And now a question about power switch...

[mr mojo]

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

[alexistheo]

i think to be extra safe you might put the fuse between hot and switch. i use a spst swich and leave one leg of the power tx primary connected to neutral. you do have the chassis connected to earth (3rd prong) right?

[mr mojo]

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

[SY]

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).

[Butterylicious]

Sy makes a very valid point. I am a licensed master electrician and I've seen some bad suff happen because of switched neutral conductors. I would lean towards a SPST switch for power, leaving the neutral hard wired.

[Butterylicious]

Furthermore, if you are using a remote power supply for high B+ circuits, you should design your secondary power cable with an extra pair of conductors so that if the cable get disconnected, the power supply shuts down. There's an example of this in my WE 121A thread.

[mr mojo]

Hey buttery,

Thanks for the info-much appreciated. I guess I should make the "switch" 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

[Butterylicious]

Look at this boiler plate schematic

This can safely be called an "American Standard". As a professional tradesman, I've often wondered about European electrical codes.

[mr mojo]

Thanks buttery, that answers it-basically hard-wiring one of the power xfrmr mains(unfused) and power cord neutral, then wiring the fused power xfrmr mains in series with power cord hot and the switch-right?

best,
mr mojo

[EC8010]

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!