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I'm curious about audio mixer inputs and outputs and have some questions.

  1. Most mixers have 2 1/4" line inputs per track, one for the left channel and one for the right. If I connect a mono device to only the left jack, would sound be output to both speakers?
  2. Would I need a preamp if my mixer already has an XLR port?
  3. How would the mixer know if the input is balanced or not? (E.g. balanced TRS or regular 1/4" plug)
  4. I've heard that professional line level inputs/outputs have a different current than home use line level controls. Does it mean that I can't connect (with good quality) a computer to my mixer or my mixer to my home amp (using 1/4" to RCA adapters)
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3 Answers

  1. Most inexpensive mixers have 1/4" stereo pairs. But keep in mind that each 1/4" input is considered a "channel", even though the stereo pairs are ganged together with a single fader. Typically, the right channel is normalled to the left, so if you plug a signal into the left input it will be duplicated on the right, just like a single channel with the pan control set to center.

  2. Cheap mixers have cheap preamps. If you are into exotic tube pre-amps (which may each cost more than a small mixer), then you will develop preferences for the preamps you want to use for a particular mic input.

  3. Most mixers that take a balanced input indicate that you will see a 3dB loss if you plug in an unbalanced signal - that's because you are only sending it half of a balanced signal. The mixer doesn't detect it, but the design of balanced 1/4" inputs allows for rough compatibility with unbalanced signals.

  4. "Home" line-level signals are, I believe, 10db lower than "pro" signals. You can plug a "home" signal into pro-audio gear, but it will be quiet. Some mixers have a 10dB boost switch to account for this. On those that don't, you can use a preamp. Or just adjust the rest of your mix accordingly.

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1) Almost no mixers have a left and right 1/4 inch line per track. Tracks are generally mono. There are rare exceptions with stereo tracks, but they are not that common and almost non-existent on cheaper boards and often use a TRS connector to have stereo in a single connection. What you may be seeing is a 1/4" line input and an 1/4" TRS insert, which lets additional signal processing be done outside the board. Pan knobs are used to do left and right alignment. In the case of an actual stereo channel, a balance knob is used, which turns down either the left or right channel's feed to the corresponding output.

2) A mixer has preamps built in. That's what the gain adjusts.

3) Generally balancing is based on the jack or will have a switch to adjust the setting. It might also be able to make a guess by looking for signal level or checking if there is a ring.

4) You can connect them, but a transformer in between them is recommended. Alternately, some boards have a tape input which normally will use RCA jacks and be a consumer line level.

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  1. Most mixers will include some kind of routing facilities which will allow you to route the audio from a given input to desired outputs (main, sub, groups, etc..)
  2. Usually, having an XLR port in your mixer will imply that there is a preamp as well. However its quality depends on the mixer quality and this is actually one of the most price defining parts at all. In some cheap mixers, the built-in preamp can be too weak (in gain sense) or too noisy.
  3. The mixer does not need to know whether you media is balanced or not. The idea of balanced lines is transmitting (and receiving) the signal on both wires but with inverse phases. The input will receive whatever was transmitted and vice versa.
  4. The most common nominal level for consumer audio equipment is −10 dBV, and the most common nominal level for professional equipment is 4 dBu. Regardless of that, you can connect your PC to your mixer however I'm not sure you'll have too much routing options if you are using the RCA inputs.
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