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From time to time...

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Posted on October 23, 2016 at 11:51:40
I listen to this 2010 project.

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Wood box construction, posted on October 23, 2016 at 12:01:52
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RE: From time to time..., posted on October 23, 2016 at 12:21:08
Chip647
Audiophile

Posts: 2633
Location: The South
Joined: December 24, 2012
Put the link in the "Optional Link URL" box below

 

That's very dangerous, posted on October 24, 2016 at 11:33:33
Triode_Kingdom
Audiophile

Posts: 10012
Location: Central Texas
Joined: September 24, 2006
It's extremely hazardous to build high voltage power supplies and amps without a power transformer. No one should be doing this.


 

RE: That's very dangerous, posted on October 24, 2016 at 14:52:28
cpotl
Audiophile

Posts: 1002
Location: Texas
Joined: December 6, 2009
"It's extremely hazardous to build high voltage power supplies and amps without a power transformer. No one should be doing this."

Hi TK. Normally, I find myself on the same side of the argument as you, but over this one, I do feel a need to disagree. There is a long and venerable tradition of direct mains connected radios and TVs. The classic AA5 radios were ubiquitous for a long period, and then direct mains-connected TVs were similarly, until the demise of the vacuum tube television. I grew up in England in the 1960s, playing around with direct mains-connected TVs, dismantling them and repairing them, from age about 10.

A couple of years ago I came across a paper by Dickie and Makovski, dating from 1954, describing an OTL amplifier with no transformers at all; direct mains connected, with the heaters connected to mains, in a string like Christmas lights, and the HT+ and HT- derived directly from the mains. I thought about it for a while, and decided I'd make a modern day copy of that amplifier. I thought about the safety aspects, and figured out a way of making it reasonably safe. The chassis is connected to 3-prong socket ground. The signal ground is (as it has to be) neutral. I use massive 35 amp back-to-back (anti-parallel) rectifiers to ensure signal ground can never deviate from true ground by more than +/- 1.5V. If it were ever plugged into one of the dreaded reverse-wired sockets, the breaker at the fusebox would blow instantly.

I think there is a tendency to exaggerate the dangers of direct mains connection. Especially in the US, the mains voltage is actually quite low, certainly in comparison to the secondary side of a power transformer for a typical vacuum tube amplifier. It is true the maximum current available from the secondary of an HT transformer is a lot less than that from the direct mains supply, but the available current is still way more than the lethal threshold.

I maintain that with care, and with thoughtful attention to possible fault modes, it is not that dangerous to make direct mains connected gear.

I would be interested in a realistic discussion of hazards with a design such as I have built, rather than just blanket assertions that it is dangerous and one should not do it.

Chris

 

+1, posted on October 24, 2016 at 15:06:12
richardl
Audiophile

Posts: 3555
Joined: September 5, 2002
x

 

RE: That's very dangerous, posted on October 25, 2016 at 08:46:35
Triode_Kingdom
Audiophile

Posts: 10012
Location: Central Texas
Joined: September 24, 2006
This is not at all like AA5 radios or directly powered TVs. Please don't make that comparison.

"If it were ever plugged into one of the dreaded reverse-wired sockets, the breaker at the fusebox would blow instantly."

Unless the neutral wire is old and corroded and maybe attached with a sheetmetal screw that may or may not be well-secured, and which wasn't itself designed to conduct electrical current. None of what you've done qualifies as safe practices. You might go years or even a lifetime without an accident (and I hope that's the case), but that doesn't mean this is a reasonable thing to do.



 

RE: That's very dangerous, posted on October 25, 2016 at 13:42:29
cpotl
Audiophile

Posts: 1002
Location: Texas
Joined: December 6, 2009
Actually, I was being a little flippant when I spoke of the back-to-back rectifiers saving the day if the amplifier were plugged into a reverse-wired socket. Yes, they almost certainly would, but of course actually I would never depend on that as the way of checking that the socket was not mis-wired. In fact, I checked, double checked and triple checked that the socket was wired correctly before I plugged the amplifier in. I have full control of the amplifier, where it is plugged in, and into which socket, in my own house, and there would be nobody else touching it or messing with it.

So for the chassis or audio ground to become live it would require two simultaneous major independent failures (neutral becoming disconnected and also ground becoming disconnected).

One could compare this with some conceivable failure modes in a more typical amplifier configuration. A common set-up involves keeping signal ground separate from true chassis ground to avoid hum loops, with, say, a 100 ohm resistor between signal and true ground. Suppose now that an insulation breakdown occurred in the main HT transformer in the amplifier. This could lead to direct mains voltage being connected through to the signal ground. The puny 100 ohm resistor would be of no use at all in preventing the signal ground from being brought up to full mains AC voltage.

Or suppose a CD player with a cheapo imported all-voltage wall-wart power supply is plugged into the amplifier. If the insulation broke down in the HF transformer in the wall-wart, again the signal ground could be brought up to full mains voltage. Does one have so much faith in the quality of construction of cheap imports?

(Personally, in my transformered amplifiers I always use big back-to-back rectifiers to ensure that signal ground cannot deviate from true ground by more than 1.5 volts or so, for precisely such reasons. But not everyone does.)

All of the above scenarios are highly improbable failure modes, and any one of them could lead to mains voltage appearing where it could be a potential hazard.

It does not seem to me that the level of hazard in a sensibly operated direct mains device is totally off scale in comparison to the other examples.

Chris

 

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