A few live sound questions.

[funky]

I've got my band's PA set up in my basement right now. I've been giving it a good listen and come up with a few questions. I'd like input from those of you with some tech knowledge.

 

Thank in advance for your input!!

 

We are running a tri-amped system, 18" Folded Horn Sub, 15" mid, horn. We are in mono with two QSC power amps. One is a USA 1300 running bridged out to the two subs. The other is a USA 800 that is running in stereo. One side powers the mids and the other side powers the horns. We have a Peavey XD crossover. The entire signal chain is wired with balanced cables.

 

1. X-over Frequencies

 

The Peavey XD crossover manual says that the sub/mid x-over point should be set to 250hz and that the mid/high x-over point should be at 1200hz. I've run the PA for a while now with these settings and it seems that way to much mid is being pumped through the horns. The result is a rather harsh top end.

 

Since I have the PA in my basement I was able to sit in FRONT of the PA speakers while tweeking the x-over. My ear says that the 250hz sub/mid x-over point is fine, but I ended up setting mid/high x-over point at 2000hz and it sounds so much better. All materials I've read tell me that this is too high.

 

Trust my ears?

 

2. Gain Structure

 

Since I've been with the band I've noticed that they always run the power amps full open, but the mixing board main output faders are always at half or less, like at least -15dB.

 

I've read a few articles where techs are saying that it's better to NOT run your power amps wide open, but that you should get a good "unity gain" output signal from the board and then turn your amps up only enough to get the desired volume.

 

Andy says he's afraid that if he doesn't have the power amps maxed, then we won't be pumping enough watts for the speakers to perform properly.

 

I need facts about this. Help?

 

 

3. Signal to noise ratio.

 

I was taught that each time you add a new gain stage to a signal, whatever gain is added at that stage also pumps up the "noise" by the same amount that it increases the signal. I wish I could word this better.

 

So my question is this. I'm confused by conflicting statements. Can someone give me a definitive answer?

 

1. You should have a reasonably hot output from your mixer and that will decrease your signal to noise ratio. Having a low output from the mixer with your power amps all the way up is gonna produce MORE noise through the system.

 

2. This is from the QSC Power Amp page FAQ.

 

http://www.qscaudio.com/support/technical_support/faq.htm#pafaq5

 

 

Where should the gain controls of my amplifier be set? We recommend that the gain controls be set between half-way and fully up. The input sensitivity of QSC amplifiers is at about 0dBV or (1Vrms). Amplifier gain controls set at a lower position require input signals to be set to a higher level to obtain suitable power levels. There are other noise and gain alignment considerations. Particularly with unbalanced input lines, the hotter your signal is at the input of an amplifier, the more noise propogation you will have into your amplifiers. Also, the gain structure of your system may become such that you will reach the maximum gain travel of a fader, at your source device, before obtaining expected power within your amplifiers.

 

 

Thanks,

 

Brett

[where02190]

You should post this in the live forum.

[funky]

Thanks, I didn't scroll down far enough. I just saw the Sound, Studio, and Stage, and thought this was the right place.

[D Charles]

Well, the thing is 15"s start beaming really noticably above 800 Hz. I wouldn't run mids in them above that.

 

Unless your horns are really large, they won't go down that far which is why when I built my live sound set up I used 4 JBL 8"s a side for mids so I could cross at 2000 Hz. They reproduce low treble just fine with no beaming on axis.

 

I'd cross subs at 80 to 100 Hz. That low mid stuff (100 to 315 Hz) sounds less crappy in the mids than in subs.

[John S. Shinal]

Originally posted by D Charles Well, the thing is 15"s start beaming really noticably above 800 Hz. I wouldn't run mids in them above that. I'd cross subs at 80 to 100 Hz. That low mid stuff (100 to 315 Hz) sounds less crappy in the mids than in subs.

 

Agree with the above. However if you are "stuck" with the 15s for the time being, walk from left to right past the stacks and see if you walk past a "hot spot" where the mids are really brash and spitty, and then sound smoother off-axis. All 15s do this to some degree, but some cabs are better than others.

At least you will know what their tendency is.

 

And STRONGLY agree to drop the subs to 100Hz or so. This should tighten things up on the lows and the mids.

 

If you have some way (crossover switch, usually ) to cut everything below 30Hz, do that too. You won't lose your kick drum, in fact it will sound better.

 

And do chack the live sound forum - great guys there - listen to their wisdom !