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Newfoundfreedom

Crossover confusion

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I've just bought some bottom PA subs to go with my 15 inch tops. 

I was advised on another thread that I would need (or at least be best served) to use a crossover. 

I was looking at something like the Behringer cx3400, which I can get locally at a decent price 

I have little to no knowledge at this point of subs and crossovers, but doing a bit of research, the ones I've seen seem to be wired, mixer - crossover - power amp - speakers. 

There's no way for me to do this because I only have an all in one powered mixer, so the signal out is already amplified. The only way I could do it would be to go mixer/amp - crossover - speakers. 

The crossover I'm looking at has a max input level of +22dBu. I have no idea what +22dBu is, but would I be right in assuming it's the level of a passive, i.e unamplified signal? 

So, can I use the crossover after the amplifier and then go to the speakers? 

If not, how do I crossover an already amplified signal? 

Edited by Newfoundfreedom

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25 minutes ago, Newfoundfreedom said:

I have little to no knowledge at this point of subs and crossovers, but doing a bit of research, the ones I've seen seem to be wired, mixer - crossover - power amp - speakers.

This is a typical setup for PA, with the addition that the crossover splits the signal into (at least) two separate feeds which would then go to different amplifiers for the tops and subs.

You're correct in thinking most crossovers like the Behringer are line level rather than speaker level and you'd do damage plugging the crossover into the speaker outputs on your mixer/amp.

Couple of options:

There's often a FX send/return loop across the mix output on combi mixer amps where you can insert EQ/comps etc. You could put a crossover at this point and send the HF signal back into the amp to run main speakers. You'd then need to get a second amp to run the subs from the LF output of the crossover.

You could run the subs from an Aux on the desk and EQ out the low end from the main PA, though you still need a second amp.

The only way to achieve a mains/subs split with only one amplifier is to put a passive crossover circuit into the main speakers with an output to run the subs. AFAIK there isn't a version in a box where you can just plug in cables?

I'd be surprised if the mixer/amp is capable of powering 2x15" tops plus 2 x Subs also...

Edited by TwoTimesBass
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6 minutes ago, TwoTimesBass said:

This is a typical setup for PA, with the addition that the crossover splits the signal into (at least) two separate feeds which would then go to different amplifiers for the tops and subs.

You're correct in thinking most crossovers like the Behringer are line level rather than speaker level and you'd do damage plugging the crossover into the speaker outputs on your mixer/amp.

Couple of options:

There's often a FX send/return loop across the mix output on combi mixer amps where you can insert EQ/comps etc. You could put a crossover at this point and send the HF signal back into the amp to run main speakers. You'd then need to get a second amp to run the subs from the LF output of the crossover.

You could run the subs from an Aux on the desk and EQ out the low end from the main PA, though you still need a second amp.

The only way to achieve a mains/subs split with only one amplifier is to put a passive crossover circuit into the main speakers with an output to run the subs. AFAIK there isn't a version in a box where you can just plug in cables?

Thanks. That's what I was afraid of. There's no way of running a passive out from my mixer. It only has 2 speaker outs, no monitor or aux sends at all. It's just a basic small band mixer like the one below.

I did find these on fleabay, but they're none adjustable and they only seem to do them in 8ohm, and I need to run two 8ohm speakers per channel (4ohm). 

https://www.ebay.co.uk/itm/2x-Skytec-2-Way-Crossover-Tops-Bins-Box-Sub-Speakers-2000-Watt-UK-Stock-/382355323416?_trksid=p2349624.m46890.l49292

Looks like I'm out of luck without having to buy a new mixer and power amp, which I currently neither have the funds or the justification for, as were a none earning hobby band, and like almost everybody else at the minute not even gigging. 😞

 

IMG_20210113_193750.jpg

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2 minutes ago, Bill Fitzmaurice said:

Thanks. 

I've already downloaded the manual, which is where I got the maximum input figure from. 

As I said above. It's no good in my case as I need a crossover which will operate after the amp, as I have no way of putting a passive signal into the crossover. 

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13 minutes ago, Newfoundfreedom said:

I did find these on fleabay, but they're none adjustable and they only seem to do them in 8ohm, and I need to run two 8ohm speakers per channel (4ohm). 

https://www.ebay.co.uk/itm/2x-Skytec-2-Way-Crossover-Tops-Bins-Box-Sub-Speakers-2000-Watt-UK-Stock-/382355323416?_trksid=p2349624.m46890.l49292

Yeah, this might work but you'd be close to the limits in terms of power handling, driving the amps and the crossover pretty hot/hard. Plus there's no way of adjusting the frequency of the crossover point or indeed the level between the speakers. Probably not worth it at this point...

