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Author Topic: 4000 xti UCS1 subs limiter question  (Read 6433 times)

Greg Percy

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Re: 4000 xti UCS1 subs limiter question
« Reply #10 on: January 26, 2013, 08:23:36 AM »

I did a little A/B/C testing---all were at sound levels where I normally use the subs

1    a Plx1602 bridged at 8ohms for each sub---OK sounding-levels were into the -10 clip to get sound I like

2     xti4000 in stereo-really had to push near/into clip to get similar sound to 1

3     xti4000 bridged at 4 ohms-daisy chained to the 2 subs-barely, if ever tickled the -10 clip---sounded a lot more "ballsy" (robust?) than other alternatives---

Does this mean if I set my limiter on xti to -3 I should be fairly safe?

Thanks
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Tim McCulloch

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Re: 4000 xti UCS1 subs limiter question
« Reply #11 on: January 26, 2013, 12:11:26 PM »

I did a little A/B/C testing---all were at sound levels where I normally use the subs

1    a Plx1602 bridged at 8ohms for each sub---OK sounding-levels were into the -10 clip to get sound I like

2     xti4000 in stereo-really had to push near/into clip to get similar sound to 1

3     xti4000 bridged at 4 ohms-daisy chained to the 2 subs-barely, if ever tickled the -10 clip---sounded a lot more "ballsy" (robust?) than other alternatives---

Does this mean if I set my limiter on xti to -3 I should be fairly safe?

Thanks

Here's the deal... when you put an amp in bridged mode, the input sensitivity increases by 6dB.  If you were to turn down the drive level (or turn down the amp input knobs) by 6dB to compensate, you'll find that #2 will be the result.

The clip lights do not indicate impending doom unless they stay on.

#1... the is no such thing a "-10 clip".  The LED flashes when the amp is around -10dB from full output.
#2... the amp is 6dB less *sensitive*, i.e. it takes more input level to drive the amp to FTB (full tilt boogie).  This does NOT reduce the amount of VOLTAGE capability available to drive the speaker, only that it will take more signal to do so (6dB more) when compared to bridged mode.
#3... the reason is sounds "more ballsy" is because the amp is getting louder, sooner, (the 6dB thing) than the rest of the rig.

If you want to make valid listening tests, you MUST match the levels of the pass band.  My guess is if you do this you'll like the sound of the stereo amp better.  It will also run cooler and live longer as a result.

But what do I know? 

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"If you're passing on your way, from Palm Springs to L.A., Give a wave to good ol' Dave, Say hello to progress and goodbye to the Moonlight Motor Inn." - Steve Spurgin, Moonlight Motor Inn

Greg Percy

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Re: 4000 xti UCS1 subs limiter question
« Reply #12 on: January 26, 2013, 01:16:33 PM »

What do you know? Clearly a lot more than I do...

Thanks for the information and staying with me on this....

GP
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Greg Percy

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Re: 4000 xti UCS1 subs limiter question
« Reply #13 on: January 26, 2013, 01:20:19 PM »

SO in scenario 2 (xti in stereo) if I were to boost my output level on my DSP (Behringer 2496) 6 db it might be a better comparison...

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Steve Montambault

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4000 xti UCS1 subs limiter question
« Reply #14 on: January 26, 2013, 01:23:09 PM »

I've always run my UCS1Bs in stereo off of a PLX3602 .. Never an issue.
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Steve Montambault
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Tim McCulloch

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Re: 4000 xti UCS1 subs limiter question
« Reply #15 on: January 26, 2013, 02:39:08 PM »

SO in scenario 2 (xti in stereo) if I were to boost my output level on my DSP (Behringer 2496) 6 db it might be a better comparison...

From a best practices standpoint, no, although it will actually work even if it's a bad idea.  Read on....

What you're doing is changing the relative balance between the subwoofer and the top box.  The balance between those pass bands is partially a matter of personal taste, genre of program material, and the intended application of the system.   What you're liking about the whole bridge mode thing is not more current, but the difference in the way the rig sounds with your choice of program material when the subs are 6dB hotter.  Turn down the top boxes by 6dB (at the DSP), run the sub amp in stereo.  You won't need any special cables or patch panels, either. ;)

Some other considerations -

One of the system engineering reasons you *don't* want a whole bunch of amplifier input sensitivity is that it amplifies the noise ahead of the amp... every EQ hiss, induced hum, console noise... is amplified MORE when the amp has higher sensitivity.  In our I-Tech powered rigs, it's common to have input sensitivity in the 4-6v range.  Considering the XTi 4000 has input sensitivity of 1.4v for FULL TILT BOOGIE output, when you give it 6dB more sensitivity, that means 0.35v  (yes, 350 millivolts) will drive the amp to provide its maximum output voltage.  See how small noises got louder?

From a practical standpoint, do you really want to make it super easy to drive a pass band to its maximum output?  If you like to see your mixer meters move (especially if you like to see red lights), having greater amp sensitivity (or hotter drive levels from the DSP) means little mistakes can have 6dB greater consequences.  Or to run your mixer at a comfortably hot level, you have to turn down the DSP outputs... the analog output noise floor of the DSP does not change, so you've effectively reduced your S/N ratio (as in, the noise is now louder).

There are lots of possible considerations, and since we can't magically teleport ourselves to your gigs and see/hear what those gigs are like and how you approach them, we have to do some speculatin'.... but if you're doing live music, the chances are if you're actually hitting a 30% duty cycle (frequent clipping) at 8 ohms, your subs are already into power compression and throwing more voltage into them will not generate more acoustic output.  This is where the extra voltage in bridge mode does you no good; it makes the amp run hotter and you'll use more electricity but get little to nothing in terms of acoustic output.

« Last Edit: January 26, 2013, 02:41:45 PM by Tim McCulloch »
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"If you're passing on your way, from Palm Springs to L.A., Give a wave to good ol' Dave, Say hello to progress and goodbye to the Moonlight Motor Inn." - Steve Spurgin, Moonlight Motor Inn

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Re: 4000 xti UCS1 subs limiter question
« Reply #15 on: January 26, 2013, 02:39:08 PM »


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