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.   
   >   
   > OK, it's just that you didn't say they were equally   
   > sensitive, or efficient.   
   >   
   > In which case, OPTs with the correct ratios and optimised   
   > for the task should make both phones look equal to the amp,   
   > and the amp should look appropriately equal to both phones,   
   > no?   
   >   
   > Damping factor, power, voltage, current, etc. all come out   
   > in the wash don't they?   
   >   
   > Aren't Sowter's trannies the same price with windings of   
   > your own spec? They were with mine.   
   >   
   >>> 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.   
   >   
   > Tell him he can't have everything, and your compromise is   
   > better than the alternatives.   
   >   
   >>> 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?   
   >   
   > Possibly, but there are EC guidelines on headphone outputs.   
   > Even if they are not statutory requirements, flouting them   
   > could count against you because it can be construed as   
   > reckless.   
   >   
   > I considered selling a head amp but meeting EC and CE   
   > requirements seemed daunting. I thought of limiting, simply   
   > by using zeners on the output or by using an expander in the   
   > gfb, but that can't be done practically without SS   
   > tarnishing the allure of classic design.   
   >   
      
   (fortunately) this client is in the US so CE does not apply but I think you   
   are right - |I'll ook   
   into it.   
      
   Cheers   
      
   ian   
   > Ian   
   >   
   >   
      
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