SP 14 hum elimination

Over time, there have been several postings about hum issues with the SP14 preamp, with many "no issue" experiences.  Having built many of these fine preamps, I wanted to share a grounding philosophy that has resulted in dead quiet units for me.  These observations are implicit in Roy's instructions, but, made explicit, may help those with issues.

Think of a ground path through the preamp, starting at the ground lug at the IEC input jack and ending at the output jacks:

-IEC grounding lug to a grounding tab fastened to the chassis
-Chassis star grounding tab connected to purple leads (shields) from power transformers
-Chassis star grounding tab to circuit board ground
-Circuit board ground to gain/volume control ground tabs
-Circuit board ground from only one shielded output cable to the jack field ground bus - only ground connection point for bus
-For each input jack cable, shield is connected to jack field bus but other end of cable shield is not connected - ground is extended along cable through shield from jack field bus


By thinking about the ground path through the preamp this way, ground loops and resulting hum issues are eliminated, at least have been for me.

Hope this helps,

Bill

Roy sent me this wonderful color drawling of his wiring plan when I was building my SP13 / PH12 combo. I found this to be very helpful. It made it so clear and easy to understand just where the grounds get connected and where they are left unconnected. It wasn't included with the original kit for some reason so he emailed it later. Thanks for putting this together Roy! I hope you make it standard issue now.

thanks guys,
I've gotten maybe 3 or 4 builders out of around nearly 500 that have had any noise problems,
but of course most people on the forum only  hear about the noise problems, not the FANTASTIC, DEAD QUIET builds !!