Power Supply Noise
  • I’m using the case pictured below. It has been working fine, until I plugged in a Braids and now there is a low hum from my headphone amp. I’m not an expert by any means in these things. I was wondering if the bus could be too long? Or if the 5v supply isn’t calibrated properly?

    I’d be grateful for any ideas of how to sort it out. So far I have considered a second getting a volts for cleaner 5v (can I add a volts to a system that already has a 5v rail?!) or a second bus for headphone amp, filter etc

    <img src=“http://mutable-instruments.net/forum/uploads/FileUpload/16/1d75d590d52c02b547a430b8861501.jpg” />

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  • There is a lot of sound advice on MW from Graham Hinton on grounding of PSUs and the importance of very low resistance on your ground/earth buss to avoid noise and hum. A flying power cable like you have is the worst solution to that issue. Maybe an Elby or L-1 power distribution board connected with very heavy duty wire (at least for the earth/ground buss) to your Mean Well? Or as Hinton recommends, to an earth binding post which is connected to mains earth and to the PSU earth and common terminals by the thickest cable you can fit in the crimp lug connectors, or even double wiring. Even better is an Amalgamod power distribution board, designed by Laurie at Elby, that uses thick aluminium bars for the busses. Overkill for the +/- 12V and +5V busses, but a good idea for the ground buss, it seems.

  • Thank you! I also have the feeling the earth might not be connected… I haven’t got it in my hands but it looks like there is nothing in that connection from this image. I didn’t build this case I should also add, I’m just trying to get it to work without the hum…

  • I had a C64 that hummed once. The ground on the power supply voltage regulator had snapped, I imagine this issue is similar? (lack of ground).

  • Connecting everything to protective earth is probably a good idea anyway, you know in case things go wrong.

  • Is the best way to put in an earth pole? Just a bit of metal and then earth everything to that? I think I will probably change the flying bus out for a bus board too… Thanks again for the help!

  • Check the photo towards the bottom of this thread here:
    https://www.muffwiggler.com/forum/viewtopic.php?t=131748&start=25

    The thread also has a bunch of useful infos on the matter.

  • First a warning: it’s life-endangering because the mains connections at the PSU screw terminals are not isolated!
    (1) And it’s not clear from the picture: does it have an IEC connector with earth contact in the middle. And is this connected to the power supply (three wires from the IEC connector to the PSU)?
    (2) Is it a Meanwell PSU? Most of these need a minimum load on the +5V rail to work properly.
    (3) Is your Braids actually an older version which needs +5V? (10 pin or 16 pin cable?)
    (4) Is your headphone amp a module or an external device?

    You could try to connect the earth (middle pin of IEC connector) to the 0-Volt (gnd) rail of your bus. But ALWAYS UNPLUG the mains cable when touching anything inside this case.

  • cj55, are you sure about #2? Do you have a link to some documentation regarding this?

  • Yes, this was also discussed in one of the PSU threads here.
    See current range in the data sheet for 5V (RT65B): 0.5A – 8A
    Without any load, the RT65B (which I own) makes some strange noise (chirping).

    Edit: also see A13 of this FAQ

  • Yes, Mean Wells (and clones) need a minimum load on every rail. Although the specs sat at least 200mA load on the 5V rail, about 100mA load seems to work OK for me. A single older Braids draws only 85mA from the 5V rail, and newer Braids draw none. You might need a dummy 5V load (or better, some more modules that require 5V). Or power something else that requires 5V DC as well.

    Yes, cj55 is absolutely correct, mains power is lethal, apply all precautions, get it checked and approved by a qualified electrician, who will tell you that the mains terminals on your PSU MUST be shrouded to prevent fatal finger-poking and loose wire accidents. I agree with cj55 that your current set-up is very very dangerous. You must construct an insulating shroud over and around all the mains side terminals on the PSU.

  • Wow, I’ll need to check on that. I am using an RT65B now and haven’t noticed any odd noises. I’ll be mounting another in a portable cabinet, probably tonight, and there is no 5V draw from any of the modules so far, should be an interesting experiment. It’s for my Anushri, Verbos Touchplate and a few other bits.

    I cover my mains terminals with electrical tape and a plastic shroud, just to be safe.

  • @cj55
    1 – Actually this isn’t an up to date picture, it does have the earth connected to the 3 pin power inlet.
    2 – it’s a t-30b I think it must be a meanwell? Or is that a copy?
    3 – actually my braids is a 16 pin, didn’t even think to check that, I assumed it was a 5v
    4 – my headphone amp is an A-139-2 from doepfer

    I tried a second bus board but it made no difference, so I think I am going to put this power supply aside based on the warnings above and get something with an external brick. I might come back to it and do an earth pole and ground the bus properly but I need to do some reading it looks like! Thanks all, it’s great to get some insight on this

  • (1) That’s good. As I said, you can try to make a connection between pin 3 (FG) and pin 6 (COM) on the PSU screw terminal. Then you have earth connected to 0V of your flying bus.
    (2) here is the datasheet of the Meanwell T-30B
    (3) 16 pin Braids is a 5V version and that is actually good, because you have some load on the 5V rail and more capacity free on the 12V rail. You may need an additional dummy load on the 5V rail for better regulation (as discussed above).
    (4) good, can’t see any problem.

    The T-30B has 1A at 12V and 0.5A at -12V, so check if your modules may need more.

    And to answer your second question of your first post:
    Getting an MI Volts will not work if your bus is already connected to the PSU 5V.

  • I am looking at a fairly simple but easy to follow wiring diagram, so I may try that. How can I get a dummy load onto the 5v rail?

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