PA Amp advice

travis1976

Hi guys, I'm aquiring a couple of speakers and need advice on the amp I need. The're 2 x 100w 8ohm full range speakers. Do I need a 200w250w 8 ohm amp? A lot of the amps/powered mixers I've looked at are rated 4ohm. I'm a bit lost.... Thanks in advance for your replies.

unclebill98

Most amps are rated at 4ohms but will run 8ohms no problem.

You will need to find what the amp pushes out in a 8ohm load. If you could give more details on the speakers you can get a better answer etc.

travis1976

The speakers specs are:
COMPONENT 10" Woofer & Piezo driver
POWER HANDLING 100W/RMS 8ohm
FREQUENCY RESPONSE 65Hz-20KHz
SENSITIVITY 93dB SPL
DIMENSION 380 x 495 x 250
WEIGHT 11 Kgs/pc

Thanks again

travis1976

Would an amp with these or similar specs do the job
•2 x 200w RMS at 4 ohms, 2 x 125w RMS at 8 ohms, 600 w MAX
•Fan-cooling
•Speaker switch-on delay
•Protective circuit with LED indication for protection against short circuit, overheating and DC voltage overlay at the outputs
•2 level controls

I'm just trying to get an idea before I go amp hunting.

cezet

Let's count,
Amplifier 125W / 8ohm can give 31V output voltage,
Your speaker 100W / 8ohm can give a full power at 28V.
As you can see, you can fry speakers at full power.
For your speakers you will need amplifier with 80W / 8ohm or 150W / 4ohm maximum power with safe power range

travis1976

Thanks Cezet,
I take it the amp would need be 80w per channel at 8 ohm or 125w per channel at 4 ohm? Apologies for my ignorance.

cezet

Yes, per channel

TroutMask

cezet wrote: »

Let's count,
Amplifier 125W / 8ohm can give 31V output voltage,
Your speaker 100W / 8ohm can give a full power at 28V.
As you can see, you can fry speakers at full power.
For your speakers you will need amplifier with 80W / 8ohm or 150W / 4ohm maximum power with safe power range

Speakers are routinely paired with amps that are more powerful than the RMS ratings of the speakers and this is the way it should be. The derivation of voltages by using Ohm's Law is misleading as it does not take into account the complex impedance behaviour of a speaker coil in motion, e.g., an 8 Ohm speaker will exhibit an impedance range of between 2 and 35 Ohms approx. as the coil interacts with the magnetic field. The reality is that you are more likely to blow the speakers with a lower powered amp as output stage clipping is more likely, and nothing blows a speaker coil more quickly than DC flowing through the voice coil. You can use a higher powered amp - 200W RMS would be OK. You can also use higher powered amps still, simply do not turn them up all the way. If you're planning on running them loud, a compressor-limiter inserted across your stereo bus can help prevent costly speaker failures.

cezet

Sometiumes, users can use a apmlifiers with a higher power than speakers, but on their own behalf.
I remember, I had some cases, where customers brought audio system to me after parties, saying that they are broken.
Some people turned the volume up fully after a few drinks and it ended with burned out speakers.
In that case the user must be careful and watch so no one touch the volume knob.

TroutMask

No doubt Cezet - that's for sure! Partygoers shouldn't be anywhere near the volume knob! For party/DJ situations, it's always a great idea to have a limiter on the PA. The Alice Level-Devil was good for this. The dBX compressors usually have a limiter on there. There are other strategies for protecting the drivers too.

cezet

I agree I never used a signal limiters before.

travis1976

Thanks for the advice guys. Sorry about the dalay is thanking you both, I was away for a couple of days.