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Old 15th October 2008, 03:04 PM   #1
MartyM is offline MartyM  United States
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Default Tube Preamp Popping on Source Input Change (?) / 6DJ8 Preamp On Chip Amps

Hi guys I am experimenting with the 6DJ8 based tube preamp ("buffer") for my LM3886 PA100 chip amps. This is the Joe Rasmussen (JLTi) tube buffer design, but I have the same issue when trying a parallel plate & cathode design.

The problem is a slight (moderate-to-low) volume pop when my FM/AM/DVD unit changes sources modes (AUX, CD, etc.). The source unit is solid state (not switched based) so I'm puzzled. This does not normally occur when the preamp is not used.

My first tube project was a 12AX7 tube preamp and popping did not exist.

Has someone seen this before or could comment? I suspect it is related to the tube in conjunction with the DC blocking capacitor.

Really hope someone can help!!
The simple schematic is here:
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Old 15th October 2008, 08:32 PM   #2
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When running this tube at such low voltages it will tend to draw a bit of grid current. This will put DC over your input pot and source selector which will induce just the sort of pops and rasps you describe. Unfortunately there are only two solutions to this. Put an input blocking cap before the 3K9 resistor, and then put a 100K resistor to ground after the cap.
The better solution is to increase the supply voltage to somewhere near 100V, which will remove the tendency to grid current.
Another possibility is to try different valves, as some will be better than others.

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Old 15th October 2008, 08:51 PM   #3
MartyM is offline MartyM  United States
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Ahhh....that explains why my +260V 12AX7 tube preamp didn't behave the same.

Thanks for the help! That was one possibility I considered but haven't tried

I'll try your suggestion and make sure my current measurements are correct (that would have been a clue maybe).

I'll report back on the results.
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Old 16th October 2008, 04:52 AM   #4
MartyM is offline MartyM  United States
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Thumbs up Seems To Be Resolved

Thanks for the help-that appears to have worked. Duplicated the same use of the head unit and all was well without any apparent effect on the audio response, at least without measurements.

Thanks again!
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