[Boatanchors] Speech Amp

[Bob Macklin]

Just hook the 70V line to the 600 ohm input and try it with a scope. Just be careful!

Bob Macklin
K5MYJ
Kent (Seattle), Wa,
"Real Radios Glow in the Dark"

  ----- Original Message ----- 
  From: Barrie Smith 
  To: J Forster 
  Cc: boatanchors at mailman.qth.net 
  Sent: Monday, November 17, 2008 2:26 PM
  Subject: Re: [Boatanchors] Speech Amp


  Thanks for the info regarding the speech amp.  However, I'm still confussed.

  Would I be better off using a regular 8 ohm output amp into a matching transformer with a 600 ohm output?

  73, Barrie, W7ALW
    ----- Original Message ----- 
    From: J Forster 
    To: Barrie Smith 
    Cc: boatanchors at mailman.qth.net 
    Sent: Monday, November 17, 2008 2:49 PM
    Subject: Re: [Boatanchors] Speech Amp


    Take a look here: 
    http://www.dvq.com/hifi/images/260a.pdf 

    AFAIK, 600 ohms is used for inputs, not outputs, and is typically balanced. 

    A 70 V line means that the amp output is 70 V RMS at the amp's rated output. The 70 V is usually stepped down at each speaker to voice coil impedance. Typical transformers have taps for the desired wattage to the speaker, so you can hook up a number of transformer/speaker combos. So on a 10 W amp, you can hook up four 1 W and three 2 W speakers. The amp looks a lot like a voltage source. 

    Since Power = V * V /R, if R=600 ohms and V=70 volts, there is only one possible power output from the amp. (4900/600 = 8.1 W) 

    Nope, 600 ohm and 70 V are NOT the same. 

    -John 

    ============== 

    Barrie Smith wrote: 

        It's my impression that the 70 volt output on a PA amp is 600 ohms. 


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