From: ruffrecords@yahoo.com   
      
   Ian Iveson wrote:   
   > Ian Bell wrote:   
   >   
   >> Indeed, one of the really big questions is just how load   
   >> does the client want the phones to be - that and the   
   >> sensitivity of the phones makes a big difference to the   
   >> output requirements. As a rule, 100mW into almost any   
   >> headphone will blow your ears off and at least 10mW is   
   >> needed to give a decent level even in the most sensitive   
   >> phones. That's a 10dB difference in power that translates   
   >> in to a voltage range of about 3 to 1.   
   >>   
   >> For this client the worst case is probably his 100R phones   
   >> that will take 3V rms. That means with the 12:1 tap the   
   >> primary must supply 102V pp into 14K or so. Alternatively   
   >> with the 6:1 tap we need 51V pp into 3600R for about 90mW   
   >> in the phones. The 541V pp is less of a problem than the   
   >> 3600R load.   
   >>   
   >> His other phones are 34R types.  For 90mW these need 1.75V   
   >> on the secondary which at 12:1 is about 60V pp- again not   
   >> too bad, this time into 4896R.   
   >   
   > Doesn't this argument omit the issue of efficiency, part of   
   > the matter of sensitivity that you originally acknowledged?   
   > If the higher impedance 'phones are proportionately more   
   > efficient, then you might be home and dry. Or not.   
   >   
      
   Depends. I define sensitivity as dB(SPL) per mW of power to the headphone - in   
   other words, how much   
   noise power you get for unit electrical power. In that case, efficiency is   
   included.   
      
      
   It just so happens that both types of headphones produce pretty much the same   
   sound pressure per mW   
   electrical input and if anything the 34 ohm ones are slightly more sensitive.   
      
   > Alternatively, it's reasonable to consider the old-school   
   > 100R to be for gentle music, and the 34R for rock. Tell him   
   > this is why god gave him two 'phones. My bet is he'll be   
   > more than satisfied with what you've already done.   
   >   
      
   Not an option at present - he wants to listen to music of any kind on either   
   phones.   
      
   > What, incidentally, if he blows his ears out? Aren't there   
   > some active safety guidelines for manufacturers? Could he   
   > sue?   
   >   
      
   That had occurred to me but I suspect it is the old health and safety thing   
   where it is sufficient   
   to say don't turn the wick up too high - and surely a similar onus must be on   
   the phones manufacturer?   
      
   Cheers   
      
   ian   
   > Ian   
   >   
   >   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)