I am a bit stumped with this one. Unfortunately there are a few variables involved, but I will do my best to ask an answerable question....

I just built a DIY synthesizer. It functions absolutely fine. This is the pcb design:


plugged into a small guitar amplifier, it functions normally, and the on/off switch functions to disconnect power from the synthesizer.

However, when I plug it into my mixing desk, any audio lead attached to it disables the on/off power switch and the synth is always powered up. The synth continues to work properly, I just cannot disconnect the power with on/off.

I had the idea that this is occurring because it is somehow providing an alternate ground (because I don't believe it can be supplying a voltage, especially not one that would not have destroyed the synthesizer) when the ground is disconnected by the on/off switch.

With that in mind, I put a Ground Lift "hum remover" in between the synth and mixing desk. Sure enough, this solves the issue and the on/off switch operates again.

The mixing desk itself is connected to an audio interface as both input and output, plus several other synthesizers, and a microphone.

is there something obvious causing this that I am missing? I am a bit stumped as to what is happening (I would assume an error in the assembly of the synth, but the issue does not exist with other inputs!)

  • 4
    \$\begingroup\$ It would be better if you provided a schematic. Not many of us are going to trace through the PCB layout to debug your issue. \$\endgroup\$ – Cristobol Polychronopolis Jan 10 '19 at 15:55
  • \$\begingroup\$ If the jacks are insert switched with contacts to ground and the power is a low side switch, that explains it \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jan 10 '19 at 16:06
  • \$\begingroup\$ While PCB's are nice to look at, to debug the circuit I'd have to reverse engineer the PCB to figure out what is going on. A schematic would be necessary to debug this, provide only the relevant parts like inputs, power and switches \$\endgroup\$ – Voltage Spike Jan 10 '19 at 16:30
  • \$\begingroup\$ yes, I'm sorry, that's fair enough - I don't have the PCB layout or I would certainly have posted that. I had until now assumed it has to be a mixer side problem because it doesn't happen with all jacks - plugging it into another source has no effect. Sunnyskyguy's answer makes sense I think - they are switched jacks.... But then I don't understand why it happens with the mixing desk, but not with plugging it into an amp.... \$\endgroup\$ – the_duckchild Jan 10 '19 at 16:30

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