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What is the difference between plus 4 or minus 10 inputs?

Audio line levels are referenced at +4 or -10 db. +4 is more common among high end studio equipment, -10 is found in high end studio equipment, and is also the standard for consumer electronics. The difference is the amount of gain. The idea is that the higher the signal gain, the lower the noise floor. Modern gear is much better quality than a few years ago, so noise is not as much a problem as it used to be. If you send a +4db signal into a -10 db input, you can overload the -10 db input with too much gain. This will cause distortion. The solution to this is to carefully monitor the signal. IIf the gain is too hot, you will need to attenuate the signal. Attenuation can be achieved with a simple volume control. Use a channel from a mixer, a pre amp, or even a compressor to do this. Likewise, a -10 db signal going into +4 db inputs will not have enough gain, and there could be noise introduced (the 2408 has -10 and +4 db outputs). If you must send a -10 db signal to a +4 db input, it's a good idea to add gain to the signal to make the levels match. Once again, a mixer channel or preamp will do the job.