SUT Ratio

[cade12]

i have a dynavector 20x2m with dcr of 150 ohms and an output of 1 mV. I'm switching some things around and would like to try a SUT into my receiver's mm input. Anyone have recommendations for this cart? I was thinking about doing something diy. Seems like I'd need something in the 1:10 ballpark, but is that going to be too much voltage into the phono input? Receiver specs are posted below...

 

[rothwellaudio]

You need a low ratio for that cartridge, about 1:3. That should give you about the right voltage into the mm phonostage and give the cartridge an easy load to work with.
You may be able to find some suitable mic transformers to do a DIY job. If you would prefer to buy something off the shelf that's guaranteed to work I do a low ratio step-up transformer.
http://www.rothwellaudioproducts.co.uk/html/sut_for_homc.html

 

[cade12]

Thanks for the heads-up. 1:3 would put the loading around 5kohm, correct? I've been running it with my Sutherland Ph3D at 1kohm.

 

[cade12]

Does the spec sheet above suggest that the MM input can handle up to 120 mV without getting overloaded? If so, wouldn't something in the 1:6 to 1:10 range be more suitable?

 

[cade12]

You need a low ratio for that cartridge, about 1:3. That should give you about the right voltage into the mm phonostage and give the cartridge an easy load to work with.
You may be able to find some suitable mic transformers to do a DIY job. If you would prefer to buy something off the shelf that's guaranteed to work I do a low ratio step-up transformer.
http://www.rothwellaudioproducts.co.uk/html/sut_for_homc.html

I saw that you have some things for sale on Audiogon. Any of these 1:3 available?

 

[rothwellaudio]

Does the spec sheet above suggest that the MM input can handle up to 120 mV without getting overloaded? If so, wouldn't something in the 1:6 to 1:10 range be more suitable?

Yes, the specs say the maximum input is 120mV before overload, but you don't want to be so close to the limit that there's no headroom left.
As for the turns ratio, yes, you could go to a higher ratio but there's a danger of compromising the bass response if the ratio is too high. This is why: the bass response is determined by the primary inductance of the transformer and the output impedance of the source. A higher source impedance requires more primary inductance to maintain the same bass response. With a typical LOMC's source impedance of about 10 ohms you don't need a lot of primary inductance to get good bass, but your cartridge has an impedance of 150 ohms and will require a lot more primary inductance than a typical LOMC. The primary inductance is determined by the number of turns of wire on the primary coil, and to get a higher ratio would mean putting more turns on the secondary winding. However, there's a physical limit to how many turns will fit on the transformer bobbin. If you want a higher ratio the options are to use thinner wire to get more turns on the secondary, or simply put fewer turns on the primary. Fewer turns on the primary means the bass response is fine with a low source impedance, but your cartridge's source impedance is at least ten times higher than a typical LOMC.
A 1:3 ratio that means more of the space on the transformer's bobbin is available for primary windings, which gives it more primary inductance and a better bass response with a 150 ohm source.
Some manufacturers specify the primary inductance of their transformers but quite often they don't. In the absence of that information it's safer to go for a low ratio.

The low ratio transformer I sell is available from the online shop here
http://www.ekmpowershop16.com/ekmps...former-for-high-output-mc-cartridges-56-p.asp
or from britaudio in North Carolina
http://www.britaudio.com/

 

[soundmig]

Yes, the specs say the maximum input is 120mV before overload, but you don't want to be so close to the limit that there's no headroom left.
As for the turns ratio, yes, you could go to a higher ratio but there's a danger of compromising the bass response if the ratio is too high. This is why: the bass response is determined by the primary inductance of the transformer and the output impedance of the source. A higher source impedance requires more primary inductance to maintain the same bass response. With a typical LOMC's source impedance of about 10 ohms you don't need a lot of primary inductance to get good bass, but your cartridge has an impedance of 150 ohms and will require a lot more primary inductance than a typical LOMC. The primary inductance is determined by the number of turns of wire on the primary coil, and to get a higher ratio would mean putting more turns on the secondary winding. However, there's a physical limit to how many turns will fit on the transformer bobbin. If you want a higher ratio the options are to use thinner wire to get more turns on the secondary, or simply put fewer turns on the primary. Fewer turns on the primary means the bass response is fine with a low source impedance, but your cartridge's source impedance is at least ten times higher than a typical LOMC.
A 1:3 ratio that means more of the space on the transformer's bobbin is available for primary windings, which gives it more primary inductance and a better bass response with a 150 ohm source.
Some manufacturers specify the primary inductance of their transformers but quite often they don't. In the absence of that information it's safer to go for a low ratio.

The low ratio transformer I sell is available from the online shop here
http://www.ekmpowershop16.com/ekmps...former-for-high-output-mc-cartridges-56-p.asp
or from britaudio in North Carolina
http://www.britaudio.com/

I'm wondering which of your SUT's would be appropriate for my Monster Cable Alpha 2? I don't seem to find a lot of information on the web about it and am not sure how much of it is really accurate. I currently have plenty of gain and adjustability with my phono pre on the Forte 2, but I'm wanting to try using the MM tube pre-amp that I have and the signal needs a substantial "step-up" to be used with that. It looks like you have one with 22dB of gain that presents at 47kOhm load to the cartridge. That may be a good choice?

