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Thread: Dealing with Hum in Consoles Driving Solid State Amplifiers

  1. #1
    mp Mezzo-Piano bnelson218's Avatar
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    Dealing with Hum in Consoles Driving Solid State Amplifiers

    I'm seeking people who have dealt with, and hopefully defeated, 60-cycle hum in solid state consoles and output systems. My setup is this: I have a TWG-based console with a Trek II, SSP3 preamp. Its G3-G4 outputs drive the balanced inputs of a 2-way, active crossover. From the console, balanced cables carry these signals to the likewise balanced inputs of the solid-state power amps that then feed the rotor cabinet's treble and bass drivers. Regarding these cables, they are grounded at the console side but NOT at the rotor cabinet side. And this has been my protocol throughout the organ: grounding shielded lines at the sending side and not the receiving side. Despite this, there is a 60-cycle hum present in both channels. I've read some white papers on this which recommend tying BOTH ends of the shield grounds to a chassis ground right at their respective entry points. I haven't tried this yet, but everything else I've tried hasn't worked.
    So, I'm hoping someone with a similar setup who has dealt with a 60-cycle hum might have a solution.
    Thanks in advance for any help!
    Over the years: Hammond M3, BC, M102, B3, four X77s and three PR-40s, a Thomas Electra and a Celebrity, three Fender Rhodes, Roland HS-10, HP-2000, HP-600, RD-600, JV-880, a thing made by Korg (?), two Leslie 910s, 122, 257, 258, 247, two 142s, and three custom-built Leslies. Wow, way too much money spent!

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    Administrator Admin's Avatar
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    Since you are correctly attaching the shields only on one end and you are using balanced ins and outs, I assume you've eliminated the possibility that the hum is originating in the console.If so, you have a ground loop somewhere.

    I wonder if you are using three wire mains connections for your console and amps? If so, try lifting those grounds. You can try connecting the shields at both ends of your cables, but that's never helped in my experience.

  3. #3
    mp Mezzo-Piano bnelson218's Avatar
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    Quote Originally Posted by Admin View Post
    Since you are correctly attaching the shields only on one end and you are using balanced ins and outs, I assume you've eliminated the possibility that the hum is originating in the console.If so, you have a ground loop somewhere.

    I wonder if you are using three wire mains connections for your console and amps? If so, try lifting those grounds. You can try connecting the shields at both ends of your cables, but that's never helped in my experience.
    The active crossover utilizes a TRS jack for the input and outputs. I can connect my little desktop CD player to the input and enjoy hum-free playback out of the rotor system. This told me that the hum is definitely console born.

    The console's been retrofitted with a 3-wire AC cord and plug going into a plug/socket at the console which has an EMI filter built in. Inside the console, the 3rd wire, the house ground, can either be connected or disconnected from the organ's ground. Either way makes no difference. The rotor cabinet also has its own 3-wire AC cord and there I can lift it from house ground with a toggle switch. Again, no difference.

    A white paper by Rod Elliot I read suggested connecting the house ground to the system ground through 1) two diodes--one forward biased and the other reverse, 2) a 100nf cap, and 3) a 10 ohm resistor. The diodes provide a protective circuit in the event of a major problem. The 100nF cap acts as a short circuit to radio frequency signals, effectively grounding them. And the resistor (a 5W wirewound resistor is suggested) isolates the low-voltage high-current ground loop circuit. I did this in both the console and rotor cabinet with no effect on the hum.


    I'm left to trying to ground the main cables at both ends. A Rane white paper states that doing so makes both chassis simply an extension of each other--a concept that actually makes sense.


    Thank you so much for your input!
    Over the years: Hammond M3, BC, M102, B3, four X77s and three PR-40s, a Thomas Electra and a Celebrity, three Fender Rhodes, Roland HS-10, HP-2000, HP-600, RD-600, JV-880, a thing made by Korg (?), two Leslie 910s, 122, 257, 258, 247, two 142s, and three custom-built Leslies. Wow, way too much money spent!

  4. #4
    ff Fortissimo Tim_at_Jonas's Avatar
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    If the crossover input is an impedance-balanced circuit instead of actively balanced or transformer balanced, the shield might be required at both ends, as an impedance-balanced input has no CMRR. Or it just might not work well with the transformer output of the Hammond for some reason I cannot fathom.

  5. #5
    pp Pianissimo delaware dave's Avatar
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    I own an L100P and run its 1/4" output into a Rolls mini mixer (to raise the output gain to an acceptable level) then take the mixer output into a Motion Sound Pro3T/LowPro. The Motion Sound front end has a tube but the amplifier is solid state. I get no hum from the leslie output.

    At one time it did hum and I took the hammond in for servicing and was told that the 1/4" connector had a powder buildup that prevented adequate grounding. The plug was cleaned and the hum disappeared. That was many years ago.
    57 Hammond B3; 69 Hammond L100P; 68 Leslie 122; Kurzweil PC3; GEM Equinox 88 and 76 key versions; Gemini desktop module; Voce V5+; Neo Vent; EV ELX112P; 67 Howard Combo Organ; www.dyinbreedband.com

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