Sennheiser MKH 416 The MKH Story - Page 2

The MKH Story Floating output and less interference due to AB powering In 1962 Sennheiser launched the omnidirectional MKH 104 as the first RF condenser microphone, which was soon followed by the cardioid MKH 404. These microphones had unbalanced signal outputs. At about the same time 12 V AB powering ("Tonader" T-powering, sometimes called T12 powering) was introduced. It provided a balanced output and shared the two cable leads for both signal conduction and powering. Blocking capacitors prevented the DC from getting into the audio circuit. The screen was not used for the current flow. Now condenser microphones could use the same cables as dynamic microphones. The MKH microphones with AB-powering were given the model code '5' (MKH 105, MKH 405 etc.). Later a 'T' was added (MKH 105 T etc.) according to the new AB powering standard. Sennheiser's first RF condenser microphone, the omnidirectional MKH 104 Although the capsule was connected to the microphone case and thus grounded, the electrical circuit was totally floating. The insulation was provided by separate windings of the RF coils. Thus AB powering in combination with the RF technique featured a transformerless balanced and floating output inherent in the design. From the beginning all MKH microphones were transformerless. This not only reduced the size of the electronics but also avoided signal distortion caused by transformers. Due to its interference-free performance, AB powering was recommended by the German Institut für Rundfunktechnik (IRT). Extended bass and less noise with smaller capsules The output impedance of the RF circuit is low and independent of the audio frequency. Therefore the inherent noise is low and nearly white, almost without flicker noise. These properties enable frequency response correction (equalising) without causing annoying noise. For instance the response of pressure-gradient microphones can be extended at low frequencies. Thus the bass response of small diaphragm capsules can be improved to an extent comparable with much larger capsules. But small capsules feature better directional characteristics at high frequencies as the onset of the pressure build-up effect is shifted towards higher frequencies. Frequency response corrections are also feasible at high frequencies, thus acoustic resonators - as used in many AF condenser microphones - can be avoided. This improves the impulse response and prevents tonal colourations. The phase response is not affected by this means because linearisation of the frequency response also corrects the phase response. This is valid for both electrical and acoustical minimum phase networks. So MKH microphones not only have a flat frequency response, they also have a linear phase response. Electrical linearization of the frequency response was utilised from the very start to improve the properties of the MKH microphones, and it was also beneficial for improving the noise characteristics. The theory was as follows: Each microphone capsule incorporates acoustic resistances for forming the frequency response and the directional characteristics. Equalising the frequency response by pure acoustical means requires quite large resistances that cause additional noise like electrical resistances. So, by reducing the acoustic resistances, the noise floor can also be reduced. This also improves the matching of the transducer to the sound field and increases its sensitivity. Furthermore, due to an appropriate acoustic design, the transducer sensitivity can be increased, especially between 2 kHz and 8 kHz where human hearing is most sensitive to noise. The higher capsule output also reduces the contribution of the amplifier noise. These effects support each other so that this 'low impedance design' improves the noise performance of the microphone considerably. The frequency response caused by this 'physiological' optimisation is no longer flat but can easily be corrected electronically. Due to this design, even the first MKH microphones had an extraordinary low inherent noise performance. An added bonus in this design is that it enables the designer to achieve a polar pattern closer to the theoretical with less off-axis anomalies. The directional performance can be designed nearly independently of the frequency response because the latter can be corrected electronically. Page 2