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 Post subject: Dynaco ST70 -- power supply design question
PostPosted: Nov Wed 15, 2006 2:45 am 
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Location: Illinois
I built a ST70 clone kit from Amplitube. It follows the original Dynaco design, and it works well.

The original Dynaco design had an ungrounded, non-polarized line cord. The fuse was on one 120-V leg, and the power switch on the other leg. Same with my clone kit.

Various people seem to feel that the chassis should not be grounded to earth through a 3-prong power plug to avoid ground loops with the pre-amp, etc.

Questions:

Should I install a polarized line cord, and put both the fuse and power switch on the hot leg, or should I leave it as the original Dynaco design?

Should I tie the chassis to ground via a 3-prong power plug?


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PostPosted: Nov Wed 15, 2006 3:13 am 
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Location: Rochester NY USA
I would use a 3-wire cord - I'd even consider putting in an IEC connector for a removable cord. And once it's polarized properly, the fuse and switch would go in the hot lead.

Hmmmm... I wonder if the switched outlets on my preamp are 3-wire... that could affect what I do with the next power amp.


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PostPosted: Nov Wed 15, 2006 10:20 am 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Albuquerque, NM 87123
I wouldn't use a grounded cord; it could cause a hum problem from ground loops. Besides, it is extremely rare for a transformer to short the primary to chassis ground and become a safety hazard.
Also, the switch and fuse should ALWAYS be on the hot side.


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PostPosted: Nov Wed 15, 2006 3:35 pm 
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Location: Detroit, MI USA
I have run into several installations where the power cord on the amplifier had to be plugged in a certain way to minimize hum, therefore the idea of a polarized plug is not a good one for this application. And have seen them where the separate ground lead provided on some turntables had to be grounded, possibly the only ground connection in the entire system.

IF you can figure out which polarity the line cord needs to be, and where/whether to ground other parts of the system to minimize the ground loop hum effect, then it might be possible to configure everything accordingly and use a 3-wire cord.

_________________
Dennis

Experience is what you gain when the results aren't what you were expecting.


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