Output levels on DCX2496

[mattwire]

Has anybody had any issues with output levels on a Behringer DCX2496 being far too high for the amps?

 

My setup (typically, though it is sometimes used with different amps/speakers):

DCX is fed by various balanced sources (FOH mixer, DJ mixer etc).

Outputs 1&2 take mono sum of A+B I/P and feed a Matrix UKP2100 in stereo mode for 2x Ohm RWS subs (~400W RMS).

Outputs 3&4 take A+B and feed a Matrix UKP1300 in stereo mode for 2x Ohm RW3 m/h's (~300W RMS).

 

The meters on the DCX have a scale from -40 to 0dbFS in five 10dB increments.

When I'm feeding a reasonable signal into the DCX I see only the first led (-40) light up, driven hard I get -30. This is with the input gain set to unity. This gives -40/-30 at the output meters as well which is more than enough to saturate the amps.

Given that the nominal max output level of the DCX is +22dBU and 0dB is +4dBU I would expect the metering on the DCX to display at least -20dBFS (given that +4dBU is -18dBFS). I would then expect the amps to clip much nearer to 0dBFS.

 

I'm thinking of putting 20dB attenuators after all the outputs of the DCX but that doesn't seem right in principle since to my mind the levels should be correct in the first place. Attenuators are also a lot easier to remove than a password lockout on the processor!

The limiters and dynamics within the dcx also expect higher levels since they have minimun thresholds of -24dbFS and -18dbFS respectively so I can't use them without increasing the levels at which point all the amps are already redlining.

 

Am I missing something obvious here? I really don't want to have to resort to attenuators or setting the input attenuators on the amps below maximum.

[Shez]

Whether or not to wind back the amp's input attenuator controls has been discussed in great depth here before. With that control fully clockwise, most amps will clip at around +4dBu; some even lower. You'll have to either drop the output levels on the DCX or wind back those controls. Which you choose will likely depend on whether the DCX is at FOH or in the amp rack.

 

Depending on the gig, I'll usually set up my gain structure so that +12dBu at the desk coincides with the limiter thresholds on the LMS. If you know what the maximum level your speakers will handle is, and you know the amp's gain, you can work out where to set the limiters and how much to drop the signal before it hits the limiters.

[TimmyP1955]

I prefer to have amps all the way up. Why? Let's say that you set up the system with the amps at 12:00, and all is well. If an amp accidentally gets turned up, your limiter setting is no longer valid. If you are pushing the rig had at the time, some drivers may well release their magic smoke.

[Shez]

That's about the only valid reason for running the amps that way. It's bad practice from a gain structure perspective though - you're wasting a load of headroom elsewhere in the signal chain and noise levels are likely to suffer as a consequence. Not so much an issue in R'n'R but it's very significant in theatre.

[mattwire]

Take it as a given that I need to run the system with the amps all the way up. It's a R'n'R rig not a theatre rig and knobs are just too easy for people to fiddle with. Plus, I'd have to remember their positions every time I run up the rig. Looks like inline attenuators may be the way to go, but I had been hoping to avoid this.

[Shez]

Looks like inline attenuators may be the way to go, but I had been hoping to avoid this.

Can't you wind down the levels on the outputs of the DBX? On my DR260, you can adjust each output level individually (though I seem to recall it's via the gains in the crossover). The limiter is the last dynamic bit of the chain so it will still do its job.

[Kevin Ross]

Looks like inline attenuators may be the way to go, but I had been hoping to avoid this.

Can't you wind down the levels on the outputs of the DBX? On my DR260, you can adjust each output level individually (though I seem to recall it's via the gains in the crossover). The limiter is the last dynamic bit of the chain so it will still do its job.

 

From looking at the circuit diagram it looks like the output gain is before the limiter and any EQ/dynamic EQ etc.

 

 

 

FYI the chain goes

 

 

 

Input source -> gain -> x-over ->EQ ->dynamic EQ ->Limiter ->polarity/phase ->delay ->DAC

 

 

 

I also use one of these units and sometimes I have to “pad down” the output level if I’m setting it up in a situation where the rig will be left unattended and I don’t want some enterprising DJ turning up the amps. For all other situations I utilize proper gain structure techniques

 

 

 

From memory you can take out 15db from each output and 15db at the input stage thus giving 30db of reduction possible before you have to get crafty with very wide EQ filters

[TimmyP1955]

I formerly used a DR480, and now use an RPM88. I've never had to gain down the amps or the DSP to get the headroom and lack of noise that I needed. The B must be a strange one.

