SUT ratio mystery

Currently looking at a 600-10k transformer offer… Can anyone help with finding out what ratio and impedances will a MC cartridge with 5 ohm coils see if this SUT goes into 47k phono?

It’s only a 1:4 ratio so loaded by 47K it’ll present about 3K.

The ratio of impedances is the square of the turns ratio. Let’s call the turns ratio N:

10,000/600 = N squared

So N = Square root of 16.666

So N is about 4.08

If you ‘look into’ this SUT when it has a 47k load you will ‘see’ 47,000/16.666 = 2,820 ohms

You could reduce the impedance by putting a lower value resistor in parallel with the input winding of the SUT. But you’d still have the same voltage step-up of 1:4.08 . If your MC cartridge delivered, say, 0.3mV you would only have 1.224mV on the secondary, which is a pretty low signal level.


Thanks, it could probably work for Koetsus at 0.5-0.6mV. Obviously it needs fiddling with resistors… I’ll pass until it cost me 10 quid. It’s a vintage Tango line step up transformer, probably made for an input stage of tube amps.

I bet you used to get extra marks for showing working out in maths :grinning:


Could it be used the other way round as a pre-out? NP206 & 406 are nice items.

I used to get it wrong if I tried to do too many steps in my head after a few ales …


Blimey, you went to a progressive school.
At my school we weren’t allowed to drink in Maths lessons, Art classes were a different matter :grinning:

I guess it can - just like someone I used to know who was using NP-406 reversely wired as a MC step up…

This post has really helped me but can someone please explain this ‘looking into’ impedance?

What is the relationship between the impedance on the coils and the impedance it looks into?

What would we like it to be and is it cart dependent? If so how do we work out what we want it to be?

So if you connect the secondary of your SUT to a 47kohm load, which is typically what an MM phono stage input socket measures, then a cart connected to the primary of that SUT will experience a 2.82kohm load. That’s the sense in which the cart is ‘looking into’ the primary and ‘seeing’ a 2.82kohm load. The SUT is ‘presenting’ a 2.82kohm load to the cart.

The cart manufacturer’s datasheet will tell you what impedance it would like to be presented with. It will also tell you what the cart’s own inherent impedance is. These are NOT the same thing. Usually the impedance that carts like to be presented with can be anything from 3 to 20 (or more) times their inherent impedance. This is why it really helps if the cart manufacturer tells you both numbers.



Yes, I keep seeing the coil impedance mentioned only. I thought I’d be able to calculate the other value somehow.

Thanks for the explanation. What I’m trying to understand though is why but maybe I shouldn’t ask that as I likely won’t understand the answer.

So one of the carts has coil impedance of 1.2ohms and states “it can be satisfactorily used for those Step up by the Step up transformer of 3-ohm general input impedance”. I suppose this is what they’re referring to?

No, this clearly doesn’t tell me what I’m after.

I suppose so too. But I agree it’s very annoying when manufacturers tell you the number you don’t need to know (the cart’s own impedance) but don’t tell you the number you do (the impedance you need to provide as a load). I have a customer right now who’s wondering whether we can reduce the brassiness of his cartridge a bit by lowering the impedance that we’re loading it with. I don’t know, and nor does he.

We need to see if we can tempt @coco and @murrayjohnson into this conversation as they know carts.


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It’s not helping that I’m trying to get my head around it all without having all the figures. I’m going to look at another cartridge and do the maths on that.

This is great though. I’m understanding that I want to match arm, cart and SUT and now have the tools to, in theory, get close enough without hearing :grimacing:.

I think Sir Humphrey would say “Courageous” … :grin:


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Had to look that up lol. I know of the program.

Still far and away the best guide to how the UK government actually works. Here’s how banking works



Someone had just sent me the following clip yesterday, so it’s strange you brought it up also.

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