Newbie PA Questions

[Doug1963]

My question is I am running my subs out of a single power amp in bridged mono and I am running 2 cabs in parallel in the input out of the cab and into the input of the other cab. THe first cab seems to have more power than the second cab. These are Peavey black widow 118 SUB HC 8 ohm cabs the Amp specs says 500 watts at 8 ohms and 1000 watts at 4 Ohms the subs are rated at 350 watts and a max of 700 watts. I was assuming that running the two split the 1000 watts to 500 watts each is that correct? Is it normal for the first in parallel to have more power?

[Sam Spoons]

If you are running two unmatched subs from the same bridged amp there are a couple of things that will cause one to be louder than the other. Firstly if they are different impedances the amp will drive the lower impedance harder so an amp
delivering 300 watts into an 8 ohm and a 4 ohm cab will drive the 4 ohm speaker with 200 watts and the 8 ohm cab with 100 watts. Next, if they are different sensitivities the more sensitive will sound louder (speaker drivers vary in sensitivity considerably).

BTW the power handling (wattage) of the speakers is not an indicator of which one will sound loudest.

HTH

[Wonks]

From the OP's description both cabs are the same.

They have jack connections (not wise at this power level and ideally should be replaced with Speakon connections if you are any good at DIY and soldering).

Yes, as long as the cabs/speakers are the same, the power should be split evenly between them.

As you state you are a PA newbie then:

1) Are you using speaker cables? If you are using guitar lead type cables then stop and don't use the rig until you've either bought suitable speaker leads or made some up yourself using two core mains cable. Guitar cable isn't designed to carry any amperage and the cable will heat up and reduce the power going in to the cab. The second cab will have further reduced power, so will seem a bit quieter. You must use proper cable designed to carry a sizeable current and voltage.

If you are using speaker cable, great, and on to -

2) The cabs are quite old and assuming they were used when bought, it's possible that one driver may have been replaced before you got them. I'd take the speakers out and check that they are the same. You'll probably have to take the grille off and drop them out from the front. They were initially fitted with Peavey Black Widow speakers. The speakers should (either on the back of the magnet or on the spider framework), have their name and impedance value.

If the speakers aren't the same, then that's very likely the cause of your loudness mismatch. As Sam has said, there can be a possible mismatch in either i) impedance, ii) efficiency or iii) impedance and efficiency. If they both state 8 ohms, then it's likely that one is more efficient than the other (the original Black Widows were pretty efficient at 100dB @ 1W @ 1m). This is operationally fine, it's just that one cab will be louder than the other. If one is 8 ohms and one is 16 then this is a Bad Thing. If one is 8 and one is 4 ohms then this is a Very Bad Thing (as the combined impedance would be below the minimum (implied) 4 ohms loading your amp needs.

If they have the same impedance then you have the choice of either living with the volume difference or trying to find a speaker that matches one of the ones in the cabs. The cabs will have been designed around the Thiele-Small parameters of the Black Widow speaker, so any speaker with different Thiele-Small parameters (i.e. just about any other speaker) won't get the best out of the cab. They may both have been changed at some point. Probably best then to find either a new or used speaker that matches the one in your loudest cab. If you can't, see if you can find one that matches the one in the quieter cab. If you can't find a matching speaker, then go back to either living with the difference or looking for a used matching cab with the correct speaker in. It probably isn't cost effective to swap out both speakers for new ones just so they match.

If they have different impedances than you really do need to either swap out the non 8-ohm speaker for an 8-ohm one or find another used cab with the right speaker in. For the reasons described above, it really should match the existing 8-ohm speaker.

You could probably get by with a different make speaker of the same impedance and the same (or slightly higher) efficiency, but you are going to get slight volume and tone differences between the cabs.

3) If the speakers are exactly the same, then use a multimeter to check the DC speaker impedance. The Peavey data sheet claims that the minimum ohmage is 7.9 ohms, so you should read around 8 ohms for each speaker. Any speaker that is much higher or lower than this has some issues e.g. 7-9 ohms should be OK but 6 or 12 probably not.

4) If the speakers are the same and they have the same DC resistance and you are using decent speaker cable, then you can only check that the woodwork inside the cabs is the same and a baffle hasn't fallen down or been removed by someone (this is a long shot but it's possible). It's a reflex port design, so the internal baffling should be the same, otherwise the cabs will sound different and have different volumes.

5) As a last resort (but do it regardless), take the jack-plate off the back of each cab and check the state of the contacts. If they've corroded over time, then they might provide a high resistance path to the second speaker. Check that the solder connections look intact and not cracked. As I've said earlier, jacks really shouldn't be used for this level of power - which is why Speakon connectors are now the norm. If the jack contacts look rusty/tarnished, then either get them replaced with new ones or modify the jack panel and fit Speakons. Speakon connectors lock, so there's no chance of them getting pulled out accidentally.

