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M-100/Leslie 760 = deep hum

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  • M-100/Leslie 760 = deep hum

    Hello again
    I hooked my M-100 to a Leslie 760 with the PreAmp ll pedal. Organ has a line-out, tapped from the speaker terminals (both, main and reverb). Turning the volume knob on the preAmp to nine'o'clock position, there is a deep hum increasing more and more, although not turning the volume knob further on. The hum seems to built up, like a feedback loop. I removed all plugs in the room (lamps, frigerator etc.). The hum persisted. So I went for a passive DI-box, putting between organ and preamp. Now the hum starts at about one'o'clock position, which is an improvement but not yet solved the problem. So what can I do to get the whole power out of the 760 amp? Any ideas what is causing this hum?
    Thank you!
    Martin
    Hammond A-100
    Crumar Mojo XT
    Leslies 46 W, 51, 330, 2 x 760

  • #2
    Hello,

    May be you have a ground loop.
    It occurs when both the organ and the pedal preamp have 3 wires power plug.

    JP

    Comment


    • #3
      Question, as I'm curious why you've done things this way.

      You have speaker level output in the organ, which is what the 760 really wants. You then cut that down to a lower level at the line out and then use the CPA to boost it back up for the 760. You may have good reasons for doing this, but wouldn't it be simpler just to connect the 760 to the M100 with the appropriate 9501 leslie kit? (Although that wouldn't mix the reverb signal in for 'spun' reverb.)

      And ditto on the ground loop suggestion.
      It's not what you play. It's not how you play. It's the fact that you're playing that counts.

      New website now live - www.andrew-gilbert.com

      Current instruments: Roland Atelier AT900 Platinum Edition, Yamaha Genos, Yamaha PSR-S970, Kawai K1m
      Retired Organs: Lots! Kawai SR6 x 2, Hammond L122, T402, T500 x 2, X5. Conn Martinique and 652. Gulbransen 2102 Pacemaker. Kimball Temptation.
      Retired Leslies, 147, 145 x 2, 760 x 2, 710, 415 x 2.
      Retired synths: Korg 700, Roland SH1000, Jen Superstringer, Kawai S100F, Kawai S100P, Kawai K1

      Comment


      • #4
        You can bypass the preamp in the CPA2 so the organs preamp goes straight to the Leslie. The CPA then just serves as a way of getting power into the Leslie and a speed controller.
        Hammond C3, M102, H112, XB3, XB5, X5, TTR-100
        Lowrey Heritage DSO-1, Yamaha E70
        Farfisa Compact Duo Mk2, Vox Continental 300, Gibson G201, Korg BX3 Mk1
        Leslie 122 x2, 145 x2, 910
        www.drawbardave.co.uk

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        • #5
          The CPA was included to the 760. These also seemed to me the easiest to hook up the leslie to the organ, just plug it in. I have no 9501 kit.

          - - - Updated - - -

          Bypassing the CPA preamp seems to be a good solution. Can you please tell me, how to do this? I have no knowledge in electric. But I think I can make it if you could give me clear instructions.
          Hammond A-100
          Crumar Mojo XT
          Leslies 46 W, 51, 330, 2 x 760

          Comment


          • #6
            Thinking about it, the 7270 'universal' kit is even simpler and does the same job. http://www.fishorgans.com/leslie_kits/007270A.pdf

            If you could do the work to bypass the CPA, then you could probably make a 7270 kit. Leslie kits are not rocket science, the technology dates back to the 1940s! OR.... buy a 7270 and then sell the CPA (you would almost certainly be in profit after the deal!).
            It's not what you play. It's not how you play. It's the fact that you're playing that counts.

            New website now live - www.andrew-gilbert.com

            Current instruments: Roland Atelier AT900 Platinum Edition, Yamaha Genos, Yamaha PSR-S970, Kawai K1m
            Retired Organs: Lots! Kawai SR6 x 2, Hammond L122, T402, T500 x 2, X5. Conn Martinique and 652. Gulbransen 2102 Pacemaker. Kimball Temptation.
            Retired Leslies, 147, 145 x 2, 760 x 2, 710, 415 x 2.
            Retired synths: Korg 700, Roland SH1000, Jen Superstringer, Kawai S100F, Kawai S100P, Kawai K1

            Comment


            • #7
              All you have to do is route the signal positive wires from the jack to the red signal wire on the 9 pin socket thus eliminating the preamp. If you are going to use both input jacks you might need a couple of resistors to mix the signals. You could disconnect the power to the preamp which is fed from the Leslie via the 9 pin socket too.
              Hammond C3, M102, H112, XB3, XB5, X5, TTR-100
              Lowrey Heritage DSO-1, Yamaha E70
              Farfisa Compact Duo Mk2, Vox Continental 300, Gibson G201, Korg BX3 Mk1
              Leslie 122 x2, 145 x2, 910
              www.drawbardave.co.uk

