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Class D Switching Power Amplifiers and Power D/A conversion

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Old 4th February 2006, 03:52 AM   #11
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Location: Uppsala,Sweden
Hi !

I have planned to recommend an ADA sub amp to a friend who plan to build a sub, thus I was curious when I saw that you had run into problems.As far as I have read about it , itīs power amp part is a half bridge class D one running at 60 kHz switching frequency, and it (definately) seems to use a non-switching power supply.
A quote from the manual:"Only use the orginal size/value of fuses when replacing blown fuses".

If the unit uses a softstart , ordinary fuses(not slow blow) should be sufficient, because the purpose of the soft start is to limit the inrush current , when powering up.

Without a softstart, the big toroid transformer represents nearly a shortcut to the mains when starting up, primarely because no electromagnetic field has yet been built up in its core.It is,as well, of importance when in the AC cycle you happen to turn it on,thus
itīs quite possible that the fuse wouldnīt blow, if you hit the "zero crossing".
My guess is, that a softstart is included, but that it may not work.
An external softstart is possible to use , but you have to implement it in a way that doesnīt void the warranty.
A working softstart is usually recognized , by a clicking sound (from a relay ) after a second or so. The relay then bypasses the
resistor,which limits the inrush current; the resistor is placed in series with the primary winding of the transformer.
Faultfinding is a process , when hypothesis often are checked
one by one, thus the worst case is when several faults exist.
So letīs hope for a "simple" fault , and keep us informed.

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Old 4th February 2006, 08:35 AM   #12
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After having look at this pictureplateamp it seems to me that no inrush limiter is used at all.

The easiest way to create one, is to add a NTC resistor in series with the primary windings of the transformer. Such a resistor has a relatively high resistance value when cold, and when it is warmed up by current running through it , the resitance drops significantly. It is not the best way , but it works.

As this plate amp is supposed to be left on all the time, another way to go , could be to add a resistor in series with the primary
winding, and manually bypass it after some seconds when powering up.

And I agree to what has been written about capacitors being charged by high current, and this is in fact a reason for using an inrush current limiting device. Capacitors donīt like high surges.

A soft start is a good thing , and if it isnīt included in your sub amp, I canīt see why.
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Old 4th February 2006, 09:02 AM   #13
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Hi Mean,

The NTC won't work with a class D amp, idle current is not high enough to keep in the low state.

I can see why it's not included.... none good.

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Old 4th February 2006, 09:39 AM   #14
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You are right, as always!
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Old 4th February 2006, 05:22 PM   #15
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if I had a 10amp breaker on the amp it wouldnt trip the 30amp house breaker, I think that would completely fix the problem.

Actually a few times the fuse didnt blow... maybe like 3 times out of the 3 fuses I went through. But eventually powering it up once more blew it.

I did leave the amp on continuosly for a while, but after not using it for some time the back was a little warm to the touch. I dont know how much idle power it uses, but my guess is over 40watts and it just seems like a waste of power to have it running ALL the time.

I'm researching these softstarts to see if I can create one myself. Thanks for all the help guys
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Old 6th February 2006, 09:22 PM   #16
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Hi !

This is an example of a current inrush limiter,others could be find
on Hypex , LC-audio , Thel Audioworld and others.
I you want to design one yourself , maybe a timer circuit like the
555 is the way to go?


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