Switch on transformer secondaries

[Axtman]

....what a dumb idea!

I'm currently working on a GK 200MK keyboard amp.

I was initially puzzled when I got the amp. I plugged it into my dim bulb current limiter and was confused why the buld was dimmly lite even though the amp was "off". I also heard faint humming when the amp was "off".

I opened it up and found that the line goes right into the primary windings of the transformer (mounted on the back wall of the amp. Luckily they put a fuse in there. The secondaries then run up to the pcb where the on/off switch is located.

The only advantage (I guess) is that a technician can work on a live pcb board without fear of getting shocked.

[mozz]

Lower voltage switch = cheaper?

[Axtman]

This reminds me of the old tube TV sets with "Instant On". Basically when you plugged the TV into the wall the tube filaments would start glowing and remain glowing until you unplugged the TV. The "On / Off" switch would just turn on the B+. This is like always having your amp on standby. Those TV did have a tendency to wear out components.

I imagine nowadays Energy Star would not allow equipment to be always on. BTW, I check and it consumes 6.6 watts while "off". Also UL would probably not allow an all metal cabinet amplifier without some means of insulation. But then what do I know? I am just a simple hobbyist.

[Helmholtz]

BTW, I check and it consumes 6.6 watts while "off"

Did you use an active-power meter?

[Axtman]

Yes I used a P3 Kill-A-Watt meter to measure power consumption.

http://www.p3international.com/products/p4400.html

[Axtman]

Ladies and gentlemen of the jury I would like to present to you Exhibit A.

[J M Fahey]

Just a couple days ago I was checking an amp schematic which not only does not switch Mains, it does not even switch lower voltage (but higher current) Secondary!!!!!!

A mosquito sized switch only mutes TDA7294 power amp ON-OFF

Most stupid idea ever, and can (literally) burn a Tech.

Wouldn´t be surprised at your switch doing something similar.

[Greg Robinson]

I once repaired a Crown? (if I remember right) ~2kW power amp that had an MOV strapped straight across the IEC power input, before the power switch. So might've only gotten used for 6 hours a night at the club it was installed in, but that MOV had to suffer every line transient 24hrs a day unless it was unplugged, slowly reducing it's breakdown voltage until it was turned on permanently and vomited its guts all over the inside of the rack chassis. I vaguely remembering finding enough of the side of the exploded disc to identify the part number and discovering it only had something like a 380VDC rating (240VAC here in Australia), so was marginal from the beginning.
The power amp was class G, had a linear supply, a great big toroid and even an electrostatic shield between primary and secondary I seem to remember, so it shouldn't have even needed an MOV.

Anyway, after scrubbing all the soot out of the chassis, I replaced it with something more appropriate and put it AFTER the switch.

[The Dude]

That would be a real PITA if the MOV shorted. You'd could be tripping a mains breaker and not even think that a powered off amp is causing it.

[Greg Robinson]

That would be a real PITA if the MOV shorted. You'd could be tripping a mains breaker and not even think that a powered off amp is causing it.

And that's exactly what happened. Club had quite a panic when they turned up before opening and a circuit was tripping, took them a while to track it down with so much equipment.

[J M Fahey]

In my view switching something OFF must mean exactly that, as drastic as *unplugging* it from Mains.
Nothing less is acceptable.