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Old 21st June 2004, 11:58 PM   #1
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Default How much capacitance?

Hi, I have a 2 channel amp that is used as a bridged sub amp only. I recently took it apart to have the heat sink powder coated and noticed that there is two empty spots for more caps. It already has two 63v 4700uf caps and that doesn't seem like nearly enough for the size of the amp.

I don't have any fancy test equipment or resistive loads, but powering a 12" sub @ 1 ohm and using a true rms volt meter and an amp clamp I measured output at about 47v and 17 amps. I'm sure it would put out much more power if it wasn't for the impedance rise of the box, but anyways.......

The amp is class AB and uses 10 2SA1694/2S4467 output devices per side for a total of 20. The supply voltage was measured at 43v.

Output device specs:

Pc(W)=80
Vceo(V)=120
Ic(A)=8
hFE(min)=50
fT(MHz)=20
Package= MT-100 (TO3P)

Now, is four 63v 4700uf caps enough? If so, with the specs I gave what kind of power should this amp be putting out bridged @ 1 ohm? (I'm a nub, please take it easy)

Thanks in advance,

Sean
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Old 22nd June 2004, 12:18 AM   #2
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I think you could easily double or triple the capacitance
and it would probably show nicely.
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Old 22nd June 2004, 12:25 AM   #3
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Thanks fo the reply Nelson. Also, would it be ok to use 50v caps instead of 63v seeing as to how the supply voltage is only 43v?
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Old 22nd June 2004, 12:29 AM   #4
Magura is offline Magura  Denmark
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50V caps would do, the voltage rating of a cap is really a matter of rating the life expectancy...to some extend like +10%. If you use caps with higher voltage rating than the nominal voltage, you will gain cap life expectancy.

If you take a look in the trading post you will find real good offers on 50v 68,000uF caps.

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Old 22nd June 2004, 08:01 AM   #5
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Quote:
Originally posted by Magura
50V caps would do, the voltage rating of a cap is really a matter of rating the life expectancy...to some extend like +10%. If you use caps with higher voltage rating than the nominal voltage, you will gain cap life expectancy.
Judging from real life (at work) the most important thing when it comes to life time is temperature! If you have less than 50 Deg C you will get 5-10 years. 40-50 deg C > 10 years for "normal" caps. RIFA caps and other "computer grade" lasts > 20 years

The the temperature goes over 60 deg C the life time is heavely reduced and at 85/105 deg C the time is only 2000-5000 hours, a couple of months!

Please note that I mean heat generated from inside (ripple currents) and ambient temperature.
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