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A switchable preamp and attenuator for the RXII HF

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  • warrenallgyer
    After testing and posting the results for Tony s new prototype receiver, I was so impressed by the performance that I wanted to see what could be done to
    Message 1 of 7 , Jun 8 4:02 AM
      After testing and posting the results for Tony's new prototype receiver, I was so impressed by the performance that I wanted to see what could be done to enhance the existing RXII HF. I have suggested to Tony that he make the preamp and attenuators selectable independent of band selection and he seems to think he can accommodate this. I wanted to see if it could be done with the current version.

      Answer: a resounding YES!

      I built a standalone version of Tony's preamp, which is a Norton amplifier (Google it) using pseudo-SMT/dead bug fusion construction. The preamp is encased in clear shrink wrap and installed along with its' relay and a companion DPDT relay with a 15 dB attenuator. Two SPST switches are added to the KM5H case. Apologies to both Tom and Tony for butchering their beautiful products. But the results are not too bad. Pix are posted in the Photos section in an album entitled "RXII Preamp and Attenuator"

      Now for the good part: 10 meter MDS was improved from -128 dBm to -137. 80 meter MDS measured a whopping -142 dBm. World class performance in both cases.

      That was the weekend: a switchable attenuator/preamp and a 25K bike ride with my wife. All in all very satisfying!

      Warren Allgyer
      9V1TD


    • Dave Sublette
      Good morning Warren, and all. Good work! Sounds like you had a good weekend all the way around. I sat for 11 hours in the shade on a 70 degree (F) day
      Message 2 of 7 , Jun 8 4:28 AM
        Good morning Warren, and all.

        Good work!  Sounds like you had a good weekend all the way around.  I sat for 11 hours in the shade on a 70 degree (F) day yesterday and listened to seven or eight of the best brass bands in the world at the Great American Brass Band Festival in Danville, KY. Friday evening, our band played for the Great American Balloon Race and it, too, was a perfect evening for music and balloons.

        But, I digress.  One question about your mod. Not to be contrary, but what good is it to have an MDS of -142 dBM on 80M when connecting just about any antenna will result in decades of noise over that level?  I understand how low noise benefits us at VHF and above.  But, I usually run my HF receivers with 6 or 12 dB of attenuation on 80 and 160, just to keep from pounding the AGC with noise.  I also usually turn the RF gain down and screw the filtering down to 250 hz on CW, all in the name of reducing noise. It works. So, even though I like the idea of having a sensitive receiver, I’m not sure what advantage that yields on 80/160.

        Dave

        On Jun 8, 2014, at 7:02 AM, allgyer@... [softrock40] <softrock40@yahoogroups.com> wrote:

        After testing and posting the results for Tony's new prototype receiver, I was so impressed by the performance that I wanted to see what could be done to enhance the existing RXII HF. I have suggested to Tony that he make the preamp and attenuators selectable independent of band selection and he seems to think he can accommodate this. I wanted to see if it could be done with the current version.

        Answer: a resounding YES!

        I built a standalone version of Tony's preamp, which is a Norton amplifier (Google it) using pseudo-SMT/dead bug fusion construction. The preamp is encased in clear shrink wrap and installed along with its' relay and a companion DPDT relay with a 15 dB attenuator. Two SPST switches are added to the KM5H case. Apologies to both Tom and Tony for butchering their beautiful products. But the results are not too bad. Pix are posted in the Photos section in an album entitled "RXII Preamp and Attenuator"

        Now for the good part: 10 meter MDS was improved from -128 dBm to -137. 80 meter MDS measured a whopping -142 dBm. World class performance in both cases.

        That was the weekend: a switchable attenuator/preamp and a 25K bike ride with my wife. All in all very satisfying!

        Warren Allgyer
        9V1TD




      • warrenallgyer
        Haha... Dave, you nailed it. There is absolutely no reason for a -142 dBm MDS on 80 meters other than to say you can do it. Or, if you happen to live in
        Message 3 of 7 , Jun 8 4:38 AM
          Haha... Dave, you nailed it. There is absolutely no reason for a -142 dBm MDS on 80 meters other than to say you can do it. Or, if you happen to live in Anarctica or an uninhabited South Seas island maybe.

          The real performance benefits of the preamp start to kick in on 17 meters and above. The other is just an intellectual exercise that comes for free.

          :-)

          Warren Allgyer
          9V1TD
        • warrenallgyer
          ..... BTW..... that is the reason for the switch! WA
          Message 4 of 7 , Jun 8 4:39 AM
            ..... BTW..... that is the reason for the switch!

            WA
          • Zack Widup
            The recent FT5ZM DXpedition reported horrible noise at times on 80 and 160. I believe this happened on Heard Island, too. :-( 73, Zack W9SZ
            Message 5 of 7 , Jun 8 8:13 AM
              The recent FT5ZM DXpedition reported horrible noise at times on 80 and
              160. I believe this happened on Heard Island, too. :-(

              73, Zack W9SZ

              On 6/8/14, allgyer@... [softrock40] <softrock40@yahoogroups.com> wrote:
              > Haha... Dave, you nailed it. There is absolutely no reason for a -142 dBm
              > MDS on 80 meters other than to say you can do it. Or, if you happen to live
              > in Anarctica or an uninhabited South Seas island maybe.
              >
              > The real performance benefits of the preamp start to kick in on 17 meters
              > and above. The other is just an intellectual exercise that comes for free.
              >
              > :-)
              >
              > Warren Allgyer
              > 9V1TD
            • warrenallgyer
              Some more performance data on the added preamp and attenuator. The Norton amplifier seems to be a marvel.... truly broadband and stable. Warren Allgyer 9V1TD
              Message 6 of 7 , Jun 8 9:00 PM
                Some more performance data on the added preamp and attenuator. The Norton amplifier seems to be a marvel.... truly broadband and stable.



                Warren Allgyer
                9V1TD
              • yl2qn_andy
                Hello, As I wrote early in this group, in my build of RXTX I m add additional front with two attenuation pads (6db and 12 db) and Norton preamp 12db. From my
                Message 7 of 7 , Jun 9 12:57 AM
                  Hello,

                  As I wrote early in this group, in my build of RXTX I'm add additional front with two attenuation pads (6db and 12 db) and Norton preamp 12db. From my experience with this mod the preamp is really need on 10m band on permanent basis. This improve sensitivity dramatically. On 15m band I'm use preamp time from time. On 20m and below - not big deal. The original sensitivity is enough.

                  Any way for good operations on 10m/15m band you need to have a good antenna. The simple Cobweb is not good enough and dramatically different for 3/4 element tribander.

                  The attenuator is good friend for low bands especially during contests. I have switched on 6db or 12db most of time. 

                  I think have this mod for RXII is really big step forward in Softrock products line.

                  Regards,
                  Andy - YL2QN
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