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Author Topic: Why are the vocal mics clipping the amps?  (Read 25124 times)

Ryan Lantzy

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Re: Why are the vocal mics clipping the amps?
« Reply #20 on: January 23, 2012, 04:35:04 PM »

I actually did dial back the input sensitivity knobs on the main amp at the last show, with no result. I had them all the way down to 12:00. It made no difference. The only thing that helped me last night was to reduce the output gain on the back of the console.

Are the other signal lights lit up (signal, -20, and -10) when the clip lights illuminate?  If not, it could be that you have a shorted voice coil in one or more of the speakers.

This would definitely cause distortion and high power levels (when singers begin singing) and according to the PL236 manual, flashing clip lights without the other signal lights being lit would indicate short or extremely low impedance load.  It could be that the short is in the mid section of the speaker (where most of the sound power would be for vocals).

Another possibility - and I don't know if this amp does this, but it could be that the amp illuminates the clip light when the inputs are over driven.  Its not very common for that to be the case, but isn't out of the question.  If so, that would explain that when you turned down the output of the console, you noticed that it went away, and that turning the amp gains down did nothing to fix it.  That said, you typically need a pretty hot signal to overdrive the input of a professional amplifier.
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Deke Lockrem

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Re: Why are the vocal mics clipping the amps?
« Reply #21 on: January 23, 2012, 06:03:54 PM »

It's 6 of one and a half-dozen of the other whether you decrease the output of the board or the input sensitivity of the power amp(s). I tend to run amps wide open so I have control at the board and so nobody can "accidentally" turn the system up while I'm tied down at FOH.
That is not true for me. Decreasing the output gain from the mono out at the board DID reduce the clipping, but turning down the input sensitivity knobs at the amplifiers did NOTHING.

I'm not exactly clear on the complete configuration of the system, but I wonder if there is another gain-stage in there somewhere, possibly in an outboard EQ or system processor.
That said, it is noteworthy that it seems to occur  only with this one outfit and may be set off by the voices.
There is no outboard gear in my signal path. Mono out of console to input of UP15 crossover....out of crossover to amplifiers.

Are there simply more vocals in this group than any of the others?  Are they being run through/processed via a sub-group?  I think more info is needed........
Yes, this particular band has the most vocal channels of any band I provide for. They are not run through the sub groups.



By the way, the range of operation of the stereo mains output pot and the mono output pot are exactly the same.  Why should they be different?
Makes sense, but Samuel is correct. The output gain dial ends at 0 for the L and R main outs. In contrast, it ends at +6 at the MONO main out.
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g'bye, Dick Rees

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Re: Why are the vocal mics clipping the amps?
« Reply #22 on: January 23, 2012, 06:10:38 PM »


Makes sense, but Samuel is correct. The output gain dial ends at 0 for the L and R main outs. In contrast, it ends at +6 at the MONO main out.

Those are just numbers.  The net output of each is exactly the same per the manual, page 17:

"Mono Output Level. This knob controls the maximum level of the Mono Output signal. The signal can be attenuated to -40 dB and boosted up to +10 dB."

"Main Output Level. This knob controls the maximum output level of both the XLR and TRS main outputs. The signal can be attenuated to -40 dB and boosted up to +10 dB."

So whether you label it "0" or "+6", they both end up @ +10dB.
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Deke Lockrem

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Re: Why are the vocal mics clipping the amps?
« Reply #23 on: January 23, 2012, 06:18:16 PM »

Do you hear just distortion on the high end or any other changes such as a lack of power in the mids etc when the distortion occurs... Are you using the same mics as before, are you using subgroups on the console(does it even have subgroups)? Is the input on your processor clipping or going into peak as well as the amps or is it just the amps? Have you tried pulling down the instruments and having just vox in the mix when the distortion occurs(if this is possible maybe during a soundcheck)?

