Moving coil

The moving coil (engl. Moving coil , abbr. MC) is a construction of a magnetic pickup for record reproduction , wherein the coils are fixedly connected to the stylus. In contrast, the magnets are fixedly mounted in the cartridge housing.

As a result, compared to moving magnet or moving iron systems, the moving masses are reduced, resulting in a more dynamic sound .

function

When the record is played, the miniature coil in the pickup experiences a deflection that is proportional to the movement of the needle. According to the law of induction, a voltage is induced in the coil. This is then mostly processed outside the turntable with an equalizer preamplifier so that a loudspeaker can be controlled via an output stage .

Output voltage

The coils of moving coil scanning systems contain microscopic coils (wound under the microscope) with only a few turns. The advantage of a comparatively extremely low mass is offset by the disadvantage of a very low induced voltage. Depending on the design, MC scanners only provide a few 100 microvolt output voltage with very low electrical resistance in the single-digit ohm range.

Conventional phono inputs designed for systems with moving magnets require an input level of 2 to 5 millivolts at 47 kOhm.

If moving coil scanning systems are connected without adaptation measures, a completely inadequate control of the amplifier is to be expected due to the gross mismatch.

Since a four-pole adapter (initially regardless of whether it is an active or passive circuit) only has one input voltage available, which is usually available on receiver antennas, and the frequency range to be processed covers several decades, the noise behavior of this four-pole has a significant impact Importance to.

Matching transformer, preamplifier

Because of the input voltage requirements of conventional phono inputs of audio amplifiers and the very low output voltage of the MC scanning systems, the quadrupole adapter, regardless of whether it is a (passive) transformer or an (active) amplifier, must increase the output voltage of the scanner by about cause 10 to 20 times that.

The higher quality solution is the step up transformer. Despite its complex structure (extremely low magnetic leakage flux for a high ratio between the upper and lower limit frequency at a given transmission ratio) and its sensitivity to ubiquitous external magnetic fields (complex magnetic shielding), it is the lower-noise and linear component. However, these ratios are reflected in a high price.

Pre-preamplifiers require careful optimization to achieve the highest level of low noise, high linearity and insensitivity to magnetic and electrical interference. When using electron tubes, the selection of tube types with extremely low microphony must also be taken into account. Semiconductor circuits have proven to be the cheapest alternative, moreover with better quality properties than a more expensive tube amplifier.