 

[rothwellaudio]

If the info I found about the Alpha 2 is correct it has an output of 0.3mV. It also says the load should be 47k, but I doubt very much that that is correct.
I would recommend a step-up ratio in the range 1:10 to 1:20. A SUT won't present a load to the cartridge of 47k because that's physically impossible with a step-up transformer, but the cartridge should be happy with about 100 ohms.
I sell three step-up transformers that would be suitable: the MC1 (ratio 1:12.9) is the entry-level model (ie, cheapest), and the MCX (ratio 1:10) and MCL (ratio 1:20) are the more up-market models.
An alternative to a step-up transformer would be a headamp - ie an active gain stage which does essentially the same job as a step-up transformer but does it with transistors. The headamp I sell is called the Headspace and it has 4 gain settings and 8 load impedance settings. It costs about the same as the MC1, so it's cheaper than the up-market step-up transformers.
http://www.rothwellaudioproducts.co.uk/html/headspace_mc_headamp.html

 

[rothwellaudio]

I think I could study that answer for a few days.

Yes, step-up transformers may appear to be extremely simple devices - two coils or wire wrapped around a lump of iron - but the more you get into the detail the more fiendishly complex you realise they really are! "The devil's in the detail" as they say

 

[soundmig]

If the info I found about the Alpha 2 is correct it has an output of 0.3mV. It also says the load should be 47k, but I doubt very much that that is correct.
I would recommend a step-up ratio in the range 1:10 to 1:20. A SUT won't present a load to the cartridge of 47k because that's physically impossible with a step-up transformer, but the cartridge should be happy with about 100 ohms.
I sell three step-up transformers that would be suitable: the MC1 (ratio 1:12.9) is the entry-level model (ie, cheapest), and the MCX (ratio 1:10) and MCL (ratio 1:20) are the more up-market models.
An alternative to a step-up transformer would be a headamp - ie an active gain stage which does essentially the same job as a step-up transformer but does it with transistors. The headamp I sell is called the Headspace and it has 4 gain settings and 8 load impedance settings. It costs about the same as the MC1, so it's cheaper than the up-market step-up transformers.
http://www.rothwellaudioproducts.co.uk/html/headspace_mc_headamp.html

Thanks for the information. My Forte has adjustable loading and the Alpha 2 does, in fact do fine with a 100 ohm loading. The Bass is a bit warmer and looser at 100 ohm, but its a subtle difference and still sounds fine. The Alpha 2 also sounds fine at 47Kohm loading, so maybe its not all the "sensitive" to impedance loading. My Alpha 2 has new ruby stylus and line contact tip from SoundSmith, but that won't impact the electrical characteristics. I understand that the Alpha-2 was designed by the same guy that designs the ZYX cartridges. Those all seem to specify a load of >100 ohms with an output impedance of 4 ohms or 8 ohms. If I recall, Years and years ago when I used this cartridge with a NYAL pre-amp, I used to load it with 1,000 ohms to good effect.

 

[rothwellaudio]

I understand that the Alpha-2 was designed by the same guy that designs the ZYX cartridges. Those all seem to specify a load of >100 ohms with an output impedance of 4 ohms or 8 ohms.

Yes, I read that too somewhere. With an output impedance of 4 ohms or 8 ohms the cartridge should be very well-suited to use with a step-up transformer.

 

[cade12]

One more question as I have another cart at my disposal... What would you recommend for 0.4 mV, coil impedance of 12 ohm, coil inductance of 25 uH, and dcr of 12 ohm?

 

[rothwellaudio]

One more question as I have another cart at my disposal... What would you recommend for 0.4 mV, coil impedance of 12 ohm, coil inductance of 25 uH, and dcr of 12 ohm?

The short answer is 1:10, or maybe a bit more (up to 1:15, roughly).

The long answer is that the coil impedance usually doesn't really matter all that much, it's the signal voltage that's important. You'll read lots of stuff on forums and various websites telling you that "you must match the impedance, the transformer is there to match the impedance of the cartridge to the phonostage, blah blah blah...) There's a grain of truth behind that, but mostly it's all been seriously misinterpreted. This is why:
If you have a low impedance source with a low signal voltage and you want to boost that voltage with a transformer (to optimise the signal-to-noise ratio), you will get the absolute maximum voltage boost by the having the reflected load equal to the source impedance. One particular turns ratio will do that. Consider the cartridge in your question as an example. It has a source impedance of 12 ohms and the phonostage has an input impedance of 47k. A step-up transformer with a turns ratio of 1:62 will make that 47k look like 12 ohms from the primary side of the transformer, so the 12 ohm source is now "matched to the load". However, bear in mind that being "matched" simply means getting the maximum possible voltage boost from the transformer. In this case the voltage boost will be 31 times and the signal voltage that your mm phonostage will see is 12.4mV. That's too high for my liking and quite a bit higher than the specified sensitivity of most phonostages. Ok, if your phonostage has adjustable gain you could drop its gain by about 8dB and be back in business, but you would be much better off by using a transformer with lower turns ratio which wouldn't give you such a high signal voltage in the first place. A lower turns ratio generally gives better performance with a wider bandwidth, so choose lower rather than higher if you can and aim to get about 5mV into the mm phonostage, not the absolute maximum voltage possible.
Following the "matched impedance" theory with some of today's cartridges can result in quite silly turns ratios. Consider an Ortofon MC Anna with source impedance of 6 ohms and an output voltage of 0.5mV. Matching the impedance to a 47k phonostage will mean having a transformer turns ratio of 1:88 and a signal voltage into the phonostage of 22mV. That wouldn't be a good idea at all.

The matched impedance idea is fine if the amplifier which follows the transformer is part of a unified design process and you're aiming for the best possible signal-to-noise ratio, but it shouldn't be taken as some kind of golden rule when the amplifier following the transformer is designed by a third party for a particular signal voltage - usually about 5mV.