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Amplifier Power test


captainbeefheart

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More equations for the math club. According to Osha, hearing loss begins when these thresholds are exceeded...

 

I. Computation of Employee Noise Exposure

(1) Noise dose is computed using this calculation

95APPATABLEG.gif

where L is the measured A-weighted sound level.

(i) When the sound level, L, is constant over the entire work shift, the noise dose, D, in percent, is given by: D=100 C/T where C is the total length of the work day, in hours, and T is the reference duration corresponding to the measured sound level, L, as given in Table G-16a or by the formula shown as a footnote to that table.

(ii) When the workshift noise exposure is composed of two or more periods of noise at different levels, the total noise dose over the work day is given by:

D = 100 (C(1)/T(1) + C(2)/T(2) + ... + C(n)/T(n)),

where C(n) indicates the total time of exposure at a specific noise level, and T(n) indicates the reference duration for that level as given by Table G-16a.

(2) The eight-hour time-weighted average sound level (TWA), in decibels, may be computed from the dose, in percent, by means of the formula: TWA = 16.61 log(10) (D/100) + 90. For an eight-hour workshift with the noise level constant over the entire shift, the TWA is equal to the measured sound level.

 

 

table-1.jpg

 

OSHA uses a 5-dBA exchange rate, meaning the noise level doubles with each additional 5 dBA. So the inverse is also true. For a 16 hour day 85 decibels is allowable. 80 decibels is the allowable level for a 32 hour period.

 

These are A-Weighted values also. That means what your SPL meter "hears" is not the same as what the human ear hears, and this value varies over the frequency band. See chart below...

 

10gfg_9.gif

 

You can see how SPL is A-weighted in total value in the chart below.

 

otm_figb.5.jpg

 

Wouldn't it be fun to know how to calculate the curve in the charts above? If you want to read the mathematic expression for the curves, follow along here. Scroll to the bottom half of the page. It's just too long to post here. 

https://en.wikipedia.org/wiki/A-weighting

 

So now we can measure our A-Weighted decibel levels at our listening position and apply that reading to this formula to get the maximum allowable time we can enjoy our music, at that level safely.

 

 

95APPATABLEG.gif

 

where L is the measured A-weighted sound level

and T is the time allowed

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5 hours ago, pauln said:

edit... The more I think about it, the more I think I may be incorrect. Maybe the -12dB level of the file is arbitrary, just a way to calculate what the listened level power would be at full scale in order to access rated power required. 

 

Bingo

 

-12db was somewhat arbitrarily chosen because yes although we could pick any value we wanted since we know how digital audio files function and can work the math out from it, but -12db or a voltage difference of 4x makes the math easy to do quickly. Just square your test reading for power into an 8 ohm load.

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It's easy to misunderstand and look at -12db as a headroom reference or anything like that. Once you understand how the test works you'll understand why it accounts for headroom.

 

A digital file cannot go past 0db.

 

The highest a signal can be on a digital recording is 0db, which strictly defines any signal's maximum amplitude can only reach this 0db. Wherever you have your volume set to doesn't matter, the largest signal amplitude peak on your digital file will always be 0db.

 

We have a sine wave recorded on a file at -12db.

 

Since we know maximum peak amplitude is 0db, and RMS maximum power of a sine wave is -3db. To get -3db when we are at -12db we need to gain +9db.

 

The voltage difference of 9db is 2.83

 

So whatever you measure with your meter multiply by 2.83, square that value then divide by load.

 

 

 

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On 2/25/2022 at 10:29 AM, LTusler said:

Here are my Klipsch Jubilee results.

Measured 60dBA, 402 Horn w/AXI 2050, .18V

Measured 82dBA, 402 Horn w/AXI 2050, 2.7V

Measured 60dBA, KPT-KHJ-LF, .14V

Measured 82dBA, KPT-KHJ-LF, 1.8V

 

 

Based on the comments above, I redid the measurements.

First I used a variac to calibrate the Motorola analog RMS meter to 1V on the 1V scale.

I set the levels for each driver to 0dB in the Xilica DSP.

Then in order to get the meter to read more in the center of the range I used more power.

