 
Moving Coil Cartridge Stepup Transformers
It is difficult to amplify the microvolt level signals
present at the output of most moving coil phono cartridges without adding
audible noise. With transistorbased
preamplifiers it is possible to do so using special techniques; it is even more
difficult to do with tubes. However, a stepup transformer can be used to increase, or in transformer lingo, "step up", the cartridge voltage admirably without adding any perceivable noise.
The moving coil cartridge is a truly low impedance voltage
generator. Because of this it is possible
to use a transformer to step up the voltage by factors of 10 or more times
without causing any impedance interface problems with conventional phono
preamplifiers. When a transformer is
used to step up a voltage, the characteristic cartridge impedance (AC
resistance) is also increased. The
mathematical relationship is such that a 10fold increase in voltage results in
a 100fold increase in impedance. In
general, the impedance transformation through a transformer is equal to the
square of the voltage transformation. It is important to take both of these parameters into account when
deciding what sort of an MC stepup transformer is required to make a given
cartridge work well with the phono preamp you have.
To make the theory real, let’s set up a hypothetical
example. Imagine an MC cartridge that
has a nominal 0.5 mV output and a phono preamplifier that has a 47K input
impedance. If we desire to increase the
output voltage of the MC cartridge to the “normal” 5 mV level associated with
most MM phono cartridges where the phono preamp gives acceptable noise
performance, then we would require a 10fold increase in signal level. To do this, we connect the primary coils on
the Lundahl LL9206 in the configuration that results in a gain of 10, or
expressed in dB, 20dB, and then wire it up to our preamp and enjoy some
tunes. (See the diagram below for an
illustration of the hypothetical connection described here) But before we drop the stylus onto the
record, we need to also consider the impedance implications of our gain
decision. Because the gain we chose was
10, we have an impedance transformation of 10 squared, or 100. So, the cartridge “sees” the 47K load
resistor at the input to the preamp through the transformer as being 470 ohms,
which is arrived at by dividing the resistor value, 47K, by 100, the impedance
ratio. Most MC cartridge manufacturers
provide a recommended range for load impedance (or resistance). Because this value is usually substantially
lower than 47K, the transformer is performing a valuable service on the
impedance front, as well. A typical MC
cartridge will sound somewhat “lightweight” when it is loaded too lightly (load
impedance is too high) and will sound somewhat dull when the load impedance is
too low. So, it’s important to get both
of these parameters, the stepup gain and the reflected load impedance, correct
to get the best sound. The gain you
choose generally has less effect on the sound quality than does the cartridge
loading. So it’s important to get the
gain in the “ballpark”, but then turn serious attention to adjusting the
cartridge loading to get good frequency balance as described above. The discussion below will guide your efforts
in this direction.
One feature of the diagram below deserves additional comment. The grounding arrangement around the transformer is appropriate for the vast majority of installations where the phono preamp has unbalanced inputs and the cabling from the turntable to the MC stepup transformer is two conductor wire terminated with an RCA plug. However, with the use of balanced cabling from the turntable, the ground connection between the primary and secondary sides of the transformer can be deleted. Further, for a phono preamp with balanced inputs, neither end of the secondary of the MC transformer would be connected to ground as shown.
You might now ask if when you need a gain of 10, are you then
“stuck” with a reflected 470ohm load for your cartridge, even if that’s not
optimum? The answer is that you aren’t
“stuck”. You can add resistance in
parallel with the customary 47K phono input resistor to lower that reflected
load impedance. Unhappily, to increase
the load impedance above 470 ohms, you either have to remove the 47K resistor
and replace it with a higher value resistor or you have to settle for a lower
stepup gain. Fortunately, it’s most
often true that the MC cartridges that have the lowest output levels also
require relatively low values of load impedance, so this is not usually a
problem.
For those of you who want to experiment with and optimize
your cartridge loading (recommended!), we have provided tables of parallel
resistance values that will help you achieve the best sound. As a starting point, I usually chose a load
impedance value of 10 times the cartridge impedance (sometimes referred to as
resistance in cartridge data sheets). The values given in the tables are referenced to the stepup gains
produced by the LL9206 and the LL1678. Note that these values don’t scale
precisely with the square of the stepup gain. The primary and secondary resistances of the transformer also figure
into the calculation. If you want to calculate
loading values yourself, send an email to kevin@kandkaudio.com
and I will email in return an Excel spreadsheet program that will allow you to
do so.
LL9206
Desired
Load Impedance
(ohms)

