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I want to build an amp selector (two amps a/b, one cab)

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  • I want to build an amp selector (two amps a/b, one cab)

    Hi Folks.

    Several times I have built the same circuit amplifier with different components... and often I have found wanting to have a box that will let me switch between the two amps while sharing the same cabinet. Now I'd like to take this on.

    In my mind I only need to create a switch that will change one amp's input (from it's speaker jack) to the output while switching the other to a... what? A 200 watt 8 ohm resistor?

    I know there are lots more to consider, this is just my first thoughts on the matter... I assume we are talking moderately lower voltage but high current here. The amplifiers in question are 100 Watt amps that will go out to an 8ohm cab/load.

    Until now I have recorded one and, powered off, switched heads, and recorded the other amp. A lot of impression is lost in that one minute and I often catch differences in the room that do not seem to come out on the recording with the same impression... for whatever reason (mic freq limitations etc).

    If you have any experience in the area, please share! And thanks!
    "'He who first proclaims to have golden ears is the only one in the argument who can truly have golden ears.' The opponent, therefore, must, by the rules, have tin ears, since there can only be one golden-eared person per argument." - Randall Aiken

  • #2
    Perhaps one of these??
    https://www.banzaimusic.com/dummy-lo...-ohm-200w.html
    "'He who first proclaims to have golden ears is the only one in the argument who can truly have golden ears.' The opponent, therefore, must, by the rules, have tin ears, since there can only be one golden-eared person per argument." - Randall Aiken

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    • #3
      Originally posted by Gtr0 View Post
      Hi Folks.

      Several times I have built the same circuit amplifier with different components... and often I have found wanting to have a box that will let me switch between the two amps while sharing the same cabinet.

      I am lazy. I would use a stereo 212 or 412 cabinet with one amp hooked to each side of the cab, then just A/B switch to the amps inputs. Use two mics (one on each side) and either record both or mix them into one channel.

      Then again if you have several times built the same amp with different components then you probably aren't lazy.

      Comment


      • #4
        Originally posted by glebert View Post
        I am lazy. I would use a stereo 212 or 412 cabinet with one amp hooked to each side of the cab, then just A/B switch to the amps inputs. Use two mics (one on each side) and either record both or mix them into one channel.

        Then again if you have several times built the same amp with different components then you probably aren't lazy.
        haha. true... however, very cool idea. I would have never thought of it. But as silly as it may sound to you, I want all variables taken out of the equasion, that includes different speakers (even same brand/model), different positions of the cab, different (again though same model) mic etc etc.

        AND at home I have a single 1x12" 8 ohm cab ;-)
        "'He who first proclaims to have golden ears is the only one in the argument who can truly have golden ears.' The opponent, therefore, must, by the rules, have tin ears, since there can only be one golden-eared person per argument." - Randall Aiken

        Comment


        • #5
          ...
          Attached Files
          "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

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          • #6
            Originally posted by Chuck H View Post
            ...
            Asking because I don't know, but do you need to be concerned for a situation where you get a speaker cable wired backwards and would then be sending one amps hot output to ground through the other amps chassis? Last time I wired a stereo/mono cabinet I used TRS switchjacks to switch both the tip and the sleeve in and out of circuit, but don't know if that was really a concern or not.

            Comment


            • #7
              Asking because I don't know, but do you need to be concerned for a situation where you get a speaker cable wired backwards and would then be sending one amps hot output to ground through the other amps chassis?
              You mean hot wire and ground wire interchanged at one end of the speaker cable? That would be a problem as it would short the output of the amp connected (via the switchbox and/or mains safety ground). So speaker cables should be tested for correct wiring.
              - Own Opinions Only -

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              • #8
                Originally posted by Helmholtz View Post
                You mean hot wire and ground wire interchanged at one end of the speaker cable? That would be a problem as it would short the output of the amp connected (via the switchbox and/or mains safety ground). So speaker cables should be tested for correct wiring.
                Yes, that is what I was thinking. You could also use a 4PDT switch so that the "grounds" are also switched between the speaker and the dummy load.

                Comment


                • #9
                  I would do it that way: a standard signal relay for assignment to each amplifier and a powerful relay for switching amps to the speaker. Both synchronized.
                  Muting the signal input in the inactive amplifier, absorption resistor is not necessary.
                  I imagine that this circuit would need the interruption of the grounding in one of the amplifiers to avoid ground loops. Using a small adapter in the plug.

                  Click image for larger version

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                  • #10
                    Muting the signal input in the inactive amplifier, absorption resistor is not necessary.
                    Are you sure that some amp might not start to self-oscillate without a load? Without the load loop gain strongly increases and tube load gets inductive and resonant at some frequency.
                    - Own Opinions Only -

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                    • #11
                      But the output of the inactive amplifier is not open. Itīs also bridged through the relay. Nothing bad should happen.
                      At least I have never seen a problem with that kind of protection.

                      Comment


                      • #12
                        But the output of the inactive amplifier is not open. Itīs also bridged through the relay.
                        You're right, if input and output of the amp are shorted/grounded, there shouldn't be risk.
                        The relays' wiring/action wasn't quite clear to me from your drawing without knowing the actual pinout, but now I understand and support the principle.
                        Last edited by Helmholtz; 10-05-2019, 10:27 PM.
                        - Own Opinions Only -

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                        • #13
                          I was referring to the common type of signal relay switching used in amps (Finder, Omron, etc.).
                          This is the scheme:

                          Click image for larger version

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                          • #14
                            Originally posted by Pedro Vecino View Post
                            I was referring to the common type of signal relay switching used in amps (Finder, Omron, etc.).
                            This is the scheme:

                            [ATTACH=CONFIG]55434[/ATTACH]
                            Clear now. Thanks.

                            Good solution!
                            Last edited by Helmholtz; 10-05-2019, 10:45 PM.
                            - Own Opinions Only -

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                            • #15
                              Originally posted by Pedro Vecino View Post
                              I would do it that way: a standard signal relay for assignment to each amplifier and a powerful relay for switching amps to the speaker. Both synchronized.
                              Muting the signal input in the inactive amplifier, absorption resistor is not necessary.
                              I imagine that this circuit would need the interruption of the grounding in one of the amplifiers to avoid ground loops. Using a small adapter in the plug.

                              [ATTACH=CONFIG]55433[/ATTACH]
                              wow! so as long as the output of the inactive amplifier goes to ground, then it is okay? and a stompbox type box would be able to support the proper grounding for this (at the chassis)?
                              "'He who first proclaims to have golden ears is the only one in the argument who can truly have golden ears.' The opponent, therefore, must, by the rules, have tin ears, since there can only be one golden-eared person per argument." - Randall Aiken

                              Comment

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