Quick amp question

[KBToys82]

So my school is looking into getting a portable sound setup with a rack mounted mixer, amps, wireless mics into a portable rack with heavy duty castors.

We currently use an all-in-one system (Yamaha StagePas 400i) for our outside needs, and while the speakers sound fantastic, we have outgrown the mixer/amp that comes with it.

I looked up the specs for the speakers, and it appears to be 200W at 4Ω's. At the same time, we have 2 floor wedge monitors that we can also mount on stands if need be (Yamaha BR 12's) that are rated at 300W at 8Ω's.

I'm not sure if I'm going to get 2 different amps to drive both speakers at the same time, since we probably won't need 4 speakers ever, except maybe our graduation ceremonies. But if I only get 1 amp to start off, I was looking at either the QSC GX3 or the Crown XLi 800 to be able to run 1 pair of them at a time. The GX3 would definitely match up to the BR 12's but double the power needed for the 400i speakers, whereas the XLi matches up nicely with the 400i speakers but be under the rating for the BR12's.

I know the simple answer is to get both amps, but I want to know what happens if they see the price tag and need me to cut it down to 1 amp.

(Also, side question, I'm getting the PreSonus AR12 Mixer, and the rack I'm looking to get has 12U on top. The dimensions for the mixer lead me to believe it would take up 9U, but I'm nervous to assume since PreSonus doesn't have that listed anywhere.


Any help is much appreciated!

 

[JD]

Well, QSC and Crown both make great amps. Go with the more powerful one. Wattage is a funny thing, you are never really driving an amp at a rated wattage. It is simply the point where driving it further will cause clipping. The higher the wattage, the more headroom you will have. Program sound is a funny thing. Generally, peak amplitudes are 5 times higher then your actual RMS output. So, for example, if you are driving your speakers at 10 watts, you would want at least a 50 watt amp to insure clean sound. A bigger amp does not mean it will be louder, but a smaller amp will limit its use. More important is the loading. If your speakers are 8 ohms, and you want to run two speakers per channel, you want an amp that works with a 4 ohm load. (Both do)

Often, in an application like yours, two speakers would be used for house sound and the other two for monitors so you can hear yourself. Unless you have a real reason to run stereo, you would simply run the house speakers off one channel and the monitors off the other, and feed them with the appropriate board outputs.

 

[KBToys82]

No reason for stereo, where I can see use for needing 4 speakers would be for our graduation ceremony, where 2 speakers pole mounted facing the audience, and 2 speakers facing the graduates/speakers on the field. When it comes to running 2 speakers off one channel, that's where I'm lost. I'm pretty sure it's impossible to do with the 400i speakers without adapters, but may be possible for the BR 12's.

 

[JD]

No reason for stereo, where I can see use for needing 4 speakers would be for our graduation ceremony, where 2 speakers pole mounted facing the audience, and 2 speakers facing the graduates/speakers on the field. When it comes to running 2 speakers off one channel, that's where I'm lost. I'm pretty sure it's impossible to do with the 400i speakers without adapters, but may be possible for the BR 12's.

Just a matter of having the right adapters. The stagepas speakers use 1/4 jacks. (Not sure how many ohms, but I assume 8) Most speakers now use NL4 connectors. Many of the QSC power amps have NL4 connectors on them. Amps usually have binding posts, NL4's or 1/4 inch. All you would be doing is getting the right adapters so that you end up with two speakers on each channel in parallel. You can find NL4 "Y" adapters on the net, as well as NL4 to 1/4 adapters.

 

[FMEng]

The Stagepas speakers are 4 ohms. Running two from one channel results in a 2 ohm load. Most amplifiers won't like that low impedance because it takes a great deal of current to drive the load. The BR12s are 8 ohms, so they can be driven as a pair from one channel. If you can get by with just three speakers, a stereo amp would do the job. Otherwise, you need two, stereo amps. Having two amps is safer because it avoids the problem of connecting the wrong two speakers as a pair on one channel. It also gives you the flexibility to use a pair of speakers for monitors with two mixes.

