TASCAM Sound Recorder (Part 5)

Part 1, 2, 3, 4, 5

As a professional equipment, the TASCAM DR40-X recorder features XLR inputs.

These are in fact combo inputs which are compatible with standard XLR connectors and 6.35 mm jack connectors . More important than the hardware, the electronics behind the connectors are very important. Most audio devices rely on unbalanced connections: the audio signal travels through one single wire. To prevent external interference, this wire is insulated and wrapped in braided wires which act as a shield. Although this method is satisfactory for short wires and standard use of audio devices, this shielding method is insufficient. The solution lies in the use of two wires for transmitting the audio signal: both wires conduct the same signal except that one is the negate of the other. This method is called the balanced mode.

Let’s see how it works with the example below:

First we start from a pure sine wave. The signal is split in two parts with same amplitude but one is negated versus the other. As they travel through the (twisted) wires, they are exposed to an unexpected burst. When the signal reaches the end of the cable, both signals are distorted in the same way. When the signal is ultimately reconstructed by adding one signal and the negated other, the burst is canceled.

Quality has a price: this method requires one extra wire, special 3 pins connectors and additional electronics. Splitting the signal and negating one is plain easy. However, this requires that some electronics is embedded in the source device along with its power supply. To prevent the use of batteries in small equipment (e.g. microphones), audio engineers had a bright idea: use the signal cable to transport energy to the source device. This energy must be a stable DC current which create a steady signal offset that we can null by means of capacitors. This power supply is called the “phantom power” as it exists but is invisible to the final listener.

Next picture illustrates the principle of the phantom power:

From right to left, a DC regulator feeds both signal lines through Re0 and Re1. At the receiver stage, Rr0 and Rr1 collect the voltage and use it to power the pre-amplifier.

The phantom power supplies comply with the CEI 268-15A rules. P48 used 48 V biasing through 6.8 k Ohm resistors, P24 used 24 V biasing through 1.2 k Ohm resistors and P12 uses 12 V biasing through 680 Ohm resistors

Then comes the problem of connecting a standard microphone to the recorder. Most microphones are electret microphones that require some biasing to operate. This biasing must be supplied by the phantom power thanks to an adapter. I chose the Rode VXLR+ adapter which is affordable (~25€), well made and looks like it can do the job.

The VXLR+ contains a small PCB which obviously care about decoupling the audio signal (thanks to the large 50 V capacitors) and some components for biasing the microphone.

This device worked perfectly well with commercial products as well as with my custom microphones.

Please note that the cheaper Rode VXLR does not contain any electronics and acts as a simple hardware adapter as shown on the picture below:

Next is a set pictures illustrating the various pin-outs involved in audio connectors:

Pinout of XLR connectors, view from the protuberant pins (male connector):

Pinout of jack connectors (3.5 mm) are specified by their number of rings, using the initial “T” for tip, “R” for ring(s) and “S” for sleeve. Next connector is a TRRS type

Various combinations exist which use jack connectors:

  • OMTP standard: T=audio left, R1=audio right, R2=microphone, S=ground
  • CTIA standard (apple): T=audio left, R1=audio right, R2=ground, S=
    microphone
  • Mono microphone: T=microphone, S=ground
  • Stereo microphones: T=microphone left, R=microphone right, S=ground

HTH

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