From: ruffrecords@yahoo.com   
      
   Arny Krueger wrote:   
   > "Ian Bell"  wrote in message   
   > news:hnp8nc$4o3$1@localhost.localdomain   
   >> Arny Krueger wrote:   
   >>> "Ian Bell"  wrote in message   
   >>> news:hnmbmi$p5e$1@localhost.localdomain   
   >>>   
   >>>> 0dB has no meaning.   
   >>> Actually, it does. In the digital domain 0 dB is   
   >>> commonly equated with FS.   
   >   
   >> No, that is 0dBFS.   
   >   
   > Yes, so your use of the word "no" must be some kind of terrible mistake on   
   > your part.   
   >   
      
   No, 0dBFS refers to full scale signal in a digital system. It is common,   
   especially in live recording to set '0dB' to -15dBFS so as to ensure   
   sufficient headroom. In  other words, 0dB can be anything you like and   
   therefore on  its own without context is meaningless.   
      
   >>> While the units are arbitrary and may be confusing to   
   >>> people who still think 100% analog, it is both   
   >>> meaningful and commonly used.   
   >   
   >> No it is not.   
   >   
   > Assertion without support, which should be immediately dismissed.   
   >   
      
   OK, then we will dismiss your original unsupported assertion that 'it is   
   both meaningful and commonly used'.   
      
   >>>> Input termination in both cases is unspecified.   
   >>> If the input is a mic input, then the usual standard of   
   >>> 300 ohms applies.   
   >   
   >> It is a power amp, it does not have a mic input. 300 ohms   
   >> is NOT the 'usual standard for a mic input'   
   >   
   > Good modern power amps are common, and do not have that much variation in   
   > their noise performance with normal variations in source impedance.   
   >   
   >>> If the input is a line level input, then the source   
   >>> impedance is usually such that system performance is not   
   >>> affected that much by probable variations.   
   >   
   >> You miss the point. How is the input terminated when   
   >> noise is measured - obviously it is not left open circuit   
   >> is it?   
   >   
   > I guess you've never measured the actual noise coming out of a good modern   
   > power amp with the normal range of source impedances. They often don't vary   
   > all that much. Remember, this is not legacy vacuum tube equipment which was   
   > generally far noisier.   
   >   
      
   A 'good modern one' should not vary much. We have no idea if the OPs's   
   device falls into this category. However, if the noise is being   
   minimised then a point will be reached when the source impedance is   
   relevant because if the amp was extremely noise free that would be the   
   ONLY source of noise.   
      
   OTOH, it will not vary much with source impedance if the amp is very   
   noisy to start with, like an old tube amp. SO precisdely the coinverse   
   of what you state is in fact true.   
      
   >>>> Output termination is unspecified.   
   >>> The noise performance of line level outputs is minimally   
   >>> affected by probable  and reasonable variations in load   
   >>> impedance.   
   >   
   >>>> No bandwidth is specified.   
   >>> Relevant and already mentioned   
   >>>   
   >>>> No weighting or not is specified.   
   >>> Relevant and already mentioned   
   >>>   
   >>>> No mention of whether the measurements are rms, peak,   
   >>>> quasi-peak   
   >>> Both SNR and DR are ratios of two measurements. It is   
   >>> most important that both measurements be done under the   
   >>> same circumstances.   
   >   
   >> In fact they generally are not since a signal and noise   
   >> have quite different characteristics. The signal will be   
   >> measured rms. The noise can be measured in several ways   
   >> each giving a different figure.   
   >   
   > While people could be stupid and compare a noise level measured in   
   > peak-to-peak volts to a signal measured in average volts, I know of no   
   > actual cases where this happens, except perhaps in your mind, Ian.   
      
   No, but as I keep saying, manufacturers will show their product in the   
   best light and judicious use of weighting and bandwidths will alter the   
   'measured' value considerably. 'A' weighting, which is pretty commonly   
   used by manufacturers will often improve and amplifiers noise spec. by 10dB.   
      
   There   
   > could be equipment noise that has a high crest factor, but nature does not   
   > usually go down that path.   What is then left is a few dB of ambiguity, and   
   > most good modern equipment (which is common) is not so noisy that a few dB   
   > is a deal breaker in actual use.   
   >   
      
   It is more than 'a few dB'   
      
      
   >>  >> Variations in the way the above are specified can make a   
   >>>> large difference to the 'measured' value   
   >   
   >>> Some yes, some no.   
   >   
   >>>>  and I bet you   
   >>>> can guess which set the marketing department will want   
   >>>> to choose.   
   >>> One other benefit of the solid state/digital revolution   
   >>> is that SNR and DR performance is often so good (> 100   
   >>> dB) that most variations in how they are mentioned don't   
   >>> matter that much.   
   >   
   >> Rubbish.   
   >   
   > Dismissive, unsupported claim, again itself worthy only of dismissal.   
   >   
      
   As I have said before DR and SNR are not the same and they have little   
   relation to the actual level of noise heard in the speaker. The OP's amp   
   for instance has an output power of 800W. Suppose this is into a load of   
   4 ohms then this requires over 56V rms output signal. The DR is 100dB so   
   the noise at the output is 100dB below 56V rms which which works out at   
   a mere -65dBu.   
      
   The original all tube Leak Point One in 1949 had a measured output noise   
   and hum of -80dB below 10W into 15 ohms which works out at just over 12V   
   rms. So its output noise is 80dB below 12V rms which works out to be a   
   mere -58dBu.   
      
   So in 60 years of development there's been only 7dB (oh sorry I should   
   have said 'just a few dB') of improvement due to the digital revolution.   
      
   >>  Once again, SNR and DR are different animals.   
   >   
   > A truism - therefore something that actually sheds no significant light. Why   
   > do you obsess over these things, Ian?   
   >   
   >> Achieving a 100dB DR is not hard but unless you run your   
   >> amp close to clipping all the time you will not achieve   
   >> that as a SNR.   
   >   
   > Another truism.   
   >   
      
   Excellent, so then you must agree that SNR is always less than DR.   
      
      
   So to get back to the OP's original problem, having a DR of 100dB is   
   irrelevant. What matters is what his SNR is. The fact he can hear hiss   
   from an 800W amp with his ear right next to it further demonstrates this   
   because that is not where listeners will be when the spekaer pumps outn   
   800W.   
      
   >> The marketing guys are as active as ever in trying to   
   >> make their product appear superior to its competitors and   
   >> will choose the measurement methods that best do that.   
   >   
   > The myth is that some equipment is head-and-shoulders better than its   
   > competition. Everybody pretty much feeds from the same trough.   
   >   
   >   
      
   In reality, marketing departments will aim to make crappy equipment   
   appear much better than it is using specmanship which is what they have   
   always done.   
      
   Cheers   
      
   ian   
      
   --- SoupGate-Win32 v1.05   
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