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Trask Amplifier for Wideband Active Loop Amp - mod suggestions?

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  • Andy Gardner
    Hi Team, I d like to try out one of these amplifiers on a 1.8m aluminium loop I have built. Not sure if the designer Chris Trask himself has time to answer
    Message 1 of 9 , Jan 29, 2014
      Hi Team,

      I'd like to try out one of these amplifiers on a 1.8m aluminium loop I have built. Not sure if the designer Chris Trask himself has time to answer questions right now, but I thought I'd put them out on the mailing list in case anyone else could answer as well, and others might be interested in the answers anyway.

      The loop I have built has the feed point at the top, and the balance point at the bottom is grounded to earth. It's up that way to get the business end as far up as possible. The bottom part is actually strapped to an alloy pole that is clamped to the centre pole of the old rotary clothesline out the back of the house. Hopefully that's fine, although she who wears the pants isn't totally impressed because it removes the "rotary" function out of the rotary clothesline.

      I'm getting fairly good results with the lz1aq amp I have installed at present, but have IMD problems with BCB stations. I'm interested in 0 to 30MHz, so would rather avoid a BCB filter. The plan is to orientate the loop so the 2 local AM BCB transmitter sites are sitting in the null. (~90 deg at 2km and ~60 deg at 5km)

      But my feedline I use is a run of Cat 5 ethernet cable, with one pair carrying the signal and the other 3 pairs sharing the +V and 0V load. So I need to reconfigure the amplifier's T6 output transformer for separate power feed, and balanced 100 ohm coming out.

      Suggestions gratefully received.

      Andy
      ZL3AG
    • Chris Trask
      ... It s actually fairly simple, and can be accomplished with a few modifications. First, break the connection between T6 and C20, then connect your power
      Message 2 of 9 , Jan 29, 2014
        >
        >But my feedline I use is a run of Cat 5 ethernet cable, with one pair carrying the signal and the
        >other 3 pairs sharing the +V and 0V load. So I need to reconfigure the amplifier's T6 output
        >transformer for separate power feed, and balanced 100 ohm coming out.
        >

        It's actually fairly simple, and can be accomplished with a few modifications. First, break the connection between T6 and C20, then connect your power line to C6, Q6, etc. to complete the supply connection. Done.

        Looking at the output side of T6, break all of the shown connections. The upper terminal of the upper winding is your + output, and the lower end of the lower winding is your - output. Connect the lower end of the upper winging and the upper end of the lower winding to ground. Done.

        The collectors of Q3 and Q4 will now see load impedances of 200 ohms instead of the original 400 ohms, so the power gain will be reduced by 6dB.


        Chris Trask
        N7ZWY / WDX3HLB
        Senior Member IEEE
        http://www.home.earthlink.net/~christrask/
      • Andy Gardner
        ... Thanks Chris! Hopefully the 6dB reduction won t be too much of a penalty. I would have to go to a different design for T6 to get the gain back? Cheers,
        Message 3 of 9 , Jan 29, 2014
          On 30/01/2014, at 2:18 AM, Chris Trask wrote:

          > >
          > >But my feedline I use is a run of Cat 5 ethernet cable, with one pair carrying the signal and the
          > >other 3 pairs sharing the +V and 0V load. So I need to reconfigure the amplifier's T6 output
          > >transformer for separate power feed, and balanced 100 ohm coming out.
          > >
          >
          > It's actually fairly simple, and can be accomplished with a few modifications. First, break the connection between T6 and C20, then connect your power line to C6, Q6, etc. to complete the supply connection. Done.
          >
          > Looking at the output side of T6, break all of the shown connections. The upper terminal of the upper winding is your + output, and the lower end of the lower winding is your - output. Connect the lower end of the upper winging and the upper end of the lower winding to ground. Done.
          >
          > The collectors of Q3 and Q4 will now see load impedances of 200 ohms instead of the original 400 ohms, so the power gain will be reduced by 6dB.
          >

          Thanks Chris!

          Hopefully the 6dB reduction won't be too much of a penalty. I would have to go to a different design for T6 to get the gain back?

          Cheers,

          Andy
          ZL3AG
        • Chris Trask
          ... Not easily and it wouldn t be worth the effort, but you could add an additional 3:2 autotransformer after it that would raise the collector load impedances
          Message 4 of 9 , Jan 29, 2014
            >
            >> >
            >> >But my feedline I use is a run of Cat 5 ethernet cable, with one pair carrying the signal and the
            >> >other 3 pairs sharing the +V and 0V load. So I need to reconfigure the amplifier's T6 output
            >> >transformer for separate power feed, and balanced 100 ohm coming out.
            >> >
            >>
            >> It's actually fairly simple, and can be accomplished with a few modifications. First, break the connection
            >>between T6 and C20, then connect your power line to C6, Q6, etc. to complete the supply connection. Done.
            >>
            >> Looking at the output side of T6, break all of the shown connections. The upper terminal of the upper
            >>winding is your + output, and the lower end of the lower winding is your - output. Connect the lower end of
            >>the upper winging and the upper end of the lower winding to ground. Done.
            >>
            >> The collectors of Q3 and Q4 will now see load impedances of 200 ohms instead of the original 400 ohms, so
            >>the power gain will be reduced by 6dB.
            >>
            >
            >Thanks Chris!
            >
            >Hopefully the 6dB reduction won't be too much of a penalty. I would have to go to a different design for T6 to get the gain back?
            >

            Not easily and it wouldn't be worth the effort, but you could add an additional 3:2 autotransformer after it that would raise the collector load impedances to 450 ohms, which is reasonably close and would actually raise the gain by about 1.0dB. I don't have a drawing set for that readily available as I've never had to describe one, but it's fairly simple.



