persistent 60 cycle hum only on tube gear
I know this topic has been addressed in the past but I'm hoping for the "latest tech" answer.
I have a persistent 60 cycle hum in my ARC tube gear. Not in my Bryston power amps or preamps- just the ARC tube pre and power amps. All XLR.
I have installed an isolated, dedicated ground system (8' copper rod driven into moist earth) , a Ground Master unit between the chassis and the ground line, I clipped the ground wire from my 20a 120v dedicated circuit, pretended to ignore the hum (that didn't work well). I even replaced the tube sets with ARC OEM tubes in the pre and power amps (sonic improvement but no hum cure) . Still the confounded hum.
Before I spend more money and failing I'd like your personal experience opinion on what worked for you.
Thanks!
@yesiam_a_pirate Does the amp make a buzz or hum without the preamp connected? If yes, Is it both channels or just one? If neither channel, The amp is OK. If in one channel only, you may have a bad (shorted) tube in that amp. Please get back to me and we can proceed.
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As you've requested, I've turned my explanations into a blog post, with better explanations about the difference between running an isolated ground wire from the panel and creating a new grounding system. I even had AI create a graphic to help with the illustration. Enjoy! |
Here are the schematic diagrams for the ARC VT130 amp and the ARC SP-15 preamp. https://www.arcdb.ws/model/VT130 (Click on VT130 schematic)
Note: It appears the circuit signal ground is connected solidly, directly, to the EGC grounded chassis. (Not good). Therein, signal ground is not connected, (for example), to a 10 ohm resistor then to the grounded chassis. B- of the DC power supplies is connected to a 10 ohm resistor (R88) then to the EGC grounded chassis. (Good design). (EGC = Equipment Grounding Conductor) . https://www.arcdb.ws/Database/SP15/ARC_SP15_schematic_and_parts_list.pdf Note the capacitors on the AC mains line input. One from Hot to EGC ground. One from the Neutral to EGC ground. Could those be leaking AC current? Especially the one connected to the Hot conductor? Note: The signal ground on the phono input runs through a couple of caps in parallel and then connect to the EGC grounded chassis. There is another cap used on the same signal ground to EGC grounded chassis by RCA line inputs. What if there is a small ac voltage present on the grounded chassis. Wouldn’t the caps pass AC current to the signal ground. Would that cause a 60Hz hum? FYI The Branch circuit EGC is lifted at the wall outlet. Also worth noting who knows what's coming in on the wire from the ground rod outdoors. There could be a difference of potential, voltage, between the IG rod and the AC mains neutral conductor. (There are stay AC voltages all over the place in the earth) . |
Erik, Your exlanation is one of the best I’ve seen, and you are performing a great service to the community. On my power cable page I warn people about defeating the ground on any power cables in their system. I’ve yet to cover isoalted grounds. In any case, I’m putting a link to your blog post on that page. Cheers, Thom @ Galibier Design |
