Mixing Magic Part 4

This week we take a closer look at some of the more important features on a mixing desk to consider during the purchase process.

• Pre-Amp - Every desk needs a Pre-Amp section on each channel, this is the input to the desk and is controlled with a gain knob. High quality Pre-Amps are desirable, but a wide gain range is more important. Another useful feature in the Pre-Amp section is a PAD button which reduces the input signal by 20dB or more. The Pre-Amp section is also key in determining the Headroom a desk has. Lots of Headroom is desirable and is measured in the amount of signal a desk can handle above full volume or 0dB. This might sound a bit odd, most people assume that 'Full Volume' is the loudest a piece of equipment can go and that it can't go louder than that. Without getting too technical, here it is in layman's terms.
• 0dB is reference point that all audio equipment uses to determine the maximum level of a signal. In theory, if everything in the system is at 0dB then its all running at full power.
• Quite a few components in an audio system are capable of handling more than 0dB, this is a good thing, because sometimes things might spike up for a split second or maybe are turned up for a blazing guitar solo, you wouldn't want the whole system to melt down in a blaze of distortion every time the guitarist turns up.
• The more Headroom you have over and above 0dB the better. Some mixing desks only have 6dB of headroom, which is not much. Others go as high as 16 to 24dB above 0dB. This doesn't mean that your system will be any louder, it just means that it will cope if individual instruments suddenly get loud. Generally we use compressors and limiters to prevent the output signal from going over 0dB and damaging your amplifiers and speakers.
• Noise Floor  - The noise floor of a desk works in conjunction with the noise floor of your amplifiers. Noise is additive, so that the noise from your mixing desk, instruments and amplifiers is added together to get the total noise for a system. This is why there is no point investing in one high quality component when the rest are crap. The noise floor of a mixing desk should be at least -100dB or better (-105dB would be better), avoid desks with a noise floor in the 90's.

Connections - The industry has standardised on the XLR connector for audio. To save cost and space, some mixers use Jack inputs and outputs with TRS connectors instead of XLR connectors. Avoid mixers that use this, they are designed so that you can also use unbalanced Jack to Jack cables, which if you have read my previous Blog, you would never do would you? Stick to the tried and tested XLR connector pictured and you can't go wrong.

Powered or Un-Powered - Never buy a powered mixer, they are heavy and if the amplifier section in the mixing desk dies or no longer suits your setup, then you have a big heavy mixing desk that you have to cart around everywhere. The amplifiers that are built into mixing desks are rarely any good.

Reputable brands of mixing desk include Allen and Heath, Soundcraft, Midas, Yamaha and Digico