[tall_phill]

Plus, I'd have to remember their positions every time I run up the rig.

 

Chinagraph Pencil!

 

Steal, er Borrow, one from the lighting crew. Put some security panels over the attenuators if you are worried about people messing with the settings.

[Matt Riley]

In this instance, surely if the OP is worried about people fiddling with the knobs, then it would be easier and cheaper just to bung an appropriate level pad in after the dcx. Sonically and electronically surely it is no different putting a pad contained in an XLR connector in, compared to using the input attenuators on the amp, which to my understanding are just variable pads?

 

Also, take the label off and hide that bit of wiring out the way at the top of the flightcase and no-one will ever know there's a pad in there! Really - it's not something people think about!

 

Matt

[cedd]

I'll confirm that the DCX unit (Behringer, not DBX as somebody above has put I think) does have extremely low metering when faced with a reasonably high input signal. I nearly sent mine back because of this - I had it connected to a mixer, was sending it a signal at a pretty hefty level, and was seeing nothing on the output meters (no speakers attached).

 

I'm pretty sure it's a problem across all of this model. I've never pushed it hard enough to find out what the actual headroom is, however I'd guess that it's less than the maximum levels shown on the meters would suggest. Other than that, great little box for the money!

[KevinE]

Ive set up a few of these and cant say Ive had any problems, we use the limiting function for nightclub systems so they have to be set accurately enough to prevent the DJ clipping the amps. The only thing I remember from the last one was that we wound down the input levels (rather than output) and then set the thresholds lower by the same amount. I simply kept an eye on the levels actually at the amps rather than the built-in output bargraphs on the Behr, and it was no problem to set gain reduction when the mixer was pushed over 0dB.

[Charlie Jeal]

Shez wrote:

"That's about the only valid reason for running the amps that way. It's bad practice from a gain structure perspective though - you're wasting a load of headroom elsewhere in the signal chain and noise levels are likely to suffer as a consequence. Not so much an issue in R'n'R but it's very significant in theatre. "

 

 

Nice to see that everyones focused on the front panel GAIN control for the amps as the problem, any amp can still reach its full output potential on any given setting of this control except - infinity when driven hard enough. IMNSHO its far better to know the input sensitivity of your amps and set crossover gains and limits accordingly to ensure the correct powering of your boxes. FWIW I've never seen big systems run with amp gain controls other than up at full, unless there's some gain shading going on within the array and in fact on some systems with proprietary dsp and sense lines e.g. Nexo Alpha NOT running the amp gain wide open causes the sense lines and therefore the dsp and limiting etc not to function correctly. Also bear in mind gain is gain wherever it is introduced in the system.

[tall_phill]

FWIW I've never seen big systems run with amp gain controls other than up at full

 

 

Off the top of my head - Billy Elliot, Melbourne, Australia. At least those parts of the system which aren't self powered (there is a lot of each).

[Charlie Jeal]

FWIW I've never seen big systems run with amp gain controls other than up at full

 

 

Off the top of my head - Billy Elliot, Melbourne, Australia. At least those parts of the system which aren't self powered (there is a lot of each).

 

 

Ok so that's one installed theatrical rig so far and as you are dealing with self powered stuff I can see why people would go fo the amp gains to match stuff up as an easy fix given the lack of parameter control offered by most powered boxes assuming that all matrixing isn't done via something like a Galileo, the same effect can still be achieved with bandpass output gains with your dsp of choice, gain is gain wherever it comes from or is subtracted from. I'll stick to my guns other than in gain shading within an array from arenas on down I've never seen people NOT running amp gains wide open. Also, as previously stated Nexo systems using sense line require you to run the amps this way otherwise the processor wont perform its function properly.

 

Strangely enough I havent seen anyone mention the amps input sensitivity yet, which also plays a huge part in these type of problems I.e if your amps have different sensitivities they will clip at different points as they will take more or less voltage to drive them into their full potential output, also wherever the amp gain is set to they can still be driven into clip.

 

IMNSHO correct setting of your DSP and limiters is far preferable to messing with amp gains where a system is constantly on the move.