The vox may just be the tipping point here, there are times where I mix 8 vocals backed by an 7 piece band and this drives out meyer rig hard. Loudness is subjective and even your meterbridge isn't a clear indication of anything, maybe you are mixing louder? You say that you have had this rig for half a year, have you run it in this venue in this season. Humidity affects high frequencies and since it may be more humid you may be running the HF drivers hotter than you have for the last 6 months?
Im not sure about the first question, to me it sounds like a loud crackling noise. When it happens, I cant hear anything else.
They are using wireless mics. At first, I thought this may be the problem. After I switched them out to wired mics from my inventory, there was no change.....no subgroups.
There are no processors in my signal chain.
I actually did try to get the mics to clip the amp at the last show during soundcheck to no avail. I did this by intentionally increasing the input gain for that specific channel and could not get the red lights to glow and there was no distortion in the main speakers.

I doubt that humidity is a factor. It is very dry here during this time of year. This has happened at 3 different venues in the past two weeks, and only with this band. Although, you may be onto something with the vocals being the "tipping point". This band is also the ONLY one that uses all 16 channels of the SL. The clipping occurs when the band is playing and THEN the vocals are added. This would also explain why I could not get the mics to clip the amp when I tried on purpose.
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Deke Lockrem

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Re: Why are the vocal mics clipping the amps?
« Reply #24 on: January 23, 2012, 06:24:47 PM »

Those are just numbers.  The net output of each is exactly the same per the manual, page 17:

"Mono Output Level. This knob controls the maximum level of the Mono Output signal. The signal can be attenuated to -40 dB and boosted up to +10 dB."

"Main Output Level. This knob controls the maximum output level of both the XLR and TRS main outputs. The signal can be attenuated to -40 dB and boosted up to +10 dB."

So whether you label it "0" or "+6", they both end up @ +10dB.
That makes no sense whatsoever. Why would they label it differently? Why did they even put that damn know there in the first place.

Since you run yours wide open at the console, do you have to attentuate the amplifiers? Have you ever set/measured the gain structure with a meter? Im only asking because that was suggested to me by a collegue, but Im not thinking that would do alot of good in this situation since there is such a small signal chain.
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Deke Lockrem

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Re: Why are the vocal mics clipping the amps?
« Reply #25 on: January 23, 2012, 06:32:33 PM »

Are the other signal lights lit up (signal, -20, and -10) when the clip lights illuminate?
Yes. 

Another possibility - and I don't know if this amp does this, but it could be that the amp illuminates the clip light when the inputs are over driven.  Its not very common for that to be the case, but isn't out of the question.  If so, that would explain that when you turned down the output of the console, you noticed that it went away, and that turning the amp gains down did nothing to fix it.  That said, you typically need a pretty hot signal to overdrive the input of a professional amplifier.
True, you do need a VERY hot signal. But again, WHY only the vocal channels? The same vocal channels that, at the console, have the input levels in check....right where they're supposed to be. Never even a yellow light on the channel meter, much less a clip light.
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Re: Why are the vocal mics clipping the amps?
« Reply #26 on: January 23, 2012, 08:04:02 PM »

Th Mic levels are always set by the meters and like I said in the OP, are never above -10.

I actually did dial back the input sensitivity knobs on the main amp at the last show, with no result. I had them all the way down to 12:00. It made no difference. The only thing that helped me last night was to reduce the output gain on the back of the console.

Deke....

I think everyone wants to know what's causing this and where the problem lies.  Stubborn bunch, us.

I quoted the above just to triple check the basic information.

1.  Regarding the input settings of "never above -10".  I never set mine above -15.  The scale on the Fat Channel meters is in dBfs so theoretically if you're really never above -10 you shouldn't be in trouble.......with one or two channels running that hot.  However, before you get to the mains, everything is run through a summing amp.  The more signals you combine where you're right at the theoretical maximum, the more likely you are to get that digital "crunch" and ugly distortion.  With a group having fewer inputs you might slip by.  But vocals are highly dynamic and there's that old tendency to sound check at a level much, much lower than performance volume.  Plus any bleed into the mics from drums and amps just adds to the problem.

Since you state that dialing back the output pot on desk seems to start to address the problem maybe you could try dropping it 5dB/channel.  If you aren't using your sub-groups for anything else, try running the vocals through the sub-group so you can see the aggregate level on the SG meters.  If your combined vocal energy is over the top there you may have identified part of the problem.

