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How to determine What VA Line Isolation transformer to get

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  • How to determine What VA Line Isolation transformer to get

    I have a Silvertone 1448 customer wants to add a Line Isolation transformer to it. I would like to know how to determine what what VA rating to purchase.
    Rather than someone just say buy this I'd like to know how to know what to buy. It looks like the amp draws 40watts by the schematic

    Thanks,
    Jason

    silvertone_1448.pdf
    Binkie McFartnuggets‏:If we really wanted to know the meaning of life we would have fed Stephen Hawking shrooms a long time ago.

  • #2
    I see 40 watts also. SO find an 80-100 watt iso.

    It is a class A circuit, so even a 60 watt would do.
    Education is what you're left with after you have forgotten what you have learned.

    Comment


    • #3
      Originally posted by Enzo View Post
      I see 40 watts also. SO find an 80-100 watt iso.

      It is a class A circuit, so even a 60 watt would do.
      So really all you do is just double the 40watts or whatever the circuit wattage draw is?

      i think I know the answer on this, I do not think the customer will buy a $65 transformer, but we will see.

      Thanks,
      nosaj
      Binkie McFartnuggets‏:If we really wanted to know the meaning of life we would have fed Stephen Hawking shrooms a long time ago.

      Comment


      • #4
        Arbitrary on my part, I am not aware of any rules. I just see no reason to buy a part and then run it right on its edge. I don't want it to get hot.
        Education is what you're left with after you have forgotten what you have learned.

        Comment


        • #5
          Originally posted by Enzo View Post
          Arbitrary on my part, I am not aware of any rules. I just see no reason to buy a part and then run it right on its edge. I don't want it to get hot.
          So is Watts the same as VA?

          nosaj
          Binkie McFartnuggets‏:If we really wanted to know the meaning of life we would have fed Stephen Hawking shrooms a long time ago.

          Comment


          • #6
            For your purposes, yes. VA is volt-amps.
            Education is what you're left with after you have forgotten what you have learned.

            Comment


            • #7
              Originally posted by Enzo View Post
              For your purposes, yes. VA is volt-amps.
              Just for clarity... "For your purposes" was prefaced earlier when Enzo recognized that it was class A, single ended. Which means the determined current won't increase significantly with signal conduction as it would with an AB or B amp.
              "Take two placebos, works twice as well." Enzo

              "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

              "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

              Comment


              • #8
                Originally posted by nosaj View Post
                So is Watts the same as VA?
                VA is the “Apparent Power” if there is phase shift between the current and voltage the “Real Power” (Watts) will be less. Watts = VA x Power Factor. The Power factor is the cosine of the phase angle. If the phase angle is zero then its cosine is 1 and VA = Watts.

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                • #9
                  Originally posted by Dave H View Post
                  VA is the “Apparent Power” if there is phase shift between the current and voltage the “Real Power” (Watts) will be less. Watts = VA x Power Factor. The Power factor is the cosine of the phase angle. If the phase angle is zero then its cosine is 1 and VA = Watts.
                  Nerd







                  "Take two placebos, works twice as well." Enzo

                  "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

                  "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

                  Comment


                  • #10
                    Originally posted by Chuck H View Post
                    Just for clarity... "For your purposes" was prefaced earlier when Enzo recognized that it was class A, single ended. Which means the determined current won't increase significantly with signal conduction as it would with an AB or B amp.
                    Ok in laymans terms if it were class AB how would I determine what VA to buy or were there any transformerless AB amps?

                    Thanks,
                    nosaj
                    Binkie McFartnuggets‏:If we really wanted to know the meaning of life we would have fed Stephen Hawking shrooms a long time ago.

                    Comment


                    • #11
                      Originally posted by nosaj View Post
                      Ok in laymans terms if it were class AB how would I determine what VA to buy or were there any transformerless AB amps?

                      Thanks,
                      nosaj
                      That's an excellent question. And one I thought might follow my post. Though I didn't hope for it

                      If I didn't already have a VA indicated on the amp or an available schematic I would do it the same way I choose a PT. I would total the current required for the filament circuit and the power amp circuit. The filament circuit/s are a known quantity listed in the tube data. A very close approximation of the power amps requirements can usually be determined by the operating conditions. Plate voltage, bias current, etc. For a PT you would also have to allocate the sums to their individual windings of course. Then, lacking the necessary education to determine "the cosine of the phase angle" (thanks Dave) I would just add some safety margin.
                      "Take two placebos, works twice as well." Enzo

                      "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

                      "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

                      Comment


                      • #12
                        Good advice Chuck! That's the way to deal with an unknown amp that you can't just run and measure. Say, one that hasn't been built yet. And I especially like the advice to add safety margin. Watts/VA in an isolation transformer are cheap.

                        In this somewhat unusual case where the amp already exists and runs (I think, from the description) the straightforward thing to do might be to just hook it up and measure the AC line current. There are some caveats to this. To get good results, you have to run the amp at maximum power draw from the AC line (as noted in the Class A versus Class AB items) and you also need to run it at the maximum AC line voltage that will be applied to the amp.

                        This last is important for really old amps where the AC line voltage used to be 110 to 112 when they were designed. Today's AC line runs 120 to 130. My house is way out on the end of the distribution network, but we commonly get 122-125, and I've measured 128. This means that transformers from the 1960s designed for 110 could be right on the edge of saturation (and hence, taking more AC line current) at 120-125. That would be a really good thing to know in choosing an isolation transformer.
                        Amazing!! Who would ever have guessed that someone who villified the evil rich people would begin happily accepting their millions in speaking fees!

                        Oh, wait! That sounds familiar, somehow.

                        Comment

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