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 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 step-up 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.

cartridge_loadingOne feature of the 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 and the LL1678. 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. If you want to calculate loading values yourself, send an e-mail to kevin@kandkaudio.com and I will e-mail in return an Excel spreadsheet program that will allow you to do so.

LL9206

Desired Load Impedance
(ohms)
Step-Up 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)
Step-Up 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)
Step-Up 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)
Step-Up 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)
Step-Up 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)
Step-Up 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)
Step-Up 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)
Step-Up 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)
Step-Up 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)
Step-Up 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)
Step-Up 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

LL1941/LL1943

Desired Load Impedance
(ohms)
Step-Up Gain
Phono Preamp
Input Impedance
Parallel Resistor
Required (ohms)
20 16 47K 6.54K
40 16 47K 14.1K
60 16 47K 24.2K
80 16 47K 38.3K
100 16 47K 59.3K
120 16 47K 94.1K
140 16 47K 162K
160 16 47K 361K

Desired Load Impedance
(ohms)
Step-Up Gain
Phono Preamp
Input Impedance
Parallel Resistor
Required (ohms)
5 32 47K 5
10 32 47K 10
15 32 47K 15
20 32 47K 20
25 32 47K 25
30 32 47K 30
35 32 47K 35
40 32 47K 40

LL1681

Desired Load Impedance
(ohms)
Step-Up 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)
Step-Up 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