Very Basic Power Amp Question

Discussion in 'Location Recording' started by AwedOne, Nov 11, 2008.

  1. AwedOne

    AwedOne Guest

    Hello Everyone.

    I should know this, and probably did at one time, but...

    when using passive monitors with a power amp, should I set the gain on the amp to full and adjust the input for a comfortable db level (85db or so), or should I set the console output for unity gain and adjust the power amp to the desired level?
  2. Greener

    Greener Guest

    Set the console output for unity gain and adjust the power amp to the desired level.
  3. AwedOne

    AwedOne Guest

    Thanks for the info.
  4. basilbowman

    basilbowman Guest

    How Come? Just as an FMI only...

  5. GeckoMusic

    GeckoMusic Guest

    Greener's got this one right.
    Gain staging.
    The mixer is going to be your biggest source of noise, so you want it running optimally. Also, the power amp is expecting line level, and it is going to be right where it was designed to operate if you feed it unity gain off your board. If your power amp is way over rated for your space, then you might want to run your board a little lower.
  6. AwedOne

    AwedOne Guest

    GeckoMusic wrote:

    I'm running a Crown D75A at 1/2 gain. The manual for my console says that unity gain for all pots is at 2 o'clock. this yields me 85db in my space, with the master fader at zero and master Vu peaking at -5db.

    My two concerns are (1) does running power amps at less than full gain causes long-term damage, and (2) am I compromising the frequency range I'm hearing, i.e., is any frequency range more prominent in the monitors when the power amp is cranked and the input is reduced?
  7. BobRogers

    BobRogers Well-Known Member

    I agree with the crowd. As a practical matter I find that the volume usually gets pushed up as the night goes on, so I usually set the board a few db below unity when setting the amps at the beginning of the night.
  8. GeckoMusic

    GeckoMusic Guest

    That sounds fine.
    1. That should be fine, posibily ideal if your speakers are toward the low ohm side of your power amp.
    2. If the red lights aren't comming on, then you should be fine.

    What is the ohm value of your speakers?
  9. bent

    bent No Bad Vibes! Well-Known Member

    +1 to the replies here.

    All of your levels should be at unity before turning the amp up, including anything after the board outputs (EQ, Comps, Xovers, etc.).

    The knobs on most amplifiers are sensitivity selectors, not volume pots.
  10. stealthy

    stealthy Guest

    so, you should always turn the amp to full power, or no?
  11. Boswell

    Boswell Moderator Distinguished Member

    You are confusing gain with output power. The knobs on power amps control the voltage gain of the system, i.e. how much input level is needed for a given output. If your amp has enough output power for the job in hand, then you should set the gain control on the amp so that when the mixer master fader is at unity (fully up) you get the amount of output power that you need for that venue. However, as Bob says, it's best to leave several dB in hand when setting up at the start of the evening in an empty venue because you need more power when the place is full of absorbent people.

    Another reason for running the mixer output levels as high as possible (and reducing the gain correspondingly on the power amp), is that if you are using a digital mixer, this drives the output DACs in the best part of their operating range.
  12. stealthy

    stealthy Guest

    ok, so you are saying to first set the mixer master fader to unity, mix the appropriate inputs on the board, and use the gain on the power amp to set the level. correct?

    now, later in the night when it needs to be louder, should you adjust the mixer master fader, or turn on the gain on the power amp?
  13. GeckoMusic

    GeckoMusic Guest

    It's going to be 6dB at most, so it really doesn't matter too much. It's easier to do on the mixer, especially if you leave yourself head room on the mixer as Bob suggested.
  14. Boswell

    Boswell Moderator Distinguished Member

    That's right. If you were to get the level right in an empty venue by adjusting the gain control on the power amp with the mixer master fader at -6dB, that should give you more than enough in hand for later.
  15. sheet

    sheet Well-Known Member

    Right. The amps controls are sensitivity. The 100W amp will put out 100W if the setting is at 25 or 100 percent. It will require more voltage on the input side to do it. Do you really want to do it that way?

    IMO, it depends on the system and demands placed on it and it's operators. It also depends on how the mix is routed.

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