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Digital noise from SOMO

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  • Digital noise from SOMO

    Hello good people.

    I've been using the SOMO II module successfully for my special FX modules, having transgressed from the Quadravox units (which are hard to get now that Milford Instruments is no more).

    I was testing a module in situ prior to dispatch when I noticed a fair amount of digital noise from the player which worsened about 2 minutes into the track. I'm using it with a 12v amp and controlling directly with a PIC16F88. I've never encountered this noise so bad before so at first I thought it was the switching PSU causing it, (as this was my first design to use a SMPS).
    After substituting the 12v and 5v PSU for a clean bench supply, and swapping the SOMO just for good measure it was still the same! I eventually tracked it down and found it was associated with the remote USB memory stick connection, I also noted that there was a fair amount of current being drawn on the 5v side whenever a memory stick was being used.

    Of course I already tried to substitute the memory stick with another FAT32 formatted stick and the noise remains!

    If anyone has experienced this and found the solution do please let me know!

    Also: During experimentation I could never get the "Key" buttons to function, that's the interface which uses resistors.

    I'll just say that The SOMO II is a versatile unit an in theory I could use the push button interface connected directly to a switch and play the single sound (which is the requirement of the things I am building - In reality you get problems with the customer holding the play button in and thus changing the volume and/or selecting another track, and of course it will pause the track if they press the button again.
    So I find the need to use a PIC to tell the somo to ramp up the volume, select one track and play it uninterrupted and also power up /shut down the auxiliary supplies, amplifier and also a relay output.

    I have uploaded a picture of one.
    Attached Files

  • #2
    Hot Dang! I believe I have found the solution, I will share it for the archives:

    There was another difference to the design which I had overlooked!
    This solution only came about when I tried using the differential speaker output with a small speaker - this would rule out any common-mode audio issues right? (Some might remember the Somo1 had this as the only output. Some of my prototypes used a nasty audio transformer to interface the amp......)

    Sure enough the differential speaker output was clean!

    So back to the solution: The Kemo amplifier module (which I've been using for years) had undergone a small specification change to the speaker( -) output shared a common ground, whereas the previous design had the speaker (+ ) floating from the + supply.
    This was enough to get me thinking about our old friend the GROUND LOOP! NOOOOOOOO!

    Yep there was enough inductance in there to create a serious RF antenna! It was simply a case of separating the ground paths along the input route, even though the supply and input ground may well be connected internally in the amp, and the Sumo has a common ground, I had created the "loop" of doom!

    A Classic benchmark scenario, and the bain of any sound engineer!

    Let it be a lesson to each and every one of us!

    And so ends todays story.
    Last edited by STEINFX; 13th March 2017, 07:16 AM.

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