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I have Behringer PMP4000 power mixer. I am considering buying two passive fullrange speakers Behringer B1520 Pro Eurolive.

According to the speakers specification they are

  • 8 ohm, with power rating 300 W RMS and 1200 W peak.

Power mixer has to "powered" outputs. According to specification (page 20) it can output:

  • 8 ohm, 300 W RMS and 400 W peak per channel
  • 4 ohm, 600 W RMS and 800 W peak per channel

Thus, to my understand the two speakers fit the setup perfectly.

However, I consider getting a passive subwoofer or a passive monitor as well (the mixer supports a special mode where the powered outputs volume can be controlled separately). Let's for an example consider this subwoofer (again, we are doing it purely for exercise purpose, I am not aiming for this model). According to the specification the subwoofer:

  • 8 ohm, with power rating 300 W RMS and 1200 W peak.

Do I understand correctly, that if connect the two speakers in parallel (that is connect mixer to one speaker, and connect the second speaker to the first one) and add the mentioned subwoofer, the power mixer should handle everything? To my understanding, two connected speakers will drop the impedance to 4 ohm while increasing the power need twice (300 W RMS * 2 = 600 W RMS). And the other mixer power output can still feed the subwoofer with 8 ohm and 300 RMS? Or both mixer outputs should have the same impedance?

Can someone also clarify the demo picture on page 14 in the mixer specification? They connect in parallel two Behringer B1800X (8 ohm, 450 W RMS, 1800 W peak) and Behringer B1220 (300 W RMS, 1200 W peak). To me it does not make much sense since the power mixer is not powerful enough to power that setup.

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    Not answering your question, thus a comment. Having a 'monitor' that comes off one side of a stereo powered mixer isn't that productive. Yes, it changes the impedance for one side, but also renders the 'monitor' uncontrollable. Far better to get a powered monitor, and feed it from 'monitor out'. That way, it's independent from the main p.a., and feedback for one won't be such a problem.
    – Tim
    Oct 15, 2018 at 10:50
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    @Tim thanks a lot for the comment. I am aware about this. The mixer has something called "MON" mode (see page 9 or page 15). Output A (one powered output) goes to monitors and is controlled via separate controls and Output B (another powered output) goes to the main speakers and is controlled via separate slider.
    – Maksim Sorokin
    Oct 15, 2018 at 10:59
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    O.k. so there are options to use the mixer as 2 separate channels, but daisychain the monitor to one speaker, or plug both foh speakers to one side, and the monitor to the other channel. That'll work, but the 1st idea makes the impedance on 1st channel 4 ohms. For the sake of a bit extra cost, a powered monitor is, in my experience, a far better arrangement. 3 separate power amps and more controllability!
    – Tim
    Oct 15, 2018 at 11:17

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Two things: 1) yes, within limits, the impedance of two identical objects wired in parallel will be half the impedance of each one.

2) As to "not powerful enough," all that means is you can't go full-blast for the given scenario. The only important thing to watch is not to let the net impedance go lower than the lowest rated impedance for your source (power amp). Even then, so long as you don't try to generate huge gain, the worst that'll happen is output distortion. If you do turn the gain way up, you risk having the amplifier trying to generate more current than it's capable of (because it tries to maintain a voltage level), leading to overheating, component stress, etc.

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