Bottlehead Forum
Bottlehead Kits => Crack => Topic started by: Dr. Soot on April 05, 2023, 04:43:08 AM
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I am in the process of rewiring my Crack and I want to replace the Neutrik/Rean headphone jack with a different jack. No reason other than I have some other 1/4" stereo jacks. My question, why are there resistors from the ground to the tip and ring lugs on the jack? I am taking a guess here and based on the wiring schematic, the three lugs nearest the 9-pin tube socket are the "active" lugs and the ones further away are all connected to the ground wire since they are not really used. But what do the resistors actually do in this case? If you had a jack with one set of TRS lugs (like the Switchcraft one I have attached for reference), would you need the resistors?
PS-is a switched jack OK vs a non-switched?
Dr. Soot
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The stock jack is a switched jack. The 2.49K resistors keep the output end of the 100uF coupling caps at DC ground potential. Without them you would have about 100V DC sitting at the headphone jack waiting for you to plug your headphones in, and you'd get a big pop and a spark, and probably one or two blown headphone drivers every time you plugged the headphones in. The switching function of the stock jack grounds the outputs when no headphones are plugged in so that this voltage can't build at all when no headphones are plugged in.
The stock jack is also insulated from the chassis plate, and if you use a jack that automatically connects the sleeve to the chassis plate, you will introduce a ground loop and the chassis plate will become part of the signal path, and this would be a serious downgrade in amplifier performance.
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OK, more proof I have no idea of what I am doing :'(. Is the stock jack the only one I should use? Or are there other options?
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The stock jack is functionally the best option. You can drill out the plate and use the locking Neutrik jack if you want, but you'll give up the shorting function by doing so.
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Understood Paul, thank you!