Measuring amplifier power used

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Thermionic Idler
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Measuring amplifier power used

Post by Thermionic Idler » Tue Feb 14, 2017 7:55 pm

Hi all,

As some of you may know I currently have plans to build a set of OTL "Beast" power amps with a configuration of 12 EL509's per channel, good for 75 watts or so.

Bruce now has the "Pinnacle" 300B-based amplifier, which as I love the 300B sound would seem to be the best of both worlds, but up until now I'd dismissed them on the basis that 6 watts per channel really would not be enough. However, with the FF225WK based Metronomes seeming to be quite efficient, I'm starting to wonder if the 300B amplifier might actually be an option. I know Steve used low-power amps with them before.

I had the idea of placing an oscilloscope on the speaker terminals to try and get an idea of our typical power usage in a listening session, should be able to work it out from the voltage displacement on the waveform.

Question is how to work it out - I know the basic equation of P = V(squared) / R, but additional complications creep in when dealing with alternating waveforms. The amp is rated at 6 watts, but I assume that's the standard RMS measurement (the 1/1.4 multiplier). According to Bruce's website, It's actually capable of making a perfect 1000 Hz square wave at 8 volts peak to peak into 8 ohms - and I make that as 8 watts?

So I'm a bit confused about what I need to be looking for. Maybe I should post this on the Transcendent forum and get input from the man himself, but I thought I'd see what you guys think.

Thanks in advance.
Deck: Garrard 301
Audio Origami PU7
Van Den Hul Black Beauty
Phono: Whest PS30RDT
Pre-amp - Scratch built AMB α24/LCDuino
Amp - Transcendent "Son of Beast" 15W OTL
Speakers - FF225WK Metronomes

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Mike H
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Re: Measuring amplifier power used

Post by Mike H » Tue Feb 14, 2017 8:37 pm

To be honest, if the same speakers will be used, in the same room etc., then all you have to do is note the Voltage the 'scope shows (and peaks are easier to watch), then you know how many Volts the OTL will have to do to produce the same loudness.

That's how I would look at it. Seemples! :D
 
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Re: Measuring amplifier power used

Post by Mike H » Tue Feb 14, 2017 8:41 pm

8 volts peak to peak into 8 ohms - and I make that as 8 watts?
Yes, 8V / 8R = 1A, x 8V = 8W.

You might be surprised how little power you actually need in your room though. 'Scoping it could be quite revealing.
 
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SteveH
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Re: Measuring amplifier power used

Post by SteveH » Tue Feb 14, 2017 11:32 pm

Do you have a TRMS DMM specced to at least 1kHz and a 10 watt or greater, 0.1ohm 1% resister?

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Thermionic Idler
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Re: Measuring amplifier power used

Post by Thermionic Idler » Wed Feb 15, 2017 12:52 am

Thanks all. Sadly SteveH I don't think my DMM matches those specs.

Earlier I had Peter Gabriel's "Kiss That Frog" playing at quite a healthy level - it has a pretty heavy bass line, and the scope was indicating up to 15 volts peak-to-peak - I make that nearly 30 watts - interesting as my present amp is rated at 15wpc.
Deck: Garrard 301
Audio Origami PU7
Van Den Hul Black Beauty
Phono: Whest PS30RDT
Pre-amp - Scratch built AMB α24/LCDuino
Amp - Transcendent "Son of Beast" 15W OTL
Speakers - FF225WK Metronomes

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Nick
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Re: Measuring amplifier power used

Post by Nick » Wed Feb 15, 2017 1:46 am

15v p/p is 3.5w

15 / 2 = 7.5

7.5 / 1.414 = 5.3v rms

5.3 * 5 3 / 8 = 3.5w

Assuming 8 ohm speaker
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Thermionic Idler
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Re: Measuring amplifier power used

Post by Thermionic Idler » Wed Feb 15, 2017 8:55 am

Thanks Nick

I'll just be over here in the corner with the dunce's cap on. :oops:

Only 3.5 watts - interesting...
Deck: Garrard 301
Audio Origami PU7
Van Den Hul Black Beauty
Phono: Whest PS30RDT
Pre-amp - Scratch built AMB α24/LCDuino
Amp - Transcendent "Son of Beast" 15W OTL
Speakers - FF225WK Metronomes

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Re: Measuring amplifier power used

Post by Mike H » Wed Feb 15, 2017 5:58 pm

Interesting, didn't think it would be that much. :shock:

I'd probably find that too loud. Mind you taking the bass into aaccount...
 
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Re: Measuring amplifier power used

Post by Nick » Wed Feb 15, 2017 6:06 pm

Remember we are talking about peaks, not steady state signals. The ratio of peak to average is one of the reasons I rant about having enough headroom in amps.
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Re: Measuring amplifier power used

Post by Mike H » Wed Feb 15, 2017 6:19 pm

Oh yes indeed, and quite right too.
 
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Re: Measuring amplifier power used

Post by Mike H » Wed Feb 15, 2017 6:22 pm

Sorry that sounds a bit like I'm taking the pee, I'm not. :D
 
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Re: Measuring amplifier power used

Post by Nick » Wed Feb 15, 2017 6:27 pm

Mike H wrote:
Wed Feb 15, 2017 6:22 pm
Sorry that sounds a bit like I'm taking the pee, I'm not. :D
It didnt, well, not to me anyway
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Re: Measuring amplifier power used

Post by Mike H » Wed Feb 15, 2017 6:48 pm

Coolio. Image
 
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Re: Measuring amplifier power used

Post by chris661 » Wed Jul 19, 2017 4:08 pm

Well, the first watt is the loudest.
It depends on a number of factors, so putting a scope on there is a good idea. That way, you can record the peak voltage, convert to RMS, and figure the minimum amplifier power you'd need.

Speaker sensitivity is one of the biggest contributing factors. For instance, a while ago I was playing with some 6.5" mini-subs used up to a few hundred Hz. They're in a small ported box and come in at about 80dB@1w. Naturally, I connected up a large amplifier - the one I used is capable of around 2KW/ch. At demo levels, the -6dB light was lighting up on every kick drum hit, indicating ~500w peaks on each channel.
A speaker that's 90dB@1w would've been at ~50w peaks, and 100dB@1w would've been at ~5w peaks.

At more sensible levels, I was getting the -24dB light occasionally, which is still a few watts.

IIRC, the big Mets come in at the mid-low 90s at 1w input, so, if you're happy with staying at moderate levels, I expect a couple of watts per side will be just fine.

Chris

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ed
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Re: Measuring amplifier power used

Post by ed » Wed Jul 19, 2017 8:19 pm

I got a little bit derailed by the title of this thread..

amplifier power used I thought was the power coming out of the wall, for which I use this:
powmeter.jpg
Title made sense to me cos the room heaters I use have power in thats not on the same continent as power out.....anyway, as you were chaps
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