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dinsdag 2 september 2014
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  Basics about Step-Up Transformers




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:
  1. Step-up / Step-down transformers
    • Signal level compatibility or matching
    • Impedance compatibility or matching
  2. Unity 1:1 transformers
    • DC blocking
    • Radio Frequency Interference (RFI) blocking
    • Ground lift and device isolation
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).



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


Where: N1: number of turns of the primary coil
  N2: number of turns of the secondary coil
  V1: input voltage of the step-up
  V2: output voltage of the step-up
  x: amplification ratio
and Z2/Z1 = N22/N12 = x2
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 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".


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..


Step-Up Transformers Specifications

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 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
  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
  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
  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
  AU 340 30,4 33 43 3
    20,0 10 470 40
  AU310 20,0 10 470 40
  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
  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


  Calculate min. Gain



MC output mV
MM input mV
X Factor
Gain dB




  Calculate Loading...

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 5 has a natural impedance and load impedance of 1880 ohms

(if your Moving Magnet input impedance is 47 kohms then:

47 kohms / 52) = 1880 ohms

Standard step-up ratios are

  • 1:5 (14dB)
  • 1:10 (20dB)
  • 1:20 (26dB)
  • 1:30 (29.5dB)
To find out the loading your cartridge sees, use the following calculator.

Phonostage: kOhms
Turns Ratio: 1:
Gain = dB
Loading = ohms



  Resistor tuning

Resistor tuning is a way to modify (to lower) the reflected load impedance directly linked to the gain of your step-up transformer.

You can lower the load impedance by adding a resistor between the + and the - of the RCA plug (out of the step-up).
Suppose you have a 1:10 gain step-up. Your actual load impedance is 470 ohms (natural impedance). You would like to test a load impedance of 100 ohms.

By adding a resistor, you're modifying the impedance of the Moving Magnet input (actually equal to 47 000 ohms - standard).

To have a load impedance of 100 ohms, you need to have an Moving Magnet input impedance of 10000 ohms

Rload = 1/(1/10000-1/47000) = 12.7 kOhms

What resistor should you put in parallel with the 47k phonostage
to change the loading to a desired value?

Note, you can only go lower in value, not higher (negative answer means unrealizable).

Phonostage: kOhms
Desired Loading: ohms
Turns Ratio: 1:
 
Gain = dB
Resistor = 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


  Search closest standard resistor value

  • E6: 20% resistors
  • E12: 10% resistors
  • E24: 5% resistors
  • E48: 2% resistors
  • E96: 1% resistors
  • E192: 0.5%, 0.25%, 0.1% resistors
  • Fill the desired value in, the closest standard value are shown in the different E serials: ohms 

    EIA Serie closest value wrongly, compared
    with theoretical value
    real value
    E6
    E12
    E24
    E48
    E96
    E192



    Links

    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 MC-step up transformer