How far should I turn my PA amps up?

[Walker]

[b]How far should I turn my PA amps up?[/b]

Our new PA system is arriving in a few days, it consists of:[list]
[*]2 Yamaha P7000S amps (each capable 750w x 2 @ 8 ohms)
[*]2 Yamaha S115V Tops (500w PGM / 1000w peak @ 8 ohm)
[*]2 x EV SB112 Subs (800w PGM / 1600w peak @ 8 ohm)
[*]DBX X-over
[/list]
I went on a live sound course last year and the tutor said run you amps at full volume and control FOH volume on the desk. But he was talking about his lovely matched Martin rig.

With my system above, how do I determine the volume the amps should be run at so I won’t damage my speakers?

Thanks very much,

Chris

[Dad3353]

Good evening, Chris...

Your tutor is doing what is typical for PA's. Before powering up, make sure all volumes are set to minimum. Turn on from the source (mics etc...) to the amps, which are last. Make sure the desk is at minimum; turn amp volumes to max. Adjust from the desk.
To turn off, it's the opposite. Turn down the amps first; switch them off. then go back in sequence to the source, turning down/off. That's how we do it; others may have variants, of course.
Hope this helps.

[bertbass]

+1 to what he said.

[Phil Starr]

Hi Chris, you shouldn't damage your speakers. The usual thing with PA is to buy plenty of power with your amps because the idea is to have no distortion coming from these. Since the distortion mainly comes at the peaks then make sure the amps can more than go above the speaker handling.

Amps and speakers are rated differently anyway, even though they are both RMS watts. The speaker rating is for how much heat it can disperse if the power is applied continuously, and the amp for how much undistorted power it can produce. Because all music has peaks and troughs the amps average power is likely to be 100th of its peak output, in practice your 1000W amp may only be averaging 10W into the speaker so it is unlikely to overheat.

If you run into distortion however then this puts a lot of power into the horns and these are the bits that blow. This is the only case where a low powered amp is more likely to blow your speaker than a high power one by the way. Fortunately your S115's have a built in protection for the horns so you won't blow these either.

If you want to be completely safe then you can by a compressor/limiter which can be set to prevent any damaging peaks reaching the speakers, but I don't think this will be a problem unless you start playing really big venues. You are far more likely to blow peoples eardrums than your speakers with this set up!

Yamaha is a good choice by the way.

Edited April 13, 2013 by Phil Starr

[Monckyman]

As above,but remember you can also attenuate the level to the amps from the crossover also.
So in effect you can calibrate your optimum desk output and send as much of that as you like to the amps by increasing or decreasing the output at the crossover and run the amps on full.
Remember that you will probably want different levels for your tops, as compared to your subs, and the crossover is where you can do this.
As to what balance, that's up to your ears and the venue but the crossover makes it easier to adjust.
HTH
MM

[Walker]

Thanks everyone, this is very useful information, appreciated. I feel far more comfortable now. In the past, I've only had consumer stereo system tothink about, but now I'm the 'sound guy' in our band, playing with all these watts, DB's and expensive kit has made me a little nervous to say the least!

My Yammy S115V's arrived (local ebay seller) - one of the tweeters is silent, so I'm negotiating.

Quick question - If I swap the tweeters over I'll be able to check that it isn't the x-over circuit in the speaker that's causing the problem - is that a good way to check? I've found a replacement tweeter on Thomann, but if the x-over is the problem, I'll send them back!

Thanks again guys,

Chris

[Monckyman]

Yes, just swap the tweeters, if the prob remains its the internal x over or the wiring attached to it.

[Jack]

If you want to make a massive difference to a middling pa system, look into aux fed subs. Really cleans up the tops and helps with clarity IME.

[Walker]

So just send an aux containing bass, kick and perhaps keys to the subs? Is that what you mean Jack?

[Jack]

Pretty much. That way it keeps all of the things that you don't want in the subs completely out. You lose all of the rumble that the guitar mics etc pick up that would normally be fed to the subs if all of the channels went to the xover. If you're xover allows it you should certainly try it.

[Phil Starr]

I think the S115V's used the eminence 2" bolt on driver for most of their life, Whatever it is they give a great vocal sound [url="http://www.bluearan.co.uk/index.php?id=EMIPSD3006&browsemode=manufacturer"]http://www.bluearan.co.uk/index.php?id=EMIPSD3006&browsemode=manufacturer[/url] you should be able to buy a new diaphragm for them if it is blown which is a lot cheaper than replacing the whole unit.

[Walker]

[quote name='Jack' timestamp='1366038453' post='2047566']
Pretty much. That way it keeps all of the things that you don't want in the subs completely out. You lose all of the rumble that the guitar mics etc pick up that would normally be fed to the subs if all of the channels went to the xover. If you're xover allows it you should certainly try it.
[/quote]
I use the hi-pass filter on all channels apart from bass and kick - would this give the same benefits as your idea Jack?

