Amplifier - wiring help required

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Amplifier - wiring help required

Post by panovice »

I help out in a house of worship where we are using a Behringer PMP6000 amplifier (mixer/amp). We have 2 speakers connected to the amp, but both going into the same output (output B). The amplifier is set to operate in bridge mode so Output A is not in use.

Manual for amp https://www.bhphotovideo.com/lit_files/84923.pdf

This does not seem correct to me and I'd like to re-do the speaker connections. So looking at taking the 2 speakers wires going into out Speakon connector and splitting into 2 speakon connectors. Can anyone point out any potential issues caused by this? Currently the amp/mixer is dropping out occasionally and before we purchase a new one wanted to try this (as we wouldn't want to use bridge mode with any new amp purchased).

I'm not sure on the speaker specs (no labels on the casing).

Any help/advice appreciated.
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Re: Amplifier - wiring help required

Post by Hugh Robjohns »

panovice wrote: Mon Mar 20, 2023 12:27 pmWe have 2 speakers connected to the amp, but both going into the same output (output B). The amplifier is set to operate in bridge mode so Output A is not in use.

On the face of it, this is not an unreasonable configuration. It's using the amp in bridge-mode to drive two speakers in series with a mono signal.

This does not seem correct to me...

Why? Have you discussed this with whoever was responsible for setting it up in the first place?

...and I'd like to re-do the speaker connections.

Why?

So looking at taking the 2 speakers wires going into out Speakon connector and splitting into 2 speakon connectors. Can anyone point out any potential issues caused by this?

The desk would then be configured to work in stereo with potential user errors which are possibly avoided with the desk in mono mode.

Currently the amp/mixer is dropping out occasionally...

Ah... it's possible that the combined impedance of the speakers is falling below 8 Ohms and that's could cause the amps to trip out due to overheating or over-current. Running the two amps separately in stereo mode should avoid that problem as each amp can handle loads down to 4 Ohms. If you run the desk in mono you should end up with much the same acoustic power into the room.
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Re: Amplifier - wiring help required

Post by panovice »

Thanks for taking the time to reply. Unfortunately don't have contact with the original installers any more.

As well as bridge mode, the amplifier can be set to Mono mode according to manual. So, splitting the speaker wires into 2 speakon connectors (hopefully no soldering involved?) and then setting the amp to operate in mono mode will work.
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Re: Amplifier - wiring help required

Post by Hugh Robjohns »

Don't see why not.

Most Speakons have screw terminals. Just make sure you wire to the correct pins. The details are in the manual (and on the back of the mixer-amp I think).
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Re: Amplifier - wiring help required

Post by Wonks »

It's very likely that any speakers will be of 8 ohms impedance each, especially if of the standard main woofer + tweeter in a box arrangement, so wiring them in parallel will give 4 ohms impedance. This is below the minimum impedance stated for bridge mode. If you just have a speaker cable going to each speaker, then this is very likely the case.

It could be possible that if there is a cable running out to each speaker, and also a cable running between each speaker from the Speakon connector, that the speakers have been wired in series to give a 16 ohm rating. This is highly unlikely though and the increased speaker resistance negates any output benefit from using bridged mode.

To wire two PA speakers in parallel for 8 ohm impedance they would have to be 16 ohm impedance speakers, and there are very few 16 ohm PA speakers around. Those that do exist tend to be small (say 5" driver maximum) and are intended for use where up to four small speakers (in say a pub or club) are fed in parallel from a single amplifier output for providing background music or making announcements.

So, provided you just have a speaker cable going to each speaker and nothing between them, you'll need to get another NL4FC connector, and rewire the mixer end of the cables so that each cable is wired into terminals 1+ and 1-. Check the speaker end of the cables to see what cable colour is connected to 1+ and 1- at the speaker end and make sure they are the same at the mixer end.

It's worth double checking that the Speakon screw terminals are tight on each end of the cable, as they can vibrate loose over time. Some need a flathead screwdriver and some a small hex key to tighten. I've had both types in the past.

Then use outputs A and B and set the mixer amp output to either mono or stereo mode. If you keep the balance pots on the mixer central, you won't tell any difference between mono and stereo mode anyway, unless you are feeding stereo inputs into the stereo line input channels from say a keyboard. In this instance, you might decide that it sounds better in stereo than mono. But otherwise there will be no difference in volume etc.
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Re: Amplifier - wiring help required

Post by ef37a »

I have been out of the PA game a long time now but around 600W per speaker seems an awful lot of power for a church?

And yes, I would guess the amp is shutting down due to the 4ish Ohm load in bridge mode.

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Re: Amplifier - wiring help required

Post by panovice »

I'm now wondering if the speakers are 16 ohm as they are in quite a small cabinet (probably 6 inches wide and roughly 14 inches tall).

