MSDI is an acronym for Microphone Simulated Direct Interface. Simply connect your amp's speaker output to the Input From Amplifier socket on the MSDI, connect your speaker to the Speaker/Load socket and take an XLR cable to your mixing desk or preamp. The DI signal has been shaped to simulate a mic'd up cabinet, and this means you can send your guitar and amp signal to a PA engineer, or even record without needing to use a mic. The MSDI employs passive circuitry, so there's no need for a power supply or a battery.
Since the MSDI's output reflects the volume setting of your amplifier, an onboard Output Level control helps tweak the level to stop your preamp overloading. A three-way tone switch offers a Reference setting as well as Hi Cut and Hi Boost, and there's even a GND (ground lift) switch to eliminate 50Hz hum from earth loops.
We tested using a Fender
Princeton and found no amplifier/speaker colouration whatsoever with the MSDI in line. We then recorded through the MSDI to compare the DI signal with the sound of the speaker and it sounded just like a well-mic'd studio amp, although a touch of spring reverb helped take away the dryness. No doubt, the MSDI gives a remarkably ‘natural' tone.
Devices of this sort are often a bit disappointing, but the MSDI surprised us. Regardless of whether the amp was set whisper-quiet or flat-out, the MSDI remained distortion-free and we had no interfacing problems whatsoever.