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Old 10th March 2009, 11:55 PM   #11
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Pilli, another interesting idea and again something to think about.

Bob, I agree about the engineers not worying about the surface. Fortunately my engineer is pretty good about surface protection, however I'm sure he's not as good as I would be if I had the tools

My alluminium plate is going to be sprayed as I'm not a fan of brushed alluminium, but it will be sprayed using the "coat with P45 filler and sand smooth" technique that custom car shops use for getting their bodywork ready for paint. A lot of effort, but it pays off for a top class finish.

I'm actually wondering whether to bother with a switch, especially with your comments about their electrical performance and ability to handle inrush currents. I'm "only" building a chip amp, but the first one I built using a 300VA torroid blows any close coupled rated fuses at switch on, when it really shouldn't.

At the moment I'm using a 4A fuse with a light bulb connected before the switch to act as a soft start circuit, but I know I'm going to have to do some form of soft start circuit when I put my new amp into the casing, otherwise I'll not be able to use close coupled fuses. So the soft start circuit should reduce the impact of inrush currents and allow me just to use the mains plug as my switch.
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Old 11th March 2009, 11:18 AM   #12
Bobken is offline Bobken  United Kingdom
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Hi Westers,

Personally, unless you intend to leave the equipment powered up almost permanently, I would avoid using a mains plug as a switch, as, of course, they are not intended nor constructed for this application.

I have gone to great lengths in my domestic set-up where mains is concerned, with a dedicated system for my audio room, and all outputs are (unswitched) rhodium-plated MK outlets. I used to work in the jewellery trade and this was easy and quite cheap to do, and using switched sockets, even the same quality MK types, are sonically less good, unfortunately.

When plugging in an unswitched piece of equipment (i.e permanently 'on') which will draw current immediately any contact is made, you will quickly discover that the brass pins on the plug become eroded and blackened and this is not desirable in my view. In my case it would ruin the plug & socket's plating, of course, which I once did during a trial when I was developing and testing an unfinished amp design! Regrettably, using toroidal transformers does exacerbate the problem, and a 300VA toroid does have quite a high inrush current as you know. You wouldn't see quite the same problems with a similar sized "E-I" or "R-core" transformer as their inrush characteristics are less severe.

Even with a soft-start circuit, I would still be wary of using the suggested *plug* to make and break any contacts, if it was myself, and I would accept the slight deterioration attendant to using at least a switched mains outlet.

Anyway, I hope it works out well for you.

Regards,
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Old 11th March 2009, 12:52 PM   #13
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Bob

Thanks for the reply.

Yes, I too wondered whether the plug would be suitable for being used as a switch and you've answered that question.

I think the easiest option is to install a switched IEC socket at the back of the amp. It's not pretty, but saves faffing about, plus it's out of sight so the ugliness will be hidden (and it can be easily reached in my simple setup).

I think my next project (a pre-amp) will have a front plate less than 8mm.
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Old 11th March 2009, 01:43 PM   #14
Bobken is offline Bobken  United Kingdom
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Hi Westers,

I think that this is a very sensible move under the circumstances, and, longer term, I am sure you will not regret this slight additional effort.
Keeping all mains contacts very clean (some enthusiasts polish their mains plug pins with Brasso, to good effect!) is important in my experience, and UK brass plug pins will not take any switching abuse without detereoration and potential problems in due course.

Regards,
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Old 11th March 2009, 02:11 PM   #15
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I almost always use EMI filters these days, mostly to keep nasty things on the outside, outside. These modules (Corcom and similar) can be had with switches, plus the line voltage switching card and fuses, and can screw into any panel, unless it s a couple inches thick. Almost any power level is obtainable, though they might be a bit expensive. IMO, if you're doing 8mm thick chassis work, cost is of little concern
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