Output voltage 0,3mV
Load impedance > 100 ohms
output impedance 40 ohms

Ideally you need a MC step-up 1:10 = gain 20 dB

Your step-up minimum position is 1:18 = gain 25,11 dB and your cart sees a load impedance of 145,1 ohms that is OK for you cart > 100 ohms, if you want increase or decrease the load impedance seen for your cart you must use the last calculator (resistor tuning) to accomodate the load impedance that you want.

I hope now this help.
Not to throw a wrench in the works or to be a butinski but I like the 103 loaded down. 60 Ohms is best to my ears and I have tried with multiple transformers. Might be a system thing, I use the cart on a SME series two with a modified (gain reduced) ARC PH3. Don’t be afraid to try it down to the source impedance of the cart.
Not to throw a wrench in the works or to be a butinski but I like the 103 loaded down. 60 Ohms is best to my ears and I have tried with multiple transformers. Might be a system thing, I use the cart on a SME series two with a modified (gain reduced) ARC PH3. Don’t be afraid to try it down to the source impedance of the cart.

It's funny you should mention that. I like my DL110 loaded with 1000R, even though Denon recommends a 47k load. The difference is night and day better loaded with 1000R. I'm going to try the DL103 into each of the three primaries of the TKS-27 (150R, 200R and 300R), with the secondary working into the standard 47k load. I guess we'll see which one I like best.

Once again, thanks to all for the answers.


I have 1:10 Tamura transformer (small cylinder like those that used for the studio equipment, model datasheet is unknown). As seller said, primary is 150 Ohms impedance, secondary is 47K impedance. DC tester shows 20 Ohms on primary and 780 Ohms on secondary.

Using the calculator from the link above, I see that DL-103 is getting 470 Ohms which is too much, and I have to add resistor of 12K to the MM-input (47K) in order to get 100 Ohms. Is this correct?
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My cart is a DL-103R. SUT is a Cinemag wired for its maximum 30db gain. Phono is 45db gain. In my system that combination offers the best dynamic performance. I've 2 other makes of SUT wired around that figure with similar results. Switching them to lower gain reduce the dynamics a great deal.I've not had opportunity to hear any of 10X gain. I'm remain skeptical of any SUT with only 10X gain or >150ohm input impedance. Its been my assumption could be more suitable for microphones but not MC cartridges for vinyl play in practice. Having said that, personally I insist a SUT with in the region of 30X ratio with 47Kohm secondary output to the next stage at 40-45db gain, nothing less.
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103R require different load than simple 103.
As for the turn ratio etc - for the moment, I have no other options. Only one I've tried - Beyer Dynamic with 1:15 ratio. Tamura performs better, exactly in dynamic, which is kind of opposite to what you are saying. Sonically they are more or less close, but Tamura shows more dynamic. I'm not sure about the correlation between ratio and overall sonic performance, never heard about it.

Some more details to add to the question above. Before MC transformers I used DL-103 with additional tube stage. Cart load was 180 Ohms resistors. After replacement to transformers I see in spectrum of the recorded sound (same record, same fragment pick), that now highs is getting a little up from the 20-22 kHz to 48 kHz, like 1...2 dB higher with transformers, than it was with the tube. In the area of 20 Hz...20 kHz spectrums are identical. Is this behaviour related to less load to the cart? That is probably not that much important, but good to know.
I think it boils down to how the thing is constructed, its size, resultant transient and frequency response that make them all sound different even though they may pose the same boost ratio. Bigger transformers offer bigger bass response in practice (of course there's trade offs in some areas). In use and to my ears, if I switch to the higher impedance, the effect is like I've turned down the bass a couple of notches.
In practice its the same with tube output transformers, each are unique in its own quality, and the associated circuitry have to be custom tuned to obtain the flattest frequency response at the output.
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