Measured 70dBA, 402 Horn w/AXI 2050, .40V

Measured 90dBA, 402 Horn w/AXI 2050, 3.20V

Measured 70dBA, KPT-KHJ-LF, .28V

Measured 90dBA, KPT-KHJ-LF, 2.1V

 

For the 90dB levels I switched the meter to the 3V setting.

So I think I am measuring correctly, but if not please advise.

BB at 90dB.jpg

Horn at 90dB.jpg

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Looks good to me, you have the switch on "3" so we are reading the bottom scale, 2.1 and 3.2.

 

That comes out to 4.4 watts and 10.24 watts for an 8 ohm load. Like I said if anyone wants to be prudent and aim for power into the lowest impedance of the speaker load the maximum power you need is a 20 watt amplifier. Or if you speakers are relatively flat across the audio band a 10 watt amp will do just fine.

 

Your average listening is 160mW.

 

 

Thanks again for participating. Let's try and get some more members to do the test, peer pressure always works👹

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OK finally got some good steady numbers. Normal level is .05 VAC and LOUD is .49 VAC. Ended up taking some banana plugs and speaker wire and making a test lead that just plugged into the amp and multimeter. Worked great.

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2 hours ago, geezin' said:

OK finally got some good steady numbers. Normal level is .05 VAC and LOUD is .49 VAC. Ended up taking some banana plugs and speaker wire and making a test lead that just plugged into the amp and multimeter. Worked great.

 

Excellent thank you for contributing to the test.

 

Your maximum steady state power into 8 ohms is 250mW. You could easily get away with a flea watt amp as even 1 watt would be more than plenty for your tastes.

 

The test is showing so far that all of us are under 1 watt majority of the time, some well below 1 watt. The determining factor is your loudest of course because some times there is an occasion where you need to really get loud, parties or what have you and still we are seeing those reading be less than 20 watts, typically much less. For your case in particular you must not crank it up loud which is good, you'll keep your hearing. My maximum was a very rare occasion but still I needed to account for it because the once or twice a year where I fill the house with music for a party I still only require a 10 watt amplifier. It's clearly showing you do not need these 350wpc monster amps with efficient speakers.

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33 minutes ago, Westcoastdrums said:

Damn, that was going to be my next build request from you. 

 

You may not always run efficient speakers and in that case could need the power. For La Scala's 80 watts should be plenty to really push those to very loud levels, but plug the same amp into 85db speakers and you won't reach the same ear bleed levels.

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12 minutes ago, captainbeefheart said:

 

You may not always run efficient speakers and in that case could need the power. For La Scala's 80 watts should be plenty to really push those to very loud levels, but plug the same amp into 85db speakers and you won't reach the same ear bleed levels.

Agreed. This will likely be my first and last tube amp. That's why I wanted once and done, go big or go home for above mentioned reasons as we discussed. 

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7 hours ago, captainbeefheart said:

 

Excellent thank you for contributing to the test.

 

Your maximum steady state power into 8 ohms is 250mW. You could easily get away with a flea watt amp as even 1 watt would be more than plenty for your tastes.

 

The test is showing so far that all of us are under 1 watt majority of the time, some well below 1 watt. The determining factor is your loudest of course because some times there is an occasion where you need to really get loud, parties or what have you and still we are seeing those reading be less than 20 watts, typically much less. For your case in particular you must not crank it up loud which is good, you'll keep your hearing. My maximum was a very rare occasion but still I needed to account for it because the once or twice a year where I fill the house with music for a party I still only require a 10 watt amplifier. It's clearly showing you do not need these 350wpc monster amps with efficient speakers.

Damn. Thought the readings were kinda low. It's a Topping PA3s I just got through a pair of Klipsch R-610f speakers. The normal was at 70-75 dB with a NIOSH app on the iPhone. Didn't bother to check the loud level 'cause the neighbor works swing shift and I think he's on graveyard this week.

 

The numbers surprise me. It's not like I listen at low volumes intentionally just where I can discern the different sounds. I do like it loud sometimes but try to be a good neighbor.

