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Old 20th December 2008, 06:16 AM   #1
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Default capacitor choice

hi, I need to know if I can substitute, in an audio subwoofer amplifier, a 4700uF cap with a 6800uF cap. My concern is that the current draw at startup will be high enough to almost short out the transformer.

Can that happen? Or am I too worried?

I don't really want to have to put a 1 ohm resistor in series with each capacitor, as that makes it a bit junky-looking. It just so happens that I'd like to use the caps I already have(two 6800uF at 50V) instead of ordering more parts...

SO thanks people,

cheers
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Old 20th December 2008, 03:46 PM   #2
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Hello gain-wire, I am guessing that you refer to replacing the main filter cap in the amps power supply. Uprating from 4700uF to 6800uF shouldn't be a problem at all. The increase in surge current demand upon power up shouldn't be enough to cause problems. Some will tell you that it will improve the sound of the bass, due to having more reserve capacity. I haven't tried uprating myself, so I won't pass judgement one way or another, but there is merit to the theory.


Happy Holidays,

Dave Gerecke

P.S. just make sure that the 6800uF caps have the same or higher voltage rating.
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Old 20th December 2008, 04:26 PM   #3
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well, thank you for the quick reply, Mr Gerecke. Obviously I kew abou the voltage rating whcih was, in fact, the problem with the previous components used in the supply. One of them failed and shorted, and the other, well, I figured it's not far behind!
these were 35V units, so I bought new 35V units, 4700uF, installed them and then measured the voltage only to find that the designers (in their infinite wisdom) chose to operate the capacitors at their maximum rating: the output voltage was 36 volts! Come on!

Yeah, well I also figured the Bass will improve (as if there wasn't already enough Bass in a 10inch subwoofer, at least for my liking) becuase of addes Energy reserve. But I also knew that it comes with a price: longer time in the conductive state, pushing the transformer for more current.

I shall install the 6800uF capacitors.

I guess if it blows the fast-acting fuse in the AC line, (that is how the unit is protected) then the 6800uF are toom uch current draw.


Thank you again!
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Old 20th December 2008, 04:48 PM   #4
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You can use a DC Current limiteer like the CL-05 or CL-06 from GE. These have a cold d.c. resistance of 7 and 10 ohms (respectively, but a hot d.c.r. of 0.16 and 0.18 ohms. I think the DK part numbers are KC0005L and KC006L.

They do heat up in operation, however (the power dissipated you can figure out...)
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Old 20th December 2008, 05:05 PM   #5
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Hmm, an intersting idea, never thought of that, but then I have to order these parts also... That's what I am trying to get away from.
You know I didn't even know this type of component existed! I only found a CL-30 on digikey, mind you I didn't search anywhere else, but I'm still going to have to order it, unless Active components here has it in stock.
Thanks for the information, much appreciated,

Gain-wire
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Old 20th December 2008, 05:56 PM   #6
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Hello gain-wire, glad to help. I agree with jackinnj about the possibility of using a DC Current limiter on the incoming AC, or on secondary of the transformer. The industry term for these is Negative Temperature Coefficient (NTC) Thermistor. They work pretty well for the application. I have multiples of a couple current ratings in my basement, if you can't find the size you are looking for shoot me an email and I will check what two sizes I have. I might have a rating that will work.
It's kind of scary that whoever designed and manufactured the amp for your sub is running the main power caps so close to the maximum. Even running them above the maximum continuous rating for voltage. That's just plain poor design. I'm kind of surprised that it made it past the Canadian safety tests. From what I have seen and heard, the Canadians are even tougher on testing than the US. Here most of it is voluntary through UL listing. The CSA(I think that's it?) is a government agency, so they have more clout! Does the amp have a UL and/or CSA label on it? If so, you may want to send some info to them about this.

Peace,

Dave

P.S. I;m also a little surprised they used a fast acting fuse, usually one would use a slow blow because of increased in rush current.
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Old 20th December 2008, 06:11 PM   #7
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check it out:
apaprently it's made by Reference audio, which could also be designed, or owned by Harman industries. the reason I am not sure is because I only have the board and back panel. The subwoofer case is in Montreal and it isn't mine. I also think it was designed in Canada (yikes) and know for a fact it predates 2003. I have to leave now but will continue this discussion if you like, later on.
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