What types of audio transformers exist?
There are two basic types of audio transformers with each having multiple functions:
- Step-up / Step-down 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
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Step-up / Step-down transformer
In a step-up / step-down 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 step-up 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 step-up 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 / step-up combo. Most of the
time, if the sound isn't good it's because of a compatibility problem.
If a step-up 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 step-up, 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 step-up modifies the impedance of the output by a factor equal to the square of
the amplification ratio. This is known as "natural impedance".
A step-up 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=47000/102).
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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 step-up 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
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Where: |
N1: |
number of turns of the primary coil |
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N2: |
number of turns of the secondary coil |
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V1: |
input voltage of the step-up |
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V2: |
output voltage of the step-up |
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|
x: |
amplification ratio |
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and |
Z2/Z1 = N22/N12 = x2 |
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Where: |
Z1: |
impedance of the primary coil |
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Z2: |
impedance of the secondary coil |
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R: |
phonostage load |
Never try to check the windings with an ordinary multimeter.
The DC-current of the meter will magnetise the core!
Another important point:
the grounding of the step-up. If you're having grounding problems, or shielding
problems, you'll probably have "hum".
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Which Step-Up Transformer for my cartridge?
If you're looking for a step-up you can buy second hand step-up transformers, brand
new or build them yourself using parts from manufacturers like Sowter,
Lundhal, Hashimoto,
Amplino, Jensen..
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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] |
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Manufacturer |
Model |
Gain (db) |
X Factor |
Natural Impedance |
Recommended Impedance |
|
Ortofon |
T5 |
26 |
20,0 |
118,1 |
3-40 ohms |
|
|
T10 |
32 |
39,8 |
29,7 |
2-4 ohms |
|
|
T10 MK2 |
28 |
25,1 |
74,5 |
2-6 ohms |
|
|
T20 |
32 |
39,8 |
29,7 |
2-4 ohms |
|
|
T20MKII |
28 |
25,1 |
74,5 |
2-6 ohms |
|
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SPU-T100 |
26 |
20,0 |
118,1 |
1-6 ohms |
|
|
T1000 |
26 |
20,0 |
118,1 |
2-6 ohms |
|
|
T2000 |
35 |
56,2 |
14,9 |
3 |
|
|
T3000 |
30 |
31,6 |
47,0 |
2-10 ohms |
|
Fidelity Research |
FRT-4 |
31 |
35,5 |
37,3 |
3 |
|
|
|
26 |
20,0 |
118,1 |
10 |
|
|
|
25 |
17,8 |
148,6 |
30 |
|
|
|
20 |
10,0 |
470,0 |
100 |
|
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FR XF-1 |
30 |
31,6 |
47,0 |
4-18 ohms |
|
|
FRT-3 |
26 |
20,0 |
118,1 |
30 |
|
|
|
31 |
35,5 |
37,3 |
10 |
|
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XG5 |
34 |
50,1 |
18,7 |
< 3 ohms |
|
|
|
26 |
20,0 |
118,1 |
3-18 ohms |
|
|
|
22 |
12,6 |
296,5 |
18-40 |
|
|
X1-M |
30 |
31,6 |
47,0 |
4-18 ohms |
|
|
X1-H |
25 |
17,8 |
148,6 |
19-40 ohms |
|
|
X1-L |
36 |
63,1 |
11,8 |
3 |
|
Denon |
AU 320 |
31,1 |
36 |
36 |
3 |
|
|
|
20,0 |
10 |
470 |
40 |
|
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AU 340 |
30,4 |
33 |
43 |
3 |
|
|
|
20,0 |
10 |
470 |
40 |
|
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AU310 |
20,0 |
10 |
470 |
40 |
|
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AUS1 |
22,3 |
13 |
278 |
3-40 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 |
|
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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 |
2-10 ohms |
|
Bryston |
TF1 |
22,5 |
13,3 |
264,3 |
5-35 ohms |
|
|
|
16,5 |
6,7 |
1052,2 |
40-250 ohms |
|
Nakamichi |
MCB100 |
26,0 |
20 |
117,5 |
2-20 ohms |
|
Sony |
HA-T110 |
26 |
20 |
117,5 |
3 - 40 ohms |
|
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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
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E6: 20% resistors |
E12: 10% resistors |
E24: 5% resistors |
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E48: 2% resistors |
E96: 1% resistors |
E192: 0.5%, 0.25%, 0.1% resistors |
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Search closest standard resistor value