              Comment


              • #8
                So if I get you right I tap the signal on the input jack of the CPA and route it to the 9 pin outlet socket. From where exactly do I get the speaker level signal andy wrote about? From the speaker terminal or from the amp output?Seems the same to me. Do I take this signal as it is? No resistors or other stuff in between? The existing line out signals come from the speaker-terminals with resistors in between. Do I have to eliminate these?
                Hammond A-100
                Crumar Mojo XT
                Leslies 46 W, 51, 330, 2 x 760

                Comment


                • #9
                  Originally posted by andyg View Post
                  You have speaker level output in the organ, which is what the 760 really wants. You then cut that down to a lower level at the line out and then use the CPA to boost it back.
                  I don't understand what makes the line out weak when it's tapped from the speaker terminal. Isn't this the speaker level output the 760 needs? Do I have to remove this resistor after the speaker terminal?
                  Hammond A-100
                  Crumar Mojo XT
                  Leslies 46 W, 51, 330, 2 x 760

                  Comment


                  • #10
                    Originally posted by Drawbar Dave View Post
                    You could disconnect the power to the preamp which is fed from the Leslie via the 9 pin socket too.
                    When disconnecting the power from the CPA does it still serve as a speed controller? Do the footswitches in the CPA work without power?
                    Hammond A-100
                    Crumar Mojo XT
                    Leslies 46 W, 51, 330, 2 x 760

                    Comment


                    • #11
                      Curious as to the design of your line out. You say you're tapping signal from both the main and reverb speaker wires. Are you using any mixing resistors to sum these, and does the _input_ to the reverb amp take its signal on the correct side of the mixing resistor? (Before the resistor, not at the point where the two signals unite)?

                      If the resistors were missing, or the reverb tap was on the wrong side, you would potentially build up feedback in the reverb circuit when cranking the volume up.
                      Current organs: AV, M-3, A-100
                      Current Leslies: 22H, 122, 770

                      Comment


                      • #12
                        I can't tell by heart how the signals are tapped. Organ is in the music-room, some miles away. I'll post pictures asap.
                        The line out was wired from the music store where I bought the organ. So I assume it's done correctly.
                        This is what I found at captain foldback:

                        Q: I have a Hammond organ with a 1/4" output that somebody made. When I connect this to my Leslie Combo Preamp there is a lot of noise and distortion.

                        A: This is because the output signal from any Hammond organ is many times as powerful as the input level accepted by the Combo Preamp. The proper way to connect a Leslie cabinet to a Hammond organ is to use a connector kit, which is often very simple. If the 1/4" way is the only practical solution I suggest that the signal from the organ is suitably attenuated with a voltage divider circuit consisting of two resistors: Take two resistors, 100k and 1k (0.5W) and connect them in series. Now connect the free end of the 100k resistor to the 'hot' signal coming from the organ and connect the free end of the 1k resistor to ground. Where the two resistors are joined together is where you should tap signal to the combo preamp. If you are handy you can fit the resistors inside the end cap on a 1/4" plug.

                        I think I will tap the signal as described and wire it to the jack socket on the organ. Then connect it to the input jack of the CPA, bypassing the pre amp. Will this work?
                        Hammond A-100
                        Crumar Mojo XT
                        Leslies 46 W, 51, 330, 2 x 760

                        Comment


                        • #13
                          Originally posted by enor View Post
                          If the resistors were missing, or the reverb tap was on the wrong side, you would potentially build up feedback in the reverb circuit when cranking the volume up.
                          Is there also feedback when the reverb is switched off?
                          Hammond A-100
                          Crumar Mojo XT
                          Leslies 46 W, 51, 330, 2 x 760

                          Comment


                          • #14
                            Sorry, I'm mixing things together. The way to tap the signal captain foldback described is for the CPA. But as I've learnt now, it's better to to feed the 760 with the direct organ output. Here is what I want: main and reverb in the 760 , CPA only as speed controler and current supply for the Leslie.
                            Can someone please describe the necessary steps to achieve this. Please in easy English (I'm Swiss), coherent to a layman in electrics.
                            Thank you so much!
                            Hammond A-100
                            Crumar Mojo XT
                            Leslies 46 W, 51, 330, 2 x 760

                            Comment


                            • #15
                              Now that you discovered the Captain Foldback site you can easily determine pins 1 and 2 on your CPA the line level
                              can be intercepted by means of a DPDT
                              switch.
                              This allows the speaker level output of your M100 to drive the 760 properly.
                              You need a switch and two small wires.
                              Not that difficult to bypass line level with a switch.
                              A100/251 A100/147 A102/222 B2/142 BV/147 BCV/145 M3/145 M102/145 M111/770 L101/760 T222/HL722 M111/770 no B3/C3!

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