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16 minutes ago, TwoTimesBass said:

Yeah, this might work but you'd be close to the limits in terms of power handling, driving the amps and the crossover pretty hot/hard. Plus there's no way of adjusting the frequency of the crossover point or indeed the level between the speakers. Probably not worth it at this point...

Yeah that was pretty much my conclusion. Plus by the time I've paid shipping and VAT on top with the whole Brexit palava they would work out expensive. Unless I was lucky enough to find some already in the EU, then I might have had a punt if they were rated at 4ohms. But at 8ohms I don't think they'd last long running 2 8ohm speakers. 

 

Edit - just found some in Germany but it still doesn't solve the 8ohm problem. 😞

 

 

 

 

 

 

 

 

 

Edited by Newfoundfreedom

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1 hour ago, Newfoundfreedom said:

It only has 2 speaker outs, no monitor or aux sends at all. It's just a basic small band mixer like the one below.

That Europower has pre out and power in in the front panel. You could connect a X-over to the pre out and put the highs to power in. From the X-over the low output could be driving the subs. If your subs are passive, an extra power amp is in need.

Active subs may have internal X-overs (LPF).

Edited by itu

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20 minutes ago, itu said:

That Europower has pre out and power in in the front panel. You could connect a X-over to the pre out and put the highs to power in. From the X-over the low output could be driving the subs. If your subs are passive, an extra power amp is in need.

Active subs may have internal X-overs (LPF).

Sorry you've lost me. There is no pre out or power in.

There's only an aux in jack socket. The only other connection is the 2 channel line in and line out, but they only relate to channel 13/14 on the mixer. 

Probably not very clear on the picture above, bit full info here.

https://www.thomann.de/gb/behringer_pmp2000d.htm?o=10&search=1610565227

 

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You say '15" tops', that suggests you have 2 of them. If that's the case, then that PMP2000 mixer can only power 2 speakers at a time. Trying to split it to 3 or more speakers will end in tears. Its quite an old style/basic mixer.

BUT it does have a CD/TAPE/LINE OUT RCA which could be used with an adapter to convert to eg XLR or 1/4". Then this could go to another power amp then onto the new (I am guessing passive) sub. Or you could swap your passive sub for an active sub. I would imagine the power amp/active sub would have enough in the way of tone controls to only amplify the bottom end and produce an acceptable result.

It won't be as straightforward to set up as a setup with a crossover though.

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21 minutes ago, Newfoundfreedom said:

Sorry you've lost me. There is no pre out or power in.

I see, my bad. I looked at the obsolete version.

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8 minutes ago, Bill Fitzmaurice said:

That's not an option.

Yup. It's looking that way. Back to plan A of just running each channel via the mixers EQ. It sounds ok and will probably do the job for what I need. 

Edited by Newfoundfreedom

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9 minutes ago, paul_c2 said:

You say '15" tops', that suggests you have 2 of them. If that's the case, then that PMP2000 mixer can only power 2 speakers at a time. Trying to split it to 3 or more speakers will end in tears. Its quite an old style/basic mixer.

 

Why would it only be able to power two speakers at a time? It should power as many speakers as I can throw at it as long as I don't go under the rated 4ohms per channel.

I'm currently running 2x15 inch 8ohm tops with built in crossovers and tweeters on one channel, and two 2x12 subs at 8 ohms each on the other channel, giving me the rated 4ohms per channel, with no problems whatsoever. 

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I can't see a way of combining wildly different styles (ie sub and full-range) speaker on the same channel without one interfering with the sound quality of the other. Sure, you could do the wiring to get the impedance the same, but I think there will be a sound quality weirdness.

Do the two channels have their own EQ? If not, then I think its always going to be a weird compromise there too.

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22 minutes ago, paul_c2 said:

 

BUT it does have a CD/TAPE/LINE OUT RCA which could be used with an adapter to convert to eg XLR or 1/4". Then this could go to another power amp then onto the new (I am guessing passive) sub. Or you could swap your passive sub for an active sub. I would imagine the power amp/active sub would have enough in the way of tone controls to only amplify the bottom end and produce an acceptable result.

It won't be as straightforward to set up as a setup with a crossover though.

Thanks. I can do this much easier just by running the tops daisy chained through one channel and EQ'd high, then running the subs daisy chained through another channel EQ'd low, which is how I'm currently running it, but I was advised on another thread that this wasn't the best way to do it and I should be using a crossover. But it sounds great to my ear, and given the cost I would have to shell out on new gear, just to run the subs I think the best option is probably just to stick with it for now. 

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5 minutes ago, paul_c2 said:

I can't see a way of combining wildly different styles (ie sub and full-range) speaker on the same channel without one interfering with the sound quality of the other. Sure, you could do the wiring to get the impedance the same, but I think there will be a sound quality weirdness.

