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Mic level vs. Line level, powered speakers

Discussion in 'Sound, Music, and Intercom' started by mnfreelancer, May 21, 2009.

  1. mnfreelancer

    mnfreelancer Active Member

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    As I may have mentioned before I work for a company that does some show/events work and a lot of rental type work. The "flagship" of our rental audio equipment at our company and just about every other like it is the Mackie SRM-450 powered speaker and Mackie 14, 16 and 24 channel mic/line mixers. These powered monitors have pre-amps built into them that enable you to plug a dynamic mic (like a Shure SM58) into them and just start talking. Thanks to Mackie in designing some versitility into their products, the mixers have an XLR output that allows you to choose line or mic level. These facts all line up and point out that as a set-up tech, you have a choice to make. Do you drive the output at line level and back off the pre-amp on the speakers, or put out a little mic level signal and let the monitors do the major amplification in the final gain stage? There is some debate within my company and between those of us on one side and those of us on the other. Here are some arguments that I've thought up for both sides...

    I fall on the "drive signal line level to the speakers" because:
    -the higher nominal voltage is less susceptible to voltage drop over long cable runs.
    -similarly, the higher nominal voltage of a line level signal is more immune to interference from electrical and magnetic fields, and RF interference.
    -the pre-amps on the mixer already did the work of getting the mic level signals up to line level where the filters and other electronics in the board can do their job at optimal parameters.
    -There is more head-room in the final gain-stage if SPL levels need to be increased.
    -If you translate the scenario to passive speakers and amp racks, almost every professional amplifier I've ever seen has taken a line level signal as an input...why should amps bolted onto the back of speakers cabinets be any different?
    -If you want to use outboard gear in "loop though" mode the signal is the right level and everything would work as it does inserted on a channel or bus (if you wanted to run a mains EQ between the mixer and the speakers)

    Those who argue for the mic level scenario say:
    -There is more gain before feedback when using open mics in close proximity (this seems counter-intuitive except than with electronics you find out quickly that every component in a circuit has a sweet spot where it operates the most efficiently and in audio this translates directly to frequency response and sensitivity...my theory is that the electronics in the amps are configured in such a way that frequencies that typically ring and feed-back in a live room are attenuated out of coincidence because of the behavior of the electronics involved.)
    -Easier to interface with house systems as most rooms have XLR "mic" jacks (this is valid but more of a point of convenience than anything else)

    So with all of this I had the following thought...the SRM-450s level knob has a "unity" setting in the middle exactly in-between mic and line level. Perhaps setting the mixer to mic level and driving the master faders a little harder (or even the pre's) could take advantage of the parameters involved on the amp side while still allowing for headroom and slightly less voltage drop / noise susceptibility. You could also set this knob at unity and drive the mixer at line level but attenuate the master faders, but you would loose some of the functionality of the meters on the board. All of this seems like a way to engineer around a problem that is much better addressed with more gear (a graphic of parametric EQ). For me, I like to start with the mics and go foward to the speakers in setting up my gain structure...that way the pre-amps are in optimal range, the faders are optimal and the output stage is optimal (with the masters at or around unity).

    I know this was kind of a brain dump but I've been thinking about this for a while and have been raring to have a technical discussion about it ... so go to it!
     
  2. David Ashton

    David Ashton Active Member

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    You follow the simple rules of gain structure, namely drive the output of the mixer to 0 db to get the best signal/noise ratio and feed it to the line level of the powered speaker and bring the volume up to the maximum level you need.
    Unless you like clipping and distortion and high levels of noise, in which case plug in to the mic level and turn up the volume and run your mixer ar 5%
     
  3. museav

    museav CBMod CB Mods Departed Member

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    Only a few of the Mackie mixers have the ability to select mic or line level output and all this is actually doing is attenuating the output signal level by inserting a pad right before the physical output. So while it is a nice feature if you have to tie into mic inputs such as for an existing room system, it seems to offer no advantage when related to powered speakers. In fact, the Mackie SRM450 manual shows using the line out from the mixer.

    I would argue there being any more gain before feedback by using the mic input on the speakers. How are you determining that there is more gain before feedback? Are you actually measuring a change in the speaker output levels? Feedback in this case is an acoustic phenomena and occurs when the loop gain of the system exceeds unity, so if the speaker, room and mic conditions remaining constant then any actual increase in gain before feedback indicates there is some manipulation of the signal going on, some filtering or phase manipulation. It could be as simple as being a result of engaging the high pass filter on the speaker or looking at the levels on the controls or mixer rather than the actual speaker output levels. Otherwise, the argument that using a mic input somehow increases the gain before feedback seems to be asserting that the electronics are not linear even over just the speech frequencies, which would seem to be something to be avoided rather than embraced.

    Making this even more interesting, the SRM450 seems to basically use the same input circuitry as the preamp in the console, meaning that any nonlinearities would be introduced twice, once at the console preamp and again at the speaker input.

    So I see numerous reasons to run line level from mixer to powered speaker while having a difficult time seeing any advantages to running it as mic level instead.
     
  4. howlingwolf487

    howlingwolf487 Active Member

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    Nevermind...found out for myself. Yay for online user manuals!
     
    Last edited: May 21, 2009
  5. mnfreelancer

    mnfreelancer Active Member

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    @Brad - this is exactly the explanation I was expecting and my sentiments exactly...the fact that there seems to be more gain before feedback when running in mic level mode defies logic (electronics logic that is) and concerns me more than puts me at ease. Thanks.
     
  6. mixmaster

    mixmaster Active Member

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    Think of the system in terms of simple math. You have a microphone that is putting out a mic level signal.
    On one hand that signal is routed to the preamp section of the powered speaker, where it is stepped up to some higher level to be sent to the main amp section. We'll call this signal level Y (more or less line level). The amp then does it's thing, however many db it is, and you get a speaker level, which goes to the speakers and gets turned into acoustic energy to produce a given SPL in the room. On the other hand, that same mic puts out the mic level which then gets routed to the board, and stepped up by the board's preamps to a line level. Either way, the signal hits the amp section at line level. By padding down your mixer's outputs to line level, you just make the preamp on the powered speaker have to re-amplify your signal, along with any noise injected into whole system.
    If you experience significantly more feedback when using the mixer at line level, that would suggest that the "line level" of the mixer is hotter than the "line level" of the pre-amp in the speaker. No great surprise there, especially given what MuseAV mentioned about the mixer and the powered speaker having the same pre-amp. The mixer would then add summing amps on the stereo buss and possibly a final stage at the output of the board, which would make for a hotter output. Which would drive the amp harder, which would result in more SPL in the room, which could result in feedback sooner. I would expect the level knob on the speaker could be adjusted to account for this slight difference in signal levels.
    For the reasons that you stated, driving the amp with the mixer at line level is the right way of doing things.
    Matt
     
  7. TimmyP1955

    TimmyP1955 Active Member

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    Line level. It would be silly to attenuate the mixer's line level signal down to mic level, then boost it back up to line level inside the loudspeaker.

    The speaker has a mic pre so one can make a noise with one mic and one speaker, with no other gear needed. (This is not likely to sound good w/o EQ, but oh well.

    The mixer has a mic level out so you can use it as a submixer and feed it into another mixer that does not have a line level input. (Such mixers are rare, but.....)
     

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