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Author Topic: Power Amp Questions  (Read 2104 times)

Justin Shive

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Power Amp Questions
« on: March 26, 2011, 01:57:55 pm »

First off, my name is Justin and I'm new to the forums. 

My church just purchased a Presonus StudioLive 16.4.2 in order to move away from our old analog Peavey board.  The old Peavey board had built-in power amp so we didn't need anything for our passive mains and monitors.  Since the StudioLive does not have a built-in power amp, my question is this:
How big of a power amp will we need?

We have 2 mains, both of which are "homerun" to the mixer.  They are Peavey(1200w Peak, 600w rms(program), 300w rms(49v rms).  8 Ohms.  **On a side note, what is the difference between "peak", "rms(program)", and "rms(49v rms)"?**

We have 3 monitors.  1 is off the Left monitor output.  Off the Right monitor output, a line runs to the first monitor, and then a line out of that to the second monitor.  The monitors are all matching Peavey(500w (program)).  8 Ohms.

Any help would be greatly appreciated.  Thanks in advance!
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Frank DeWitt

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Re: Power Amp Questions
« Reply #1 on: March 26, 2011, 04:10:49 pm »

Well the easy answer is that if you had enough volume with the Peavey power mixer, then you can use the same power it had.  Just research the model, and then buy an amp with the same, or next step up in power.

BTW  You should be able to use the powered mixer as an amp, at least temporarily.  Run a cord from your line out of the new board to the Power Amp input of the old board and you are set for mains.  Another cord from your new board aux out to the second power amp in on your old board and you have auxes.

There are at least two ways to rate an amps and speakers.  RMS power is the power the amp or speaker can run at all day long with a simple tone in.  Example, connect it to a 8 ohm speaker and crank it up to 300 watts and it lives.  Physics says that at 300 watts and 8 ohms it will show 49 volts RMS across the speaker terminals.

All amp and speaker mfg. would like to print a higher number so they claim that you don't listen to a steady tone. Music has loud moments and quiet moments, therefor they can hit a loud for a moment, then cool off and hit it again.  That is where they get the higher program number.  The peak number is what they can hit once in a while for an instant (example, big finish with drums)  The number that you can easily move form brand to brand and that you can measure and test is RMS.

BTW Note the power in your amp does two things. It makes music at the speakers, and it heats up the wires to the speakers.  Now that you are separating the amps from the mixer, you can move them closer to the speakers.  That's good.

Frank


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Thomas Lamb

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Re: Power Amp Questions
« Reply #2 on: March 27, 2011, 11:16:39 am »

To add to this for the money the qsc rmx amps are a good bang for the buck.


Well the easy answer is that if you had enough volume with the Peavey power mixer, then you can use the same power it had.  Just research the model, and then buy an amp with the same, or next step up in power.

BTW  You should be able to use the powered mixer as an amp, at least temporarily.  Run a cord from your line out of the new board to the Power Amp input of the old board and you are set for mains.  Another cord from your new board aux out to the second power amp in on your old board and you have auxes.

There are at least two ways to rate an amps and speakers.  RMS power is the power the amp or speaker can run at all day long with a simple tone in.  Example, connect it to a 8 ohm speaker and crank it up to 300 watts and it lives.  Physics says that at 300 watts and 8 ohms it will show 49 volts RMS across the speaker terminals.

All amp and speaker mfg. would like to print a higher number so they claim that you don't listen to a steady tone. Music has loud moments and quiet moments, therefor they can hit a loud for a moment, then cool off and hit it again.  That is where they get the higher program number.  The peak number is what they can hit once in a while for an instant (example, big finish with drums)  The number that you can easily move form brand to brand and that you can measure and test is RMS.

BTW Note the power in your amp does two things. It makes music at the speakers, and it heats up the wires to the speakers.  Now that you are separating the amps from the mixer, you can move them closer to the speakers.  That's good.

Frank
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bigTlamb

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Tom Young

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Re: Power Amp Questions
« Reply #3 on: March 27, 2011, 02:33:21 pm »

To add to this for the money the qsc rmx amps are a good bang for the buck.

Agree 100%.

I use RMX series amps on most of my church and performance space projects and have yet to have any problems. Some off them have been in service for about 7-8 years.


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Brad Weber

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Re: Power Amp Questions
« Reply #4 on: March 28, 2011, 07:59:51 am »

There are at least two ways to rate an amps and speakers.  RMS power is the power the amp or speaker can run at all day long with a simple tone in.  Example, connect it to a 8 ohm speaker and crank it up to 300 watts and it lives.  Physics says that at 300 watts and 8 ohms it will show 49 volts RMS across the speaker terminals.
There are a multitude of ways to rate amplifier power, one reason it is so confusing.  However, "RMS" power is a misnomer, there is no such thing as RMS Watts and what is really usually being referenced is long term or continuous power.  However, "continuous" does not necessarily represent all day long, it may be 100 hours or 2 hours.  And the testing typically uses a specially shaped and manipulated broadband pink noise source rather than a simple tone.
 
To the OP, "peak" power ratings come from the fact that the test signal usually used for speaker power testing intentionally has peak levels that are 6dB greater than the RMS level, thus it is assumed that the level the speaker can handle for momentary peaks in the signal is 6dB greater, or four times, the continuous levels.  Most music is somewhere between being all peak and pink noise, so the "Program" level is a rating created to very loosely represent the expected power handling with heavily compressed music.  Note that only the long term or continuous power is actually measured, the peak and program power are simply calculated based on that that result.
 
In relation to the topic, you'll also note that amplifiers have different ratings into different impedances and in different modes, for example stereo mode and 8 Ohms per channel.  The mode refers to the way the amplifier is operating.  The most common is probably stereo mode where each channel effectively operates independently.  For parallel mode the outputs are independent but they share a common input.  Bridge mode is basically a way of operating a two channel amplifier as a higher output single channel amplifier.  The impedance rating should generally match the speakers to be associated with that channel.  For example if you are planning on having one 8 Ohm speaker on each channel then you would want to look at the 8 Ohm ratings.  If you have one channel driving two 8 Ohm speakers that are "daisy chained" or wired in parallel then that equates to a 4 Ohm total load and yu would want to look at the 4 Ohm ratings for the amplifier and since the speakers both have an 8 Ohm rating, assume that half the amplifier's output goes to each speaker.
 
There are two ways to approach your question.  One approach is basically what power is needed to get the most out of the system while still maintaining reliability.  For systems used for a range of purposes this is often a good approach.  However, for systems more dedicated to a single purpose the goal is often a bit different and approached as what power is required to get the system performance desired in that specific application?  The first approach typically involves some general 'rule of thumb' guidelines such as an amp rated twice the continuous power rating of the speaker.  Teh second approach requires looking at the specifics of the application such as the desired sound levels, the dynamics of the sources, the sensitvity of the speakers and so on.
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