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Old 3rd January 2009, 04:08 AM   #1
OMNIFEX is offline OMNIFEX  Jamaica
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Question Does Pink noise follow the V x V/R method?

Hi.

I am having difficulty achieving the correct sum when comparing amplifier published wattage versus maximum RMS voltage swings on a particular model. The numbers do not match although having both figures displayed on the specification sheet.

Possibly my math is incorrect?

How can you achieve 425 watts in an 8-ohm load per channel, with 0.05% Total Harmonic Distortion, both channels driven using 1 kHz from a Maximum RMS Voltage of 71?

My calculations arrive to 315 watts per channel in an 8-ohm load.

Would pink noise measurements play a factor why the numbers do not correspond?

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Old 3rd January 2009, 04:17 AM   #2
OMNIFEX is offline OMNIFEX  Jamaica
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I am attaching a photo of the specification sheet.

Click the image to open in full size.


Cheers!
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Old 3rd January 2009, 10:03 AM   #3
syn08 is offline syn08  Canada
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Default Re: Does Pink noise follow the V x V/R method?

Quote:
Originally posted by OMNIFEX
Hi.

I am having difficulty achieving the correct sum when comparing amplifier published wattage versus maximum RMS voltage swings on a particular model. The numbers do not match although having both figures displayed on the specification sheet.

Possibly my math is incorrect?

How can you achieve 425 watts in an 8-ohm load per channel, with 0.05% Total Harmonic Distortion, both channels driven using 1 kHz from a Maximum RMS Voltage of 71?

My calculations arrive to 315 watts per channel in an 8-ohm load.

Would pink noise measurements play a factor why the numbers do not correspond?

Your calculation is correct, 315W RMS power.

It wouldn't be the first time when manufacturers are defining their own "watt", or they are flat misleading. Here, they don't say anything about the output power being RMS so God knows what they are talking about.
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Old 3rd January 2009, 11:37 AM   #4
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How did u get 315W per channel?

If it *really* can produce 71V RMS on each channel, that would be
(71*71)/8 = 630W PER CHANNEL.

But in real life, due to the internal impedance of the power supplies etc, it will not achieve a full 71V swing with both channels driven into a load. So taking into account some voltage drop, it may be able to put out the claimed 425W per Ch.
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Old 3rd January 2009, 11:44 AM   #5
AndrewT is online now AndrewT  Scotland
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I think they are telling the truth, But they don't specify the duty cycle of the test signals.

look at the peak output voltage required for each of those (assumed sinewave) power figures and compare them to a maximum output sinewave of 71Vac (~=100Vpk) into what is probably an open circuit load.
425W 8r0 82.5Vpk
500W 8r0 89.4Vpk
700W 4r0 74.8Vpk
825W 4r0 81.2Vpk
1000W 2r0 63.2Vpk
1325W 2r0 72.8Vpk.

It appears that these claimed output voltages are all consistent with an amp running ~+-105Vdc on the PSU.

It also appears that the PSU is severely undersized for a 2ohm capable amplifier due to the 3dB drop in output voltage from the 8ohm single channel output figure.
Even the 4ohm power output is compromised by an undersized PSU. It drops >1.5dB when a drop of -0.4 to -0.8dB is easily achievable.
Based on the data provided I suspect this is a robust 425+425W into 8ohm amplifier that will perform poorly into 4ohm loads. Forget about trying to use it on 2ohm speakers.

If the data provided by the manufacturer is accurate and clearly specified, one can decipher a lot about the way it may perform into real loads.

Let's go a bit further. Look at the single channel power figures.
500W 8r0 and 825W 4r0.
The 40hm power has dropped by -0.84dB. This indicates that the PSU is struggling to supply a single channel 4ohm load.
I suspect that if the amp were run in single channel mode the extra voltage on the output stage may destroy the transistors. It would be my guess that to keep reliability in this amplifier they have deliberately underrated the PSU to ensure the resultant voltage sag on the supply allows the output and driver devices to stay within their limited SOAR capability. Don't beef up the PSU and don't run it in single channel mode into 4ohm loads until you have properly investigated the output capability of the devices used.
As I said earlier it is probably a good 8ohm amplifier.
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Old 3rd January 2009, 11:47 AM   #6
syn08 is offline syn08  Canada
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Default Re: Re: Does Pink noise follow the V x V/R method?

