AMB beta22 - hum issue, no idea where it comes from

  1. Sathimas
    Hi there,

    I just finished my one-case 2ch beta 22 and have some annoying hum on the output.
    Also with the volume turned down completely there's some kind of hiss.

    I followed AMBs wiring scheme correctly I think...

    The RCA-jacks have no contact to the case and are isolated from each other.

    If I have only connected the inputs to the boards (no ps-connection)
    (RCA In/G -> Alps pot -> board)
    there's no connection between the boards ground and case-ground

    Here's all the wiring concerning the s22/power

    2x V+/G/V- -> ß22 boards
    G (s22) -> Neutrik locking jack
    G (s22) -> GLB -> encloruse (bottom)

    AC ground -> enclosure (bottom)

    AC ground an GLB are not connected to the same point

    When I measure betwenn the s22 ground and AC ground I get 10 ohms,
    as is resistor value of the GLB is 10ohm. Same reading from the body
    of the neutrik jack to the sleeve contact.

    Where's the mistake?

  2. tomb
    I can't address your hiss, but that toroid looks as if it's close enough to be touching the Sigma 22 board. You'd be much better off turning the Sigma 22 board 90 degrees and move it up as close to the front of the case as you can get. That will maximize the separation distance between the toroid and the Sigma 22 board. I could be wrong, but those heat sinks could be acting like huge antennas for the flux around the toroid and reflecting that into the amp as 50-60 Hz hum.
  3. Speedskater
    a] Connect the Safety Ground to the chassis near the IEC receptacle.
    b] Connect both RCA jacks to the chassis.
    c] Connect the DC supply common to the chassis very near the RCA jacks.
    d] Connect the audio circuit common to that same point as the DC supply common.
  4. Zashoomin
    Even though the transformer is fairly far away from your amp boards it might be the cause of some of the hum. I recommend turning the transformer and seeing if that increases or decreases the hum. If it does keep turning it til you get minimal hum or none at all. Also if you are using sensitive headphones it will pick up the transformer hum in a single case, at least from my experience.
  5. penmarker
    Just try unscrewing the standoffs from the chassis floor and push the boards away from the transformer (not while its turn on, turn it off and wait for everything to discharge, move them, and turn it on again). See how it goes. If it works and you want better protection, look for transformer cans. Mu metal works best but they're expensive. Make sure you get a magnetic can.

    Have you added the 0.1uF capacitor parallel to the resistor for the ground loop breaker?
    Last edited: Sep 22, 2017
  6. Sathimas
    Ground loop breaker contaings the cap, of course.

    I also installed Zobel networks on the outputs what helped to get rid of the hiss and
    make the hum a little more quiet.

    I also got a new transformer which is shielded but did not have the time
    to install that yet - I'll report as soon as I've done that.
  7. gabriel-dan
    Try grounding the alps pot. Wire one of the metal screws at the back of the pot to ground on the one of the boards.
  8. Sathimas
    I already did that - made no difference.
  9. Sathimas
    Just installed the new torroid - silence!

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