33000 uF capacitance on input of LM338 regulated supply for LM3886

[bikehorn]

I'm about to get underway building my first chip amp (ESP P19, stereo LM3886)and I have decided to try the CarlosFM regulated supply. I noticed that the original Pedja Rogic supply has a pair of 4700uF caps on the input while the CFM version has a single 4700uF. It happens though, that I have a couple of 33,000uF 40V computer grade electrolytics handy and I'd like to use these.

I know everyone's already seen it, but for posterity, here's the Decibel Dungeon page with the schematics.

http://myweb.tiscali.co.uk/nuukspot/...ainclone6.html

I'm just wondering if this will cause any trouble or degrade performance in some way. I will be using a 25-0-25 250VA Avel Lindberg toroid, and a very heavy duty power switch to prevent any welding. Is it inadvisable to omit a soft start circuit with this setup?

thanks

[drummer_Dave]

Lets do some math

25 VAC * 1.4 = 35 VDC

Your Caps are rated at 40 VDC so mathamatically they should be fine, but just barely. However if your Line voltage increases (which does happen) to lets say 125VAC

115 VAC = 25 VAC
125 VAC = X

x = (125 * 25 ) / 115
x = 27.17
VDC = 27.17 *1.4
VDD = 38.38 VDC

Ok so voltage fluctuation probably wont put it over 40 volts, and even if it did the cap ratings have probably at least a 10% safety margin.I know I was taught that it's better to be safe then sorry, but Id say if you got them Use them.

However I cant tell you if that value will affect the sound quality there are other threads discussing this fact and i will leave this up to you to try.

So after all this B.S. I just wrote If it was me Sure i'd try them out. Just be safe make sure they arn't pointing at you when you first power them up.

Good Luck lets us know how it turns out

Dave

P.S. o darn i forgot to calculate the 1.4V drop across the Bridge. o well lol

[suvatn]

You' re also need to care of maximum input voltage of LM338.

[AndrewT]

Hi,
the transformer is specified as 25-0-25Vac.
This is centre tapped and will not suit a dual polarity supply using twin positive regulators.
You need a twin secondary transformer.

Is the spec voltage measured with the transformer loaded to it's max current rating or is it the opencircuit voltage when supplied with the correct mains voltage?

Measure the actual mains voltage and the opencircuit secondary voltage of your transformer.
Now multiply up the secondary voltage to the maximum tolerance mains supply voltage.
Here in the UK we buy 230Vac transformers and they are rated to be OK on 220 to 240Vac mains supplies, but our tolerance is +-6% giving a maximum of 254Vac into a 230Vac transformer. That must be taken into account when checking maximum voltages in the downstream electronics. Particlularly dissipation in regulators!!!!
Then apply the 1.4 factor to obtain the peak voltage at the rectifier. Finally subtract 0.7V for each diode.
Your 25Vac transformer could easily overstress 40Vdc smoothing caps if the amps are disconnected and similarly overheat/stress the regulators when the regulator voltage drop rises to a maximum.