Amp Power Specs - Help Translating?
hey i’m looking at getting a PA setup, but am a little confused at the power specs. if anyone could explain this, it would be greatly appreciated.
for example:
Power Specs:
8 ohms stereo - 150W
4 ohms stereo - 225W
…
8 ohms bridged - 450W
4 ohms bridged - 650W
Ok the ohms is the impedance of the speaker; each speaker will have it’s impedance rating, usually 8ohms and 4ohms. (it’s determined by coil design, or multi driver) When you bridge an amp, you basically connect both the left and right channel amplifiers to create a more powerful mono amp.
Now about a couple of things you need to know:
Buy a amp that can exceed your speaker requirements by at least 25% (i personally recommend 50%). It’s much safer to over power a speaker than it is to under power it! If you buy lets say a 100w amp to drive a 100w speaker at full power; at the upper volume levels the amp will begin to clip. Instead of sending a smooth waveform signal to the speaker the top portion will clip sending a square wave resulting in burn out voice coils and crossovers. by getting an amp that has extra power to spare ensures not only a clean signal but usually a greater dynamic range. Not only that, the amp will run more efficient saving both power and improving reliability.
Impedance. Get an idea of what and how you will be using first! You can usually connect multiple pa speakers in series circuits. If you do remember this: (1/2Rt= 1/2R + 1/2R) 8ohm + 8ohm=4ohm or 4ohm+4ohm= 2ohm
Every time you connect 2 identical speakers you decrease the impedance in half.
So for lower impedance setups not only is more power needed but the ability for the power amp to be able to drive the lowered resistance efficently.
There’s more stuff like slew rate, torridial vs chip, rms, peak, but that should get you started. I hope i didn’t screw up your brain. Im sorry but im extremely tired since i had to stand watch (navy thing) and im finding myself falling asleep typing this. Will clearify later.
ah thanks for the reply, no worries on the explanation, i got most of it. that helps a lot. i always thought against putting too much power to the speakers, not wanting to blow them. thanks again brother.
I know it was a real shocker for me too when i first found out. my first day working at GC i had to setup the live room with all of the speakers, amps, snakes. I still remember trying to connect 2 JBL SRX728s to a single QSC RMX5050. I remember Rob giving me the blank “are you retarded” stare before telling me that amp barely had enough power for one of those subs. It was pretty scary since the amps weighed in at 80lbs and ran off of 220V with a massive 6AWG power cord!
that is great info.
Here’s a pretty complete guide on power amplifiers that explains all of what was stated here in better detail.
http://www.sweetwater.com/shop/live-sound/power-amplifiers/buying-guide.php
One thing different. Sweetwater suggest a power amp that’s double the rated power requirement of your speakers. That’s good but the 125%+ rule still can apply. It’s hard to judge how a amplifier will perform in different situations where music, ambient temperature, length and gauge of both speaker cables and power cables, Power quality and ripple, and amount of time using the system can change actual power amp efficiency. TO SUM IT UP JUST OVERPOWER THE SPEAKER. I personally like at least 150%, Crown rep / Guitar Center suggested dead min of 125% and Sweetwater suggests 200% power ratings.
And for you guys that think I’m speaking nonsense and buy a amp that is too little or equal to the power rating. Your gonna fry your voice coil! it probably won’t happen in your house, but at the first outdoor, or paid gig you do! I’ve had people that didn’t take head of my advice while working at GC and bought massive dual 15" JBL JRX speakers only to find out they powered 2 of them by some ghetto mackie 200w power amp. (cringes) I told them before selling the speakers, and so did 3 other guys, that that amp won’t work right. And they did fry them and during a paid outdoor event! All of us were pretty pissed when they came back and started bitching about how poorly made they were and how we didn’t tell them they needed more power and how incompetent we were (funny the three of us can remember but they can’t) So we charged them restocking fees and i made a note in my card file to never hook em up and try to rape them whenever they came back. So don’t be that guy! lol If it breaks, just bring it back and don’t yell at the sales manager. I’ll hook you up nicely with a new free pair and an awesome power amp for cheap (I.e. dead cost, no profit for us and even can remove added cost like our shipping and storage fee for keeping it in our warehouse and even take the old amp as a trade in.)
2nd the good articles on sweetwater… the cleaner and less distorted the better.
For instance:
My 2 subs handle 1200 watts program each and I run them with a xti4000 amp which is double than what the speakers can handle. but there isn’t a lick of distortion. all clean unadulterated power and the bass is very tight. and my subs are over 10 years old.
The only thing distortion and clipping does is generate heat and eventually causes voice coil failure
Another thing is that your only running the amp at say 50 - 60 percent of its gain = Less strain on the components, Better cooling efficiency, and will last you a very very long time
Another sound tip when running multiple subs on a single amp is that its to best to run speaker cables from the amp to individual cabs and NOT daisy chain from one cab to the next… its all about the “Damping Factor” the amplifiers ability to actually control the movement of the speaker(s)
For less then 15 foot cable runs I use 12ga speaker cable
If i have to run longer i usually up to either 10 or 8 gauge
@ chex: hey did you see where crown just unveiled their new XTi-6000, The thing looks like a beast
It does look like one sexy piece! I would of loved setting that up instead of the RMX5050 because of weight alone.(25lbs vs 75lbs nice!) My back still aches from that qsc! Also love the fact it runs on 115VAC, a 2u vs 4u, and has all of the awesome feature of all of Crown’s xti series. If by any chance i do get an insane sub like a srx728s, that’s the amp I’ll use for it! But right now i just want a nice pair of active 12" speakers to replace the old srm-450s i used. I just love the new QSC K12. Once you go active you never go back!
Yea that Qsc is one heavy piece of machinery.
I had a couple of Peavy 1.3K’s years ago and i think my back still hurts from lugging them around, with the enclosures they were over 100lbs each
I Would like to see the qsc plx units they look to also be solid performers,
I have to say both Crown audio and Qsc have always been major payers when it comes to sound reinforcement, I still love my old crown csl, it pushes just as hard as the day I got it.
We have a sound company locally here that i am good friends with the owner that has the newer JBL VRX line array series and they power them all with the crown I-techs, one of the outdoor concerts they did recently they used close to somewhere to almost 100,000 watts of total amp power.
:eek:
That had to been a massive show! About how many people? I can imagine around 20,000 with something like that!
I don’t remember the crowd specs but it was for a country music singer travis tritt for a motorcycle rally, very big annual event