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Old 9th July 2011, 02:23 PM   #1
dobias is offline dobias  United States
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Default AC/DC umbilical cord

Will AC imprint on a DC wire in a 7 wire umbilical cable from a PS to the amp? Should I locate the rectifier in the amp chassis?
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Old 9th July 2011, 03:35 PM   #2
AndrewT is offline AndrewT  Scotland
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If you locate the rectifier in the amp chassis, then you will bring AC to the amp chassis along the umbilical.

If you locate the rectifier in the transformer chassis and locate the smoothing bank in the amp chassis then you bring pulsed charging currents into the amp chassis. These pulsed charging currents peak at ~10times the average AC current around the charging loop.
The average secondary AC current is many times the average Primary AC current, for a 230:35Vac+35Vac transformer the ratio of secondary to primary average currents is ~ 3.3Times.

For the above reasons moving the transformer out of the amp chassis bring very small if any gains in noise reduction on the low voltage supply lines.

To gain that benefit, the whole PSU must be remotely located and the separated system but be designed, firstly to be Safe and secondly to gain that electrical benefit.
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Old 9th July 2011, 09:21 PM   #3
DF96 is offline DF96  England
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You could put the reservoir cap with the rectifiers and transformer, then put the smoothing cap with the amp. At least then you only have ripple in the umbilical, not charging pulses.
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Old 10th July 2011, 02:14 AM   #4
dobias is offline dobias  United States
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DF96,
I'm considering a CLCLC filter with the 6.3v transformer, rectifier, (C-1) 10,000uF cap & an (L1) 2mH choke in the PS chassis.The amp chassis would have a (C2) 10,000uF cap, an (L2) 2mH choke & a (C3) 10,000uF cap.
Frank
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Old 10th July 2011, 03:39 PM   #5
DF96 is offline DF96  England
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You won't get much smoothing from 2mH. Cutoff will be 35Hz, and PSU impedance will be low so there is a risk of ringing too. There is more to PSU design than slapping in the biggest cap you can find/afford.
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Old 10th July 2011, 06:08 PM   #6
dobias is offline dobias  United States
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At the risk of showing my ignorance (again), would it be worthwhile to use AC on the 6SN7/7N7 gain tubes? I gather that the 6SN7/7N7 drivers don't benefit from DC,anyhow.
I would follow the best practices of twisting the wires in the PS & the amp chassis.
Would it also be beneficial to twist & braid the umbilical wires?
I'm thinking the worst that would happen would be to add proper rectification & filtration later on if the AC doesn't work out. Is that a reasonable approach?
Frank
PS: 2mH chokes are cancelled. The 10,000uF caps are original to the Moondog build.
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Old 17th July 2011, 06:12 PM   #7
dobias is offline dobias  United States
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Everything I've read told me to leave the filament & heater trans. in the amp chassis.
I did cut down on the congestion by using one 6.3v trans., rectifier & DC filter for both gain tubes. The drivers are on AC. I'm down to 0.3mV of hum, so far.
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