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Author Topic: I need help understanding ratings of this power amp  (Read 343 times)

Steve Crump

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I need help understanding ratings of this power amp
« on: June 11, 2019, 05:38:27 pm »

I have been looking at buying a couple new amps for my setup, but I am not really sure how to interpret the manufactures specs. I did contact them via the website and no reply.
 
Lab Gruppen IPD2400, 1200 watts @ 4ohms per channel max, 70VRMS, 100V peak. My understanding is that 100volts at 4ohms is more than 1200 watts peak? Is the 1200 watt rating more of a continuous rating and not peak? 


 

Corey Scogin

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Re: I need help understanding ratings of this power amp
« Reply #1 on: June 11, 2019, 05:58:05 pm »

Lab Gruppen IPD2400, 1200 watts @ 4ohms per channel max, 70VRMS, 100V peak. My understanding is that 100volts at 4ohms is more than 1200 watts peak? Is the 1200 watt rating more of a continuous rating and not peak?

Here's an attempt at an explanation: The peak voltage of 100V references the top of the waveform relative to ground. The 70Vrms just happens to be right at the value you get when converting peak voltage of 100V to RMS voltage for a sine wave. The RMS value is more representative of how much work the voltage has the potential to do since it's AC not DC and so is useful for power calculations.

Power = Voltage2 / Resistance
1225 = 702 / 4

*The language above when referring to "work" isn't exactly accurate but I think it gets the point across.

« Last Edit: June 11, 2019, 06:04:02 pm by Corey Scogin »
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Ivan Beaver

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Re: I need help understanding ratings of this power amp
« Reply #2 on: June 11, 2019, 07:46:58 pm »

Corey got it right.

People often confuse "peak" with "maximum RMS".

The RMS and the peak of a sine wave are different values that are 3db apart, or .707 or 1.414 of each other.

It all depends on how it is used.

Sometimes it is a way of "cheating" specs. 
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Len Zenith Jr

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Re: I need help understanding ratings of this power amp
« Reply #3 on: June 11, 2019, 09:30:34 pm »

Amplifiers are current limited voltage amplifiers. Regardless of their voltage output they will eventually run out of current capacity (at low impedances) based on their internals (power supply, etc). Since power = v x i, when you run out of i (current), you are out of power (and usually trip internal circuit breakers).
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Frank Koenig

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Re: I need help understanding ratings of this power amp
« Reply #4 on: June 12, 2019, 11:54:32 am »

First bear in mind that the output power specs on modern amplifiers are what they can sustain short-term, typically a few tens or hundreds of ms. This is fine for most music which has a high crest factor. If you're running EDM subs, a shaker table, or using the amp as an AC power supply, you need to de-rate it considerably. In a "serious" application you likely should talk to the manufacturer's engineers for guidance. The other simplification is that pretty much all specs are given for a resistive load. Loudspeakers are reactive and different amplifiers may behave slightly differently even though they have the same spec. (Back in the day some amplifiers would even become unstable with too large a capacitive load.)

Audio amplifiers are indeed voltage amplifiers and have an essentially zero output impedance in the small-signal or linear sense. (The exception is certain amps that allow for a negative output impedance to compensate for wire resistance on subs.) The available peak voltage (large-signal),  however, tends to sag a little with increased current. This is because there are resistive losses inside the amplifier that cannot be compensated for by the negative feedback that is used to fix the small-signal output impedance. You can see this on the power vs load-impedance specs that are not exactly consistent with constant voltage even when the amplifier is not current limited. This small sag can be ignored if all you're trying to do is get an idea of the optimum load impedance, which will occur when the voltage and current limit simultaneously.

Zopt = Vmax / Imax

then

P = Vmax * Imax

--Frank
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Steve Crump

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Re: I need help understanding ratings of this power amp
« Reply #5 on: June 13, 2019, 11:41:31 am »

Here's an attempt at an explanation: The peak voltage of 100V references the top of the waveform relative to ground. The 70Vrms just happens to be right at the value you get when converting peak voltage of 100V to RMS voltage for a sine wave. The RMS value is more representative of how much work the voltage has the potential to do since it's AC not DC and so is useful for power calculations.

Power = Voltage2 / Resistance
1225 = 702 / 4

*The language above when referring to "work" isn't exactly accurate but I think it gets the point across.

Yeah, that works and thanks for your reply. I did get a reply from Music Group basically with the same explanation. I am trying to gain as much knowledge as I can in order to understand how to get the most out of my system with out damaging my drivers. When the real confusion started is when I looked at some Powersoft specs....135V peak, 1500 watts at 4ohm, then the math and my lack of amp design collided. Based on the Powersoft specs 135 * .7 =94.50V....94.50x94.50/4=approx. 2200w (not perfect math, just using round numbers).  The Lab Gruppen numbers were much easier to reconcile...lol.

Steve Crump

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Re: I need help understanding ratings of this power amp
« Reply #6 on: June 13, 2019, 11:53:47 am »

Thanks to everyone who replied.
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