Moving Coil Cartridge Step-up 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 transistor-based preamplifiers it is possible to do so using special techniques; it is even more difficult to do with tubes. However, a step-up 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 10-fold increase in voltage results in a 100-fold 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 step-up 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 10-fold increase in signal level. To do this, we connect the primary coils on the Lundahl LL9226 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 on the right 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.


One feature of the above diagram 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 step-up 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 step-up 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 step-up gains produced by the LL9206, LL9226, LL1678, LL1931/LL1933, LL1941/LL1943 and LL1681. Note that these values don’t scale precisely with the square of the step-up gain. The primary and secondary resistances of the transformer also figure into the calculation.
Desired Cartridge Load Impedance
(Ohms)
Desired Step-up Gain
Phono preamp input impedance
Required parallel resistance,
(Ohms)
1600
5
47K
374000
800
5
47K
40200
400
5
47K
15800
200
5
47K
7870
100
5
47K
4290
50
5
47K
3010
400
10
47K
374000
300
10
47K
97600
200
10
47K
40600
100
10
47K
15800
60
10
47K
9310
40
10
47K
6650
100
20
47K
374000
80
20
47K
133000
60
20
47K
62000
40
20
47K
31600
20
20
47K
14000
LL9206
Desired Cartridge Load Impedance
(Ohms)
Desired Step-up Gain
Phono preamp input impedance
Required parallel resistance
(Ohms)
1600
5
47K
309000
800
5
47K
37400
400
5
47K
14000
200
5
47K
6570
100
5
47K
3500
50
5
47K
2100
400
10
47K
309000
300
10
47K
88700
200
10
47K
37400
100
10
47K
14000
60
10
47K
7870
40
10
47K
5360
100
20
47K
309000
80
20
47K
107000
60
20
47K
52300
40
20
47K
26100
20
20
47K
10700
LL9226
Desired Cartridge Load Impedance
(Ohms)
Desired Step-up Gain
Phono preamp input impedance
Required parallel resistance
(Ohms)
600
8
47K
287000
450
8
47K
88700
300
8
47K
38300
200
8
47K
21500
150
8
47K
15400
75
8
47K
7870
150
16
47K
287000
125
16
47K
121000
100
16
47K
66500
80
16
47K
43000
60
16
47K
27400
40
16
47K
16500
40
32
47K
374000
30
32
47K
133000
20
32
47K
62000
10
32
47K
31600
LL1678
Desired Cartridge Load Impedance
(Ohms)
Desired Step-up Gain
Phono preamp input impedance
Required parallel resistance
(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
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
LL1931/LL1933
Desired Cartridge Load Impedance
(Ohms)
Desired Step-up Gain
Phono preamp input impedance
Required parallel resistance
(Ohms)
160
16
47K
360000
140
16
47K
162000
120
16
47K
93100
100
16
47K
59000
80
16
47K
38300
60
16
47K
24200
40
16
47K
14100
20
16
47K
6540
40
32
47K
360000
35
32
47K
162000
30
32
47K
94100
25
32
47K
59300
20
32
47K
38300
15
32
47K
24200
10
32
47K
14100
5
32
47K
64900
LL1941/LL1943
Desired Cartridge Load Impedance
(Ohms)
Desired Step-up Gain
Phono preamp input impedance
Required parallel resistance
(Ohms)
250
13
47K
1430000
200
13
47K
174000
150
13
47K
73400
100
13
47K
35700
75
13
47K
24100
50
13
47K
15600
60
26
47K
649000
40
26
47K
84500
30
26
47K
47000
20
26
47K
26100
LL1681
NOTE: Parallel resistor values have been 'standardized' to easily available values, we use Mouser as the supplier, so all indicated parallel resistor values should be available from Mouser.