Phono MM/MC

Designing an amplifier for signals from a record player is probably the most demanding task in analog audio technology. In the extreme case, for output at studio level of +21dBu, a total amplification of 73dB occurs, even reaching 93dB for MC; 57dB or 77dB of this occurs in the phono module. This means that in the extreme case (MC and +10dB pressed), the input signal is amplified by a factor of about 45000 (!). In the simplest case (MM and normal output level), this is still about a factor of 1400. The signal voltages coming from the cartridge of the record player are really minimal, especially with MC systems.

The first challenge is to amplify only the useful signal and not any interference. But because the useful signal is so small, any kind of interference quickly becomes problematic. In addition, the inherent noise of the amplifier must be low. Both of these factors speak in favour of maintaining our fundamentally balanced signal routing, even in the phono module, and thus lead directly to the decision in favour of input transformers. Since a transformer is always a perfect “differential amplifier”, and this in contrast to many electronic circuits even in case of high asymmetry, it provides a high degree of noise suppression at the input. For MC amplifiers, transformers are not uncommon and these also provide the first 20dB of gain, which is approximately the usual difference between MM and MC systems. As far as we know, however, the idea of using input transformers even for MM is completely new. Because the source impedance of MM is much higher than that of MC, here no up-conversion takes place; the signal passes through one-to-one.

In both cases, we get the balanced signal needed for further processing very elegantly, even if the record player is connected unbalanced via RCA. However, almost all MC systems and also some MM systems can also be connected balanced via XLR. This is highly recommended!

The input transformers, including the corresponding matching elements for capacitance (MM) and impedance (MC), are mounted on small plug-in modules. Thus, it is possible to switch between MC and MM by exchanging the transformers.

The following stages are then identical for both variants, MM and MC.

Since the widely used active RIAA equalisation has decisive disadvantages in terms of dynamic behaviour and only measures better than it sounds, we rely on a classic passive RIAA equalisation in several stages that are separated by linear amplifier stages. This provides inherent noise far below that of the medium record and at the same time a great speed to optimally follow the dynamic events in the music.