Do the two channels have their own EQ? If not, then I think its always going to be a weird compromise there too.

Yes. The two channels are totally separate and EQ'd accordingly for highs or lows, and only speakers of the same type (subs or tops) are on each channel. 😉

Edited by Newfoundfreedom

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You have 2 two tops, each 8 Ohm, and 2 subs, each also 8 Ohm?

My understanding is that if you plug one 8 Ohm sub and one 8 Ohm top into one of these Skytec passive crossovers you end up with an overall load for that system of approx. 8 Ohms. Which you can plug into one channel of your Behringer, you'll get about 165W RMS according to the spec less a bit in the crossover.

http://www.caraudiohelp.com/newsletter/speaker_impedance.html

 

 

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The challenge will be running 3x or more subs on one channel then. It might work, but you are running into the problem of "everything is a tone control" and also the mismatch of the amp's power output with the speakers power handling. In other words, because there is no more power output, there is no improvement in simply moving a greater quantity of cones.

I guess you're placing the subs each side too, rather than one group in the middle. You will get "power alley" from that.

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Just now, paul_c2 said:

The challenge will be running 3x or more subs on one channel then. It might work, but you are running into the problem of "everything is a tone control" and also the mismatch of the amp's power output with the speakers power handling. In other words, because there is no more power output, there is no improvement in simply moving a greater quantity of cones.

I guess you're placing the subs each side too, rather than one group in the middle. You will get "power alley" from that.

Yeah I'm using the subs as stands for the tops. I will never be running more than 2 subs or two tops so running any more won't be a problem. I know this is the absolute limit of what this little PA can do, but it should do the job for anything we need it for (small pubs etc, if and when we can ever get out gigging again) it was more about filling out the sound and adding a bit more bottom end, which it seems to do perfectly well with the current set-up. I know it's not ideal, but it will suffice on a very tight (none existent) budget for a bunch of weekend warriors. 

Thanks for taking the time to advise. 👍

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3 minutes ago, paul_c2 said:

Google "power alley" though.

Yeah I know the logic behind it. I just don't think I'll ever be playing anywhere big enough for it to make a difference. 😆

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Power Alley is most problematic in small venues, not large. Unless they're separated by at least 15 meters subs should usually be clustered, and wall loaded. You can't wall load a sub that's under a main that has to be in front of the band to prevent mic feedback.  

But to be frank your entire setup is AFU. Fifteens are for subs, not tops. Tops should only be run above 100Hz, and above 100Hz the only thing fifteens give you is poor midrange dispersion from cabs that are too big and too heavy. Then there's the matter of driver displacement and cab size. The demands of both double with each octave lowering of the pass band, so to match up with a pair of 2x15 mains you'd need at least eight 1x15 subs. It's just like an iceberg. What you can see above the water is the mains, what you can't see below the water is the subs.

Of course one wouldn't expect that you'd make wholesale system changes at this point, but since you now have a pair of subs the next logical move is to get rid of the 2x15 mains in favor of a pair of 1x12 mains.

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1 minute ago, Bill Fitzmaurice said:

Power Alley is most problematic in small venues, not large. Unless they're separated by at least 15 meters subs should usually be clustered, and wall loaded. You can't wall load a sub that's under a main that has to be in front of the band to prevent mic feedback.  

But to be frank your entire setup is AFU. Fifteens are for subs, not tops. Tops should only be run above 100Hz, and above 100Hz the only thing fifteens give you is poor midrange dispersion from cabs that are too big and too heavy. Then there's the matter of driver displacement and cab size. The demands of both double with each octave lowering of the pass band, so to match up with a pair of 2x15 mains you'd need at least eight 1x15 subs. It's just like an iceberg. What you can see above the water is the mains, what you can't see below the water is the subs.

Of course one wouldn't expect that you'd make wholesale system changes at this point, but since you now have a pair of subs the next logical move is to get rid of the 2x15 mains in favor of a pair of 1x12 mains.

Thanks. But I'll stick with what I have now. I know everything you say is true on paper, but it all sounds just fine to me, and I'm not a professional muso playing to a paying audience, so at the end of the day that's all that really matters. I don't have the money to be buying more gear. I saw some cheap bass bins local. Cobbled them together with what I have, and they sound ok. That's good enough for me. 

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8 hours ago, Newfoundfreedom said:

But it sounds great to my ear, and given the cost I would have to shell out on new gear, just to run the subs I think the best option is probably just to stick with it for now. 

Plan. It sounds good, keep practicing.

You are looking at a complete do over of your system to get a proper biamp out of speakers that are less than ìdeal. You might be better trading out of it wholesale as a going concern.

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