If they are 16 ohm, could I do any damage by connecting them individually rather than bridge mode? By default I think the amp supports 4 and 8 ohm speakers.
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Re: Amplifier - wiring help required

Post by ef37a »

panovice wrote: Mon Mar 20, 2023 3:27 pm I'm now wondering if the speakers are 16 ohm as they are in quite a small cabinet (probably 6 inches wide and roughly 14 inches tall).

If they are 16 ohm, could I do any damage by connecting them individually rather than bridge mode? By default I think the amp supports 4 and 8 ohm speakers.

Hmm, I know small speaker power ratings have increased substantially in the last couple of decades but I would say such small speakers would be in danger connected to such a potent amplifier. Even if they are 16 Ohms that amp could put 300-400 watts into them and I dare bet that is above their rating.

You can check their impedance with a digital multimeter. Speakers read about 60% of their nominal impedance as a DC resistance. If you don't have a digital meter...GET ONE! Nobody should even think about messing with speaker wiring without one.

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Re: Amplifier - wiring help required

Post by Wonks »

You obviously don't need to use all the power, it's nice to have headroom available. And that 600W is into 4 ohms, whilst 300W into 8 is a more typical usage.

But probably it was the cheapest way to get a powered mixer with the required input channel count. The power goes up the more channels you have with that range.

I don't know if it was the same model, but I had a very similar Behringer powered mixer maybe 15+ years ago, for a very short while. It couldn't pull the skin off a rice pudding. I think they got the internal gain structure very wrong as despite its 600W per channel rating, you could hardly hear it.

I needed more inputs than my then current Spirit Powerstation 600 (2x 300W into 4 ohms) had. My Peavey 75W+75W monitor amp fed from the Behringer main output jack totally blew the Behringer away.

If you fed the Behringer with an external signal into the power amp input sockets it got quite loud, but set up the desk as normal with PFL channel signals peaking on the 0 level on the output meter, and pushing the channel fader and main fader right up, you could talk over the top!

It went back to be replaced with a standard mixer and a big power amp.
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Re: Amplifier - wiring help required

Post by Hugh Robjohns »

panovice wrote: Mon Mar 20, 2023 3:27 pm I'm now wondering if the speakers are 16 ohm as they are in quite a small cabinet (probably 6 inches wide and roughly 14 inches tall).

That sounds like a standard old-school line array speaker intended for speech amplification. It could be almost any impedance depending on how many drivers there are and how they are wired. 16 Ohms is a distinct possibility.

By default I think the amp supports 4 and 8 ohm speakers.

You won't damage the amp with a 16 Ohm speaker load, you'll just get less output power. The risk for solid-state amps is if the load is too low an impedance which is why they state a 4 Ohm minimum (8 min in bridge mode).
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Re: Amplifier - wiring help required

Post by Wonks »

panovice wrote: Mon Mar 20, 2023 3:27 pm I'm now wondering if the speakers are 16 ohm as they are in quite a small cabinet (probably 6 inches wide and roughly 14 inches tall).

If they are 16 ohm, could I do any damage by connecting them individually rather than bridge mode? By default I think the amp supports 4 and 8 ohm speakers.

Size doesn't denote impedance, so they may still very well be 8 ohms. They would be far less at risk though if they were 16 ohms and the mixer fed each one from its L+R outputs than if they were connected in parallel and fed from a bridged output. But the fact the amp is cutting out indicates 8 ohms to me.

If you have a multimeter, you could measure the coil resistance and be sure.

Easiest to unplug the mixer Speakon connector and then unscrew its cover. Measure the resistance between the 1+ and 2+ terminals. As the two speakers are in parallel you'll get half the individual speaker value. For 8 ohms you'll read somewhere between 3 and 4 ohms; for 16 ohms you'll read between 6 and 8 ohms.

If they are 16 ohm speakers, then when wired in parallel and fed from the bridged mode output, the mixer will output up to 1200W, or 600W per speaker. If fed from individual L+R outputs, then the wattage should reduce to 150W per speaker.

If they are 8 ohm speakers, then when wired in parallel and fed from the bridged mode output, the mixer will still output up to 1200W, or 600W per speaker (it's maximum output which it can't really exceed), but because it's feeding into 4 ohms it will be trying to put out more and failing, causing its protection circuitry to operate. If fed from individual L+R outputs, then the maximum wattage should reduce to 300W per speaker.

So regardless of the speaker ohmage, running from the L+R outputs will be a much better solution, and you should still have more than enough power.

The difference between 300W and 600W (providing the speaker can take it) is only a 3dB increase in the SPL produced, just enough to be noticeable but not a major change. And I'm sure you are running the system at far less power than maximum.
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Re: Amplifier - wiring help required

Post by Wonks »

One other possibility is that the speakers are 100v line fed speakers and between the mixer amp and the speakers is a matching transformer to drive the speakers. The Behringer's output doesn't feed into any intermediate device, does it?
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