I did a comparison test of power supplies for the QC.
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First of all, thanks for taking the effort and doing such a test. In your scenario, you have the QC connected to a powered Marshall speaker. It’s not very clear from Marshall’s website but I assume, the speaker is powered by a grounded wall plug and has an internal power supply. Is that right or does it have an external power supply in the same way as the QC?
It’s quite interesting that in your case the noise gets stronger when you touch the strings. Although you don’t mention it, it’s kind of obvious that you have the volume knob of your guitar turned up, right? Did you do any tests with the volume pot at zero? This could be useful when testing for noise due to absent grounding and it typically appears when people are using the QC with headphones only without any other (grounded) equipment connected. In this case, however, the noise or hum usually disappears when you touch the strings.
Thanks for the comments.
The Marshall has an internal power supply, but is not grounded (just 2 wires going into the back).
I didn’t try it with the guitar volume down. Thinking about it now it would have been a good thing to do.
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That’s a crucial information, IMHO. Thanks.