• Re: ?? Can I drive the S.E.X. amp from a solid state crossover? - Paul Joppa 15:11:26 01/09/05 (3)
  In Reply to: ?? Can I drive the S.E.X. amp from a solid state crossover? posted by mistergroovy on January 9, 2005 at 14:48:35:

  Since the SEX input impedance of 100k is larger than the minimum load impedance of 2k (!) there should be no problem.
  
  

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  • 2k or 50 Ohms? - Jim Carlon 16:16:25 01/09/05 (2)
    In Reply to: Re: ?? Can I drive the S.E.X. amp from a solid state crossover? posted by Paul Joppa on January 9, 2005 at 15:11:26:

    I'm not sure what 'output load capability' means exactly.
    
    

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    • Re: 2k or 50 Ohms? - Paul Joppa 10:30:33 01/10/05 (1)
      In Reply to: 2k or 50 Ohms? posted by Jim Carlon on January 9, 2005 at 16:16:25:

      A great source of confusion - good question!

      Any signal source functions as a voltage source in series with a resistance, called the output resistance. The load has a resistance also. In modern designs, the source resistance is made small relative to the load resistance. This preserves most of the voltage, independent of the actual load resistance.

      The source can put out a certain maximum voltage, and it also has a certain maximum current capability. To stay within the current capability, the load resistance must be larger than some value. For example, the Foreplay can produce a maximum of 20v, and its cathode follower cannot produce more than 2mA, so its specified minimum load impedance is (Ohm's Law) R = V/I = 10k ohms. This minimum load resistance is still much larger than the equivalent internal source resistance, 800 ohms for the Foreplay. The Marchand is specified to have 50 ohms internal resistance, and be able to drive a load of 2000 ohms or greater.

      Note that all this has nothing to do with "matching" resistances! That idea comes from the early days of the telephone business. If the source has no current limit, then the most power that can be extracted from it occurs when the load resistance equals the source resistance. This was important when telephones did not have amplification - the power generated by the microphone was the only power available for the earpiece, so you had to couple as much as possible to the earpiece in order to hear anything. Once the telephone system acquired tubes, that was no longer necessary, but at the same time lines became longer. Long lines (over a mile or so) have reflections unless the source resistance, line characteristic resistance, and load resistance are all identical. So matched resistances were still needed by the phone system.

      Hope that helps!
      
      

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      • Whew! Thanks! - Jim Carlon 11:30:44 01/10/05 (0)
        In Reply to: Re: 2k or 50 Ohms? posted by Paul Joppa on January 1, 2005 at 10:30:33:

        OK, that spec makes sense now! Basically, they are using different phrases to mean 'output impedance' and 'minimum load impedance', if I follow you correctly.

        I have read a bit of history on the early telephone systems, and always marvel how those engineers figured out the balanced line, transmission line principles. impedance matching, etc., way back then. I guess necessity is a mother!
        
        

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