StepUp
Gain

Phono
Preamp
Input
Impedance

Parallel
Resistor
Required
(ohms)

1600

5

47K

377000

800

5

47K

40600

400

5

47K

15700

200

5

47K

7900

100

5

47K

4290

50

5

47K

3040

Desired
Load Impedance
(ohms)

StepUp
Gain

Phono
Preamp
Input
Impedance

Parallel
Resistor
Required
(ohms)

400

10

47K

377000

300

10

47K

98200

200

10

47K

40600

100

10

47K

15700

60

10

47K

9300

40

10

47K

6600

Desired
Load Impedance
(ohms)

StepUp
Gain

Phono
Preamp
Input
Impedance

Parallel
Resistor
Required
(ohms)

100

20

47K

377000

80

20

47K

134000

60

20

47K

62300

40

20

47K

31300

20

20

47K

14000

LL9226
Desired Load Impedance
(ohms)

StepUp Gain

Phono Preamp
Input Impedance

Parallel Resistor
Required (ohms)

1600

5

47K

307000

800

5

47K

37200

400

5

47K

14000

200

5

47K

6570

100

5

47K

3500

50

5

47K

2100

Desired Load Impedance
(ohms)

StepUp Gain

Phono Preamp
Input Impedance

Parallel Resistor
Required (ohms)

400

10

47K

307000

300

10

47K

89000

200

10

47K

37200

100

10

47K

14000

60

10

47K

7900

40

10

47K

5300

Desired Load Impedance
(ohms)

StepUp Gain

Phono Preamp
Input Impedance

Parallel Resistor
Required (ohms)

100

20

47K

307000

80

20

47K

108000

60

20

47K

52300

40

20

47K

26000

20

20

47K

10800

LL1678
Desired
Load Impedance
(ohms)

StepUp
Gain

Phono
Preamp
Input
Impedance

Parallel
Resistor
Required
(ohms)

600

8

47K

285000

450

8

47K

88900

300

8

47K

38400

200

8

47K

21500

150

8

47K

15300

75

8

47K

7870

Desired
Load Impedance
(ohms)

StepUp
Gain

Phono
Preamp
Input
Impedance

Parallel
Resistor
Required
(ohms)

150

16

47K

285000

125

16

47K

122000

100

16

47K

66500

80

16

47K

42900

60

16

47K

27400

40

16

47K

16400

Desired
Load Impedance
(ohms)

StepUp
Gain

Phono
Preamp
Input
Impedance

Parallel
Resistor
Required
(ohms)

40

32

47K

377000

30

32

47K

134000

20

32

47K

62300

10

32

47K

31300

LL1931/LL1933
Desired
Load Impedance
(ohms)

StepUp
Gain

Phono Preamp
Input Impedance

Parallel Resistor
Required (ohms)

600

8

47K

221000

400

8

47K

60400

300

8

47K

34000

200

8

47K

18200

100

8

47K

8060

80

8

47K

6340

40

8

47K

3240

20

8

47K

1780

Desired
Load Impedance
(ohms)

StepUp
Gain

Phono Preamp
Input Impedance

Parallel Resistor
Required (ohms)

160

16

47K

348000

140

16

47K

165000

120

16

47K

90900

100

16

47K

60400

80

16

47K

37400

60

16

47K

24300

40

16

47K

13700

20

16

47K

6340

LL1681
Desired Load Impedance
(ohms)

StepUp Gain

Phono Preamp
Input Impedance

Parallel Resistor
Required (ohms)

250

13

47K

1440000

200

13

47K

175000

150

13

47K

73000

100

13

47K

35300

75

13

47K

24100

50

13

47K

15600

Desired Load Impedance
(ohms)

StepUp Gain

Phono Preamp
Input Impedance

Parallel Resistor
Required (ohms)

60

26

47K

648000

40

26

47K

85300

30

26

47K

47200

20

26

47K

26100

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