From: ruffrecords@yahoo.com   
      
   Audio Empire wrote:   
   > On Wed, 24 Mar 2010 13:24:45 -0700, Ian Bell wrote   
   > (in article ):   
   >   
   >> Audio Empire wrote:   
   >>> On Wed, 24 Mar 2010 03:54:59 -0700, Arny Krueger wrote   
   >>> (in article ):   
   >>>   
   >>>> "David Nebenzahl"  wrote in message   
   >>>> news:4ba97b4f$0$2381$822641b3@news.adtechcomputers.com   
   >>>>   
   >>>>> I'd say that since a differential amp practically   
   >>>>> eliminates common-mode noise, there's no reason *not* to   
   >>>>> use one (which, after all, was the question the OP asked).   
   >>>> Transformers are an effective means of elimination common-mode noise. They   
   >>>> are as a rule more effective than differential amplifiers. If you have an   
   >>>> input transformer, you don't need a differential input and you pick up   
   >>>> some   
   >>>> inherent protection against EMI and casualty losses.   
   >>> Now that's interesting. My sources all say that transformers have a lower   
   >>> common-mode rejection ratio than can differential amplifiers.   
   >>>   
   >> 'Have' and 'can (have)' are not identical conditions.   
   >>   
   >> Cheers   
   >>   
   >> Ian   
   >>   
   >>    
   >   
   > Well, of course, we're comparing best-case scenarios here. A transformer is a   
   > transformer, and while there are a number of makers of microphone   
   > transformers, they are all very similar in design and all have transformer   
   > characteristics in the same proportion, both good and bad. Differential   
   > amplifiers, OTOH, come in a myriad of configurations from cheap 709 style   
   > op-amps to very good ones like the latest National Semiconductor LM 497XX   
   > series to differential amps made with selected discrete components to potted   
   > gain modules made specifically for the pro recording industry. A transformer   
   > can be better than a old-school 709 or 741 era op-amp, but newer op-amps are   
   > much better than that wrt common mode rejection ratios.   
   >   
      
   Actually, I just noticed you said 'differential amplifiers' rather than   
   operational amplifiers and to that extent I agree with you when you use   
   selected descrete components. However, when it comes to op amps, they   
   are not the prime determinant of achieved CMRR. It's all the other   
   components around it, and by the time you have included the dc blocking   
   caps and zener diodes, series inductors etc to prevent RFI and damage   
   from phantom power and so on necessary for a mic preamp, on a PCB then   
   actual measured CMMR across the audio band will be little better than a   
   good transformer with phantom power fed through a primary centre tap.   
      
   Cheers   
      
   Ian   
      
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