Quote:
Originally posted by syn08


Your calculation is correct, 315W RMS power.

It wouldn't be the first time when manufacturers are defining their own "watt", or they are flat misleading. Here, they don't say anything about the output power being RMS so God knows what they are talking about.
I stand corrected. 71V is RMS (not peak as I was assuming), which makes 625W RMS into 8ohm.

Sorry for the confusion. Andrew's analysis is correct.
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Old 3rd January 2009, 12:19 PM   #7
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Cheers Mates!


Quote:
Originally posted by Steerpike
How did u get 315W per channel?

If it *really* can produce 71V RMS on each channel, that would be
(71*71)/8 = 630W PER CHANNEL.


It does not say 71 V RMS on each channel it says 71 V RMS.

I arrived to the same sum as well however, divided it into two so it would be 315 watts per channel in an 8-ohm load or 630 watts @ 8 ohms total.

Quote:
But in real life, due to the internal impedance of the power supplies etc, it will not achieve a full 71V swing with both channels driven into a load. So taking into account some voltage drop, it may be able to put out the claimed 425W per Ch.

Would that be sine wave or pink noise?

This company (Peavey) made an amplifier (CS 800) 30 years ago that offered more RMS Voltage Swing than what this amplifier (A CS 2000) can offer yet, it was rated only 240 watts per channel in an 8-ohm load. However, it was tested using sine waves. I’ve examined the Total Harmonic Distortion amongst the two and they are too close to make such a boost in the output in this (CS 2000) model.

Would using pink noise fall into your "real life" why these figures "may be able to put out the claimed 425W per Ch."?

Cheers!
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Old 3rd January 2009, 09:18 PM   #8
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Amplifiers' power is not determined only by the voltage swing they can achieve. The voltage swing puts an upper limit on the point at which they clip, but an important factor is the heat generated internally while they are doing this.

Sinus or pink noise ought not to make much difference in an amplifier. The basis of the calculation is measuring the RMS voltage, and computing the power based on a resistive load.
The waveshape doesn't change anything, and in an amplifier, unlike say a speaker, there is no division of duty according to spectral content. The amp works just as hard, no matter what the signal.

Measuring the RMS value of pink noise is a bit more iffy - you need a rather high-tech meter to do it accurately.

Amplifiers that will swing to 70V but not into an 8 ohm load DO have an application, since some PA distribution systems are based on a 70V audio line voltage, In those systems, the speakers are transformer coupled to the line, and present a fairly high impedance. So some amplifiers are designed to be able to do double duty in both types of system.

As mentioned, I think you'd find that with an 8ohm load, the 71V RMS value is not achieved. That is likely a no-load condition, or light load.
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Old 3rd January 2009, 09:53 PM   #9
llwhtt is offline llwhtt  United States
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The 71VRMS is meaningless since no load is specified. It only takes a hair over 58VRMS to make 425 Watts into 8 Ohms, that's under the 71VRMS max. so it looks like the specs may be achievable.

Craig
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Old 4th January 2009, 05:00 PM   #10
OMNIFEX is offline OMNIFEX  Jamaica
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Thank you.

I guess "peaks" would be the best way to describe those wattage figures. For the more I look at them, the more I noticed things missing (Continuous Average Power for example) that guaranteed the wattage could be delivered on a long-term basis.

There is even a disclaimer that states, “2-ohm power is time limited by magnetic circuit breaker” that I clearly overlooked. Now that would be something I would really enjoy encountering when conducting a thirteen-hour rave and needed to load eight 8-ohm midrange drivers in the worst-case scenario.

I would imagine a magnifying glass is in order on how amplifiers are measured these days compared to 20-30 years ago.

Cheers!
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