Amp advice

[gafftaper]

Disclaimer: I'm am a competent sound op but not an expert in the noise department.

We were advised by a theater sound system designer to purchase a QSC CX502 to power a JBL ASB6118 Sub.

Amp:
Bridged Mono at 8 ohms 1100 Watts

Sub Specs:
Transducer (AES) Power Rating:1200 W (4800 W peak), 2 hrs
Long-Term System Power Rating:800 W (3200 W peak), 100 hrs

We cranked it up and BAM distortion on all the big bass notes. The installer says theres not enough head room and it's distorting when it peaks, so I need a bigger amp.

I'm hoping to return it and upgrade to a QSC CX902 which hits 2000 Watts at 8 ohms, bridged. Does this sound to you like it will fix the problem?

[avare]

Looking at the specs for the speaker and into consideration you wrote about a theater sound system designer, I have one question immediately. How loud are you driving the system?

Going from 1,200 W to 2,000 W is less than a 3 dB increase. If 3 dB is the difference, try first reducing the output by 3 dB and see if stops the major distortion.

Andre

[David Ashton]

You have to make sure the gain structure is set correctly all through the system, only then will you be sure where the distortion is entering the system.90+% of distortion problems I meet are gain structure related.

[DaveySimps]

I agree that proper gain structure is key to a good sounding system. As a rule you want your amps to be 1.8 to 2x the watts of your speakers RMS rating. For instance, if you have a speakers rated at 500W than your amp should be at least 900W or 1000W so that you can handle the peaks in your program material. If you go to big with your amp, you greatly increase blowing your speaker.

Looking at your spicific case, I would inclined to get a QSC CX1102 (if you want to stay in that line of amps). It offers a wattage in the range I listed above. The CX902 might still be a little weak on those peak program signals. The 502 that was recommended to you is definately under sized for your usage in my opinion. This could definately cause the distorted sound you were hearing on the "big bass notes" since these represent more peaks in the program material you were sending to the speaker.

~Dave

[avkid]

Originally Posted by DaveySimps

As a rule you want your amps to be 1.8 to 2x the watts of your speakers RMS rating.

I tend go on the lower end of the scale at 1.5-1.8 x RMS.

[DaveySimps]

Even based on your preferred formula, that amp that was recommended is still undersized. If you go with the 1.8 value, he will find a workable solution.

~Dave

[gafftaper]

Just got word that they will take it back with a 15% restocking fee but I need to upgrade within the QSC product line. I'm really happy because it's been sitting here waiting for install for 6 months so I was sure they wouldn't take it back. I'm checking on the price for that 1102... that's double the power and one of the most powerful amps QSC sells. Sounds like every watt helps.

[DaveySimps]

Originally Posted by gafftaper

Sounds like every watt helps.

It especially makes a difference when you are talking about subs.

~Dave

[museav]

As Andre noted, if you change to the CX1102 you are talking about a maximum 3dB increase and that is only relevant to the maimum peak level if you really push the sub.

Before you go getting a different amp, I would first verify some other potential factors. Are all the amp settings correct and is it wired correctly, especially in regards to the bridge mode operation? Has the installer set the system gain structure? Are you clipping the amp input or anywhere else in the signal chain? Have they tuned the system and can someone confirm that appropriate filtering and processing, including high pass filtering for the sub, has been applied? Any of these could also be related to the distortion you noted and you probably want to eliminate these potential causes before spending more money on a different amplifier.

Also, just because you can go potentially go that loud does not mean that was the intent. You might want to find out if there was some target level that was defined, it may be that the system is working exactly as intended.

[gafftaper]

Originally Posted by museav

As Andre noted, if you change to the CX1102 you are talking about a maximum 3dB increase and that is only relevant to the maimum peak level if you really push the sub.

Before you go getting a different amp, I would first verify some other potential factors. Are all the amp settings correct and is it wired correctly, especially in regards to the bridge mode operation? Has the installer set the system gain structure? Are you clipping the amp input or anywhere else in the signal chain? Have they tuned the system and can someone confirm that appropriate filtering and processing, including high pass filtering for the sub, has been applied? Any of these could also be related to the distortion you noted and you probably want to eliminate these potential causes before spending more money on a different amplifier.

Also, just because you can go potentially go that loud does not mean that was the intent. You might want to find out if there was some target level that was defined, it may be that the system is working exactly as intended.

We were running it loud, but speaking loudly, I could still be heard by someone three feet away. Yelling I could be heard 30 feet away in the booth. I would say it was no louder than you would hear in a movie theater. It was definitely not concert volume. The audio being played was a Blueman Group 5.1 audio DVD so it had some big bass in places, but also wasn't loose hip hop bass... we heard the "bad sound" while listening to "Hotel California" too. The technician doing the install was quite certain it wasn't his gain settings.

The installers are bringing a bigger amp in to test in the next day or two. Just to verify what's wrong.

Thanks guys.