Matching PA Speakers to Power Amp

aggieactuary · #1 10-01-2010, 03:44 PM

I got a deal on a Peavey CS500A power amp. So, now I'm looking for PA speakers to use with the amp.

The specs from the manual read:

"Music Power (IHF-202)":
4 ohms, 1kHz, 1% THD: 275W RMS per channel
8 ohms, 1kHz, 1% THD: 165W RMS per channel

Continuous Power:
4 ohms, 1kHz, 1% THD: 200W RMS per channel
8 ohms, 1kHz, 1% THD: 130W RMS per channel

Rated Power:
4 ohms, 10Hz to 20kHz, 0.03% THD: 190W RMS per channel
8 ohms, 10Hz to 20kHz, 0.02% THD: 120W RMS per channel

This is going to be used for vocals for now.
What rating should I look for in speakers?

I've heard the rule of thumb that the power amp should be twice as powerful as the speakers, but I've also read that you should match the speaker and amp with a little room to spare in the amp. Also, which rating should I use to match against: music, continuous or rated?

I'll probably look for 4 ohm PA speakers.
Should I look for 4 ohm speakers rated around 150W continous?

Thanks for any help. Let me know if I left anything out.

somegeezer · #2 10-01-2010, 03:57 PM

"I'll probably look for 4 ohm PA speakers."
You mean a single 4 ohm or 2 4ohm? Cos by the sounds of it, your amp only goes as low as 4 ohm... Unless it's 2 separate 4 ohm outputs? 150 watt speakers sounds like a good match for 200 watts of power though.

Vinny D · #3 10-01-2010, 04:01 PM

I hope by good deal you meant free.....because that amp has very minimal power for anything but maybe monitors....maybe..

aggieactuary · #4 10-01-2010, 04:26 PM

It is a two channel power amp. So, it'll be 4 ohms per channel as referred to in the specs.

It's for vocals at rehearsal. Right now we're running the vocal mic into a 65watt guitar amp, and it's almost ok for our rehearsals. So I think we'll be ok without whatever expensive PA you have vinny.

Vinny D · #5 10-01-2010, 04:55 PM

Quote:

Originally Posted by aggieactuary

It is a two channel power amp. So, it'll be 4 ohms per channel as referred to in the specs.

It's for vocals at rehearsal. Right now we're running the vocal mic into a 65watt guitar amp, and it's almost ok for our rehearsals. So I think we'll be ok without whatever expensive PA you have vinny.

It will only be 4ohms per channel if you run (2) 8ohm cabs together or can find a 4ohm PA cab.
Pretty much the only 4ohm PA cabs that you are going to find are something like a Yamaha S215V cab.

As I said, it will probably be fine for monitors but other then that you will be asking way too much of that size amplifier.

Sorry if it's not the answer you were hoping to get

Bufalo · #6 10-01-2010, 04:55 PM

Just about any pair of 12" mains or monitors would be driven fine by that amp. I would say anything with at least a 200W rating would suffice. I expect you'll find mostly 8Ohm rate speakers, and that's what you want. In a practice environment, you could reliably drive a pair of floor monitors on each channel and give every guy in your band something to listen to. You'd be stressing neither your speakers nor your amp, and that'll give you plenty of volume for practicing.

Your rule of thumb about having twice as much power into your speakers as they're rated for is probably not a good one to follow. The fear of over driving a power amp into speakers is something that I've never witnessed outside of the bassist community. Most of the world worries about dumping too much power into a speaker and doing damage that way. Yes, an amp which is clipping is probably the worst sort of signal a speaker of any power rating can receive, but if you keep the RMS / continuous power rating of each about the same, while giving the speakers a little extra headroom, you'll be better off in the long run.

I've had good luck, which surprised me to be honest, with the Kustom 12" monitors. They're 8 Ohm, you could daisy chain two of them off each of your amp's channels, and they do a pretty OK job of putting vocals (and I run guitars through them at gigs) in front of your face where you need it.

Vinny D · #7 10-01-2010, 05:02 PM

Quote:

Originally Posted by aggieactuary

Rated Power:
8 ohms, 10Hz to 20kHz, 0.02% THD: 120W RMS per channel

That is the spec. you should be looking at.

MNAirHead · #8 10-01-2010, 05:17 PM

I'd be looking for 8 ohm cabinets.. then mildly searching for a power amp with more headroom.

You're fine (depending on how big/style/band) you're doing.

For cheap and light.. I've never known a set of impulse speakers to blow.

aggieactuary · #9 10-01-2010, 06:40 PM

Thanks for the info. I didn't realize that most PA speakers were 8 ohm.

If I ran two pairs (4 cabinets) of 8 ohm cabinets what should they each be rated?

Each pair would be effectively running one channel at 4 ohms and ~200 watts, right? Does that mean each individual cabinet gets 100 watts (assuming 4 cabinets each rated 8 ohms)?

Bufalo · #10 10-01-2010, 07:03 PM

Quote:

Originally Posted by aggieactuary

Thanks for the info. I didn't realize that most PA speakers were 8 ohm.

If I ran two pairs (4 cabinets) of 8 ohm cabinets what should they each be rated?

