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Lindsayt
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Unread post by Lindsayt » Sat Mar 18, 2017 1:26 pm

Can anyone with a few minutes to spare please read over what I've written in this thread: https://www.avforums.com/threads/do-dif ... e.2090058/ and please tell me if I've written anything wrong or explained myself badly?

I for the life of me can't understand where andy1249 is coming from. I really don't understand what he means when he says that I have been using the wrong units.

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Dave Cawley
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Unread post by Dave Cawley » Sat Mar 18, 2017 2:37 pm

I think the question they are really asking is, just how are you measuring such a low power which is less than 1mV into 8 ohms ? If the measurement device is not connected directly to the amplifier/speaker then I would have doubts too.

And there is no reason why THD should not be measured at that level, none at all.

Dave
"if it measures well and sounds good, then it is good" : "if it measures badly and sounds bad then it is bad" : "if it measures badly and sounds good, then it could be improved" : "if it measures well but sounds bad then it is bad" © Dave Cawley

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Lindsayt
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Unread post by Lindsayt » Sat Mar 18, 2017 4:19 pm

I'm not measuring the power. I'm measuring the sound in dbs with my sound meter. From there I can make a reasonable estimate of the power levels from my amplifier if I know the sensitivity of my speakers.

It's simple deduction. My speakers are 12.5 % efficient at turning electrical energy into acoustic energy when fed with 1 watt (2.83 volts). I think it's a fair enough assumption / estimate to say that my speakers will be 12.5% efficient at turning 1 milliwatt or 1 microwatt of electrical energy into acoustic energy.

With the db scale of sound, as measured by sound meters, a reduction of 10 dbs from the reading represents one tenth the acoustic and therefore electrical power.

I don't listen much at 103 db volume levels. I do listen late at night at volumes where the music will be jumping from 30 something dbs to 60 something dbs. Therefore, the THD+N might be of some interest to me at the micro-watt power level - especially when it's possible (from extrapolation of Stereophile graphs) that this figure might be of the order of 10% to 100%!

Surely there's just as much, if not more, reason to measure THD+N at power levels resulting in volumes of 33 to 93 dbs than there is in measuring it at power levels of 93 to 123 dbs?

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Dave Cawley
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Unread post by Dave Cawley » Sat Mar 18, 2017 4:41 pm

I get that. But you did specifically mention 1/10 of a uW. And that is probably not a reasonable estimate given all the variables. Me; I don't care, but you asked.

THD will be measurable and if extracted from the noise, as a THD measurement would be, it could easily be well under 1%. But if you throw in noise, which is not THD then it will "appear" greater. TBH not many people would be interested in THD+N measurement at that level, and if it's a class A/B amplifier it will be firmly in class A so should be OK. In fact at that level why THD+N when the noise will be dominant and not just measure THD ?

Anyway I believe I have explained what the other guy is saying, and that was your question. Personally I'm not that interested in debating this as I'm confident of the outcome. However if you want to drop by we can do all and any measurements here in real time.

Dave
"if it measures well and sounds good, then it is good" : "if it measures badly and sounds bad then it is bad" : "if it measures badly and sounds good, then it could be improved" : "if it measures well but sounds bad then it is bad" © Dave Cawley

_D_S_J_R_
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Unread post by _D_S_J_R_ » Sat Mar 18, 2017 6:31 pm

Dave, thanks :grin:
...I have renounced the worldly bustle and live in peace at a quiet place,
I live alone in this my heaven, in my love (and) in my songs.

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Lindsayt
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Unread post by Lindsayt » Sat Mar 18, 2017 9:58 pm

Thank-you Dave.

So would you agree with me that this statement is incorrect / misleading?

"At any lower volume THD + N will be less."
Even if it's just because the noise added by the amplifier will become more significant as a percentage as the volume is decreased?



And would you also agree that this statement is :hbs: ?

"100mw RMS continuous output power into 8 Ohm load in an amp is barely audible, if at all!"

100mw would be loud through my Patricians and a pretty generous volume through 87 db/2.83v/1m speakers.

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Unread post by Dave Cawley » Sun Mar 19, 2017 12:48 am

Life isn't black and white and it isn't my argument.

My iPhone directly into a pair of Klipschorns is almost as load as a HMV 150.

Dave
"if it measures well and sounds good, then it is good" : "if it measures badly and sounds bad then it is bad" : "if it measures badly and sounds good, then it could be improved" : "if it measures well but sounds bad then it is bad" © Dave Cawley

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Lindsayt
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Unread post by Lindsayt » Sun Mar 19, 2017 7:51 am

Oh yes, the grey area as to whether 100 milliwatts produces about 93 dbs at 1 meter from my Patricians or a sound that is barely audible. :-?

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Unread post by Dave Cawley » Sun Mar 19, 2017 10:34 am

Lindsayt wrote:Oh yes, the grey area as to whether 100 milliwatts produces about 93 dbs at 1 meter from my Patricians or a sound that is barely audible. :-?
Yes probably. I really don't know your speakers, perhaps you could give me a link as I found more than one Patrician 800 ? Initial research would indicate that with 100mW you would get an OK listening volume, as the iPhone example above proves. BTW this started off about advice on terminology that I was happy to give. Now it's somewhat changed and as it involves old classic loudspeakers I'm very happy to continue !

Remind me where you are ? It might be interesting to do the amplifier measurements and also some SPL measurements from a calibrated source. We might even turn it into an article for HiFi World ?

Dave
"if it measures well and sounds good, then it is good" : "if it measures badly and sounds bad then it is bad" : "if it measures badly and sounds good, then it could be improved" : "if it measures well but sounds bad then it is bad" © Dave Cawley

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Lindsayt
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Unread post by Lindsayt » Mon Mar 20, 2017 11:07 am

I've come to the conclusion that andy1249 is a moron.

He is unable to grasp the nature of the logarithmic db sound pressure scale and how that relates back to the amount of power fed from an amplifier to a pair of speakers, as shown here: http://myhometheater.homestead.com/splcalculator.html

He thinks that 0.1 watt fed from an amplifier to a pair of speakers is barely audible. He's made his mind up and isn't going to change it regardless of the evidence to the contrary. He thinks that amplifiers are incapable of feeding power levels as low as 1 micro-watt to a pair of speakers! :angry-steamingears:

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