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# AC power meter for HiFi audio system

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(@lobster)
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Joined: 3 years ago
Posts: 6
Topic starter
Hello,

not sure if you can help me.
My goal would be to build an AC power meter for my HiFi audio system (1 for each channel).
The problem is that the power depends on the frequencies of the signals.
So the measurement should somehow take this into account of the calculation.
As the frequencies can change rapidly it should additionally have a kind of delay
of the output (LCD with I2C).
Is that possible ?

Thanks
andy

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(@mike650)
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Joined: 2 years ago
Posts: 13

Hi, I can help.

A power meter should be flat over the entire frequency range.  It only needs to show the product of the AC voltage and AC current at any frequency.

Another way commonly used is to measure the voltage across a fixed resistance, square it, and display the value as power.

Power should NOT be a function of frequency.

Mike

(@zander)
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It sounds like what you really want is a display of freq on the X axis and power on the Y axis.

Arduino says and I agree, in general, the const keyword is preferred for defining constants and should be used instead of #define
"Never wrestle with a pig....the pig loves it and you end up covered in mud..." anon
My experience hours are >75,000 and I stopped counting in 2004.
Major Languages - 360 Macro Assembler, Intel Assembler, PLI/1, Pascal, C plus numerous job control and scripting

(@lobster)
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Topic starter

Thank you guys,

well, I knew that the power is AC voltage times AC current. The problem here is the fixed resistance as the resistance (of the speakers) changes with the frequency.

But lets assume the resistance IS fixed. How/where would I measure voltage and current. There are some power meters to buy. For my knowledge it has to be at the output of the amplifier WITH the speakers  connected. Right?

Rons answer could be it if I would know how to to do it with an arduino (hardware, sketch).

andy

(@will)
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Experience is what you get when you don't get what you want.

Ron reacted
(@zander)
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@lobster I don't understand how 'resistance' can change. Let me admit at 80 it's been over 60 years since I learned ohm's law but I don't remember any relationship to frequency. Perhaps you mean reactance? I don't know but that's my best guess.

HOWEVER, looking back at your original question, the power is related to the frequency because the area under the curve changes with frequency. I recall something called RMS voltage, I wonder if that is what is causing the frequency related power change you are thinking of. That should be measurable.

Arduino says and I agree, in general, the const keyword is preferred for defining constants and should be used instead of #define
"Never wrestle with a pig....the pig loves it and you end up covered in mud..." anon
My experience hours are >75,000 and I stopped counting in 2004.
Major Languages - 360 Macro Assembler, Intel Assembler, PLI/1, Pascal, C plus numerous job control and scripting

(@zander)
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@will That looks interesting, but wasn't he talking about audio frequencies? Your reference is for 60Hz, will it scale to 10,000Hz as a midpoint? I think it may depend on the MPU processor speed, buyt that's just a hunch.

Arduino says and I agree, in general, the const keyword is preferred for defining constants and should be used instead of #define
"Never wrestle with a pig....the pig loves it and you end up covered in mud..." anon
My experience hours are >75,000 and I stopped counting in 2004.
Major Languages - 360 Macro Assembler, Intel Assembler, PLI/1, Pascal, C plus numerous job control and scripting

(@will)
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The video I referenced is aimed at calculating the power of an AC device. It would allow the power used by the entire device as a whole, but probably wouldn't provide a satisfactory result for an individual speaker.

Measure total power with the speakers disconnected, then reconnect them and measure again for the same music. The difference should be the total power delivered to the speakers.

Experience is what you get when you don't get what you want.

(@zander)
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@will I suspect that is not his interest, I suspect he wants to know the power at different frequencies. I can imagine someone wanting to adjust the power for various frequencies so a meter is needed or a scope or similar.

Arduino says and I agree, in general, the const keyword is preferred for defining constants and should be used instead of #define
"Never wrestle with a pig....the pig loves it and you end up covered in mud..." anon
My experience hours are >75,000 and I stopped counting in 2004.
Major Languages - 360 Macro Assembler, Intel Assembler, PLI/1, Pascal, C plus numerous job control and scripting

(@will)
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It certainly wouldn't be the first time I guessed wrong at what somebody is asking for 🙂 Nor the last.

Experience is what you get when you don't get what you want.

(@zander)
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Joined: 2 years ago
Posts: 4440

@will Mind reading isn't my strong suite either.

However, my HS electronics teachings taught me that 10 x power is 10 db sound which the human ear interprets as 2 x louder BUT I do not recall if the freq has a bearing but this article hints at a relationship https://www.britannica.com/science/sound-physics/The-decibel-scale

Arduino says and I agree, in general, the const keyword is preferred for defining constants and should be used instead of #define
"Never wrestle with a pig....the pig loves it and you end up covered in mud..." anon
My experience hours are >75,000 and I stopped counting in 2004.
Major Languages - 360 Macro Assembler, Intel Assembler, PLI/1, Pascal, C plus numerous job control and scripting

(@will)
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Posts: 2286

I don't know squat about audio and I can't figure whether he really means resistance or reactance or impedance, so I'm going to just wander off now 🙂

Experience is what you get when you don't get what you want.

(@zander)
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Joined: 2 years ago
Posts: 4440

@lobster Check this out as well https://www.britannica.com/technology/sound-level-meter

Arduino says and I agree, in general, the const keyword is preferred for defining constants and should be used instead of #define
"Never wrestle with a pig....the pig loves it and you end up covered in mud..." anon
My experience hours are >75,000 and I stopped counting in 2004.
Major Languages - 360 Macro Assembler, Intel Assembler, PLI/1, Pascal, C plus numerous job control and scripting

(@mike650)
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Joined: 2 years ago
Posts: 13

@will  Those AC voltage and AC current sensors appear to be meant for power line frequencies,  (50 or 60 Hz)  It's  unlikely that they will be accurate for a wide frequency range such as audio (20 to 20 kHz).

A speaker's impedance (AC resistance) will vary over frequency.  With a good speaker, say a 3 way system you should have a fairly constant impedance over frequency but it won't be perfect.  Especially if there's a resonance- the impedance will rise at the resonant frequency.

The RMS voltage squared divided by the speaker's estimated impedance will be the estimated power. Anything better would be hard to measure accurately because the impedance is not perfectly constant over frequency.

(@lobster)
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Joined: 3 years ago
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Topic starter
Posted by: @will

I don't know squat about audio and I can't figure whether he really means resistance or reactance or impedance, so I'm going to just wander off now 🙂

you are right! what I meant was impedance.

Ron reacted
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