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Thread: Powering different devices with single 9V AC transformer

  1. #1
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    Powering different devices with single 9V AC transformer

    Hello,

    I'd wish to use a single 9V AC transformer to power several audio effect units. Is this safe and correct? Some Q.:

    - some units haven't a full bridge rectifier but a simpler rectification, sending one of power input contacts to ground. Two different units may create a short on the transformer, but I can simply invert the contacts on a specific barrel to avoid shorts and have it right, is it correct?
    - how to evaluate if the transformer load is too much? May I mesure the voltage drop when I add the next load? How much drop is correct?
    - how much temperature increment can be acceptable?
    - may I need to scope the secondary voltage to check if the sine wave is distorting under too heavy load?

    Many thanks
    m.p.

  2. #2
    Old Timer olddawg's Avatar
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    Just get a One Spot. They work great, are inexpensive, and will power several pedals.

  3. #3
    Supporting Member Jazz P Bass's Avatar
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    Man, what a can of worms.

    Each separate pedal has a certain voltage & power requirement & power plug pinout.

    If the pedals require Voltage ac, then you can do what you said.

    If they require Volts dc, then a transformer by itself will not work.

    Also each pedal will 'draw' a certain load from the power supply.

  4. #4
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    To me a 9v AC transformer outputs AC. I think because you're referring to rectification you mean a 9v DC power supply?

    The starting point is the power supply and its rating. Many of these units are not regulated and have an output voltage that's only provided under a specific rated load. So your 9v supply may give 12v or even more when unloaded and this drops to 9v as the load approaches its rated output. If the load is increased further the output falls even more and the unit gets hot.

    These types of supply are generally unsuitable for powering pedals as they have a high level of ripple or other noise that gets through into the audio path. Ideally you need a regulated supply and these have a stable output voltage that is practically noise-free.

    Directly connecting together the outputs of different power supplies is not recommended unless you are fully aware of how each supply 'sees' the other. You can divide up your pedals and run one half off one supply and the other half of another. Just make sure the polarity is correct.

    A simple way to get some idea of your total power requirement is to add up all the rating plates on each pedal, or refer to the documentation for the consumption. Or you could break into a lead and insert a meter set to mA in series and measure each pedal individually. If you want to measure voltage drop on an unregulated supply make a note of the unloaded voltage and then add pedals one-by-on until the voltage gets to 9v. In fact, a regulated supply will read 9v unloaded so is a good indication (even if it's a simple zener regulator). If the supply is a higher-current unregulated unit (say, 2A to 5A) you may not be able to load it sufficiently to give 9v.

  5. #5
    Old Timer J M Fahey's Avatar
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    Please read the original post.

    Quote Originally Posted by Marco Pancaldi View Post
    Hello,

    I'd wish to use a single 9V AC transformer to power several audio effect units. Is this safe and correct?
    It is safe and correct IF they are designed to be powered from 9VAC.
    A single pedal including its own bridge rectifier will work with any floating/independent 9VAC winding.
    Two or more similar ones will not, because bridge rectifiers require floating independent windings, the bridge itself has one corner grounded, not the winding, so plugging 2 different pedals will make sparks fly (or plain burn transformer)-
    - some units haven't a full bridge rectifier but a simpler rectification, sending one of power input contacts to ground.
    Those will work.
    One end of winding is grounded at each pedal, interconnecting many won´t cause trouble because "all grounds are the same"
    The other end of the winding may make different things in different pedals:
    * a single diode and cap will give you raw +12V which can be easily filtered and regulated for +9V, just what the doctor ordered.
    * another diode and cap, but reversed, can give you raw -12V for your Germanium Fuzz Face, Booster, etc.
    * both combined can give you symmetrical 12V , good for Op Amps and way better than plain +9V
    * 9VAC can power a 12AX7 filament. At least Real Tube does exactly that.
    * 9VAC can feed a voltage doubler and give you almost raw 24V, so guaranteed clean +18V (or +15 or +12).

    So 9VAC is a very versatile wall wart solution, but each and every pedal fed from it must be specially designed for that, if you want to Mod conventional pedals for it you NEED to add at least 1 diode, one (largish) cap and some extra filtering or regulation, at EACH pedal.
    Two different units may create a short on the transformer, but I can simply invert the contacts on a specific barrel to avoid shorts and have it right, is it correct?
    Inverting contacts does NOTHING for you , the problem is what you do with grounds and how you rectify said 9VAC.
    - how to evaluate if the transformer load is too much? May I mesure the voltage drop when I add the next load? How much drop is correct?
    - how much temperature increment can be acceptable?
    - may I need to scope the secondary voltage to check if the sine wave is distorting under too heavy load?
    Those are minor problems, you first must have (or add) proper internal rectification and ground one end of winding.
    And yes, ground the same end in all.
    Juan Manuel Fahey

  6. #6
    Senior Member mozwell's Avatar
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    Also, be aware that a lot of stomp boxes operate from a 9VDC supply, and many have a reverse polarity diode across the power supply input. You can NOT use a 9VAC to power these. Others have a series diode to block negative DC getting into the unit, but generally the input capacitor is NOT designed to filter an AC input supply. It is there only to provide a small extra filter on an existing DC supply.

    rather than just make blanket statements about your question, and in order so we can help you better, please list the products you wish to power, and what the rating plate says for each one, ie unit 1 9VAC 0.4A, unit 2 12VDC 0.1A, unit 3 15VAC 1.0A
    J M Fahey likes this.

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