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Old 1st March 2011, 08:11 PM   #11
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I have a couple of 600V 10X probes.

With inputs shorted:


At C5 ripple is 250mv PP (around 88V RMS) @ 120Hz. C5 is a 180u Pana TS-UP bypassed with a 100u/370VAC motor run. Disconnecting the 100uf motor run yields 350mv PP at C5.

At C7 ripple is 60mv PP (around 21V RMS)....freq reading on my Dig scope was all over the place.

Last edited by boywonder; 1st March 2011 at 08:21 PM.
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Old 17th March 2011, 03:56 AM   #12
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Default It wasn't filament hum at all......

I finally figured out the problem and it's fixed. I'm running a 9 conductor umbilical between the power transformer chassis and the amp chassis. The safety ground from the power entry module in the power transformer chassis is tied to a ground stud in that chassis, then on to the amp chassis to another ground stud which serves as the star ground for pretty much everything in the amp chassis.

Initially, to save on umbilical conductors, the power transformer CT was tied to the ground stud in the transformer chassis, resulting in the safety ground and signal ground sharing the same conductor in the umbilical (apparently not a good idea).

I ended up with one extra umbilical conductor (not used) in the initial build, so I used this conductor as a dedicated line to bring the power transformer CT up to the amp chassis ground stud.

Now with the inputs shorted I'm measuring about 1.5mv rms at the speaker terminals and the hum is barely audible on my 95db sens speakers with my ear right next to them.

And finally..one correction to my prior post: The C5 and C7 ripple rms values should be 88mv and 21mv, not 88V and 21V.
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Old 17th March 2011, 08:08 AM   #13
Ian444 is offline Ian444  Australia
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Ha! Well done! Good find!
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Old 17th March 2011, 06:43 PM   #14
rknize is offline rknize  United States
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Glad you figured it out. Ground loops are a pain. Just to make life easier, I've been using Pete Millett's "ground loop breaker" trick in my new amps. Makes my setup much less finicky about what is plugged into what and which outlet that are plugged into, etc.
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