H9 Daisy Chain Power Rail Hum

[nickrose]

It does sound as though you have some kind of ground loop, but without seeing your setup it is hard to be sure.

There is a tiny tiny chance that a hardware fault in the H9 is creating your hum, but you say that "the issue does not occur when I use a separate supply for the H9" – this means that the H9 is not at fault.

The above confirms to me that you have a ground loop, probably through your common power supply. You should either remove any superfluous grounds, or if you cannot, run the H9 on its own separate power supply.

 

[raphaelchang]

My H9 seems to have a power rail hum injection issue that I’m not sure if it is a ground loop. When running off a daisy chained power supply, the other pedals on the chain begin to hum when the H9 is powered on. The hum is definitely introduced by the H9 because I can hear the tone of the hum change as the H9 goes through its startup text changes. However, I tried to check if it was a ground loop by unplugging all audio cables from the H9 and the hum still remained. There was still a hum even when the only ground connection from the H9 to the rest of the system was the single power cable, so I suspect that it is not a ground loop. The issue does not occur when I use a separate supply for the H9. Is it possible that the H9 is injecting noise into the power rail? 

[raphaelchang]

Let me clarify: the hum comes from the other pedals, even when all audio cables are unplugged from the H9 so that the H9 is not in the signal chain. The only connection of the H9 to the rest of the system is through the daisy chained power cable, so it’s reasonable that the problem does not occur when that cable is removed and replaced with a separate power supply cable. Because the only ground connection is through a single cable I don’t think it can be a ground loop.

[tstern]

How confident are you that your power supply can provide enough current to power all of your pedals?  If you're hearing the hum vary as the H9 starts up (drawing more or less power through the process), it might well be that you're overdrawing your power supply.  If you remove power from other pedals, does the hum go away or change?

[raphaelchang]

I’m confident that the power supply can supply enough current, but it does look like unpowering other pedals causes some change to the hum. I tried unplugging all audio cables and listening to each pedal individually, and some pedals hum more than others and respond differently to other pedals powering off. Most of them still respond most signifcantly to the H9 being powered off though, as in the hum always goes away without the H9.

[tstern]

Have you actually added up the max current draw for your pedals compared to your power supply? Depending on what else you're using, it's likely the H9 is at the high end of the power usage for your signal chain.

If you have a multimeter, you can also check that the voltage your power supply is providing is stable; overdrawn power supplies and underpowered pedals can appear to function normally but misbehave in subtle ways (like noise).

[raphaelchang]

I don’t have many other pedals so the sum of the current draws according to the manufacturers is less than the power supply limit. I scoped the 9V line but it looked flat… maybe the resolution isn’t high enough because there has to be something present that’s causing the humming. I think I will switch to an isolated supply, but I just wanted to make sure this was expected behavior of the H9 and I did not receive a defective unit.

[tstern]

If the hum changes tone as you add and remove pedals that aren't the H9, then it probably is the power supply causing problems rather than the H9.  The H9 might just be the largest current draw that pushes the power supply past its capabilities.

[raphaelchang]

The H9 is the largest current draw, but I only have 4 other pedals, where one of them draws 250mA and the others draw <100mA, and my power supply is rated for 1.7A. Also, I should note that the frequency of the hum changes when the text on the H9’s LCD screen changes, almost like it’s playing a melody.

[bohan]

You may want one of this. This should fix the hum issue because all its outlets are isolated.

https://store.eventideaudio.com/collections/featured-pedals/products/powermax

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