John Curl's Blowtorch preamplifier part II - Page 2359 - diyAudio
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Old 31st May 2012, 05:06 AM   #23581
1audio is online now 1audio  United States
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Originally Posted by Wavebourn View Post
Which Medical version do you mean? I have not seen filters without that Y-caps, so I never used them.
Read the datasheet. The B version is without Y caps. The medical leakage requirements are in line with good audio. . .
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Old 31st May 2012, 05:50 AM   #23582
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Originally Posted by 1audio View Post
Read the datasheet. The B version is without Y caps. The medical leakage requirements are in line with good audio. . .
Thanks!
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Old 31st May 2012, 02:00 PM   #23583
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I prefer power transformers with internal shields between primaries and secondaries.
And just what do you do with this shield? I take it you mean an insulated conductive foil (so as not to act as a shorted turn) with a drain lead attached.

The first issue is to identify on the primary which lead goes to the inside of the coil and be sure that is connected to the neutral. (The neutral is actually grounded in the U.S.) This means that you have minimum coupling of the AC line directly to the metal of the transformer core and through it's mounting into your chassis.

Now when you have a transformer where the secondary is wound on top of the primary you will have the "Hot" lead coupling better into the secondary! Placing a shield here is the old method of reducing the issue. However if you connect the shield to the common or ground then you are coupling the noise into your circuit. If you connect the shield to the safety ground wire, then you have to float the chassis or ground. But any noise or RFI that comes in on the ground line will have greater coupling into the secondary. So you basically are trading hum for RFI!

If you connect the shield to the neutral you violate code and will have real fireworks if it ever gets plugged into a incorrectly wired outlet.

Now with a split bobbin you have a bit of capacitance from primary to secondary. However as the dual secondary is wound at the same time you get a differential mode that cancels much of the coupled noise!
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Old 31st May 2012, 03:27 PM   #23584
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Hi Ed;

connecting something means creating path for current. All depends on which path, and is this path common also for small signals, and how big is voltage drop created by this current in respect to signal voltage. And how they are coupled by fields. Search for your optimum. I search for mine.

Anatoliy
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Old 31st May 2012, 03:35 PM   #23585
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Well said, Ed Simon. Where, oh where, do we connect the shield wire? A fundamental and often 'fatal' question.
Bonsai, we normally use more 'modern' transformers, today, especially when the transformer is located near the audio electronics. In the CTC Blowtorch case, we designed the power supply chassis to be located away from the audio, so we could use these more traditional transformers, especially designed to be low capacitance between the input and output windings. Parasound preamps use a R core (I'm pretty sure) that works pretty well, and has less stray field.
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Old 31st May 2012, 04:55 PM   #23586
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Test and measure, guys. Words tend to be empty
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Old 31st May 2012, 05:04 PM   #23587
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Test and measure, guys.
I prefer to add "Think" to this loop.
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Old 31st May 2012, 05:32 PM   #23588
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Originally Posted by john curl View Post
Well said, Ed Simon. Where, oh where, do we connect the shield wire?
Scott's magic finger?

Or otherwise, it's a thing I suggested before: why not have one safety ground also used for shielding, and a separate floating signal ground? Following Class II requirements for the innards, there should not be any safety issues.

Wouldn't work with RCA cables, but XLR with one hot and one signal earth wire would. No pin 1 problem possible, since signal earth and shielding earth have no connection.

vac
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Old 31st May 2012, 05:36 PM   #23589
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With XLR I don't need even to think about anything floating except balanced signal wires.
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Old 31st May 2012, 05:48 PM   #23590
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I prefer to add LISTEN to this group. '-)
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