            Chris Trask
            N7ZWY / WDX3HLB
            Senior Member IEEE
            http://www.home.earthlink.net/~christrask/
          • Andy Gardner
            Well, my bulk buy of BC549C s has arrived, so one more question before I get started on the amplifier... How closely do I need to match the transistor pairs?
            Message 5 of 9 , Feb 21, 2014
              Well, my bulk buy of BC549C's has arrived, so one more question before I get started on the amplifier...

              How closely do I need to match the transistor pairs? Can I just go by HFE reading on my cheap digital multimeter, or for best results do I need to break out something a bit more serious? If so, what exactly? :)

              Can I just use my 8405A and check that there's exactly 180 deg phase and equal amplitude on each side?

              Thanks,
            • Chris Trask
              ... I never did go to the trouble of determining if hfe imbslance affected the performance, and it s less important for common base amplifiers than it is for
              Message 6 of 9 , Feb 21, 2014
                >
                >Well, my bulk buy of BC549C's has arrived, so one more question before I get started on the amplifier...
                >
                >How closely do I need to match the transistor pairs? Can I just go by HFE reading on my cheap digital multimeter, or for best results do I need to break out something a bit more serious? If so, what exactly? :)
                >
                >Can I just use my 8405A and check that there's exactly 180 deg phase and equal amplitude on each side?
                >

                I never did go to the trouble of determining if hfe imbslance affected the performance, and it's less important for common base amplifiers than it is for common emitter. But, my feeling is that a match of 5% or less would be prudent.


                Chris Trask
                N7ZWY / WDX3HLB
                Senior Member IEEE
                http://www.home.earthlink.net/~christrask/
              • Andy Gardner
                ... Thanks Chris. I ve just started winding the transformers. I ve only got AWG30 wire rather than AWG32 so I ve had to use 2873000202 instead of 2873002402 as
                Message 7 of 9 , Feb 24, 2014
                  On 22/02/2014, at 1:57 AM, Chris Trask wrote:

                  > >
                  > >Well, my bulk buy of BC549C's has arrived, so one more question before I get started on the amplifier...
                  > >
                  > >How closely do I need to match the transistor pairs? Can I just go by HFE reading on my cheap digital multimeter, or for best results do I need to break out something a bit more serious? If so, what exactly? :)
                  > >
                  > >Can I just use my 8405A and check that there's exactly 180 deg phase and equal amplitude on each side?
                  > >
                  >
                  > I never did go to the trouble of determining if hfe imbslance affected the performance, and it's less important for common base amplifiers than it is for common emitter. But, my feeling is that a match of 5% or less would be prudent.
                  >

                  Thanks Chris.

                  I've just started winding the transformers. I've only got AWG30 wire rather than AWG32 so I've had to use 2873000202 instead of 2873002402 as I an't fit 4 turns of it on the smaller core. Hopefully I can cram it all on the board I'm using.

                  I still have to sort out some low ESR caps, but I've pretty much got everything I need to build one, finally.

                  Cheers,

                  Andy
                  ZL3AG
                • Chris Trask
                  ... Go for it! In case you re interested, there s a seller on eBay who is selling the MPS6521 at pretty reasonable prices. I bought two lots of 25 for $10
                  Message 8 of 9 , Feb 24, 2014
                    >
                    >>
                    >> >
                    >> >Well, my bulk buy of BC549C's has arrived, so one more question before I get started on the amplifier...
                    >> >
                    >> >How closely do I need to match the transistor pairs? Can I just go by HFE reading on my cheap digital
                    >> >multimeter, or for best results do I need to break out something a bit more serious? If so, what exactly? :)
                    >> >
                    >> >Can I just use my 8405A and check that there's exactly 180 deg phase and equal amplitude on each side?
                    >> >
                    >>
                    >> I never did go to the trouble of determining if hfe imbslance affected the performance, and it's less
                    >>important for common base amplifiers than it is for common emitter. But, my feeling is that a match of 5%
                    >>or less would be prudent.
                    >>
                    >
                    >Thanks Chris.
                    >
                    >I've just started winding the transformers. I've only got AWG30 wire rather than AWG32 so I've had to use 2873000202 instead of 2873002402 as I an't fit 4 turns of it on the smaller core. Hopefully I can cram it all on the board I'm using.
                    >
                    >I still have to sort out some low ESR caps, but I've pretty much got everything I need to build one, finally.
                    >

                    Go for it! In case you're interested, there's a seller on eBay who is selling the MPS6521 at pretty reasonable prices. I bought two lots of 25 for $10 each from him, which will satisfy my needs. They're basically a 2N2222 with twice the current gain. It's a shame that they were never made in SMT so that they might still be available.


                    Chris Trask
                    N7ZWY / WDX3HLB
                    Senior Member IEEE
                    http://www.home.earthlink.net/~christrask/
                  • tmg2304
                    I just checked Mouser and they have them in stock.
                    Message 9 of 9 , Feb 25, 2014
                      I just checked Mouser and they have them in stock.
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