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A quote from Nelson Pass' Firstwatt web page. With our horns the first watt is the most important not how many watts an amplifier has that will never be used. 

 

"Dick Olsher famously remarked that “The first watt is the most important watt.” This sentiment has also been expressed by others as “Who cares what an amplifier sounds like at 500 watts if it sounds like crap at one watt?” With this in mind, I created First Watt in 1998 as a "kitchen-table" effort, exploring unusual low power amplifiers with an emphasis on sound quality.

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I've seen mentioned in other threads the idea that amplifiers have a "sweet spot" that is usually more toward the high end of their power output range rather than at the bottom. Can you more knowledgeable folks address that? If it is true, it seems a flea watt would actually be a superior choice for high efficiency Klipsch speakers.

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2 hours ago, CWelsh said:

I've seen mentioned in other threads the idea that amplifiers have a "sweet spot" that is usually more toward the high end of their power output range rather than at the bottom. Can you more knowledgeable folks address that? If it is true, it seems a flea watt would actually be a superior choice for high efficiency Klipsch speakers.

That is usually mentioned in the context of class AB amplifiers, in which one device amplifies the positive half of the waveform and another device amplifies the negative half, with a small region of overlap in-between. There is a slight anomaly in the region where one device hands-off to the other; it is called "crossover distortion" for intuitive reasons. That crossover distortion is constant, regardless of the amplitude of the waveform being amplified. So at low power levels the amplitude of the crossover distortion is larger, relative to the amplitude of the audio waveform, than it is at high levels. Therefore, as a percentage of the total output, the crossover distortion gets lower as the power gets higher.

 

As the power output rises, at some point the amplifier starts to clip, at which point distortion starts to rise again. So there is a point where the overall distortion percentage, from all causes, is at a minimum. That is the "sweet spot" that you mentioned.

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2 hours ago, CWelsh said:

I've seen mentioned in other threads the idea that amplifiers have a "sweet spot" that is usually more toward the high end of their power output range rather than at the bottom. Can you more knowledgeable folks address that? If it is true, it seems a flea watt would actually be a superior choice for high efficiency Klipsch speakers.

 

 

Edgar nailed it, the vast majority of amplifiers will have worse distortion figures at low power vs high power. This means if you have a 120 watt amplifier and you live under 1 watt of power with your highly efficient speakers then it's a large possibility you are listening to the worst your amp can offer in that low power region vs the best it can offer.

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4 hours ago, CWelsh said:

I've seen mentioned in other threads the idea that amplifiers have a "sweet spot" that is usually more toward the high end of their power output range rather than at the bottom. Can you more knowledgeable folks address that? If it is true, it seems a flea watt would actually be a superior choice for high efficiency Klipsch speakers.

 

 

I had posted here some info regarding the Quad current dumping principle. It combines reasonable power with the finest low power signals.

 

This article below in the link describes the circuit by its inventor, Peter Walker, some of you may know his electrostatic speakers as well. The document refers to the first attempt, the Quad 405, my Quad 306 otherwise does not have the need to use the op-amp in the signal way. I have purchased two used units and I am happy to let them work with my Underground Jubilees soon. Beside the ultra low low level distortion is this 306 quad extremely dead quit noise free even with 108 dB speakers.

 

https://www.dadaelectronics.eu/uploads/downloads/05_Other-Quad-Documents/Current-Dumping-explained-by-Peter-Walker-1975.pdf

 

 

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3 hours ago, CWelsh said:

I've seen mentioned in other threads the idea that amplifiers have a "sweet spot" that is usually more toward the high end of their power output range rather than at the bottom. Can you more knowledgeable folks address that? If it is true, it seems a flea watt would actually be a superior choice for high efficiency Klipsch speakers.

While I have no doubt this is true at least regarding distortion measurements, I have owned very powerful amps which sounded great at low volume levels.

Both a Phase Linear 400 II and a QSC PLX1804 (900WPC @ 4 Ohm) sounded good at any volume.

I am using the QSC amp on Quarter Pies at less than a watt on average. Overkill never sounded so good.

Neither was quite as good as my 25 WPC class A DIY Pass amp, but that is IMHO in another class.

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