QSC PLX 2402 and Pre-Amp Compatibility

Tobias Wilcox

Tobias Wilcox

Aug 18, 2006
789
392
4,831
London, ENGLAND
#1
Just got tis QSC power amp to go with my SWR Grand Prix Pre. I intend running it in stereo so that I'm putting 700w into my 610.

The SWR manual states-

"For a power amp to be compatible with the Grand Prix while using the unbalanced out jacks, its sensitivity must be 7V RMS or LESS and have an input impedance of greater than 600 ohms."

Will the 2402 be driven hard enough by the Grand Prix?

Is it best to run the 2402 with the attenuator fully up and control from the pre?
 
#2
Yes, 7 volts is about 9dB more than is necessary to drive the 2402 to full rated output.

Start with the attenuators fully up, and if you find that you have more than enough drive signal you can reduce them, but I would recommend leaving the limiters of the PLX on and reduce the attenuators by 3dB which will get you 6dB of limiter compliance before the output driver of the preamp distorts. I would also recommend the HPF's on and set for 30Hz.

If you plan on bridging, you will need to look up the rated sensitivity, sometimes it's the same, sometimes more sensitive and sometimes less sensitive depending on the way the internal bridging circuitry is designed. Usually it's the same, resulting in 6dB additional power amp voltage gain (will sound louder, but just more sensitive for the given output power).
 
  • Like
Reactions: BasturdBlaster and Bob Lee (QSC)
#4
deepbluesuk said:
Hey, thanks for that! Sounds as if there shouldn't be a problem there. I don't plan on bridging as the PLX 2400w @ 4 Ohms and my cab being 900w may not enjoy that?
Click to expand...
Nice to see a good application of common sense here. I think you will be happy with the results.

(I toured with a few dozen PLX amps up until a few years ago, one of the most reliable SMPS based power amps of its day. I still have maybe 10 or 12 that I use for one-offs).
 
  • Like
Reactions: deepbluesuk
#6
murphy said:
It was not a good amp for my Alembic F2b
The SWR stereo 800 power amp was heaven
Click to expand...
The output impedance is very high on the F2b which results in a lot of loss (and some loss of low frequency) when driving a 22k ohm input impedance at high (line) level like the QSC PLX (IIRC).
 
  • Like
Reactions: murphy
#7
agedhorse said:
The output impedance is very high on the F2b which results in a lot of loss (and some loss of low frequency) when driving a 22k ohm input impedance at high (line) level like the QSC PLX (IIRC).
Click to expand...

Yep. Here's a spitball LTspice model of the loading effect on the F2B of changing the power amp input impedance downward from 50K ohms to 5K in 5K increments:



F2B_loading5K_50K.jpg
 
  • Like
Reactions: JGR, murphy, BasturdBlaster and 1 other person
#9
deepbluesuk said:
I was after a SWR Power 750 and I'm beginning to wish I'd held out for one. Not feeling the "whump" that I was hoping for from the QSC after running a couple of pre's through it.......
Click to expand...
Not due to the power amp, but perhaps due to the driving capabilities of the preamp, or the input module setting on the power amp. Where are the HPF's filters set on the power amp? Are you bridging?
 
#13
Shouldn't make any difference between stereo and parallel.

What cabinets are you using? When you say you miss the whump, did you have it with these specific cabinets or others?
 
#15
deepbluesuk said:
Ashdown 610 - yes, definitely there before and ran the SWR pre through the FX return of a couple of different 500w amps, absolutely loads of clout.
Click to expand...
I don't know why you aren't getting what you want. I am out of ideas, sorry.
 
#17
deepbluesuk said:
Thanks for all your suggestions. i guess that pre just isn't compatible.
Click to expand...
Maybe the amp you used before is not flat response, but has a small rising low frequency response. Some MI power amps do voice their power amps in a way that suits their marketability.

It's really hard to say, but I'm confident that it's not just the amp itself (I have just too much experience with them). There's something else going on, I just don't know what.
 
You must log in or register to reply here.
Top Bottom
Back