From: zekfrivo@zekfrivolous.com   
      
   In article , Randy Yates  wrote:   
   >Hi Arny et alius,   
   >   
   >Thank you all for your comments. They have been very helpful.   
   >   
   >FYI, the reason for my question is that I am attempting to review the   
   >design of the front-end of a heart rate monitoring device. This design   
   >is based on the SSM2167 (e.g., from National).  My thinking is that this   
   >is the wrong part for the job since the sensor is fed to the preamp via   
   >a run of cable perhaps 4 feet in length, and there could therefore be   
   >lots of common-mode noise.  Also, I believe the AGC action is, in this   
   >stage, not appropriate.   
   >   
   >This appears to be an electret type of microphone in that DC power is   
   >fed to the device, so the poster who discussed how to feed phantom power   
   >and simultaneously receive the input differentially was right on the   
   >money.   
   >   
   >Thanks again for your input and suggestions.   
   >   
   >--Randy   
   >   
      
   Hope your using battery power.   
   Isolated preamps must be used when attached to the line.   
   They usually use switching and transformers.   
      
   greg   
      
      
      
   >"Arny Krueger"  writes:   
   >   
   >> "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.   
   >>   
   >> If you've ever had a grounding problem that a standard active differential   
   >> input can't handle, you know what I mean.   
   >>   
   >> Trnasformers  also provide galvanic isolation, or isolation of signal   
   >> ground.  We would probably be a little less critical of the configuration of   
   >> safety grounds in audio if we were still in the days of transformers.   
   >>   
   >> Transformers would probably rule the inputs of professional grade gear were   
   >> it not for costs. While it is hard to make transformers have as flat of   
   >> frequency response and as have ultimately low distortion as differential   
   >> inputs, it is possible to make them perform well enough to be sonically   
   >> innocious.   
   >>   
   >> Transformers are still the preferred means for obtaining the best possible   
   >> freedom from grounding problems in areas where the cost issues are not so   
   >> severe, such as analog and digital communications.   
   >>   
   >>   
   >   
      
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