Amp input sensitivy

S

Steve Ferreira

Sophomore
Jan 12, 2011
189
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16
Toronto, Canada
Hi guys,
I had an issue with my IT8000s. I noticed that when I made the switch over to these amps, the console didn't have to be driven as hard as before to get "loud". I just basically opened SA and went straight to the x-over, limiters and eq section. I just checked if it was setup for analogue and was done with it. After scratching my head for sometime, and just finally getting to play with the amps at home I realized that the input gain was set to 39db.

Now I know that some amp manufacturers like Crown and LabGruppen offer different input sensitivies via dsp or dip switches.
My question is? What input sensitivity do most of you run? All my mains amps are the same, so I will set both the same. I have heard 32,36.....

All comments are welcome.

TIA
 
Re: Amp input sensitivy

The I-Techs have configurable input sensitivity in System Architect. Don't just turn down the front knobs, as you'll significantly sacrifice input headroom, because with the I-Techs you have to watch for both input and output clipping.

Ideally, all your amps should use the lowest gain possible to achieve maximum (or required) output, so as to keep the signal to noise ratio high. Does anyone do this? Not really. Most people run amps wide open, and keeping all amp gains the same makes it easier to calculate limiters. From what I've seen, 32dB is most common.
 
Re: Amp input sensitivy

Silas Pradetto said:
The I-Techs have configurable input sensitivity in System Architect. Don't just turn down the front knobs, as you'll significantly sacrifice input headroom, because with the I-Techs you have to watch for both input and output clipping.
Click to expand...

This is not unique to I-Techs. On nearly any amplifier turning down the input gains can cause input stage clipping. Additionally, most amps will have no visible indication of this condition. Not to mention that the these knobs may not be as accurate as setting the gain in the crossover.

Ideally, all your amps should use the lowest gain possible to achieve maximum (or required) output, so as to keep the signal to noise ratio high. Does anyone do this? Not really. Most people run amps wide open, and keeping all amp gains the same makes it easier to calculate limiters. From what I've seen, 32dB is most common.
Click to expand...

Most do run amps wide open, but most amps have several 10s of decibels more S/N ratio to start with. A really easy way to check and see if an amp (running WOT) is the cause of noise is to just unplug the signal from the amp. If it's silent, then your noise source is elsewhere.

If that is indeed the case, turning down the output of your crossover will yield the same result as turning down the input of your amp.
 
Re: Amp input sensitivy

While I am not an amplifier designer, I'd say that a DSP amp such as the I-Techs are more sensitive to input overloading, since the line inputs feed an AD stage rather than analog circuitry. In the case of the I-Techs, the front panel knobs are digital gain controls, and have no effect on reducing the analog input level. So for every dB of gain reduction in the digital realm, there is that much less overall headroom. Not good. The input sensitivity selection in the System Architect menu actually changes the analog input sensitivity, which is the right thing to do.
 
Re: Amp input sensitivy

I use 32dB on my lows and subs. I use 26 on my mids and highs. I found it a little better sounding with the mids/highs on 26 dB but of course I had to take this into account when I set my limiters.

I also have a bunch of amps for my small rigs that have either 0dB, 2dB or 1.4volt sensitivities rather than a constant gain. Just more math involved.
 
Re: Amp input sensitivy

Even amplifiers with DSP built in have an analog input stage. Only an idiot would design the input stage that would overload and go right into hard 0dBFS. I don't have schematics for an I-Tech, but I believe you're very mistaken.

Most likely, the input stage of the amp is not adjustable at all in the analog realm. I probably samples the input and has some very conservative max input above 22 dBu. After it samples it, and it's in the digital realm that's where any adjustment in gain would take place. Again, I don't know this for sure, but I would bet this is the case. Therefore, I think the exact opposite is true and a low gain setting would very likely not affect the headroom as far as the input is concerned.

I.e. if changing the input sensitivity actually changes the gain in the analog input stage, then this could cause input clipping. If the knobs turn down some other digital gain stage, there will be no clipping.
 
Re: Amp input sensitivy

Shouldnt all the input gains in the different amps for each band pass be the same. Your cross over points expect each gain to be the same for summing of under laped / over laped points do they not.
 
