Calculate min. Gain  Calculate Loading  Resistor tuning 
What types of audio transformers exist?There are two basic types of audio transformers with each having multiple functions:
and 
Stepup / Stepdown transformerIn a stepup / stepdown transformer, the primary and secondary have a different number of windings, thus they have different impedances. Different impedances cause the signal level to change as it goes through the transformer. If the secondary has a higher impedance (more windings) than the primary, the signal level at the secondary will be a higher voltage than at the primary. A transformer with multiple taps provides access to multiple impedances and to different signal gains or losses. Unity 1:1 transformerOften called an isolation transformer, it has the same number of windings on each coil. As the impedance is identical for the primary and secondary, the signal level does not change. A unity transformer allows an audio signal to pass unmodified from the primary to the secondary while blocking DC voltage and radio frequency interference (RFI). Also, since the primary and secondary are insulated from each other, a unity transformer will electrically isolate different pieces of equipment. This can solve hum problems by isolating ("lifting") the grounds of different devices. Moving Coil CartridgesThe characteristics you should take care of for this kind of cartridge to be correctly exploited by your system are:
At the same time, the stepup transformer adapts the impedance of the signal produced by the MC cartridge to an impedance that can be "seen" by your Moving Magnet input. For this reason, check the compatibility of your MC / stepup combo. Most of the time, if the sound isn't good it's because of a compatibility problem. If a stepup has a gain of 1:10, this means that the output of the cartridge will be multiplied by 10. If the output out of the cartridge is 0.3 mV, you'll have, after the stepup, 3 mV, perfect for a Moving Magnet input. The Moving Magnet input has its own impedance generally equal to 47 Kohms. The signal emitted by the MC cartridge must be in accordance to that 47 Kohms. A stepup modifies the impedance of the output by a factor equal to the square of the amplification ratio. This is known as "natural impedance". A stepup with an amplification ratio of 10 has a natural impedance and load impedance of 470 ohms (if your Moving Magnet input impedance is 47 kohms: 470 = 47000/10^{2}). 
For one channel, there are two coils: a primary (the 'input': for the signal emitted by the cartridge) and a secondary (the 'ouput': the amplified signal will go out to your Moving Magnet amplifier stage). You will see that there are less turns on the secondary than on the primary. This is not a strange idea from the designer of this scheme. Your amplification ratio is a direct result of the turns ratio (primary/secondary). This explains why, on a stepup that can be used with several internal impedances cartridges, the gain differs according to the internal impedance you're selecting. 
Where:  N1:/td>  number of turns of the primary coil 
N2:  number of turns of the secondary coil  
V1:  input voltage of the stepup  
V2:  output voltage of the stepup  
x:  amplification ratio/td>  
and  Z2/Z1 = N2^{2}/N1^{2} = x^{2}  
Where:  Z1:  impedance of the primary coil 
Z2:  impedance of the secondary coil  
R:  phonostage load 
Another important point:
the grounding of the stepup. If you're having grounding problems, or shielding problems, you'll probably have "hum". 
Which StepUp Transformer for my cartridge?If you're looking for a stepup you can buy second hand stepup transformers, brand new or build them yourself using parts from manufacturers like Sowter , Lundhal, Hashimoto, Amplino, Jensen 


Example:You have a Moving Coil Cartride with 0.5 mV output and you would like to obtain 5 mV in your Moving Magnet Phono Amplifier input, so you need a 1:10 transformer. You would like to have a load impedance of 40 ohms. With a gain of 1:10 ( 20dB ), you need an impedance of 4000 ohms on yor Phono Amplifier input ( Transformer Ratio^2 * Desired Loading = 10^2 * 40) The resistor must have a value equal to: Rload = 1 / (1 / Desired Loading  1 / Phonostage) Rload = 1/(1/4000  1/47000) = 4372 ohms 
Cartridge
loading by Jim Hagerman 
Jensen provides a lot of
information 
Sowter provides help to choose the correct
product you need
for your cartridge 
Lundahl
Transformers 
K&K Audio specialist in
Lundhal
transformers 
SACThailand SILK Supermalloy MCstep up transformer 