 

[JD]

The Stagepas speakers are 4 ohms. Running two from one channel results in a 2 ohm load. Most amplifiers won't like that low impedance because it takes a great deal of current to drive the load. The BR12s are 8 ohms, so they can be driven as a pair from one channel. If you can get by with just three speakers, a stereo amp would do the job. Otherwise, you need two, stereo amps. Having two amps is safer because it avoids the problem of connecting the wrong two speakers as a pair on one channel. It also gives you the flexibility to use a pair of speakers for monitors with two mixes.

4 ohms. Interesting. So that little mixer/amp that they supply with the system runs at 2 ohms. Indeed, most amps are fussy (and not even rated) at 2 ohms.
The other way around that is to put the Stagepas speakers in series, that would give you an 8 ohm load. It reduced the available power and knocks the output down several Db, but would allow you to run all 4. As long as you had the headroom, should be fine.

 

[Chris15]

Why not use the stage pas's mixer amp to effectively create one or two powered speakers?

 

[FMEng]

Why not use the stage pas's mixer amp to effectively create one or two powered speakers?

Chris makes an interesting point. Your new mixer can drive the Stagepas mixer. I did a little more investigation and made a discovery. The manual shows that the Stagepas mixer/amp is using DSP EQ and limiting to compensate for the diminutive size and cost of the speakers. That would explain how they are eeking out lower bass than expected from an 8" woofer (at modest sound levels), and why the speakers have not exploded doing PA for a large outdoor crowd. My guess is the speakers and processing are tailored to work very closely together. Driving those speakers from an amp without the processing might sound rather disappointing and be prone to failure.

Another option would be to buy some self-powered speakers, such as QSC K12.2, Yamaha DXR12, or DSR112.

 

[venuetech]

4 ohms. Interesting. So that little mixer/amp that they supply with the system runs at 2 ohms.

That is a stereo amp that comes with the stagepas systems.
I would agree with the suggestion to use the stagepas amp for the speakers.
Electro-Voice has the ELX112P to consider along with the others suggested.

 

[KBToys82]

For graduation I used a mixer to feed the StagePas mixer for the audience speakers and used the monitor out to feed the press box amp for the stadium speakers facing the field for the graduates. I was really hoping to be able to not need the StagePas mixer and use a rackable amp for more convenience by putting everything in a portable rack.

 

[TimmyP1955]

A friend has a StagePas for use on solo gigs. When he needs infills (wide stage and dance floor) or outfills (wide wedding reception hall) he feeds his Presonus into the StagePas mixer - it does well so long as the gain structure is right.

I very much like the Yamaha DSR12 and DXR 12 powered boxes. And the Behringer XR series mixers.

 

[Ancient Engineer]

When selecting amplifiers for a speaker load expressed in Watts RMS, a 2:1 ratio is usually a VERY safe route.

As stated above, not having enough power leads to clipping and speakers strongly dislike DC voltages except VERY VERY VERY intermittently.

Also, putting a 5000W amplifier on a 350W rated speaker is a recipe for the drivers to make excursions that are also unsafe. Eventually somebody will grab the gain pot on the amp and crank it past the safe power output level.

If you stay with a 2:1 ratio you end up with the RMS of both devices being reasonably close (depending on program material) and your peaks are able to whip them diaphrams for that big cymbal crash or kick hit. Like a 700W amp on that 350W RMS Speaker mentioned above.

Subs require extra caution regarding program material. 2:1 will get you happy for most situations, but if the program material is a droning bass line for hours (think trance types of music) then you might have to actually go with a 1:1 ratio.

Essentially if the "peaks" never cease then it is not a peak any more... And the drivers will crap out. The only exception I can think of is the (very) old-school Servosubs which as long as the amp matched the dissipation of the servo you could drive them all night with your signal generator and they just kept kicking...