[Walker]

[quote name='Phil Starr' timestamp='1366039267' post='2047586']
I think the S115V's used the eminence 2" bolt on driver for most of their life, Whatever it is they give a great vocal sound [url="http://www.bluearan.co.uk/index.php?id=EMIPSD3006&browsemode=manufacturer"]http://www.bluearan....de=manufacturer[/url] you should be able to buy a new diaphragm for them if it is blown which is a lot cheaper than replacing the whole unit.
[/quote]
Cheers - I'll certainly investigate!

[Jack]

[quote name='Walker' timestamp='1366039463' post='2047590']
I use the hi-pass filter on all channels apart from bass and kick - would this give the same benefits as your idea Jack?
[/quote]

No. Even the HPF aren't dead cuts, they're (sometimes steep, sometimes not) roll offs. If 7 or 8 mics are picking up the kick drum (say, all the ones on the drum kit and one of each of the amps next to it) then that's still a hell of a lot of kick making it past the HPFs and going through the PA. If the xover is set at say 100Hz and the desk HPFs at 80Hz, that produces a very steep cut to get it out of the subs. [url="http://www.prosoundweb.com/article/a_detailed_explanation_of_the_aux_fed_subwoofer_technique/"]A helpful guide.[/url]

Edited April 15, 2013 by Jack

[Walker]

Thanks Jack - extremely helpful stuff!

[Monckyman]

Good article that.
I've been using an aux to feed our subs for a bit and at first I didn't like the extra thought and work involved, also losing an aux.
But it really does clean everything up, let's you use those HPF less dramatically and allows separate EQing and treatment of the subs output.
For my lot, a wedding /party band, it's the kick and the bass that get sent and that's it.
Sounds a bit of a waste doesn't it?
But it's really handy being able to wind in that low end excitement if the floor fills up and people want to dance.
If you do try this, just remember you need a POST fade aux setting so the sub signal goes up or down relative to your main foh signal.

[mdc]

Chris, I reckon your amps and speakers are a good match. You don't say if your crossover has a built-in limiter but if it does then (by my calcs) you'd set it at 3.22dBu for the S115V and 5.27dBu for the subs and then you can run the amp inputs fully up - that's if you want to stay within the RMS speaker ratings. For running at 'program' the values would be 6.23dBu and 8.28dBu respectively. That's based on RMS ratings of your speakers being half the program figures (e.g. 250W in the case of the S115V), and P7000 amp gain of 32dB according to the spec.

Bear in mind that wherever you set the amp input controls, you can still increase the input level (and therefore the output level up to the max the amp will produce) by sending more to it, e.g. by turning up the mixer output.


And +1 for aux fed subs...

Hope this helps.

Mike

Edited April 19, 2013 by mdc

[Walker]

[quote name='mdc' timestamp='1366415212' post='2052474']
Hope this helps.

Mike
[/quote]

Thanks Mike, that does help. I appreciate you taking the time.

Chris

[Walker]

2 more (probably daft) questions coming up!

My amp's output power at 20Hz-20kHz is750w x 2 @ 8ohms and 950w x 2 @ 4ohms.

Is it better to connect 2 speakers to one side of the amp to present a 4ohm load, or one 8ohm speaker in each side?

Also, on the [url="http://www.yamahaproaudio.com/global/en/products/poweramps/ps_series/specifications.jsp"]Yamaha spec' page[/url] it says that the amp's output power at [b][u]1kHz[/u][/b] is750w x 2 @ 8ohms and 1100w x 2 @ 4ohms. Why the different output power at 4ohms when compared to the 20Hz-20kHz figures?

Thanks again for your help gents, this stuff is fascinating!

Edited April 23, 2013 by Walker

[Phil Starr]

First of all the relationship between watts and sound level is slightly complex. Our ears pick up quiet sounds really well but have to cope with sounds a million times louder so they work on doubling the sound level only giving a small increase in the volume we hear. (actually 3dB) So, increasing the amps output from 500W to 1000W only gives you a small but noticeable increase in sound. To go from 750W to 950W is just about going to be a noticeable 1dB increase. Run one speaker a side. You did want stereo anyway, right?

The amps power is limited by two things. The voltage swing it can provide and the power of the power supply inside the amp.

Let's imagine your amp can swing 32Volts, the voltage squared divided by the resistance gives the power so that is 1024/8 or 128W for 8ohm speakers and 1024/4 or 256 for 4ohm speakers. You get double the power into half the ohms theoretically. So why doesn't your Yamaha give 1500 into 4ohms? The other limit is the amount of current your power supply can give. Twice the power demands twice the current and sooner or later the transformer inside your amp reaches its limit. in your case this is when the impedance drops below 8ohms.

In fact the current limiting tends to be slow, the power supply might be able to provide a high current for a fraction of a second so your amp may give peaks well in excess of the 950W into 4ohms.

In truth your 750W a side amp is probably going to be more than loud enough for any gig you play and you have bought something you can plug in and let run without worrying.

[Walker]

Fantastic Phil, thank you.

So much to learn so little time!