Re: Amp input sensitivy

Travis Watson said:
Shouldnt all the input gains in the different amps for each band pass be the same. Your cross over points expect each gain to be the same for summing of under laped / over laped points do they not.
Click to expand...

Your crossover points expect whatever gain they're designed for; most all manufacturers that provide tunings don't care what the gain is, as long as they're all the same. Of course, the gain will matter when setting limiters.

If you're rolling your own tunings, or adapting manufacturer tunings to a custom configuration, you can do what you want, as long as the numbers are juggled around properly.

Yes, bandpass gain would have a lot to do with where the acoustic crossover lands.
 
Re: Amp input sensitivy

Silas Pradetto said:
Your crossover points expect whatever gain they're designed for; most all manufacturers that provide tunings don't care what the gain is, as long as they're all the same. .
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That was my point they should all be the same. IF you have the botom amp with a input sensitivity of 26 and the mid amp is 39 and the highs are 32 and the manufactures tuning is expecting them to be the same your just going to have a mess. And limiters as well.
 
Re: Amp input sensitivy

Travis Watson said:
That was my point they should all be the same. IF you have the botom amp with a input sensitivity of 26 and the mid amp is 39 and the highs are 32 and the manufactures tuning is expecting them to be the same your just going to have a mess. And limiters as well.
Click to expand...

I wasn't quite sure what you meant at first; you're absolutely right.
 
Re: Amp input sensitivy

My rack has 2x IT8000s for my tops and subs, and 2xXTIs for monitors. I was just concerened with the 8000s, as they where set to 39db, the system seemed to get loud with very little input.
 
Re: Amp input sensitivy

Steve Ferreira said:
My rack has 2x IT8000s for my tops and subs, and 2xXTIs for monitors. I was just concerened with the 8000s, as they where set to 39db, the system seemed to get loud with very little input.
Click to expand...

The I-Techs are set to maximum input level of 15dBu by default, they should be changed to 21dBu if you have the drive voltage.
 
Re: Amp input sensitivy

Silas Pradetto said:
Originally Posted by Steve Ferreira View Post
My rack has 2x IT8000s for my tops and subs, and 2xXTIs for monitors. I was just concerened with the 8000s, as they where set to 39db, the system seemed to get loud with very little input.
Click to expand...
The I-Techs are set to maximum input level of 15dBu by default, they should be changed to 21dBu if you have the drive voltage.
Click to expand...

The issue is that the XTI have a fixed 1.4v. input sensitivity. For the XTI4000, that's a voltage gain of 34.2dB. Changing the I-Tech input sensitivity to 2.82v. will give it the same voltage gain as the XTI (play with it in System Architect). Personally, I think 34db is still too much.

You can take another 6db at the XTI input attenuators, but the inputs will be clipping when the outputs clip.

Have fun, good luck.

Tim Mc
 
Re: Amp input sensitivy

The I-Techs have both peak voltage and RMS ("power") limiters, and they offer different types of protection. The PVL will prevent cone over-excursion on peaks (kick drums, dropped mics, that kind of thing) whereas the RMS limiter is designed to keep you from applying more long-term power than your voice coils can dissipate.

FWIW, we primarily use the PVL (they're preset in the VerTec V4 files). In the VerTec setting the RMS limiter is not engaged, nor are the numbers that appear S.A. correct for the pass bands.... I think that tells us what JBL considers important. We haven't burned up a coil to date.

If I were in your position, I'd knock down the XTI inputs by 6db (internally, using System Architect) and set the I-Tech sensitivity to 28db.

While I would normally agree with Silas' comment about setting the max analog input level to +21, in your case you'll never get close before you're clipping the XTIs, so you can safely leave it at +15.

Have fun, good luck.

Tim Mc
 
Re: Amp input sensitivy

Tim McCulloch said:
FWIW, we primarily use the PVL (they're preset in the VerTec V4 files). In the VerTec setting the RMS limiter is not engaged, nor are the numbers that appear S.A. correct for the pass bands.... I think that tells us what JBL considers important. We haven't burned up a coil to date.
Click to expand...
My money's more on a) You guys are pros and don't run your rigs to the ragged edge b) JBL was worried if they put in reasonable power limiting numbers nobody would use them because they'd seem too low.
 
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