SUT matching question

[Nagraboy]

I have a Rega P8/Apheta 2 which I’m very happy with. After some thought, I’m thinking of using it with a SUT and MM phono stage.

My question is what SUT do I need? My cart’s output is 0.35mV and Rega suggest 100 Ohms to load it. Most of the SUTs I’ve seen are either 1:10 or 1:20 (eg. Rothwell, Music First) giving 3.5mV and 7.0mV respectively. I thought around 5.0mV was the norm for MM carts?

I currently use a Croft RIAA and the specs quote 2.0mV input for a 0.5V output. So does this mean the 7.0mV output of a 1:20 ratio SUT would be too high? I also notice that most SUT with 1:20 ratios have lower impedance figures which would be right for the Apheta 2, whereas the 1:10 ones have higher figures (eg. Music First 632 has 430 Ohms, way above Rega’s advice).

So what do I need for my Apheta 2/Croft interface? 1:10,1:20, 100 Ohm, or do Ohms not matter?

 

[Arkless Electronics]

I have a Rega P8/Apheta 2 which I’m very happy with. After some thought, I’m thinking of using it with a SUT and MM phono stage.

My question is what SUT do I need? My cart’s output is 0.35mV and Rega suggest 100 Ohms to load it. Most of the SUTs I’ve seen are either 1:10 or 1:20 (eg. Rothwell, Music First) giving 3.5mV and 7.0mV respectively. I thought around 5.0mV was the norm for MM carts?

I currently use a Croft RIAA and the specs quote 2.0mV input for a 0.5V output. So does this mean the 7.0mV output of a 1:20 ratio SUT would be too high? I also notice that most SUT with 1:20 ratios have lower impedance figures which would be right for the Apheta 2, whereas the 1:10 ones have higher figures (eg. Music First 632 has 430 Ohms, way above Rega’s advice).

So what do I need for my Apheta 2/Croft interface? 1:10,1:20, 100 Ohm, or do Ohms not matter?

1:20 should be your best bet out of the two and give very close to the recommended 100R load (117.5R). Don't worry about the 7mV input to the Croft... it will very easily handle it.

 

[Nagraboy]

@Arkless Electronics

Thanks Jez. I don’t really understand it : Music First told me 1:10 would be fine, but you say 1:20 is best. Also, why does Croft quote their figure for 0.2mV if I’m looking at using over three times that output (0.7mV) into the Croft? I don’t know what the overload margins are,and I don’t know if I’ll end up with only a very small amount of volume control to use.

 

[Arkless Electronics]

@Arkless Electronics

Thanks Jez. I don’t really understand it : Music First told me 1:10 would be fine, but you say 1:20 is best. Also, why does Croft quote their figure for 0.2mV if I’m looking at using over three times that output (0.7mV) into the Croft? I don’t know what the overload margins are,and I don’t know if I’ll end up with only a very small amount of volume control to use.

I'm guessing you mean 2 and 7 rather than 0.2 etc?

1:20 gives the best match to the cart if it is supposed to be at its best with a 100R load.

Phono stages have to have considerable overload margin anyway but in the case of a valve phono stage the high voltages used to supply the valves mean they have a huge overload margin... generally. 5mV is the generic figure usually used as typical for a MM cart but 7mV isn't a vast amount more. Yes you will end up with less usable vol control range before it gets "too loud" but I would not expect this to be to the extent that it made things difficult setting the volume to a comfortable level.... if you currently have it at say 1 o clock it may need to go down to 11:30 - 12 o clock ish.. that kind of order of difference.

 

[Arkless Electronics]

BTW you could also use a 1:10 SUT and add loading resistors of 150R at the SUT primary end ie across the output from the cartridge. This should give you the right load to the cart.