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  1. #1
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    Default Configuring Gain

    Good day,

    I have some questions about configuring gain. In my preset setup, I believe that I have six to seven potential volume adjustments - depending on what instrument I happen to be using at that time.

    Please see the following document for further information: Configuring Gain
    PS: The document is a total of three pages, with four questions that I am seeking answers to.

    Thanks to all who contribute, and I am looking forward to your answers.

  2. #2

    Default Re: Configuring Gain

    Quote Originally Posted by mr_es335 View Post
    Good day,

    I have some questions about configuring gain. In my preset setup, I believe that I have six to seven potential volume adjustments - depending on what instrument I happen to be using at that time.

    Please see the following document for further information: Configuring Gain
    PS: The document is a total of three pages, with four questions that I am seeking answers to.

    Thanks to all who contribute, and I am looking forward to your answers.
    Question 1: Where does one begin?

    Start at the very beginning; a very fine place to start.


    Question 2: What do you do at each point in the signal path?

    Maximize the signal-to-noise ratio and minimize distortion as best you can. This typically means setting the gain so that the average signal level is as hot as can be without distorting at this stage -- or at the input to the next stage ("overdriving" the input). Usually, if the signal is nominal, you should set the gain control to its nominal or "unity gain" position.

    Problems tend to arise in situations where A) the signal is lower or higher than expected (than nominal) and/or B) there's a level mismatch between gain stages (e.g., if you send a line-level signal into a mic-level input you'll get distortion; if you send a mic-level signal into a line-level input you'll hear a poor S/N ratio and low output level).

    You can usually attenuate something that's too loud for a certain input (by turning the gain down or adding an attenuator (pad) before the input), but once you've distorted or added noise to a signal, it's too late.

    Impedance matching, as has been described, can also be important. That's beyond my mental grasp. But my experience is that if you don't depart from what you know to be conventional interconnections of gear, you should be fine.

    With your setup you should get a feel for the output levels of your guitars vs. what your audio interface is expecting to "see." (read the manuals) That's the most likely place to introduce problems. The Behringer appears to have lots of input options (mic/line/pad), so lots of flexibility, there.

    Also the output of the interface into your Bose system is a point of interest, but, again, I doubt there would be trouble. My self-powered QSC K12's have a mic/line switch on the inputs, so I can ready them to take either level signal. Looks like your Bose accepts line level. The Behringer looks like it has line level outs, so you should be fine. It seems a little odd that the Bose has a "trim" control for the input, but then no gain control for the amplifier (unless that's somewhere else). You have the choice of using the "trim" as your master P.A. volume -- or the Main Out knob on the Behringer. I'm not sure which would be more handy, ergonomically, in your setup; but each approach may have different resulting noise floors (S/N ratio). Or it could be a non-issue. Something to experiment with and decide on your preferred approach.


    Question 3: Should I be using the SAC Input Attenuator?

    If it's helpful, sure. Bear in mind, though, that it will only allow you to optimize your bit-depth usage for a source once the signal is in the digital domain. What it won't do is take away distortion or noise that's already been added to the signal. Those issues are typically solved before the audio hits the A/D convertor. As such, I find myself using the attenuators in SAW (and I'd suspect it's similar in SAC) rarely. If I can bring audio in at a nominal, optimal level after optimal gain staging between the source and the convertor, no tweaking of the attenuator is needed.

    I don't use SAC, though, so others may have scenarios in mind that I'm not aware of.

    Actually, there is a way that I use the attenuators in SAW, sometimes. Sometimes I want a source to sit low in a mix, but I prefer to keep the channel fader near it's nominal position of 0 dB, where the scale resolution is best. In that case I'll use the attenuator to "calibrate" the main channel fader to that 0 dB position. This may have even more utility in live sound, come to think of it.

    UPDATE: I looked over the manual for the Bose. Looks like you need the ToneMatch mixer accessory thingy to get a separate master volume control. The manual does say, I noticed, that there's an LED indicator that let's you know when the main input is getting enough signal. Of course, at that point it looks like the Bose is playing at full volume, so it still seems a little odd to me that you'd use a trim control for your overall P.A. volume, or, more accurately, that there is no P.A. volume adjustment. It's always on full blast as long as you're feeding it a nominal input signal. OR using the ToneMatch mixer accessory instead of the dedicated input.
    Dave "it aint the heat, it's the humidity" Labrecque
    Becket, Massachusetts

  3. #3

    Default Re: Configuring Gain

    Dell,

    The most critical thing in your situation is the gain knob on the UMC. You really don't want to see the red lights there. Adjust the input gain to just below the point where the light flashes on the loudest signal applied to that input. Red lights can mean digital distortion that is very nasty. S/N is a problem of the analog world and is not much of a consideration after the A to D conversion.

    SAC has a lot of headroom through the engine. I usually run my main outputs at unity and control balance and output level with my input faders. In my situation with multiply acts many days I use the input attenuator often (I run input gains cold and often add makeup gain).

    If your channel pans are centered you don't need to worry about the mono switch on the output. That would be used if you set up a stereo image on your main outputs but want a mono signal for another feed like a center fill or an overflow listening area.

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