6ba5438a   
   From: stoatgobbler@minasmorgul.demon.co.uk   
      
   emailaddress@insightbb.com wrote:   
   > On Feb 21, 7:12 am, Stoatgobbler   
   >  wrote:   
   >> Hi there,   
   >>   
   >> I'm building a JL Hood hifi amplifier and to construct the power   
   >> supplies I've mounted two toroidal transformers in a case, one on top of   
   >> the other. Each toroid has two secondary windings which I'm going to use   
   >> to create two separate, independent, regulated +22v - 0 - -22v power   
   >> supplies. To make these power supplies truly independent (and to reduce   
   >> the chances of earth loop hum) I've "lifted" each power supply 0v common   
   >> from the chassis earth via 60ohm resistors.   
   >>   
   >> On testing the power supplies I've noticed that the greater the current   
   >> I pull out of each power supply the greater the potential difference   
   >> that appears across the 60ohm resistor (chassis earth to power supply 0v   
   >> line). when I pull 1.1A from the power supply then I have nearly 17v   
   >> across the 60ohm resistor. This voltage does not seem to vary with   
   >> raising the upper toroidal away from the lower so I don't think the size   
   >> of this voltage is to do with interactions between the two (but would   
   >> stand to be corrected).   
   >>   
   >> I've never "played" with this, I did expect a voltage across it (due to   
   >> eddy currents within the transformer normally "lost" in heating the iron   
   >> core in a conventional E I type transformer?) but I thought toroidals   
   >> minimised this and the voltages resulting from this would be   
   >> nominal....17v across 60ohm is not nominal.   
   >>   
   >> Is this normal or is something going on?   
   >>   
   >> Any advice or comments would be appreciated.   
   >>   
   >> Stoat   
   >   
   > The part you're overlooking is these transformers have primary   
   > connected to neutral, but secondaries are floating (until you connect   
   > them to ground with the resistor).   
   >   
   > That means it is merely coincidence if you measure 0V difference   
   > relative to ground when the circuit isn't powered, as there's simply   
   > nothing generating a charge to make it different than ground.  Once   
   > you put current through the circuit, the return is through the ground,   
   > or center of the secondaries.  It's normal, as it's carrying current.   
   >   
   > Now what you might want to do is remove that 60 ohm resistor and use a   
   > more conventional grounding arrangement with a smaller value resistor   
   > and a capacitor.  Ground the chassis directly to the ground wire from   
   > your outlet through the power cord, assuming grounded outlets and 3   
   > wire cord which it should be as a metal chassis is being used.  Use a   
   > ground loop-breaker config between amp ground and chassis ground, that   
   > being a roughly 5-10 Ohm 5W resistor in parallel with a 0.1uF XY   
   > rated, generally about 250V /110VAC line or ~630V/220VAC line   
   > capacitor between amp ground and chassis.  The point at which this   
   > resistor and capacitor are taken from amp ground to the chassis should   
   > be a connection made in the middle of the star grounding system we'll   
   > assume you are using.   
      
      
   Many thanks for this, you refer to this as a more conventional   
   methodology though this is a new idea to  me (putting a capacitor into   
   the ground lift). Would you mind pointing me in the direction of any   
   literature on this please, I'd like to read up on it.   
      
   Many thanks.   
      
   Stoat   
      
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