[Mike Stranks]

Hi Doug and welcome!

On the question of 'power' and 'loudness... I'm assuming that you mean that one sounds louder than the other? Does it make any difference if you daisy-chain them in a different order? Does one still sound louder if you parallel then at the amp rather than through the paralleled sockets on the cabs?

If all of the above still results in one sounding louder than the other and if the cabs seem to be identical and are both badged the same then I'd suggest that the drivers are different. Have you had them from new or is one/both second-hand? Drivers are often replaced and sometimes not with a direct factory-spec new unit. If you'd had both from new then it could be than one has suffered some damage so that the coil is now touching the magnet for at least part of its travel.

Edit: Wonks got there first! And a good call on connecting cable...

[Exalted Wombat]

My question is I am running my subs out of a single power amp in bridged mono and I am running 2 cabs in parallel in the input out of the cab and into the input of the other cab. THe first cab seems to have more power than the second cab. These are Peavey black widow 118 SUB HC 8 ohm cabs the Amp specs says 500 watts at 8 ohms and 1000 watts at 4 Ohms the subs are rated at 350 watts and a max of 700 watts. I was assuming that running the two split the 1000 watts to 500 watts each is that correct? Is it normal for the first in parallel to have more power?

If you are running two 8 Ohm cabinets is parallel, the amp will "see" 4 ohms. These are pretty much "only numbers" (as are the power output figures, and the "350w continuous, 750w program" specs of the speakers) and aren't particularly relevant to your problem anyway.

Obviously check the integrity and power-handling capability of the cables. Is there a short cable from amp to one speaker then a long one to the other? There might be quite a few ohms in the longer cable! You don't need magic cable, but you do need something better than bell wire or an instrument cable. 6amp 2-core mains cable is often used.

Then, if one cabinet is louder, it must be different in some way. I think these will be quite old units? They can deteriorate. Also, the perceived volume from a speaker, especially a LF one, can vary considerably with room position. Were they assessed standing next to each other, or in a performance setup?

[Doug1963]

Thanks for the replies, I did find out that the lower volume speaker is probably in need of a reconing job as when I crank them up a little I can hear the rattle on that speaker. I took the speaker out and I do not see any markings so I will remove the other and check as well to see if they are mismatched. I also am using 14 gauge speaker wires and wired in the speakons on the amp side. So maybe there is a mismatch or because the other has a bad voice coil could be giving me the problem I am having. I will look further. I am a newbie but have a full understanding of the ohms thing. Again thanks for the quick replies. Im just setting up my sound for the band for future gigs it consist of 4 EV SH1512's, 2 Peavey 118 SUB HC's, a Carvin RX1200L 1600 Watt Powered Mixer, and a Behringer(sorry tight budget) I-Nuke 1000 Power amp for the subs. Looking to add some floor monitors as well and 2 More subs and another Power Amp for them.

[Wonks]

Great that you're up to speed with all of that. There's a good chance that that speaker has been replaced and maybe it hasn't got the power handling capacity of the other one, hence possible damage to the voice coil through over-exertion. Your power amp is in the right ballpark for supplying sufficient power to the speakers, so once you've got the speaker sorted out, you should be good to go.

[Sam Spoons]

Just a point, the DC resistance of an 8 ohm (impedance) speakers likely to be around 6 ohms measured with a DC multimeter.

[Wonks]

Black widdow spec sheet says 7.9 ohms. Could be less for replacements.

[Doug1963]

I purchased these second hand one sounds great and the other rattles a bit I also purchased along with them 2 EV SH 1512 ER top speakers that sound great! They may be old school but sound awsome. I also picked up 2 more matching EV tops sound great as well my goal is to pick up some wedge monitors and 2 more of the matching subs for our ACDC tribute band. I hope they will suit our needs for small outdoor shows

[Sam Spoons]

Black widdow spec sheet says 7.9 ohms. Could be less for replacements.

Are you sure that's not impedance Wonks? the DC resistance of the 18" shown on this page is 6.5 ohms?

[Wonks]

I purchased these second hand one sounds great and the other rattles a bit I also purchased along with them 2 EV SH 1512 ER top speakers that sound great! They may be old school but sound awsome. I also picked up 2 more matching EV tops sound great as well my goal is to pick up some wedge monitors and 2 more of the matching subs for our ACDC tribute band. I hope they will suit our needs for small outdoor shows

So Dirty Deeds Amplified Dirt Cheap.

[Wonks]

Black widdow spec sheet says 7.9 ohms. Could be less for replacements.

