You do not necessarily need a transformer. There are also head amps, or pre-preamps, that will provide the necessary gain to allow an MM phono stage to work with a low output MC cart. Many hear will tell you that there are inherent issues iwth a step-up transformer that would point you to an active gain stage (aka head amp, aka pre-preamp). whatever you get should have provisions for optimizing the loading to get the most of your new cart. And if your physical space requires long cabling for the xformer, that is also a factor, as the multiplication factor also "applies" to the cables.
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I think you will not find a consensus on which way to go. Some people claim SU transformers cause dynamic compression and others praise them because of their low noise and tonal neutrality. The BENT AUDIO TX-103 step-up device as described by Arthur Salvatore here gets his highest praise. It uses the Stevens and Billings TX-103 transformer. But this gets low marks from Michael Elliot, the designer of the famed Counterpoint SA9/SA11 phono/line stages of the late 80s/early 90s. He is in the process of designing a new preamp as detailed here. If you scroll to the bottom of this report, his findings on the TX-103 indicate a rather dull presentation. So there is not even a consensus on the performance of this one SU transformer.
One additional problem with SU devices, unless they are embedded within the preamp itself such as is the case with the Elliot designs, is that yet another pair of ICs is required. This too can be expensive and can also result in much of the magic lost that would have been attained until that extra cable was added.
And then there's the all solid state or all tube phono stage followers. Again, it's the ultra low noise vs. harmonic textures, decays, etc.
I have had very good results with all of these but currently an all tubed phono stage is providing me with the most 3-dimensional sound I have had in my phono setup. At very high volume, the little bit of noise (on the softest of passages) is easy to ignore because of these other strengths. You have to try some products of each of these designs that you can afford and go with the one that brings you the closest to the music.
I use a Koetsu Rosewood Signature and I have found the 1k load to be ideal. Anything lower and too much is lost in the upper frequencies.
I use a pair of Ortofon T-5 mini transformers which bump my preamps 40db up to 66db which is perfect for the 0.26mV output of the Dynavector Karat 17D2 Mk2. You can find a gain calculator on KAB's site http://www.kabusa.com/ on the home page choose Phono Preamps from the yelow buttons at the top, once there select Gain Computer from the list on the right. Kevin has posted a lot of information on his site and is a great resource. Enjoy
I would agree with Jafox that you will probably not get any general concensus on this. That being said, I would not want to be shelling out big money (ie. $700-$1000) on tranformer(s)/stepups realizing you are probably going to have to do another high quality interconnect as well. Do a search at Vinyl Asylum and perhaps post there on cost effective transformers such as the Cinemag if you have a decent extra IC hanging around.
Once you get into spending $700-$1000, I'd want to be very sure that the step-up/CJ combination was going to be extremely good and for that to happen you're going to have to have: a) great step up b) great phono stage in your CJ c) great extra interconnect and d) synergy with all the above. I'd be looking at some stand alone phono stages in that price range (and obviously trying to audition them in your system).
The stand alone phono stage will not only grossly simplify things, if you look at the right stages you will have greater flexibility (at least in terms of ease of use or "plug & play") down the road should you change cartridges again.