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Old 9th October 2009, 03:57 PM   #1
TheGimp is offline TheGimp  United States
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Default Power Measurements at normal listening levels

From the thread "Wanting to start a DIY tube amp"...

Wanting to start a DIY tube amp

Quote:
Originally Posted by wicked1 View Post
I will.. tell me how
I've a scope, and a synthesizer (lets call it a "signal generator"). Obviously a multimeter, too, but for ac it only measures hundreds of volts. If there's a way to do it w/out lugging the synth up here, all the better. (Its a giant modular synth I built)
If you want to measure power at normal listening levels, you don't need the synthisizer, just play whatever you like to listen to at your normal settings.

Then measure the peak and average voltage levels at your amp speaker terminals with your oscilloscope and asume your nominal speaker impedance to calculate power (we used different equiptment, but a scope should be good enough). If your scope can't do average, etc, you will just have to guess where average level is.

Since these are rough measurements you can leave you speaker hooked up and not switch to a resistive load which would give better lab numbers. (The resistor is easier to compare results since the speaker impedance varies with frequency and someone with different speakers will get different results, but that is the whole point. We expect to get different numbers.).

What you listen to has an impact on level, especially peak values. I think we were listening to Freddy Hubbard, but I can't find what I thought was the album on the web (Sky Dive?).

Please post your results along with the amp you are using and your speaker type and efficency.

Steven

Last edited by TheGimp; 9th October 2009 at 04:00 PM. Reason: add link
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Old 9th October 2009, 06:28 PM   #2
wicked1 is offline wicked1  United States
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Ok, Ive taken my measurements, and what is the math? Just ohms law? so voltage/resistance?

-edit-I think my scope is in bad need of calibration, so just ignore the rest of the message!! but let me know if Im on the right track. I'll go calibrate my scope.

so, 1 volt is loud.. .5 volts is about average (I'll take some better measurements later)

so, lets do 1... 1/.8 = 1.25, so 1.25 watts is loud? and thats my computer speakers which are tiny 3" ff85k fostex @ 88db sens.
thats from a st-35 clone (one of the diytube PCB's)

- ok, I dont know why I used .8 for the ohms, I guess that should be 8! so that's .625 watts?

I'll get some measurements off my set and 96db sens. fostex speakers later.

So, is that correct for the math, or is there more to it?

Last edited by wicked1; 9th October 2009 at 06:37 PM.
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Old 9th October 2009, 06:46 PM   #3
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Well you got the right answer but with the wrong equation.

Power = V^2 / R.

Fortunately for you the answer is 0.625W.

But this is pretty inaccurate. The speakers will be more like 6ohms at DC with big humps in the mid band and trying to guess the average level of music is next to impossible.

I would say you're within a factor of 2 (somewhere between 0.3 and 1.2W)
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Old 9th October 2009, 06:56 PM   #4
wicked1 is offline wicked1  United States
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well I calibrated my scope, and yeah.. It's still in the area of .5 volts, w/ some 1v peaks on the really loud hits, for normal listening. That's .03 watts!?! Insanity! Why aren't all amps 5 watts or less?
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Old 9th October 2009, 07:12 PM   #5
adason is offline adason  United States
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I got pretty much the same results when I measured power on my speakers, just few hundred milliwatts, with peaks somewhere up to one watt for normal listening.
I used oscilloscope too.
I use 5-7 watts tube amps for midrange and 40-70 watts solid state for bass.

Now things get pretty crazy once you start turning the volume up.
It's easy to clip the solid state amp! In one of my systems the solid state amp is AKAI with nice and fast fluorescence display measuring the power. I can hit the red (100 watts) with dynamic music (very loud).

But again at normal listening levels the power stays below 1 watt and I have to switch x100 on the display to see it.
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Old 9th October 2009, 07:42 PM   #6
AndrewT is offline AndrewT  Scotland
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Hi,
the average level for listening is around 60 to 90dB.
Let's settle for 70dB, about equal to a conversation.

If the speakers are 85dB/W/m, then a pair are ~80dB/W @ 2.5m
Input ~100mW to the speaker and the SPL ~70dB.
So about 100mW into 85dB/W/m speakers results in an average level roughly equivalent to a conversation.

Now look at the dynamic range of the music you are listening to. It could be 60dB from peak to quiet. Most recordings will have a peak to average of <20dB, that leaves the quiet at >40dB below the average level or about <=30dB SPL.

That +20dB figure is ten times the signal voltage.
Yes, your 900mV average signal can go as high as 9Vpk when listening to dynamic sources at conversation level.

Now turn up the volume to loud domestic, about 80dB. That results in peak voltage levels of ~30Vpk

Turn up the volume to party levels approaching 90dB. That requires a peak signal capability of ~90Vpk That is 500W into 8ohms.
Yes, we listen to average levels of 100mW to 1W but the amplifier and 85dB speakers are required to handle 500W in a domestic listening environment.

95dB/W/m speakers will require one tenth of the quoted power levels for the same SPL conditions, i.e. 50W from a chipamp driving 95dB 8ohm speakers is good for most domestic listening conditions
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Old 9th October 2009, 07:51 PM   #7
wicked1 is offline wicked1  United States
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I guess age is an important part of that equation, too. Because 10 years ago, a party meant 50,000 watts of bass in your face, where as now I can have my house packed, and don't turn the volume up much past where I took the measurements earlier, so 1 watt. and Chardonnay has replaced MDMA
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Old 9th October 2009, 08:23 PM   #8
wicked1 is offline wicked1  United States
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-and basically what im trying to say, is I can't believe anyone would ever need 500 watts for a home, or even 100! My most powerful amp is an old 100w per channel sansui, w/ some early 80's consumer speakers (no idea on the specs) but I never turned that thin up past 3 or 4 on the dial.. and thats the amp I was using in the mid 90's in my wilder louder days.
I can't imagine an adult every needing more than 10 watts!

I guess it's the difference these high efficiency speakers make.. why hasn't the commercial market focused on high eff. speakers?
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Old 9th October 2009, 09:59 PM   #9
TheGimp is offline TheGimp  United States
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Some of the market has. Look at Klipsch, the K-Horns (3 horn sections) is rated at 105dB/W-M and have been built from the late 30s on.

Drive them with a 1W amp and it better be clean or you will hear every defect in it.

Dissadvantage - Very expensive, very large, require moderate large room for proper setup, require corners with unobstructed walls as speaker extensions for proper loading of the horns.

On the other hand LaScala (Three horn sections 105dB/W-M)works well in corners without being as sensative as K-Horns. Still quite a bit of ka-ching.

Cornwals (Base Reflex with horn loaded mid and high 102dB/W-M) were (to me) too obtrusive, although better sounding than the Herasey (96dB/W-M, Heresy III is 99db/W-M but I'm not about to buy new ones).

I havn't looked at the speaker market in 30 years, but I am willing to bet the average efficency has crept up a bit, but you still have to pay for efficency and quality.

I'm sure there are other companies making speakers as good or better, and possibly at lower cost.

The Heresy never had the greatest specs as far as frequecy response went, but they sounded good to me when I got them in 77, and they still do sound good.
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Old 10th October 2009, 02:12 AM   #10
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a few more factors....

An amplifier rated 100W is usually several percent THD at that power. If you want clean, low distortion peaks then you need a 500W amplifier to deliver a clean 100W.

Also, if you have exotic speakers/crossovers with wild impedance curves then you need more spare power to control the complex load.

BTW, nice synopsis AndrewT!
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