What types of audio transformers exist?
There are two basic types of audio transformers with each having multiple functions:
 Stepup / Stepdown transformers
 Signal level compatibility or matching
 Impedance compatibility or matching
 Unity 1:1 transformers
 DC blocking
 Radio Frequency Interference (RFI) blocking
 Ground lift and device isolation

Stepup / Stepdown transformer
In 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 transformer
Often 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 Cartridges
The characteristics you should take care of for this kind of cartridge to be correctly
exploited by your system are:
 The output level, in millivolts
 The internal impedance, in ohms
 The load impedance, in ohms
The function of a stepup transformer is to raise the output of the MC cartridge
in order to be correctly handled by a Moving Magnet phono section.
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.
In a perfect transformer: N2/N1 = V2/V1 = x
Where: 
N1: 
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 
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 
Never try to check the windings with an ordinary multimeter.
The DCcurrent of the meter will magnetise the core!
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..

Data in bold type are manufacturers specifications, other data was recalculated:
X Factor 
= 
10^[( gain )/20] 
Gain in db 
= 
20*ln(X Factor)/ln(10) 
Natural impedance 
= 
47000/[(X Factor)^2] 
Manufacturer 
Model 
Gain (db) 
X Factor 
Natural Impedance 
Recommended Impedance 
Ortofon 
T5 
26 
20,0 
118,1 
340 ohms 

T10 
32 
39,8 
29,7 
24 ohms 

T10 MK2 
28 
25,1 
74,5 
26 ohms 

T20 
32 
39,8 
29,7 
24 ohms 

T20MKII 
28 
25,1 
74,5 
26 ohms 

SPUT100 
26 
20,0 
118,1 
16 ohms 

T1000 
26 
20,0 
118,1 
26 ohms 

T2000 
35 
56,2 
14,9 
3 

T3000 
30 
31,6 
47,0 
210 ohms 
Fidelity Research 
FRT4 
31 
35,5 
37,3 
3 


26 
20,0 
118,1 
10 


25 
17,8 
148,6 
30 


20 
10,0 
470,0 
100 

FR XF1 
30 
31,6 
47,0 
418 ohms 

FRT3 
26 
20,0 
118,1 
30 


31 
35,5 
37,3 
10 

XG5 
34 
50,1 
18,7 
< 3 ohms 


26 
20,0 
118,1 
318 ohms 


22 
12,6 
296,5 
1840 

X1M 
30 
31,6 
47,0 
418 ohms 

X1H 
25 
17,8 
148,6 
1940 ohms 

X1L 
36 
63,1 
11,8 
3 
Denon 
AU 320 
31,1 
36 
36 
3 


20,0 
10 
470 
40 

AU 340 
30,4 
33 
43 
3 


20,0 
10 
470 
40 

AU310 
20,0 
10 
470 
40 

AUS1 
22,3 
13 
278 
340 ohms 

AU300LC 
20,0 
10 
470 
40 
Audio Technica 
AT700T 
34 
50,1 
18,7 
3 


26 
20,0 
118,1 
20 


23 
14,1 
235,6 
40 
EAR 
MC4 
29,5 
30 
52,2 
3 


27,6 
24 
81,6 
6 


25,1 
18 
145,1 
12 


20,0 
10 
470,0 
40 

MC3 
29,5 
30 
52 
4 


26,0 
20 
118 
12 


20,0 
10 
470 
40 
Supex 
SDT 3300 
28,5 
26,6 
66,4 
210 ohms 
Bryston 
TF1 
22,5 
13,3 
264,3 
535 ohms 


16,5 
6,7 
1052,2 
40250 ohms 
Nakamichi 
MCB100 
26,0 
20 
117,5 
220 ohms 
Sony 
HAT110 
26 
20 
117,5 
3  40 ohms 


Example:
You have a cartride with 0.5 mV output and you would like to obtain 5 mV
in your Moving Magnet 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, you need an impedance of 4000 ohms on yor Moving
Magnet input (10^2*40).
The resistors must have a value equal to:
Rload = 1/(1/R1  1/R2)
Where
 R1 is the impedance you want for your Moving Magnet input
and
 R2 is the actual impedance of your Moving Magnet input
Rload = 1/(1/4000  1/47000) = 4372 ohms

E6: 20% resistors 
E12: 10% resistors 
E24: 5% resistors 
E48: 2% resistors 
E96: 1% resistors 
E192: 0.5%, 0.25%, 0.1% resistors 