Edit: 

From the good old Internet,

"There is no decibel to dBFS converter
 
Notice - Comparing dBu and dBFS: There is really no fixed
world standard like e.g. −20 dBFS = +4 dBu = 0dBVU.
The digital peak scale is not equivalent to the analog RMS scale.
 
You can never match dBFS and dBu."

This may be contributing to what's happening.  The dBFS metering on the channel just doesn't mean the same thing we were used to with a MixWiz or whatever preceding analog units we learned on. 

Here's a link to a discussion of the "problem".  I do not say that it offers any answers, just that it shows where problems may occur in switching over to digital.

http://www.soundonsound.com/forum/showflat.php?Cat=&Number=46284&Main=45794

If after all this we ASSume that -12dBFS converts to an analog console meter reading of +4, then your -10 setting is getting well into the "yellow" range of the old analog console LED'S.
Like I said about the arbitrary numbering of the levels on the output pots on the rear of the SLive, numbers themselves are meaningless until you understand what they mean in voltage.  I don't pretend to understand the answer, but I do think I understand the problem...... 
 

2.  You say lowering the input sensitivity on the amps didn't do anything, but you also state that you turned it down only as far as 12 o'clock.  Did you try turning it down until the clip lights quit illuminating?  Going only as far as "straight up" doesn't really get to it.  You've only reduced the input sensitivity by approximately half, assuming linear response.  It is entirely possible that you could reduce the input level further and get to the point where the problem goes away or is manageable.

It does seem to me that the first thing to check is the consistency and overall levels through the signal chain.  What works for fewer inputs and a less demanding band may be fine.  But if you're already on the edge before getting to the larger ensemble it would seem possible that going back and reducing signal levels right from the start would be a logical attempt to rein things in.

Not to criticize, just trying to help.

Best.

DR   
« Last Edit: January 23, 2012, 08:23:02 PM by dick rees »
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Paul van Dort

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Re: Why are the vocal mics clipping the amps?
« Reply #27 on: January 24, 2012, 08:29:26 AM »

Deke,
do you hear distortion in the headphone of mains?
Cheers,
Paul
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Ryan Lantzy

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Re: Why are the vocal mics clipping the amps?
« Reply #28 on: January 24, 2012, 08:33:49 AM »

Yes.  True, you do need a VERY hot signal. But again, WHY only the vocal channels? The same vocal channels that, at the console, have the input levels in check....right where they're supposed to be. Never even a yellow light on the channel meter, much less a clip light.

Just because you don't see a yellow/amber light or a clip light on the console doesn't mean you can't overdrive an amp. 

The input sensitivity of a PL236 at 4 Ohms is 1.07 V.  That's  ~2.8dBu.  Not sure where 0 dBFS is on your mixer but I'd imagine if it's like most gear it's around 22 dBu, which would mean you could clip the amps at -19 dBFS on the mixer.

As to why it's only the vocal channels?  Well, I'd imagine that is just perception.  During the musical parts, the rig is just humming right along right below clip, and then the vocalists sing, and it puts you over the edge. 

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Mike Christy

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Re: Why are the vocal mics clipping the amps?
« Reply #29 on: January 24, 2012, 11:33:20 AM »

I think Dick's and Ryan's posts have a lot of meat to them.

For example, on my 01V96 the first yellow LED on the main output scale is almost 0db on my PL236, the 3rd LED certainlly is clipping. There are several more main scale LEDs that never light.

Depending on the input signals, I could have the main output fader at 5 10 or 15, depends on my gains, compression, channel faders, room, number of peeps... it depends.

Can you run a test sine wave through your desk and to the amp ( speakers unplugged ) to verify and correlate when the amp clip LED illuminates, in reference to the mixer output?

It could be that the LEDs on that mixer design are not fast enough, they just show you an average (RMS)  value, and youll never see those quick clip peaks.

Mike


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Re: Why are the vocal mics clipping the amps?
« Reply #29 on: January 24, 2012, 11:33:20 AM »


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