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Toroid Wiring Question

geode

geode

F4 Phantom Seat Jockey
Subscriber
My tube preamp SP14 has two toroid transformers in it. It has an IEC connector that joins chassis and earth grounds together. The build calls for attaching the purple shield wires to the chassis ground, but I have seen pics of the same kit build that connects the shield straight to the earth ground lug on the IEC connector. What would be the preferred way to wire the toroid, does it make a difference since both chassis and earth are connected internally in the IEC. Thanks.
 
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Chassis is fine - this wire carries very little current, so won't be a noise source. Any current on the shield comes from capacitance between the primary winding and the shield, perhaps a few hundred pF at most. It should be a solid connection though, as it's the ground return path if the transformer were to short.

Where you need to be careful about grounding is there there's a high AC current. So the first filter cap after a rectifier should always be connected directly to the rectifier minus or transformer center-tap. Don't let that ripple current flow in the chassis! If you use a busbar ground, connect components to it in the right order, ground it at one point. In a power amp, it's usually OK to ground anywhere if you get the power supply right. In a preamp, can be trickier.
 
generally the IEC ground is going to tie to chassis with a fairly short, heavy wire too so there isn't much practical difference.
 
Thanks Tom and Gadget. I'm trying to eliminate a hum in my preamp. I thought I had the hum down to where it is barely audible, until I changed up the room furniture. I had a very stuffed futon about 5 feet from the speakers. After moving the futon back to about 10 feet from the speakers the hum is much more audible. I can now hear it from the listening position. Pretty sure it is a ground loop hum 60 Hz and doesn't change with volume control. Good grief :confused:
 
With the Khozmo attenuators there's an eyelet for grounding the case to the PCB. Areb the EIZZ grounded to the case?

At this point it sounds like you have a ground loop between the preamp and amp. What's the amp, and have you tried playing with lifting the AC ground on one unit or the other?
 
zackthedog said:
With the Khozmo attenuators there's an eyelet for grounding the case to the PCB. Areb the EIZZ grounded to the case?

At this point it sounds like you have a ground loop between the preamp and amp. What's the amp, and have you tried playing with lifting the AC ground on one unit or the other?
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Yeah, I think ground loop too. The Eizz case is chassis grounded. The amp is VTA ST120 which is dead quiet. The pre has a 3 prong plug, the amp has a polarized two prong plug. Nothing but pre and amp connected there is hum. Tried different interconnects, bought a Furman power conditioner, nothing stops the hum. Also used cheater plug on preamp and ran extension cord to different circuit. Still hums.
 
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Are you sure you have the hot and neutral side connected properly at the IEC socket and with the polarized plug power cord on the ST-70? Maybe one is backwards?
 
Lavane said:
Are you sure you have the hot and neutral side connected properly at the IEC socket and with the polarized plug power cord on the ST-70? Maybe one is backwards?
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The IEC in the preamp is correct. The small blade of the plug (Hot) connects in the right position of the outlet and then to the fuse. The ST120 I'm not sure. I will need to open it up and see which way the builder wired it up. I can say the ST120 is dead quiet, no hum or hiss.
 
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