Dear Audio people,
I am working on a project to build a loop switcher for Guitar effects. I have a crazy problem I couldn't understand and need help to find a solution. Let me explain:
I use a LM7805 regulator with the suggested circuit from the data sheet. I have a 10uF and a 100nF cpacitor on the input side (9V supply voltage) and the same on the output side (regulated 5V). With the regulated voltage I power a PIC microcontroller which switches some relais (TQ2-5V). The audio signal is running through the switches of the relais and should therefore be isolated from the rest of the circuit. All digital stuff works really well.
My problem is, that the audio signal is crazy noisy. You clearly hear the typical 50Hz hum from the power source. The LM7805 is powered by a regulated and isolated 9V voltage from a multi-Output effect pedal supply. If I power the board with the 5V of a USB port (behind the LM7805 - so this is not active) , the signal is nearly clean. The audio ground is not connected to the board. It goes straight from the input to the output jack. The audio signal itself is connected to the relais and so should also be isolated from the digital side of the board. I hear the sound of my guitar so all should be connected correctly. I can't understand why I hear this crazy hum, because the audio signal should have no relation to the main circuit. Even if this would be the case, I guess the LM7805 should do its job quite well and produce a clean supply voltage. The hum is not a digital noise which is usually higher in the frequency range. If I power the digital parts with the usb supply voltage all works quite well, too. So my knowledge does not help me anymore.
I want to ask, if you may have any suggestions where I should find the solution of this problem? It seems to be the DC regulator or one of its components. Can a bad capacitor cause this kind of problem? Is it possibly an EMI problem? I would try to use another voltage regulator, but have to buy one.
Thank you for your help...
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