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single sub PA - phase issue

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single sub PA - phase issue

Postby Dave71 » Sun Mar 08, 2020 10:54 pm

Hello all

Think I’m making a school boy error so maybe an embarrassing question but I’m first to stick my hand up when I’m stuck

So...first dress rehearsal with the band today using my qu-16, Rcf 422 tops and recently acquired Rcf 8003 as sub.

Took L&R out of the desk into the subs 2 xlr inputs, then linked 2 tops out of the sub with crossover set at 120hz.

Fired everything up and hooked up my iPhone for some tunes to have a tinker with the eq. Phone inputted into 2 mono channels, linked to create a stereo input. Sounded great after some tweaking.

Hooked up the band for a test run and no bottom end from the kit or bass guitar, probably same with the keys but never got that far.

Kit is a Roland tdk into 2 mono inputs, bass into a mono channel, keys top and bottom into 2 mono channels.

Basically, taking the L&R out of the desk is causing phasing on the sub inputs, cutting out the bottom end. The signals are going out to the tops fine and we’re getting the slight stereo imaging as requested by the drummer but without any bottom end.

So, with the drums I changed the right ch input polarity in the desks input section which cured that but no joy with the bass guitar. Removing one of the subs inputs cured that instantly.

I’ve worked with a couple of bands who have used a more or less identical setup with no issues, but somethings not right here.

I feel like I’m missing something obvious / doing something very wrong. didn’t have much time to investigate at rehearsal and cant exactly rig everything up at home.

Please enlighten me. Thanks.
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Re: single sub PA - phase issue

Postby Hugh Robjohns » Mon Mar 09, 2020 12:39 am

One of your XLR cables between the desk and sub is wired up backwards -- polarity reversed?
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Re: single sub PA - phase issue

Postby MarkPAman » Mon Mar 09, 2020 12:53 am

+1

I guess the cable you used to test with your phone is also incorrectly wired - or you had the polarity switched on one of the inputs you used.
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Re: single sub PA - phase issue

Postby OTE2020 » Mon Mar 09, 2020 9:20 am

Quickest way to tell if in ''in-phase'' or not is to turn one of the subs around on the floor (180 degrees).... if you gain 3dB your subs were out of phase / cable out of phase if you gain nothing then it could easily been a whole list of things but my guess like everyone else here is a phase issue
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Re: single sub PA - phase issue

Postby Wonks » Mon Mar 09, 2020 9:38 am

OTE2020 wrote:Quickest way to tell if in ''in-phase'' or not is to turn one of the subs around on the floor (180 degrees).... if you gain 3dB your subs were out of phase / cable out of phase if you gain nothing then it could easily been a whole list of things but my guess like everyone else here is a phase issue
But as clearly stated, the OP only has one sub!
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Re: single sub PA - phase issue

Postby OTE2020 » Mon Mar 09, 2020 9:58 am

Then it will make no difference if its in-phase or out of phase - You might have some odd standing waves in the room it was in, at which point move the sub about / have a play with different HPF as well which can affect the phase / might gain some level OR if you need a little ''boost'' throw it in the corner lol
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Re: single sub PA - phase issue

Postby Wonks » Mon Mar 09, 2020 10:11 am

The sub has a quoted frequency range of up to 120 Hz. However, the frequency response chart of the sub shows this to be an untruth , as at about 105Hz the frequency response is 30dB down on the peak output (125Hz figure is probably 50dB below peak.

https://www.rcf.it/c/document_library/g ... upId=20195

The available xover settings are 60, 80, 100 and 125Hz, so I'd definitely set the Xover to 80Hz maximum to avoid leaving a hole in the frequency response. The xover has a 24dB/octave slope so is very steep. So set to 125Hz, the tops won't be putting out anything much below 120Hz and the sub hasn't got a lot above 80Hz, so it may be that you are simply hearing that big frequency hole due to the crossover frequency setting. It would certainly be very noticeable.

18" subs are great for the really low end, but really need a 15" sub as well to do the upper bass if you want the tops to only handle down to 125Hz. The tops will certainly handle down to 80Hz with no issues, but you aren't maximising their sound output.

So certainly change that crossover frequency. You may well also have a cable phase issue, but sort out the crossover first and then consider phase if you still lack low-end punch.
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Re: single sub PA - phase issue

Postby Wonks » Mon Mar 09, 2020 10:14 am

OTE2020 wrote:Then it will make no difference if its in-phase or out of phase - You might have some odd standing waves in the room it was in, at which point move the sub about / have a play with different HPF as well which can affect the phase / might gain some level OR if you need a little ''boost'' throw it in the corner lol
Yes it will if one of the input signals is out of phase to the other. Summing to mono for the sub signal will normally cause most of the low-end signals to cancel out.
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Re: single sub PA - phase issue

Postby OTE2020 » Mon Mar 09, 2020 10:26 am

My point was if the driver is wired out of phase this will make NO difference to the output / many companies wire drivers out f phase for many reasons like JBL & d&b Audiotechnik also run the HF out of phase on the older Q & C series boxes
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Re: single sub PA - phase issue

Postby Dave71 » Mon Mar 09, 2020 10:50 am

It never entered my head that an xlr would be wired incorrectly, Half an hour with multimeter in hand and found just that. 1 rouge lead.

Hooked up the mixer and sub at home this morning and recreated the issue using the faulty cable then swapped it for a new lead. Sorted.

My sub is the older 8003as, not the as-ii so only 80 & 120 available on the crossover. We only had the hall for 3 hours so were rushed. Booking a full afternoon next time so will have more time to experiment and fine tune.

Thanks for the prompt and informative replies.
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Re: single sub PA - phase issue

Postby Hugh Robjohns » Mon Mar 09, 2020 11:04 am

Dave71 wrote:It never entered my head that an xlr would be wired incorrectly, Half an hour with multimeter in hand and found just that. 1 rouge lead.

Where do I collect my prize? :lol:

Commercially manufactured leads should be okay, obviously, but they get broken and repaired and that's when errors creep in... and home-made ones should always be treated with suspicion until proven good. :D

And this is why a cable tester is an essential accessory....

H
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Re: single sub PA - phase issue

Postby shufflebeat » Mon Mar 09, 2020 11:11 am

Dave71 wrote:1 rouge lead.

Daring.

Sunbaked terracotta sounds more integrated.
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Re: single sub PA - phase issue

Postby MarkPAman » Mon Mar 09, 2020 11:59 am

Hugh Robjohns wrote:
Dave71 wrote:Commercially manufactured leads should be okay <snip>

And this is why a cable tester is an essential accessory....

H

Though no always!

When I needed a longish 16-way XLR loom in a hurry, I bought one from a usually very good supplier, though it was from their budget range. Turned out all 16 were wired wrongly.. Once I knew that, I was able to just press the polarity buttons on the desk I was using- so no big problem, and the supplier took it back for a full refund, so a good result for me in the end.

So a quick test is always worth doing.
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Re: single sub PA - phase issue

Postby Dave71 » Mon Mar 09, 2020 4:55 pm

shufflebeat wrote:
Dave71 wrote:1 rouge lead.

Sunbaked terracotta sounds more integrated.

Rouge is all the rage, black is.......so last season!

Hugh Robjohns wrote:
Where do I collect my prize? :lol:

H
Just send me your full name, address, date of birth and bank details and a surprise will emerge very soon ;)
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Re: single sub PA - phase issue

Postby Hugh Robjohns » Mon Mar 09, 2020 4:58 pm

Dave71 wrote: Just send me your full name, address, date of birth and bank details and a surprise will emerge very soon ;)

Excellent... :bouncy: :think:
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