Capsules/Preamps

While most Binaural heads use small 5mm omni capsules to accurately capture the ear canal the KU 100 uses a much larger 22 mm omni capsule. This gives it a much higher signal to noise ratio and allows for better loudspeaker reproduction. The downsides of this size come from fitment and smooth air flow from the much small ear canal affecting frequency response. To reproduce the KU 100 I used a stereo pair of MicParts (Roswell pro audio) SDC pencil condensers. These microphones are 22mm in diameter with an omni pickup pattern. and the internal preamp is based on the transformer less CM5C Schoeps circuit.

10 Db pad

The KU 100 has a single switch for a stereo 10Db pad. this allows for recording of higher signal sources without overloading the circuitry. there are a number of simple ways to create a 10Db pad. many condenser microphones use a resistor and capacitor to ground. some create a pad by connecting the phase flipped outputs using resistors. Thankfully in the build instructions for the SDC preamps it states that adding a capacitor in parallel with the preamp coming out off the capsule you can change the effective capacitance of the mic capsule. this results in lower signal reaching the preamp and creates a pad. according to the instructions a 150pF axial styrene capacitor will result in a ~10Db pad. for my build I was able to acquire 163pF Siemens axial styrene capacitors and a 100V rated switch to connect the grounded capacitors to the capsules signal wire. this results in ~10Db switchable stereo pad.

Pinnae/Ear Canals

For Binaural recording Neumanns research shows that having the full pinnae and and the first 5 mm of the ear canal modeled provided almost all of the spatial encoding information. having full ear canals does provide a more accurate Binaural recording but it substantially reduces the quality of loudspeaker reproduction. this downside would make the microphone much less versatile. Almost all commercially available binaural microphones use this methodology and only mold the first 5mm of the ear canal. The pinnae and ear canal are also responsible for the microphones pickup pattern and frequency response since the KU 100 and my recreation both have 22mm capsules which are much larger than the ear canal there is a substantial affect on the polar pattern and frequency response. According to Stephan Peus (The lead designer for the KU 100) Binaural microphones should be diffuse field equalized to achieve the most accurate capture of the room they are in. this was change between the KU 80 and the KU 81 which was achieved by adding a conical tube to expand the smaller diameter of the ear canal up to the larger diameter of the microphone capsule. the centre cone of this was made hollow to act as a Helmholtz resonator to help the frequency response of the microphone which was tuned based of of their finding with the protoype they made where they could tune the Helmholtz resonance. Since my head, ear shapes, and microphones parts are different from Neumanns I couldn’t perfectly recreate this however I was able to create an approximation which significantly improved the frequency response, sensitivity, and stereo imaging. In addition to this Stephan Peus notes that for the KU 81 they made a pair of perfectly symmetrical ear pinnae and canals to make the binaural head more universally accurate. To get the most accurate ear shape I could imagine found online models of ears based off of the IEC 60318 submission for artificial ear testing and used those to build my microphones ear assembly.

Frequency response and equalization

A binaural head should be diffuse field equalized (flat frequency response in a live room). since I do not have the resources to properly test and engineer mechanical solutions. I will be recreating approximations of Neuamnn’s designs and doing final adjustments using linear phase MS and stereo EQ digitally. This will allow me to tweak the heads response and get it as close as possible to the KU 100.

High Pass filters

the KU 100 has a switchable HPF of 12db/octave at 50Hz and 150Hz. this slope tells us that the HPF circuit being used is a second order circuit. looking at other Neumann microphone schematics they consistently use multiple active components to achieve a 2nd order circuit. because these microphones require phantom power you can’t use a first order RC or RL circuit because it will either block the phantom power or short it to ground. this meant that to implement any HPF’s I would need a complete understanding of the preamp circuitry and how to modify it to create the circuit. Because of this level of complexity and the time required the high pass filters were cut from the spec sheet to minimize complexity and meet the deadline.

Power

Condenser microphones require DC power to power the internal preamp and get signal from the capsule. normally this is achieved through 48V phantom power supplied form your board. some microphones (the KU 100) have internal batteries to supply power in remote locations. the KU is also unique in that it will accept power from an external power supply via a switch. Most microphones don’t actually require the full 48V to function. this is how battery packs work. almost all condensers will function with a nominal voltage of 9. the KU 100 uses 6 AA Batteries in series to achieve this voltage. the downside of using low voltage power is increased electrical noise and a reduction in maximum SPL. This battery pack supplies the power in an identical fashion to phantom power from a board. by using an RC circuit to block the voltage from going down the cable and supplying positive voltage to pins 2 and 3 of the microphone circuit you give it power. Because the voltage is lower you do have to adjust the values of the Resistor and capacitor of the RC circuit to minimize impedance issues. While this would have been relatively simple to implement do to space constraints and time the internal battery supply was cut from this project as well.

All sources and research information is provided at the bottom of this webpage.