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This one has me stumped. Any electronics wizards out there?

Discussion in 'Pickups & Electronics [BG]' started by ric426, Jan 16, 2014.

  1. ric426

    ric426 Supporting Member

    Mar 27, 2002
    Southeast Michigan
    Here's the scenario:
    I've got several different 3 band on-board preamps (Aguilar OBP-3, Bart NTMB and Nordy 3 band) mounted in pedal style boxes with bypass footswitches so I can do direct comparisons and also use them with passive basses that I don't want to modify. I've been running them each on their own 9v battery and everything works fine. I recently put them all on a makeshift pedalboard and wired them to a common 9v supply, so that I can leave them all wired up without running the batteries down.

    Here's the problem:
    They all work fine in that configuration, but for some reason the Nordy preamp has a lot of hiss and noise on the output, even with the preamp gain trimmer and the EQ pots on minimum, but only when it's running on a 9v supply. It's well below the bass signal level, but very noticeable when I'm not playing. It's nice and quiet on a battery. I've tried two different supplies, various value filter caps on the 9v line to the preamp board, checked my wiring several times and called Nordstrand. The guy I talked to had no idea what would be causing it because they've always run them on batteries. They're very busy getting ready for NAMM, so Carey wasn't available and I didn't want to be a pest over relatively minor problem that's probably being caused by me using it in a configuration for which it wasn't designed. Like I said, it's a minor problem, but it's bugging me that I can't correct it.

    Anyone have any ideas on what could be causing it and what could be done to correct it?
  2. Power supplies generally aren't as clean as batteries. Are you using transformer-based supplies, or switched-mode supplies? Any ground issues?

    It is common for pedalboard power supplies to be isolated so that every pedal has it's own supply. This prevents ground loops, and other issues related to feeding multiple sources.

    You might also consider filtering the power supply with more than just a capacitor. Try something like a Pi filter, with an inductor to reject ripple current.
  3. ric426

    ric426 Supporting Member

    Mar 27, 2002
    Southeast Michigan
    Thanks for the suggestions.
    I tried both kinds of supplies with the same result. Same also if the preamp is the only load on the supply and the input grounded, so I don't think it's an isolation or hum issue. I've had situations where some of the switching noise from a supply has leaked into the signal path, but it doesn't sound at all like that. It just sounds like a significant increase in circuit noise with no apparent hum or switching noise. When I get more time to work on it I'll dig my scope out and see if I can trace the noise back to the source. It's really just a tinkering project, so if I don't find a simple solution I'll just use it with a battery.
  4. walterw

    walterw Supportive Fender Gold Supporting Member Commercial User

    Feb 20, 2009
    what supplies have you tried?

    you may need a proper isolated supply like the voodoo lab or the T-rex fuel tank Jr, where each output is not only not switching, but is individually isolated from the others.

    otherwise, try lifting the ground connections between the pedals, either on the signal cable side or the DC power side. you might be getting a ground loop from having both ground paths connected at the same time.
  5. ddnidd1

    ddnidd1 Supporting Member

  6. bassbenj


    Aug 11, 2009
    I agree. You really want to test the Nordy alone with batteries and supply. It may be an isolation thing (possibly ground loop).

    I"m not sure what you mean by "hiss and noise" (we get some very strange descriptions of noises here). Hiss is like escaping steam. That is usually a semiconductor thing and should NOT be affected at all by a power supply unless it's triggered by supply voltage.

    One thing you may want to look for is parasitic oscillations. These are an oscillation in the preamp circuit at frequencies higher than you can hear. These can come and go depending on supply impedance etc. To find them you have to probe around with an oscilloscope because they are too high frequency to hear.
  7. ric426

    ric426 Supporting Member

    Mar 27, 2002
    Southeast Michigan
    Problem solved.

    Once I got my scope out it was pretty easy to track down the problem. It was clear that both switching supplies had a little bit of switching noise on their outputs and apparently the Nordy doesn't have much in the way of supply filtering. They probably figured it wasn't needed since it's designed to be used with a battery. With the transformer based unregulated supply there was enough hum on the 9v line that it was showing up on the output as well. For whatever reason, the switching noise that was apparent on the output sounded like white noise through the amp which led me to think the circuit was doing something weird. I put a 100 ohm resistor in series with the 9v followed by a 0.1ufd cap to ground and the switching noise is all but gone on the 9v input of the preamp and no more white noise on the output.

    Thanks for the helpful suggestions.
  8. carlitoswhey


    Apr 3, 2009
    You have all 3 on one pedal board?!! Dear sweet baby James Jamerson !! please make a you tube video and update this thread
  9. Troph


    Apr 14, 2011
    Kirkland, WA
    I'm surprised that such a small 1st order filter cap removed noise substantially. Doesn't speak well of the design of the power supplies...