Components

Stage Tec’s patented TrueMatch technology enables optimum audio quality to be achieved. The analogue input stage is an integral part of the 32-bit converter. This architecture confers many benefits since all signal processes such as subsonic filtering, phase inversion, clip limiting, and gain adjustment are performed entirely in the digital domain. All of these digital functions can be applied with independent settings for each of the four splitter outputs on each of the eight channels.

The XMIC+ includes a detector circuit, which automatically mutes the audio whenever phantom-powered condenser microphones are connected or disconnected.

The input stage features a noise level of –132,5 dBu (at 0-ohm impedance) and can accept signal levels of up to 24 dBu, thus providing an overall dynamic range of 156,5 dB. In theory, this dynamic range could have been achieved using only 27 bits. One advantage of Stage Tec’s strategy of using the 32-bit conversion method on the XMIC+ board is that the intrinsic noise acts as a »natural« dither, which moves the quantisation noise far below the noise level of the connected source.

XMIC+ microphone A/D converter features in brief:

• The TrueMatch converter dynamic range exceeds the usable dynamic range of any conventional microphone.
• Due to the generous 32-bit resolution, the microphone’s overall dynamic range, from the capsule’s intrinsic noise level to the threshold of clipping, can be transferred without gain switching.
• Featuring a maximum input level of 24 dBu (at 0-dB gain), it is almost impossible to overdrive the converter. 24 dBu is also the maximum output level of standard studio devices. As the XMIC+ board can handle such levels without any problem, it can also be used as line-level input board.
• Even low levels are digitised with high resolution.
• The XMIC+ outputs a 24 bit signal to the Nexus router. 24 bits are capable of representing a theoretical dynamic range of 144.5 dB, which is at least 10 dB more than conventional microphones can produce. The XMIC+ digital gain adjustment determines where the 24 bit output „window“ is positioned in the 32 bit output of the converter stage. No dithering or noise shaping is applied since the microphone self-noise performs the same function more naturally.
• Gain and signal-processing takes place in the digital domain after the A/D conversion, thus having minimal effect on signal quality. The settings merely tweak the signal level to make it suitable for subsequent processing and recording. When the board is combined with an AURUS or CANTUS system the only level changes necessary are when signals are summed onto mix busses and to accommodate recording devices. Thanks to 40-bit floating point processing the AURUS / CANTUS DSP engine maintains the outstanding dynamics from the XMIC + throughout the signal chain.

Target Formats

The NEXUS system is adaptable to suit all standard digital and analogue target formats. Internally the system always uses 24-bit resolution, which can be down-converted, for example, to 16 bit for CD productions.
When using this procedure it is important to realise that the dynamic range will be significantly downgraded. Only 96 dB of dynamic range is available when using 16 bit. Working with NEXUS and especially when using the XMIC+ board, it is recommended that the conversion to 16 bit is only undertaken at the end of a production after all recording and mixing has taken place.
With this strategy, the recording process is much easier and the signal quality much higher. In fact, the signal fed from the microphone can be recorded with no extra processing. Fine gain adjustment, equalisation, and subsonic filtering can be shifted to the post-recording stage. Digital clipping spoiling recordings is now a thing of the past, making the XMIC+ perfect for live recording, live interviews and reports, and theatres — not to mention OB vehicles.

Transformer-Insulated Input Stage

Thanks to its innovative and sophisticated circuit design, the XMIC+ board features transformer-isolated balanced inputs that avoid the disadvantages of conventional solutions.
Compared with conventional circuits based on input transformers or on electronic balancing, XMIC+ boards provide the following benefits:

• Insusceptibility to magnetic fields
• Low THD&N. at both low and high levels and even at low frequencies
• Low input impedance
• Excellent balancing
• True galvanic isolation

Limiter

To avoid clipping of amplified signals, a limiter is provided that can be enabled as required. This ultra-fast limiter (attack time: 0 samples) only responds at a level of -0.06 dBFS, so impacts on audio signals are impossible at normal levels.

Subsonic Filter

The subsonic filter can be adjusted up to 155 Hz in 5-Hz steps. Currently, the control software supports settings of 20, 40, 60, and 80 Hz.

Microphone Splitter Amplifier

The XMIC+ board can either be used in the traditional manner with a single output, or as a splitter amplifier with up to four independent outputs per microphone input.
The splitter-amplifier functionality is available only on NEXUS systems running Matrix 5 software or later. The NEXUS system is configured in such a way that each user can see and access only one of the four splitter outputs.
Each splitter output is controlled independently: the other outputs are never affected.
For phantom power, the software uses an »OR« function: microphones are supplied with phantom power only if at least one user has enabled it.
The microphone splitter amplifier is available as an option.

Variant Types

The following A/D converters can be supplied for the AURUS, CANTUS, and NEXUS systems:

• XAD+ (8-channel version, 24-dBu input level)
• HXAD (8 stereo channels, 15-dBu max input level)

RJ45-XLR adapter

The RJ45-ADP adapter PCB is available for connecting signal sources requiring XLR1 ports. This allows low-cost connecting lines to be specified using S/STP multicore cable.

Operating Notes

The NEXUS XMIC+ board features fully galvanically isolated inputs acting like ideal audio transformers. These inputs are isolated and can be used for balanced floating sources, actively balanced sources, as well as for unbalanced sources. Active is used in some sections of the industry in this context but runs the risk of confusion with electronic versus transformer balancing.
External conversion of signals from unbalanced to balanced is not necessary. Thus, the inputs can even offer DI box functionality.

With microphone inputs, any upstream electronic components (passive splitters, active splitter amplifiers, and passive as well as active DI boxes) introduce significant signal degradations such as harmonic distortion, noise, and hum into the transmission chain. Some active DI boxes continue to draw phantom-power current until the collapsing voltage level of the phantom power corresponds to the supply voltage of the DI box. This can trigger the phantom-power anti-surge fuse.

Stage Tec recommends that external DI boxes should not be placed ahead of microphone inputs as this may distort the signal or can even render it useless. Experience has shown that the sound quality is significantly higher without the use of an additional external DI box. All Stage Tec inputs already include DI functionality.

Gain adjustment is not required as the XMIC+ inputs handle levels of up to 24 dBu (approx. 12 V RMS).
A simple ¼" Jack to XLR adapter is entirely sufficient. On request, all XLR input ports can be equipped with a combo jack accepting both XLR and ¼" Jacks before shipment.

Wiring Diagram:

Leave pin 1 not connected for ground-lifting.

If an earth connection is required, the shield of the ¼" plug needs to be wired to pin 1 of the XLR plug. (In this case, there’s a risk of an earth loop!)

With the XRL-port version, only the ports of channels 1…4 are made available on the front panel. The ports of channels 5…8 are made available on an additional 8-HP front panel that is connected to the board via an internal RJ45 line.