Calculation MC transformer: The ideal solution for adapting MC pickups to MM phono amplifiers
Introduction
MC (moving coil) cartridges have a special place in the world of vinyl playback because they offer a finer and more detailed reproduction than MM (moving magnet) cartridges. However, MC cartridges have the disadvantage of providing a very low output voltage, usually in the range of 0.2 to 0.5 mV. Since conventional phono amplifiers designed for MM cartridges require an input sensitivity of around 6 to 8 mV, additional amplification is necessary. This is where MC transformers come in. They help to raise the low voltage of an MC cartridge to a suitable level while also matching the impedance.
The history of MC transformers
MC transformers have a long history that is closely linked to the development of MC cartridges. When the first high-quality MC cartridges were developed in the 1950s and 1960s, it quickly became apparent that the low output levels were a hindrance. Since high-gain transistor amplifiers were not yet widely available at the time, many engineers resorted to using transformers to increase the voltage of the cartridge. These transformers, now known as MC transformers, offered a passive and efficient solution for impedance matching and amplification.
MC transformers fell into the background for a while in the 1970s and 1980s when transistorized phono preamps became more and more popular. Today, however, they are experiencing a renaissance as many vinyl enthusiasts appreciate their sonic advantages. MC transformers also offer passive and low-distortion amplification.
How an MC transformer works
An MC transformer works on the basis of the electromagnetic principle. It consists of two coils: a primary coil that is connected to the MC cartridge and a secondary coil that is connected to the MM phono amplifier. The number of turns of the primary and secondary coils determines the transformation ratio of the transformer, which amplifies the output voltage of the cartridge.
The basic idea is simple: a low voltage at the input (from the MC cartridge) produces a proportionally higher voltage at the output (to the MM amplifier), depending on the gear ratio. For example, if the gear ratio is 1:20, the voltage is increased by a factor of 20. An MC cartridge with an output voltage of 0.4 mV (like the Koetsu Black Goldline) is thus boosted to 8 mV, which is in the optimal range for the MM phono amplifier.
The optimal gear ratio for different MC pickups
The transformation ratio of an MC transformer must be carefully matched to the specific MC cartridge, based in particular on its output voltage and internal resistance. For the Koetsu Black Goldline, which has an output voltage of 0.4 mV and a coil impedance of 5 ohms, the transformation ratio must be chosen to raise the voltage to the required level of about 6 to 8 mV for the MM phono amplifier.
- Low output level (< 0.3 mV): MC cartridges with very low output levels require higher gear ratios, often in the range of 1:20 to 1:30, to raise the voltage to the required level for the MM phono amplifier.
- Medium output level (0.3 - 0.5 mV): For pickups in this range, gear ratios of 1:10 to 1:20 are suitable.
- High output level (> 0.5 mV): Pickups with higher output levels require lower gear ratios, usually between 1:5 and 1:10, to bring the voltage to the desired level.
Calculation example for the optimal gear ratio
Let's consider the Koetsu Black Goldline MC cartridge, which has an output voltage of 0.4 mV. To bring this voltage to the typical range of about 6 to 8 mV for the MM amplifier, the optimal gear ratio is calculated as follows:
The next best gear ratio would therefore be 1:15. In this case, the output voltage of the MC cartridge would be increased to 6 mV, which is ideal for an MM phono amplifier.
However, if a gear ratio of 1:20 were chosen, the result would be:
A transformation ratio of 1:10, on the other hand, would result in an output voltage of 4 mV:
The gear ratios of 1:15 - 1:20 from our examples are therefore to be preferred.
Calculating the connection impedance of a transformer
A crucial aspect when selecting and adjusting an MC transformer is the connection impedance. This must match the impedance of the MC pickup to ensure optimal signal transmission and sound quality. Ideally, the impedance that the MC pickup "sees" should be between 10 and 20 times the internal resistance of the pickup. For the Koetsu Black Goldline, which has an internal resistance of 5 ohms, the effective impedance should ideally be around 100 ohms.
The connection impedance of the MC cartridge is determined by the secondary side of the transformer and the input impedance of the MM phono amplifier. Since MM amplifiers typically have an input impedance of 47 kOhm, this impedance acts back on the primary side of the transformer, influenced by the transformation ratio of the transformer.
The effective impedance that the MC cartridge “sees” can be calculated using the following formula:
- : The effective connection impedance that the MC cartridge sees
- : The input impedance of the MM amplifier (typically 47 kOhm)
- Transformation ratio: The ratio between the turns of the secondary and primary coils of the transformer
Example: A transformer with a transformation ratio of 1:20 and an MM input impedance of 47 kOhm results in an effective impedance for the MC cartridge of:
This means that the MC cartridge "sees" an impedance of around 117.5 ohms. This is close to the optimal range for the Koetsu Black Goldline, which should see around 100 ohms. A gear ratio of 1:20 would therefore be a good choice to ensure ideal impedance matching and thus optimal sound quality.
conclusion
MC transformers are an elegant and sonically excellent solution for connecting MC pickups to MM phono amplifiers, provided the connection impedance and transmission ratio are consistent. Especially in today's world, where analogue music lovers have the highest demands on playback quality, MC transformers offer a sonically and musically appealing way to get the best out of a high-quality MC pickup.