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Old 12th January 2009, 10:46 PM   #391
Dr.X is offline Dr.X  United Kingdom
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Quote:
Originally posted by westers151
However where is the soft start circuit at DD? I've looked everywhere and can't find it at all.

Thanks

,,,about 3/4's down this page... lightbulb tester
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Old 13th January 2009, 12:57 AM   #392
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You don't need a soft start circuit with GC.

Quote:
from Nuuk's site
So for a stereo chip amp with one power supply I would suggest the transformer is rated at 160 VA. You can go much higher of course,with extra cost and physical size being the limiting factors. But I would not recommend going above 300VA per channel because you would be unlikely to get any further sonic advantage, and you would be into the area where you would need to consider a soft-start system that adds a whole lot more complexity to the supply. Even if you use a 300VA for each channel, I would suggest switching them on and off with separate switches.
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Old 13th January 2009, 07:31 AM   #393
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by Peter Daniel
You don't need a soft start circuit with GC.
Peter,
he just told you the T1.6A blows at switch on.
On the UK 240Vac mains, this fuse is large enough for 384VA.
The fuse is big enough, the start up current is too high, so the fuse blows.

Westers,
Don't fit a bigger fuse, instead fit a soft start, the fuse and transformer combination needs it.
After doing the soft start you may find that a T1.2A or T1A will suffice.
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Old 13th January 2009, 09:00 AM   #394
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Not wanting to start an argument/heated debate, I can see where you are both coming from.

Peter, you're right that in theory my 300VA primary shouldn't need a soft start, but I'm getting through fuses like there's no tomorrow.

A bigger fuse could be the answer as you say, but, as Andrew points out, this could cause other issues elsewhere in the circuit.

Andrew, a question for you. Having thought about it I can see how the light bulb tester acts as a soft start for the transformer, but once the testing is completed and I remove the bulb, then surely I face the same problem everytime I switch on the amp - the fuse blows because there is nothing to throttle back the current?
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Old 13th January 2009, 12:26 PM   #395
AndrewT is offline AndrewT  Scotland
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Yes, the bulb tester is only for safe start up. It is a Positive Temperature Coefficient (PTC) soft start. works but is designed to NOT tolerate any big load. That's why it lights up and reduces the supply voltage to the equipment.

You need a Negative NTC soft start or constant resistance soft start to allow sensible and safer fuse use, once you know the amp is wired correctly.
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Old 13th January 2009, 02:11 PM   #396
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Hi Westers.
Any chance of a photo? How did you have time to get readings? You said earth to ground, is that IEC to case?
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Old 13th January 2009, 02:57 PM   #397
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Yes - IEC earth to the case, although I've got everything mounted on square bit of MDF so there's no real case to earth. To be safe I've drilled a hole and attached the IEC earth lead to a bolt through the board (using a crimp on circle connector to go over the bolt). That way there's an earth if something happens.

I'm guessing that there's no real need for an earth as there's no casing, but I thought it good practice to get into the habbit of creating an earth.

As for getting time to get readings, good question. The first time I switched on I got an 11.2V reading on both + and - rails. That caused me to scratch my head and think there was a problem, which lead me to look at the fuse. The glass fuse had clearly blown (it was black inside), so I replaced it, but this time the reading was about 1V (+/-) and when I looked at the fuse it was even blacker than before.

I've pulled all the connections apart so I can do the soft start light bulb test to check the switch, but also make sure I get secondary voltages that are correct (35V). However, looking at the connections (both primary and secondary to the Bridge) they're all connected up correctly and nothing is shorting out.

I even pulled the switch apart to see if something had gone wrong there (I couldn't think what would have gone wrong as it's just a method of making and breaking contacts) and that was fine.

So, it must be the sudden rush of mains current is very high and the fuses don't like this.

Assuming the above is correct then it possibly doesn't surprise me. The house is prone to blowing light bulbs, especially if one blows and is not replaced straight away then often we get a number of other bulbs blowing. I know it's a totally different circuit, but I think the house was a 1980's Friday afternoon job, judging by some of the other workmanship I've had to fix.
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Old 13th January 2009, 03:06 PM   #398
AndrewT is offline AndrewT  Scotland
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Default Take Care

Have you checked your mains voltage at various times of the day and night?
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Old 13th January 2009, 03:14 PM   #399
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HI Weesters,
The fuse probably blow quickly and you are just measuring the voltage left in the caps.
Seems like it is just the inrush blowing the fuse and the lightbulb tester will at least allow you to test your boards. Then you must decide between Peter or Andrews solution.
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Old 13th January 2009, 03:18 PM   #400
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No, I haven't checked my mains voltage - I wouldn't know how to, although I'll google it to see how to.

No caps on the bridge - I'm using Peter's kit and haven't installed the optional caps included for the bridge - I wanted to hear what it sounded like before adding those.

All will be revealed when I build the light bulb tester tonight.
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