Guitar preamp to unplugged MacBook has hum

Thread Starter

blah2222

Joined May 3, 2010
582
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
 

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#12

Joined Nov 30, 2010
18,076
You should be aware that the jfet is a high enough impedance to pick up power line hum. If using a battery doesn't fix this, put it in a metal box to shield it.
 

Thread Starter

blah2222

Joined May 3, 2010
582
Replace the wall wart with a 9V battery.
I know that would be the easiest solution, I just don't want to waste batteries at this point.

You are using shielded cables for the audio connections, correct ?
Shielded patch cord from guitar to breadboard and breadboard to laptop adapter.

I think it's a noisy wall-wart tbh but I guess the breadboard doesn't help.
 

GopherT

Joined Nov 23, 2012
7,983
1) Make double sure that your circuit ground is going to the ring on the connector.

2) Do not try to cover left and right channels by connecting left and right channels in series and leaving the ground floating.

3) if you want left and right channels, just connect both of the left and right channel rings together but make sure the circuit ground goes to connector ground on the plug (usually the ring closest to the cord (but check - apple loves to set up unique pin-outs just because they can).

If ground is reversed (even with battery powered circuits), you will get power line hum.
 

wayneh

Joined Sep 9, 2010
16,102
Replace the wall wart with a 9V battery.
+1

No way you'll avoid wall wart hum without doing this.

Well, using a voltage regulator circuit would help a lot too, if you don't want to waste batteries. Look up 7805 regulator IC. Or power with USB from the Macbook.
 

tubeguy

Joined Nov 3, 2012
1,157
You can automatically switch the battery on/off by using a RTS (stereo) jack
as one of the jacks on the preamp. Connect the circuit grounds to the gnd on the jack, but connect the battery gnd to the ring connection.
Using a regular mono plug will connect the battery.
 
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