From: jl@glen--canyon.com   
      
   On Fri, 14 Nov 2025 16:20:54 +1100, Bill Sloman    
   wrote:   
      
   >On 14/11/2025 2:25 am, john larkin wrote:   
   >> On Thu, 13 Nov 2025 15:44:26 +1100, Bill Sloman    
   >> wrote:   
   >>   
   >>> On 13/11/2025 5:02 am, john larkin wrote:   
   >>>> On Wed, 12 Nov 2025 17:28:04 GMT, Jan Panteltje    
   >>>> wrote:   
   >>>>   
   >>>>>> john larkin wrote:   
   >>>>>>> On Wed, 12 Nov 2025 16:11:03 +1100, Bill Sloman    
   >>>>>> wrote:   
   >>>   
   >>>    
   >>>   
   >>>> The problem is to find an audio spectrum analyzer with -150 dB   
   >>>> distortion.   
   >>>   
   >>> Not if you filter out almost all of the fundamental before you feed the   
   >>> residual signal into the spectrum analyser.   
   >>   
   >> You just shifted the problem to finding a filter with -150 dB   
   >> distortion.   
   >   
   >So you wouldn't use ferrite-core inductors to make it, and you might   
   >need two made with different components to check if they produced the   
   >same result. Twin-T narrow-band filters can be pretty sharp, but you   
   >would probably need to twiddle a pot to keep them at the right frequency.   
      
      
   Google   
      
   resistor voltage coefficient   
      
   Some common resistors are -1000 PPM/volt.   
      
   The best you can buy are around 1 PPM/v. Pots are usually cermet,   
   really bad.   
      
   Spice has excellent resistors.   
      
   Caps are worse.   
      
      
   John Larkin   
   Highland Tech Glen Canyon Design Center   
   Lunatic Fringe Electronics   
      
   --- SoupGate-Win32 v1.05   
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