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Allen RCA Cables and ADC Audio Mixer

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    Allen RCA Cables and ADC Audio Mixer

    Background: I have finally moved far enough along on putting together my VPO that I'm starting to work on how to run the VPO audio through my organ's audio system. I received a schematic of my organ's ADC Audio Mixer with my organ (that has several notes and edits specifically for my organ). I've also run a few experiments to verify the schematic. Currently, I have three (unused) inputs (one expresses with the solo) on the ADC Audio Mixer that would add the external audio alongside the Allen audio on three of the four output channels. I could use these inputs to supplement the native allen voices with some external VPO audio. I could also replace the six input channels of Allen audio with four channels of external VPO audio. These six channels are under expression and using them would eliminate the need to send expression data to the VPO. I'd only use four because the six channels get mixed down to four outputs anyway. There are a couple of other inputs that output to more than one channel. The two reverb inputs output to two channels each and the tape input sends to all four output channels. I would like to run several cables from the outside of the organ (where they would be easy to reach) to the ADC Audio Mixer. I might also design a pcb to turn off the Allen audio when I plug my computer in.

    Issue: In my experimenting, I have encountered a lot of hum when I've plugged additional cables and devices into the ADC Audio Mixer.

    What I've tried so far: I've followed John's advice in this post: https://organforum.com/forums/forum/...580#post649580 of connecting a clip lead to a couple of the input grounds. That eliminated the hum when something is plugged into the organ. (i.e. if I plug my phone or computer into the ADC Audio Mixer and the device is powered on, then the hum disappears. There is still hum if the device is off or unplugged. Currently, I have an audio cable (headphone jack to RCA) that runs from my computer to a couple of inputs on the ADC Audio Mixer.

    My current guess at the issue is that my audio cable is not adequately shielded. The thing that is odd is that the cable does not hum when plugged into any of my other audio systems (car, stereo, guitar amp, etc.) regardless of whether an input device is connected or not.

    Am I on the right track? Have I missed something obvious? Does anyone have any other suggestions? Thanks.

    p.s. Are the Allen RCA cables significantly better shielded than regular RCA cables?

    Sam
    Home: Allen ADC-4500 Church: Allen MDS-5
    Files: Allen Tone Card (TC) Database, TC Info, TC Converter, TC Mixer, ADC TC SF2, and MOS TC SF2, ADC TC Cad/Rvt, MOS TC Cad/Rvt, Organ Database, Music Library, etc. PM for unlinked files.

    #2
    Very unlikely this is a shielding problem. It is a grounding problem, likely a ground loop. A ground loop occurs when various ground points are at slightly different potential from one another. This causes current to flow between grounds resulting in hum. The clip lead trick is effectively insuring that J500 and J512 grounds are at the same potential eliminating the hum.

    Things you can try:
    • If any of the equipment in your chain is using a three prong power cord, try lifting the ground pin. Easiest way to do that is to use one of those three prong adaptors.
    • Likewise, for two prong power cords, reversing the plug in the socket may help. Guitar amps often have as switch for this purpose.
    • Make custom audio cables and connect the shield to one end of cable only. Since the shield is connected at one end only, there can be no ground loop.
    Note that messing with mains grounds can result in a shock hazard, so I'm only mentioning it here as means of identifying the problem, not as a solution.

    I've experienced this problem many times when interconnecting computers to audio gear and it can be difficult to fix. A last resort would be to use 1:1 audio transformers in the signal path to eliminate any direct connection between the two devices.
    -Admin

    Allen 965
    Zuma Group Midi Keyboard Encoder
    Zuma Group DM Midi Stop Controller
    Hauptwerk 4.2

    Comment


    • samibe
      samibe commented
      Editing a comment
      I will look through my equipment and try the stuff you suggested.

      What is strange to me is that the hum only occurs if a cord is plugged into the ADC Audio Mixer. Just organ = no hum, organ + cord = hum, organ + cord + external device = no hum.

    #3
    Ok, I'm slightly confused. Earlier you said that you had hum when the external device was connected but was off. I'm assuming that's what you mean in your comment?

    I would expect hum if you plug a cable into the mixer and leave the other floating and connected to nothing. That is normal. The unshielded tip of the connector is picking up the hum. If you short the RCA plug on the floating end, the hum should disappear. Now, if the hum is present when a device connected, but not powered on, it means that as far the input to the mixer is concerned the cable is still floating. Turning the device on lowers the source impedance seen by the mixer's input eliminating the hum.

    That being the case, the solution is to either disconnect the cable going to the external device when it's not being used or unplug the device and short the cable at the disconnected end.
    -Admin

    Allen 965
    Zuma Group Midi Keyboard Encoder
    Zuma Group DM Midi Stop Controller
    Hauptwerk 4.2

    Comment


    • samibe
      samibe commented
      Editing a comment
      You were right. If I ground the inputs on the cable the hum is eliminated. I think I may make a box to switch inputs on/off so that I don't have to open the organ each time I want to switch inputs. If I use a DP switch for the inputs, I can ground the inputs when I turn the switch off. Thanks admin.
      Last edited by samibe; 01-18-2020, 01:36 PM.

    • Admin
      Admin commented
      Editing a comment
      You could wire 1/4" switching phone jack, commonly used to mute speakers by opening a contact when headphones are plugged in, to short the connection when nothing is plugged into it. Plugging something in would open the contact removing the short.

    • samibe
      samibe commented
      Editing a comment
      I'll do that for a couple of the supplementary inputs. I'll have to use some relays to turn off the cage inputs and use the external inputs instead.
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