Hi all,
I wanted to get started with basic guitar recording in GarageBand on my MacBook Pro, so I picked up a 1/4" to 1/8" adapter to access the line-in port.
Reading Apple's spec, input resistance was stated at minimum 20K so I figured I would need to build a quick little JFET preamp to give my signal a bit of a boost and lower its output impedance.
After wiring everything up, the sound quality and volume level is great but unfortunately there is 120Hz hum. I am using a breadboard so a bunch of stray inductance and capacitance is around but I am certain it is coming from the wall-wart (no batteries for testing).
The part that is confusing me is how it is so prevalent in my signal as it appears with no change when I plug-in and unplug my laptop from the wall. The wall-wart is the only component grounded (two-pronged) so I can't see how there would be a ground-loop problem.
I've been scratching my head on this one, and wondering if anyone can spot something that I am not picking up on.
Attached is a rough sketch of my schematic.
Thanks again,
JP
I wanted to get started with basic guitar recording in GarageBand on my MacBook Pro, so I picked up a 1/4" to 1/8" adapter to access the line-in port.
Reading Apple's spec, input resistance was stated at minimum 20K so I figured I would need to build a quick little JFET preamp to give my signal a bit of a boost and lower its output impedance.
After wiring everything up, the sound quality and volume level is great but unfortunately there is 120Hz hum. I am using a breadboard so a bunch of stray inductance and capacitance is around but I am certain it is coming from the wall-wart (no batteries for testing).
The part that is confusing me is how it is so prevalent in my signal as it appears with no change when I plug-in and unplug my laptop from the wall. The wall-wart is the only component grounded (two-pronged) so I can't see how there would be a ground-loop problem.
I've been scratching my head on this one, and wondering if anyone can spot something that I am not picking up on.
Attached is a rough sketch of my schematic.
Thanks again,
JP
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