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    Grounded Power Cord

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    Grounded Power Cord

    Post by Guest on Fri May 20, 2011 5:05 pm

    Hi

    I've been doing some repairs on my ST-70 and I was thinking about changing the power cords on the ST-70 and my PAS-3 to the grounded type. Will this cause any problems? They will be plugged into the same grounded outlet in the house so I don't think I would have any issues with ground loops.

    Chris

    Bob Latino
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    Re: Grounded Power Cord

    Post by Bob Latino on Fri May 20, 2011 5:17 pm

    Hi Chris,

    As long as you don't have another component with a 3 wire cord in your music system you should be OK. If you do have a 3 wire cord on another component you could set up a "ground loop" and pick up some hum. If this does happen just float the ground on either component with a 3 to 2 "cheater plug" and the hum should go away.

    Bob

    stevenally

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    Grounded line cord on a ST 70

    Post by stevenally on Thu Dec 26, 2013 9:40 am

    I am nearly done rebuilding my ST 70. I am replacing the original line cord with a IEC socket and 3 wire cord. My question is should the neutral go to the fuse or the switch? Common sense tells me to put the neutral to the switch (the part I will be handling most often) but I just want to make sure.

    arledgsc

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    Re: Grounded Power Cord

    Post by arledgsc on Thu Dec 26, 2013 12:48 pm

    "My question is should the neutral go to the fuse or the switch?"
    Fuse the Hot lead immediately out of the IEC jack.  The other side of the fuse goes to the switch.  Neutral connects directly transformer.  Theory being in N. America if your house is wired properly Neutral is near ground potential since it is grounded at power entry to the building or main fuse box.  This is the way UL likes to see chassis wired for best safety concerns.  

    Here's a schematic that illustrates the wiring (lower right).  "L" is the Hot, Live, or Line depending up where you live.  "E" is Chassis GND or Earth.
    https://www.keepandshare.com/doc/1782400/jtm50-schematic-custom-spec-a15-1-76k?da=y&dnad=y

    GP49

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    Re: Grounded Power Cord

    Post by GP49 on Thu Dec 26, 2013 1:02 pm

    You are right. For safety, FUSE THE HOT SIDE, switch the neutral.

    Some people will point to manufacturers having fused the neutral, but for the most part that was back in the day prior to three-wire power cords, and many were GUITAR AMPLIFIERS with nothing on the outside that would be touched by a user.

    Also you may find hum from ground loop problems, once you replace the two-wire power cord on a Dynaco with a three-wire cord grounding the chassis.


    sKiZo

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    Re: Grounded Power Cord

    Post by sKiZo on Thu Dec 26, 2013 8:17 pm

    So, what you're saying is, if I'm not getting tossed across the room whenever I touch the metal chassis, I got it right?  What a Face 

    Still a little bummed I let yall talk me out of the "floating" ground plate I was gonna add to my ST120 build. Now I'll never know if it would have worked.



    On the bright side, it does make a nice back scratcher ...

    Avinunca

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    Re: Grounded Power Cord

    Post by Avinunca on Fri Dec 27, 2013 9:36 am

    GP49 wrote:You are right.  For safety, FUSE THE HOT SIDE, switch the neutral.

    Some people will point to manufacturers having fused the neutral, but for the most part that was back in the day prior to three-wire power cords, and many were GUITAR AMPLIFIERS with nothing on the outside that would be touched by a user.

    Also you may find hum from ground loop problems, once you replace the two-wire power cord on a Dynaco with a three-wire cord grounding the chassis.


    You seem to be saying the OP should not switch the live and to switch the neutral instead.
    I would never ever use anything in a metal chassis without a safety earth and I would  switch both live and neutral with the live passing through a fuse first.

    Typical fault scenarios come to mind - neutral and live being reversed in the house wiring or some extension lead. Live touching a chassis part in the amplifier - you become the earth when you touch it.

    I would urge you to follow the advice of arledgsc but consider a double pole switch for live and neutral.

    arledgsc

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    Re: Grounded Power Cord

    Post by arledgsc on Fri Dec 27, 2013 12:09 pm

    "Typical fault scenarios come to mind - neutral and live being reversed in the house wiring or some extension lead"
    That is a very good point.  My garage was wired reversed until I corrected the wall sockets.  With Neutral only switched or a reversed line even when the amp is turn off you still have voltage on the power transformer input winding.  Any leakage at all may pose a shock hazard.  So switch both if unsure of your wiring or for maximum safety.

    An ungrounded chassis is actually "grounded" in use by the audio input cabling.   So any potential difference between two chassis causes current to flow in the ground leads of the audio cable.  I burned up an oscilloscope ground lead once working on an ungrounded chassis.  The AC grounded scope and unit under test were on different phases of the AC line and perhaps an air conditioner kicked on and shifted the ground potential of one of the phases.  Poof!  So if you use an ungrounded chassis keep all equipment on the same wall plug.  I never have any noise issues with my grounded chassis ST-120 and I have a noisy AC environment - CFLs, light dimmers, Insteon automation, wireless Ethernet, solar panel inverter, etc.

    PeterCapo

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    Re: Grounded Power Cord

    Post by PeterCapo on Fri Dec 27, 2013 12:51 pm

    I actually get a lot of hum from my St-70 unless I lift its chassis ground, and I also have to lift the ground on the preamp.  Lifting both grounds eliminates the hum entirely.  Thinking out loud here... I wonder if this speaks to an issue with the wiring in my St-70?  I have the power wired as described here by arledgsc...  Everything except the TV is on the same power strip...  But, I also sometimes use Nordost interconnects, which do not have a ground shield...  I wonder if removing the 10-ohm resistor in the St-70 would help, as I would like to keep the safety ground in use all the time...

    peterh

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    Re: Grounded Power Cord

    Post by peterh on Fri Dec 27, 2013 1:20 pm

    I used an IEC well with built-in fuses, one on each lead. This saves panel space, i could
    mill the hole for the IEC right over the existing fuse.

    As regards to hum , it is more quiet when all stuff that has a ground inlet is grounded. My thorens
    do not, i don't care.

    arledgsc

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    Re: Grounded Power Cord

    Post by arledgsc on Fri Dec 27, 2013 3:36 pm

    " I wonder if removing the 10-ohm resistor in the St-70 would help, as I would like to keep the safety ground in use all the time..."
    That was common practice 15-20 years ago to isolate the chassis ground from the signal/ power grounds by a small value resistor.  Sometimes a ferrite bead was used in place of the resistor.  This causes a lot of EMI headaches and the practice nearly abandoned for a central or star ground scheme.   The ST-120 does this nicely with all the signal/power grounds tied together near the quad cap.  The 10 ohm resistor could be lifting the chassis potential so when three 3-prong plug is used you have a voltage difference between the amp and preamp.  And when there is a voltage difference you have current (on the interconnects).  I never liked the resistor isolation method as if it ever fried and failed you lost your ground protection.

    PeterCapo

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    Re: Grounded Power Cord

    Post by PeterCapo on Fri Dec 27, 2013 3:54 pm

    Thanks. I will put it on my list of things to do.

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