- yeahforbes
I've found that many amps, when the attenuators are wide open, clip with an input signal of about 0 dB (+4 dBu), which explains how you can run them with significant attenuation and just a hotter input and always achieve full wattage.
But if you open up the attenuators and you set your DRPA to limit just below that point, possibly around 0dB (figure this out with the speakers disconnected), you may actually be in good shape. You may say, "have the DRPA limit at 0dB? then I'm never even using the top 20dB of the DRPA's output converters!" And indeed, you would be running your mixer only in the negative numbers as well. But actually that allows you the entire positive range on the mixer output and DRPA input for unexpected peaks to be properly squashed. And with 24-bit digital, ~18dB headroom is actually pretty standard (many calibrations involve -18dBFS when 0dBFS is the clip point). If you had the limiters way higher (say, +15) and the attenuators down to compensate, then you need to run your mixer at +15 for full output and an unexpected surge of merely 5dB clips the DRPA input.
If you find that running the amp's attenuators wide open produces undesirable hiss, back them off until the additional hiss is indistinguishable from the always-present hiss, and then set the limiters for that point. And mark the attenuator positions with silver sharpie or something.
But if you open up the attenuators and you set your DRPA to limit just below that point, possibly around 0dB (figure this out with the speakers disconnected), you may actually be in good shape. You may say, "have the DRPA limit at 0dB? then I'm never even using the top 20dB of the DRPA's output converters!" And indeed, you would be running your mixer only in the negative numbers as well. But actually that allows you the entire positive range on the mixer output and DRPA input for unexpected peaks to be properly squashed. And with 24-bit digital, ~18dB headroom is actually pretty standard (many calibrations involve -18dBFS when 0dBFS is the clip point). If you had the limiters way higher (say, +15) and the attenuators down to compensate, then you need to run your mixer at +15 for full output and an unexpected surge of merely 5dB clips the DRPA input.
If you find that running the amp's attenuators wide open produces undesirable hiss, back them off until the additional hiss is indistinguishable from the always-present hiss, and then set the limiters for that point. And mark the attenuator positions with silver sharpie or something.
September 1, 2011 @04:56am
My PA (at least the relevant parts) consists of a Mackie Onyx 24.4 mixer, a DriveRack PA, and the QSC GX7 powering two pairs of JBL JRX115 speakers. (I'm also running a subwoofer amp and speakers, too.) The crossover has been properly set at 180hz 24db-LR curve. I got the GX7 specifically because it provides 1000w on each channel at 4 ohms, and each speaker pair consists of two JRX cabinets which are each rated 250w continuous, 1000W peak with an 8 ohms load. The math works out, yes? (500w to each speaker, twice the continuous rating.)
Anyways, I just bought the GX7, and have not been able to fully set up and tune the system in a live space, but I connected the DriveRack and amps to the mixer to set the gain structure and amp gains as best I could in my apartment, sans speakers. (No, I haven't observed it with a load yet.)
I followed dbx's instructions on setting gain structure (found on their website) as closely as possible, just as I would in a performance space, using proper pink noise and setting limiters as proscribed at the end of the procedure. However, the gains on my GX7 only ended up being turned to the "15" mark on the amp (a little past 9 o'clock) before they would start clipping. The DriveRack seems to have a beefy signal being put through it, kept short of internally clipping the DRPA via the dbx road to proper gain structure.
Am I wrong feeling a little disappointed that my amp must be dialed down so low, and should I worry that I'm not getting nearly as much power, headroom, and sig/noise ratio as I should be? I realize having the amp opened all the way is not ideal, either, but what should my end goal be? (For the record, my sub amp [Crown XLS5000 driving a pair of Turbosound TCS215] managed to gain up to about 3 o'clock or so.)
The club I work at uses a similar system, with a Midas Venice console fed through a DRPA into an XLS802 amp, and I notice that the meter lights on their DR show a much softer signal going to it than on mine, and they have their amps opened all the way up. Granted, I don't know how their DR is programmed, and I haven't sent actual pink noise through it yet, but it still makes me wonder what makes my Onyx's signal different from the Midas. (I would have guessed they both use +4db standards, but I haven't confirmed that.)
Anyways, I'll quit rambling. Thanks for reading!