silent running

I'm in the middle of designing a simple PA system for a small hall and need to use an existing amp that's already available. It's a hifi Marantz, a mains powered integrated amp that just does its job, and as a hifi amp no doubt does it very well. Output is stated at 2x 50w RMS into 8 ohms or 2x 70w RMS into 4 ohms.

So can someone just confirm to me that if instead of running a single pair of 8 ohm 150w RMS speakers, I hook up a parallel pair of them on each side, I'll get the equivalent of 4x35w rather than 2x50w? i.e. another 40 RMS watts for free?

Also, can someone confirm that going to the next speaker size up (300wRMS) isn't going to get me more volume out of the system unless the dB efficiency is higher? I'm pretty sure bigger speakers won't give me more volume without a bigger amp, but just need to check!

Just running it past those of you on here with more recent working electronics experience than me, maybe I've overlooked something really obvious? Any advice would be appreciated.

Northern Nick

iTrader: (1)

Running 2 speakers in parallel or series, you aren't going to gain any extra volume, the amp will only put out 50watts at 8ohms, 70watts at 4 ohms, you add speakers, your only going to decrease the quality and volume of each channel.

Regarding speakers, you far better off buy/using 'matched' speakers for the amp.

OK, what you amp is doing is physically pushing and pulling the speaker cone right, idf you fit a speaker cone designed to work with 300watts rms and connect it to a 50rms amp, your amp hasn't got the power the push or pull the cone. Basically, your speaker should be 50watts rms, 60watts max.

Have you ever heard 50watts rms at full tint? Its Foooking loud!!!!, the lads at work just fixed a Fender 125watt valve amp and Iain was having a strum on the Guitar, that Fender made you feel sick if you sat within 5 feet of it

2x 50watt rms speakers on a 2x 50watt rms amp will easy be heard in a small hall, if it is RMS its rated at

Power Junkie

iTrader: (1)

It also depends on the sensitivity of the speakers, If you have some that are rated at 80db@1m 1w and some that are 90db@1m 1w the 90s will be quite a lot louder, they would also give the amp a much easier job, how big is the hall and what will the PA be used for??

(This sort of thing is my job )

silent running

Well it's for a medium sized school hall but it's one of those things where budget is very limited and anything will be better than what we have at the moment - a couple of 6x4 oval drivers in unbaffled open chipboard boxes rated at 10w/16 ohms. They seriously look like they last saw service in a WWII underground bunker or something.

The most action this system is going to see is amplifying a wireless mic or line level CD or DVD audio in stereo. We're not talking about 'School of Rock' or anything. After a bit of testing at home with the Marantz and my own Monitor Audio floorstanders, it seems that the amp is one serious bit of kit - not mega loud, but distortion free until virtually right round the dial. So it'll deliver a clean signal, and from the sounds of it, a genuine 50w RMS per channel.

Power Junkie

iTrader: (1)

ok, If you can find a good pair of hifi speakers (maybe floorstanders) with a good sensitivity you will most likely get a better result than lowend Pa speakers, it will also sound much better. Unless you can find a cheap Pa amp and speakers I would say the hifi route would be your best option.

Labman

I agree with Power Junkie, keep it simple. Use a decent quality set of loudspeakers and you'll be fine.
Running 2 sets in parrallel off each channel will in theory give you another 3dB (which is just about the smallest sound pressure level change we can detect) but will probablty sound awfull, you'll get all sorts of intermodulation distortion, lots of nodes and anti-nodes in either the horizontal or vertical axis, depends how you have the speakers oriented. basically as you move about the room the mid and HF will sound loud then quiet then loud, then quiet again.

As for power, I agree with Northern Nick about the amp only putting out 50W into 8ohms but I think he is slightly misguided about the power rating of the speakers. If you should connect a 300W rated pair they will work just as well as the 50W rated pair. The power rating of the speakers tells you nothing about how loud they will go, how good they will sound, or what amp you should be connecting them to. The power rating is exactly what it says on the tin, i.e. the amount of power the speakers are designed to handle continously before you melt the voice coil.
And don't read to much into the sensitivity, it can be misguiding at best and downright wrong at other times. We often test LS sensitivities and they are often wide of the mark. Also the sensitity is often measured at one single frequency (1kHz) so tells you nothing of how it will perform at the other 15,000 frequencies we hear.
Hope this helps.

silent running

Absolutely perfect, thanks for all the advice. I hadn't thought of the intermodulation problems with two sets of speakers. One last thing. I've heard a rumour, and I've often toyed with the idea myself, that in a big installation with long cable runs you might as well just use mains cable instead of fancy speaker cable. Sounds sensible to me...anyone else heard this?

Power Junkie

iTrader: (1)

For cable just use good cable, you don't need fancy stuff if you can get a reel of 1.5mm that will see you fine, lots of this cable talk is a farce really.