Are you sure that's not impedance Wonks? the DC resistance of the 18" shown on this page is 6.5 ohms?

You're right Sam. My brain turns off more often these days. Min impedance of 7.9 ohms.

[Doug1963]

Another question is on my mixer it has a single sub out that I am using to my power amp. What if I wanted to add 2 more subs with another power amp. How would I connect that up? Y cables from the sub out on the mixer? Its an XLR out that would have to y into 2 1/4" patch cables if that even possible.

[shufflebeat]

Black widdow spec sheet says 7.9 ohms. Could be less for replacements.

Are you sure that's not impedance Wonks? the DC resistance of the 18" shown on this page is 6.5 ohms?

You're right Sam. My brain turns off more often these days. Min impedance of 7.9 ohms.

Ha, +1, so does

[Wonks]

Another question is on my mixer it has a single sub out that I am using to my power amp. What if I wanted to add 2 more subs with another power amp. How would I connect that up? Y cables from the sub out on the mixer? Its an XLR out that would have to y into 2 1/4" patch cables if that even possible.

First, it's worth seeing if in bridged mode, the input signal to the amp is connected to the other input socket. It probably is in mono mode - and might possibly be in bridged mode. In that case, you can just run and XLR-XLR or TRS-TRS cable to the next amp.

If that doesn't work, yes you could make up a splitter lead - but it's rather awkward to use, and if it's lost or damaged then you're stuck until you can make another one.

A fairly cheap solution, although its not quite an exact fit, would be a Behringer MX882 rack mixer and use it to actively split the signal.

Alternatively use some DIY skills and make up a metal box with an input (female) XLR and two (or three or four) output (male) XLRs mounted on it, with all the pin 1's connected, all the pin 2's connected and all the pin 3's connected together. You can dasiy-chain a few times without too much problem (except maybe a slight drop in the signal level to the amps), but doing so reduces the input impedance the mixer sub output is driving into, so after a several chains, the overall current requirement may exceed the mixer sub output's capacity to provide it; the voltage drops and the signal starts to get distorted. It's often not an issue - as active speakers are often simply daisy-chained together, but an active split is generally safer.

The Behringer unit was the first one I came across and is relatively cheap. Unless you already have the parts and the skill to drill or punch large holes in a metal box and rivet or nut & bolt the XLR chassis in place, the Behringer option is probably as cost effective as a DIY unit. You may already have another small mixer which you could put to use here - just put the sub output into an input, pan it centrally and just take a feed from the mixers left and right main outputs. Or you could look round yourself for a similar small mixer/line splitter.

[damoore]

Or perhaps something like this looks like it might do the job too.

[Mike Stranks]

Splitters are a good idea and just... work. ART generally produce good stuff...

[Wonks]

Splitters

[damoore]

splitters

[Sam Spoons]

Black widdow spec sheet says 7.9 ohms. Could be less for replacements.

Are you sure that's not impedance Wonks? the DC resistance of the 18" shown on this page is 6.5 ohms?

You're right Sam. My brain turns off more often these days. Min impedance of 7.9 ohms.

Ha, +1, so does

And mine chaps

[Doug1963]

Thanks very much, I found a couple rack mounted 4 channel splitters that look like they will do fine :crazy:now I hope I have enough speakers and power to do a show:

1 Carvin 1600 watt(4x400) powered mixer
4 EV SH1512 ER Tops 2 each side in parallel running off powered mixer
2 Peavy 118 SUB HC Subs running thru a 1000 watt bridged Power amp

Good for a small club? Small outdoor? I would assume I would need more for outdoor

[damoore]

So you are planning to run a pair of the tops in parallel off one channel on the mixer?

That will work, but there is no real advantage in doing that and leaving the other two channels unused, other than perhaps saving you a couple of cable runs.

Those tops seem rather heavy at 75lbs each for what they pump out, about 124db each. But they will probably work for you in small to mediumish venues as long as the band is prepared to help schlepp.

[Doug1963]

The other 2 channels are going to be used for stage monitors

[Mike Stranks]

The other 2 channels are going to be used for stage monitors

Ah! You've opened a new can of worms now!

Do you mean side-fill monitors ('normal' speakers at the side of the stage pointing back to the musos) or the shaped monitor boxes on the stage in front of musos? Stage monitors (in front of musos) are not normally given a feed of the front of house mix or any other 'audience' mix. They usually have their own mix(es) fed from the pre-fade auxiliaries on the desk/board. And note: PRE-fade. If you work POST-fade (which is what you're intending) then any slight adjustment of the sound on the faders will be reflected in what is going to the on-stage monitors. This can be most disconcerting for the musos.