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Old 28th May 2012, 03:17 PM   #1
JGAN is offline JGAN  United States
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Question Buzzing in preamp circuit?

Hey guys,
I finally finished my tillman preamp circuit I will be posting more on it later, but it seems to be helping the tone quality already. However, I do have a problem with buzzing/humming from the circuit. It happens whenever plugged in and switched on, and I don't think its the battery. The buzzing does go away when I short the input though. Could I have bad jacks?

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JGAN
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Old 28th May 2012, 03:30 PM   #2
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... or improperly wired or shielded.
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Old 28th May 2012, 03:49 PM   #3
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I don't think I did anything wrong when soldering, but how do I go about shielding?
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Old 28th May 2012, 04:46 PM   #4
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Which is the input impedance of such stuff?
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Old 28th May 2012, 04:54 PM   #5
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Discrete FET Guitar Preamp

3.0M Ω input impedance.
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Old 28th May 2012, 05:06 PM   #6
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3Megohm is a very high input impedance. So a good shielding is imperative. Shield al conections from board to the input plug, and tie the shield to gnd of the circuit at only one point. Also, I suggest to add a source follower at the output of your circuit to come from low impedance to a cable then to the amp.
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Old 28th May 2012, 05:10 PM   #7
JGAN is offline JGAN  United States
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I actually may have built the circuit wrong. For a n-channel JFET transistor, which is the correct schematic (drain vs source)?
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Or...
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Old 28th May 2012, 05:13 PM   #8
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You may be creating a ground loop with the input equipment. Have you been careful with power and signal grounding in your pre-amp to avoid power noise running along with signal ground?
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Old 28th May 2012, 05:49 PM   #9
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No, most JFET have D & S interchangeable, ans in the particular case yours not, it will not work properly, but isnīt the cause to hum. Distortion may be, but not hum/noise.
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Old 28th May 2012, 09:16 PM   #10
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Quote:
Originally Posted by doozerdave View Post
You may be creating a ground loop with the input equipment. Have you been careful with power and signal grounding in your pre-amp to avoid power noise running along with signal ground?
I'm not exactly sure what that means?
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