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Powering EHX BMS

Discussion in 'Effects [BG]' started by BroKenSticKs, Apr 15, 2010.

  1. BroKenSticKs


    Jan 11, 2010
    On the pedal it says not to use with a daisy chain but why is this? I have a 1-spot to power my pedals and i'm wondering why i cant use it with the synth.

    Sorry if this is a dumb question.
  2. Jazz Ad

    Jazz Ad Mi la ré sol Supporting Member

    I don't know the technical reason but more than often it simply won't turn on when used with a daisy chain. It works fine with my BBE SupaCharger though.
  3. Don`t know why but it is true. You can not daisy chain BMS. I have own 1-spot for BMS.
  4. Silent Fly

    Silent Fly Supporting Member Commercial User

    May 8, 2006
    London - UK
    Owner/designer [sfx]
    I haven't analysed the circuit of the BMS but I think the reason is related to the internal power supply system.

    My guess is that the BMS is powered by 9V but internally the pedal has a circuit that increases the operating voltage. This has the advantage of allowing the use of a 9V PSA instead of a higher voltage one (e.g. 24V).

    Unfortunately, EHX designed the internal powers supply system in a peculiar way...

    The ground used by the main circuit of the BMS (input and out jacks included) it is not at the same level of the ground of the power supply. In other words there is a voltage that is not zero between the outside of the input/output jacks and the ground of the power supply connector of the BMS.

    The majority of the other pedals (e.g. Boss pedals) have the signal-ground and the power-supply ground that are physically connected. Consequently, there is always 0 volt between the in/out jacks and the centre of the power supply connector.

    If we connect 2 Boss pedals with a jack cable, the signal-grounds are connected. The power-supply-grounds are also connected through the jacks. When we daisy chain the power supplies everything works nicely.

    If we connect a Boss pedal to a BMS with a jack cable, the signal-ground of the two circuits are connected. The power-supply-ground of the Boss pedal is also connected to the signal-ground but the BMS power-ground isn’t. The power supply system of the BMS forces a voltage between the signal-ground and the power-supply-ground. In other words, the BMS forces a voltage between the centre of the power supply connector and the in/out jacks.

    If we daisy chain the power supply of the Boss pedal and the BMS, the two power-grounds are connected by the daisy chain cable, the signal-grounds are connected with the jacks and the Boss pedals connects the two grounds together.

    In a nutshell, the BMS forces a voltage between signal ground and power ground. The daisy chain cable short circuit it. This stops the BMS from working, causing a potential damage to the BMS.

    It there a way to design an internal power supply that increases the voltage without causing ground level conflicts? Yes. Why didn’t EHX use it? I don’t know. Cost and circuit complexity I guess.
  5. BroKenSticKs


    Jan 11, 2010
    Interesting! Thanks for explaining that. Never would have figured that out on my own, lol.

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