Each pair would be effectively running one channel at 4 ohms and ~200 watts, right? Does that mean each individual cabinet gets 100 watts (assuming 4 cabinets each rated 8 ohms)?

Yeah, at the most basic level, you're putting 100W into each of the cabinets on each channel.

Pete · #11 10-02-2010, 01:39 AM

You're doing this backwards. Speakers then power. That amp is so small no speaker will be a good match unless you really don't need much to begin with. The amp for horns I use is 260 @ 8, I wouldn't use anything less.

Most all PA speakers are 8 ohm, unless they use a dual mid driver (2 15's and a horn). Some sub boxes use it but that trend is about over. Find a pair of speakers that work well for you then get the right amp. My rule of thumb is minimum 300w per main @ 8 ohms (600w @ 4) for both main and monitor apps.

aggieactuary · #12 10-03-2010, 06:28 PM

For rehearsals, are monitors better than normal speaker cabinets?

Vinny D · #13 10-03-2010, 07:37 PM

I always prefered monitors for rehearsal, it seemed to make it easier to keep volumes down.
If you buy a decent monitor cab it most likely can be used as a main PA cab also, just check and see if it has a pole mount on the cab.

aggieactuary · #14 10-04-2010, 03:12 PM

Thanks Vinny.

All other things being equal, which speaker would have the potential to be louder?:

Speaker A: sensitivity 95db, rated 100watts
Speaker B: sensitivity 92db, rated 150watts

Assume they're properly matched to an amp.

My guess is Speaker A would be louder at 100watts than Speaker B at 150watts. If I use the old rule of thumb that it takes twice the power to increase 3db, then Speaker B would need to be rated 200watts to be as loud as Speaker A at 100watts, correct?

Vinny D · #15 10-04-2010, 03:36 PM

I don't put a whole lot of faith in what speaker mfg's put down for there spec's.
There is no standard set in stone for how every mfg arrives at there said published spec.
So unless both speakers are from the same mfg and the same series of speakers there really is no certain way of telling which one would be louder other then connecting both of them and just listen.

What is your budget for a cab?
that would be the easiest way to determine the best bang for your buck.

My general rule is by the best speaker you can afford, you can always upgrade the amplifier later.

Pete · #16 10-04-2010, 03:43 PM

correct. Speaker should be rated to have a certain sensitivity, a margin, and a freq response with a -3db down and -10dB down. Ex:
98db 1w/1m 60hz-16K +/- 3dB, useable 45hz -10dB point.

I'm making these specs up but that's the kind of info you want from a speaker. Beware of speakers that claim really high or low freq extentions (30hz to 25k). And without a margin there could be a 12dB spike @ 1k and you'd never know until after you've bought them. And speakers with a very low sensitivity (88-92ish) are most likely rebranded car audio garbage.

Wattage is another myth too. Thermal and mechanical limitations, bandwdth, program material all effect the numbers a comany will use to rate thier drivers. 100 watts what? Peak? RMS?

Ballpark I'd look for a good monitor that has real published meaningful specs in the 300w RMS range, an amp in the 300w @ 8ohm range, Nuetrik speakon conectors and pole mountable for use as mains.

aggieactuary · #17 10-04-2010, 03:47 PM

I'm guessing you're not going to like the answer based on our discussion about power amps. Right now, I'm looking at $100 budget, but I might stretch up to $200 for a great deal. This system is not going to be used for shows. It's a basement rehearsal only system.

I know most decent PA speakers are rated at least a 200-300 watts. If I got speakers that are rated really high, wouldn't they clip with the power amp I have?

Vinny D · #18 10-04-2010, 05:46 PM

I would search your local Craigslist for some used Yamaha Club series cabs (SM12V or SM15V), They usually always appear on there sooner or later for cheap money.

Although I am not a huge fan of Carvin's PA gear they make one cab that I have actually heard a few times and they are pretty decent, just slightly out of your price range but it may be worth saving an extra few weeks:
http://www.carvinguitars.com/product...p?product=LM15

aggieactuary · #19 10-04-2010, 06:34 PM

Would a SM15V clip with my amp if I run the amp bridged (mono)?

The amp (Peavey CS500A) specs say- Continuous Power: 8ohms, 1kHz, 1% THD, 400 Watts RMS Bridged

It doesn't give the bridged "rated power" (which is defined by them as 10 Hz to 20 kHz, 0.02% THD) , but from the rated power in stereo mode it's 120 Watts per channel at 8 ohms. Can I assume that bridged I should be safe to run a Yamaha SM15V?

The Yamaha SM15V specs only give Program power at 500W. But I've found that's usually about twice the RMS power, right? So, I'd be looking at 250W RMS for the SM15V, right?

aggieactuary · #20 10-05-2010, 03:52 PM

For my price range, I can get a pair of new Kustom KSC 12's. That looks like about all I can do if I want to run it stereo.

Or I can run the power amp bridged, and that should give me about 380 watts at 8 ohms. Then, I could grab one of these speakers used (in my price range):

- Yamaha S112V
- Yamaha S115 IV
- JBL SF 15
- EV SH1502
- Peavey Impulse 200
- Peavey PR15

Which way should I go?