• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

And now a question about power switch...

mr mojo

Member
2005-06-06 3:58 pm
Iowa
Well I had a friend who is a fellow audio and electronics DIY enthusiast stop by to check my first DIY amp for mistakes, and he thought I wired the power switch wrong so I wanted to get some members opinions.

I'm using an earth ground with a star ground arrangement and a double pole switch. All xfrmr ct and output xfrmr grounds are grounded at the star point as well as the green/ground for power cord.

The white/neutral and black/hot of the power cord go to the bottom terminals of the switch and the power xfrmr mains are going to the top terminals with one of the xfrmr mains fused between switch and xfrmr.

I like a two pole switch because in the unlikely event the switch shorts the leads are physically separated. His point was if there was a short on the power xfmr main that wasn't fused, the chassis could become hot since that mains doesn't share a terminal with the white/neutral of the power cord.

Can anybody shed some light on which way is right and why?

Thanks for any and all help,
mr mojo
 

mr mojo

Member
2005-06-06 3:58 pm
Iowa
Alex,

Yup, green from power cord is grounded to the chassis where it enters and forms the only star ground in the amp.

What you're describing is exactly what my friend recommended, but to my thinking, if the power xfrmr CT is grounded to chassis and one side of the power xfrmr mains is fused, then no matter what happens, a short goes to ground through the chassis, or blows the fuse if the short draws more than 3 or 4 amps-am I right on that?

Best,
mr mojo
 
Getting that solid earth ground on the chassis is 99% of all the safety you could want.

I've seen rather convoluted arguments against double-pole switching. The argument basically is that if the switch fails shorting on the Hot side, the user may think the amp is off, starts poking around, and gets a nice mains-level jolt. If you have a single pole switch on the Hot side, with Neutral unswitched, the same fault will keep the amplifier on, thus warning the user not to poke around without pulling the plug (which should be done routinely, anyway).
 

mr mojo

Member
2005-06-06 3:58 pm
Iowa
Hey buttery,

Thanks for the info-much appreciated. I guess I should make the "switch":D to a SPST switch rather than the DPST I've got now.

So I'd have the black/hot of the power cord and the fused mains lead of the power xfrmr on the separated terminals of the switch and the other side of the mains lead of the power xfrmr and the white/neutral of the power cord hard-wired to the remaining third terminal-have I got this right?

Best,
mr mojo
 

EC8010

Ex-Moderator
2003-01-18 7:57 am
Near London. UK
At the risk of confusing the issue...

In the UK, we have 240V with neutral connected to earth. Provided that it is not possible to swap live and neutral (for instance with one of those horrible "shaver plugs" that CD players often have), an SPST switch in the live wire is all that it needed. The fuse should be the first thing that power entering the chassis sees. The best place for it is integral to the IEC inlet.

My understanding is that the USA and France have 220V centre-tapped to earth entering the house. In the USA, you run your cookers and washing machines off 220V, and because it is centre-tapped to earth, you must have a switch in both legs - double pole switching. You run your Krell Class A amplifiers off 110V, which effectively has a neutral connected to earth, and one live, so single pole switching in the live is perfectly adequate.

In Germany, they have three-phase entering each house!
 
EC8010 is basically correct about US wiring. We operate under NEC (National Electrical Code) guidelines. Sometime in the 60's, they added an article stating that residences shall not have voltages higher than 150V to ground. Some older homes here still have 120/240V 3 phase delta wiring with the 3rd phase being a high leg. This is potentially very dangerous if miswired. In my apprenticeship, I smoked a Coke machine with a freak leg (nick name). Larger office building have 3 phase 277/480V wye power. I could go on & on about wiring.

As for the fuse being in front of the power switch, there are 2 schools of thought. I for one like to have a disconnecting means in front because I always have my paws inside of stuff. I'm not certain what UL specifies. If you do put the fuse in front, and it is a user servicable install (ie twist cap), make 100% certain you get your line and load connections correct, otherwise you can get bit trying to change a good fuse.
 
Mainland Europe?

Interesting thread!

EC8010 wrote:
"My understanding is that the USA and France have 220V centre-tapped to earth entering the house. In the USA, you run your cookers and washing machines off 220V, and because it is centre-tapped to earth, you must have a switch in both legs - double pole switching. You run your Krell Class A amplifiers off 110V, which effectively has a neutral connected to earth, and one live, so single pole switching in the live is perfectly adequate.

In Germany, they have three-phase entering each house!"

I doubt if France has centre-tapped 230V? Anyway, Sweden, and as
I understand all of Europe minus UK, has 230V "single-ended"
house mains (and yes, 380V three-phase for stoves, central
heating, etc.).
But the thing is, there is typically no three-prong, safety-earthed
outlets, except in the kitchen (very new homes excepted)! And the
European mains plugs (barring Britain), whether two- or three-
prong, can be turned either way!
Hence, there is no point in considering one lead "live" and the other
"neutral", as you never know which way the plug will be in the
mains outlet! (Safety earth, if present, always connects up right,
of course, as there are two prongs opposite sides on each plug.)
So the question to those in the know that naturally follows is:
for us in mainland Europe, single or double switch? one fuse or two?
and where do(es) the fuse(s) go??

Morgan
 

EC8010

Ex-Moderator
2003-01-18 7:57 am
Near London. UK
Interesting. If you can't be certain that one leg of the mains is at earth potential (neutral), then you have to treat both legas as potentially live, and that means double-pole switching. The fuse should still be the first thing the mains hits as it enters the equipment, and, following the logic of double-pole switching, you'd really like a fuse in each leg that could be guaranteed to break simultaneously so that a fault broke both legs. That's impossible, of course, but you might be able to find 3A double pole breakers that could be used instead. Why not have a look at the precautions adopted by manufacturers of commercial equipment sold in you country?

Edit: I know there are IEC inlets with double pole fusing, so perhaps they are intended for use in such systems. It would be the only way to protect against a live to chassis short within the equipment...
 
Thank you, EC!

Commercial equipment here always (?) has double-pole switching,
but I've never seen double-fuse IEC connectors (well, for sale, but
never in equipment). Anyway, most hi-fi gear uses fixed two-prong
leads anyway, being nominally Class 2 (double insulated). Every
black-box amp or CDP I've owned, borrowed or seen had a two-
prong cord. And one fuse...
The only advantage I can see is that you can try the mains plug
both ways, to determine which "sounds best" (may affect the
transformer magnetic field, it is claimed). Hopefully (if it matters),
the manufacturers of gear made for US, UK, and maybe other
markets (where polarity is fixed) got the xfmr connection the "right
way"...
Oh, and without the safety earth you never get ground loop
problems! But maybe shocked. :hot:
I have a Micromega CDP that has an ICE inlet and grounded
cord - but the green/yellow safety ground is not connected
to the chassis (in fact, the pin is missing)!!
I had a hum problem from my turntable, which could only be cured
by running a wire from a grounded outlet (kitchen, so wire runs
across the whole flat!). So now my stereo is grounded after all,
but don't tell the SEMCO inspectors! (Not recommended - the
cable TV ground seems to be a completely different potential,
so TV cannot be connected to DVD player without removing the
earth-ground connection. Guess I need an isolation xfmr.)
Anyone from France, Germany, etc, who can tell us if your mains
is similar to Sweden or not?
Sorry if topic is drifting!

Morgan