Re: really dumb question re direct line powering and safety

Naah, this is not a dumb question – in fact, this is one of the
coolest posts on this newsgroup for weeks.

I don’t see why your idea would not work but I admit that it does seem
overly complex and presents quite a many drawbacks and issues that
would have to be solved. For instance, there would be a need for an
audio transformer per each jack so features such as FX loop and DI
outputs would become more complex and expensive to add. >Good< audio
isolation transformers are not cheap either. On top of that, the BIG
important point is: The transformers would have to be ones that can
handle the task of isolating very high voltage. The way I see it, most
of those things are only designed for simple ground loop interruption -
type tasks and are not up to the requirements of mains isolation. As a
side note, an alternative for transformers might be the use of
optocouplers.

Since the circuit would lack the step-up and step-down conversions
provided by a power transformer, in order to get higher power the (in
tube amp scale) low mains voltage should be pumped up with some
cascade circuits. Not difficult but much more complex than just having
the right voltage to begin with. You would also need step-down
circuits to derive bias voltages and stuff like that.

Then, the whole chassis construction and all conductive parts would
have to be double insulated. This means that you would likely have to
encase the amp in a plastic cabinet or something and take great care
that cases of potentiometers, switches and those sorts of things were
either isolated properly or cannot become in touch with the user of
the amp. Compared to convenient construction that can use the exposed
and safety grounded metal chassis as the mounting platform this kind
of thing would be quite arduous thing to build.


By the way, plenty of guitar amps, that were powered from the mains
without isolation, actually were built. Sure, it's damn dangerous but
they did that anyway. In 1930's - 1940's these kinds of devices were
quite common but what is most amazing is that the practice continued
even to late 1960's. For example, Teisco introduced an output
transformer coupled SS amp that was powered directly from the mains.
Plenty of cheap Silvertones and alike tube amps out there as well...
It seems to me that those cheap designs are always the ones with least
contribution to adequate safety. That's just stupid. The way I see it,
a cost of one transformer (be that cost either weight, money or both)
doesn't seem so high anymore after you electrocute someone or
yourself. I wouldn't neccessarily go with your idea, but nevertheless
it is an interesting one. Are you familiar with Viking amps? I wonder
what's up with those since they lack the power transformer as well.
.

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