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Old 6th December 2008, 08:36 PM   #1
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Question Power supply Capacitor, bad design?

Hi people,
I have this Reference Audio subwoofer amplifier that blew the TDA7294 output chip and the negative power supply capacitor.
The caps are 4700uF 35V "Jun fu" capacitors, or at least they were, until I replaced them with smae value Xicon caps.

The problem is the DC measured voltage I get is 36volts! This is what I'd call a "borderline design"!

I've attached a schematic of the power supply section in hopes that someone can clarify this, is it really a bad design, or should I be expecting another voltage? According to my calculations, the output voltage of this Full Wave rectified circuit is what it's supposed to be: (formula: 2*Vm/pi)
So,
Vpeak AC= 40V x 1.414= 56.56V

Voutput= (2 x 56.56V)/pi = 36V

which is exactly what I am measuring! except on EACH capacitor, meaning the total volatege is 72volts!

help, please?
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Old 6th December 2008, 09:13 PM   #2
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They play this game with all consumer items. Sanyo , Sony..etc.
all these will use a 63V cap with 55-57v supplies (push it to
the limit) I always derate , replace them with a 80V units.

Disgusting , isn't it.? This is the reason I no longer listen
to consumer items , only DIY..
OS
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Old 6th December 2008, 11:34 PM   #3
lineup is offline lineup  Sweden
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For 2x36 VDC I would use caps rated 50 VDC.
Two of them.
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Old 7th December 2008, 04:43 AM   #4
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Thank you guys! you know as as I was writing the post, I started remembering how I measured the voltage and I could have been measuring it across the positive cap only. which still makes it bad but not 72 volts as I said earlier.
My head must be in Mississipi!
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Old 7th December 2008, 03:05 PM   #5
AndrewT is offline AndrewT  Scotland
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Hi,
the voltage of a transformer is measured when it is supplied with it's nominal input voltage and it's output is loaded to deliver the full VA spec of the transformer.

When you measure it, the supply voltage will be different and the output load is missing.

You must correct for these errors in measuring or your time is being wasted.

You must also correct for worst case voltages that come out of the rectifier.

Read up on how to calculate the worst case voltage and then you can select the correct voltage rating for your capacitors.
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Old 8th December 2008, 02:58 PM   #6
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you didn't attach a schematic.... but, a lot of designs are redlined to keep costs down. not the design engineer's fault, but the manufacturing engineer's fault. the manufacturing engineer's job is to keep costs down. never heard of "Jun Fu" caps, but my guess is they're listed on badcaps.com. if you smell fish oil in those caps, they belong in the garbage.
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Old 8th December 2008, 05:36 PM   #7
Apex Jr is offline Apex Jr  United States
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This is an interesting post:

I first checked my Apex "Senior" amplifier by a friend
and it checked out great, so I ordered my 1st batch
of 250 units. Also with the Jun-Fu caps 6800Uf 80Volts
what we didn't expect was the rail votage went above
the 80 Volts and I had failures and needed to replace these
caps.
Normal USA made caps have a leeway to go above the
rated voltage.

The company I buy my new batch of amplifiers still use
the Jun-Fu brand but now all the Apex "Seniors" have
been retrofitted with 6800 Uf at 100 Volts and no problems.

Upgrade your caps

Steve @ Apex Jr.
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