Minimum voltage for capacitor rails

[jmillerdoc]

Hello,
I am in the process of acquiring parts for an Aussieamps NX150 stereo amp. I have decided to go with 63v rails from a 45+45 600VA transformer. I was wondering if I could safely get by using 80v rated caps on the PSU or if I should go with 100v rated caps.

I would also like some suggestions for the best cap I could use here too. I have been looking at Cornell Dublier, Elna, Kendeil, and of course Nippon.
Jeff

[Mooly]

80 volt is fine, well within ratings even allowing for mains variations. Definitely no need for 100 volt rating.

Which caps... they are all good names. It's far far more important to ensure the wiring and layout is correct.
A good amp (high PSRR) correctly wired should be imune to it's PSU unlessthere is something grossly wrong.

[ostripper]

I use 8- 80v nichicon 10,000uf caps . They were manufactured in 1992(datecode) , were used with 76VDC rails till 2002 in the original amp. Upon gutting the original , they were ESR tested with a "cap wizard" .. very low reading. I have now used the hell out of them in my new amp for 14 months .. absolutely no problem.

By actually testing these 18 year old Nichicons , I am amazed they tested so good only being derated by 4 volts. In fact I have 2009 4700uF caps that only test .1 ohm better on the ESR meter. VERY impressed with this quality.

If 80v units worked for 18 YEARS @76v , 63v would be nothing.


OS

[Junm]

I also have this Nippon chemicon stuff and used for my amps and no problem at all. They are aged by still rugged.

[AndrewT]

Originally Posted by jmillerdoc

I have decided to go with 63v rails from a 45+45 600VA transformer. I was wondering if I could safely get by using 80v rated caps on the PSU

do the arithmetic.
a 230:45+45 transformer running on a 240Vac that is at max 6% tolerance gives out 240*1.06/230 * 45 =49.77Vac
Now add on the transformer regulation to find the peak voltage before the rectifier.
Assume the regulation is 5%.
Output voltage is 1.05*49.77 =52.26Vac.
Peak voltage is sqrt(2) times Vac
Vpk = 1.414*52.26 = 73.9Vpk
The rectifier voltage drop at near zero current and with fully charged capacitors on minimum leakage current will be ~0.5V.
The worst case capacitor voltage is 73.4Vdc.
75 or 80V capacitors can safely be used.

I would advise reforming any old capacitors to their rated voltage before starting them on line.

[jmillerdoc]

So even a 75v cap could be used then. Hmmmm, tempted to go with the 75v if the cost is even lower yet. Thanks for everyones help!
Jeff

[AndrewT]

do the arithmetic for your supply voltage and for your transformer.

The numbers are different for each set of circumstances.

is it 110:45+45Vac, or 115:45+45Vac, or 120:45+45Vac?
is it 3.5% regulation or 4% or 4.5% or.....?
What is your nominal mains supply voltage?
What are the tolerances on mains supply voltage from your supplier?

If you do not know this information, play safe and use 250Vdc capacitors. Well that's as good a guess as any other.