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-   -   PS voltage without load (http://www.diyaudio.com/forums/power-supplies/108372-ps-voltage-without-load.html)

duke_tj 12th September 2007 07:50 AM

PS voltage without load
 
Hi i'm really new to diy audio and i'm thinking to start a few projects, so i'm preparing a Power Supply for them. I have remove the power supply from an old amplifier with a pretty basic configuration - transformer, full bridge rectifier and two 6000uF capacitors as filters- it has an output voltage of +44 / -44 volts.

i would like to buil a chipamp with a lm4780 bridged and seems like +/- 30v is needed for safe operation of the chip. i would like to know if i can use the mentioned PS, how much drop in voltage i have to spect from the 44 volts with load?

any coments or advice?

mitwrong 12th September 2007 11:13 AM

Hi, Gent:
If you want to use your PS+44,-44V for chip amp with +30,-30V. You have to drop 14Von both. In this case, the best way is to use regs.
for details, you better consult our professor Walt Jung, I think he will willing to give hand, he is an expert in linear, the PS he designed is especially for audio.

AndrewT 12th September 2007 08:02 PM

Hi,
how much does the +-44Vdc vary with changes in the mains supply voltage?
The highest supply voltage will give the highest PSU voltage.
The regulator (if you decide to go with this option) will have to dissipate the output current times the maximum difference between the PSU and amplifier voltages.
Starting from +-44V is probably asking too much of the regulator.
Then you will have to meet the peak demand of the amplifier which could be around 7Apk to 11Apk. Where will this come from? A regulator will probably not be able to source a current demand at this high a level.

duke_tj 14th September 2007 09:05 AM

Hi guys thanks for the reply.

ive been doing some testing and after a cleaning and several power ups the rail voltage climb to +- 46vdc i have puto some load on it and only drop like one volt, so i think i need to make a choice.

-i can go with a aksa or a p3a amplifier which are friendlier to plus +- 40vdc, but i feel they are more complicate and somenthing afraid of my skills for a first project, any experience with them??
-go with the lm338 regulator wich can do 5a on the output if not more than 15v diference betwen vin - vout, but dont know how big the heatsink needs to be, any ideas??

lastly i dont want to go with a new supply right now, like i said before this is my first proyect and i like to see a few positive results before going into big spending, after that maybe i build a completely new PS (i promess:D)

thanks again for the replys

AndrewT 14th September 2007 09:57 AM

Quote:

Originally posted by duke_tj
amplifier which are friendlier to plus +- 40vdc, but i feel they are more complicate and somenthing afraid of my skills for a first project, any experience with them??
-go with the lm338 regulator wich can do 5a on the output if not more than 15v diference betwen vin - vout, but dont know how big the heatsink needs to be, any ideas??

Hi,
if you are unsure of your first project abilities, then adding a regulated PSU to your requirement is multiplying the complexity.
Keep this first one simple.

If the voltage does not suit a simple amplifier then temporarily lay aside this PSU and do a simple project first.
When you are ready for a discrete amplifier then bring back your high voltage PSU.

5A regulator supplying a ClassAB power amplifier will probably tick over with an average output current of less than 1A, unless you're a headbanger that likes listening to continuous distortion.

15V times 1A = 15W.
A dual polarity PSU will dissipate twice this.
Assume a 1C/W sink. Assume worst case ambient of 30degC.
Heatsink temp rise (deltaT1) is 2*15W *1C/W * derating factor. use a DF of 1.2 for first guess. deltaT1=36Cdeg.
Regulator temp rise (deltaT2) is 15W times Rth c-s, assume Rth c-s=1C/W i.e. deltaT2=15Cdeg.
For your 1A of output current, the regulator runs at about 30+36+15=91degC.
Check the regulator de-rating for this Tc.
You WILL require a BIGGER sink. And this is just the heat dissipation calculations.
BTW, 2A output gives Tc~=125degC (DF=1.1), 0.5A gives Tc~=65degC (DF=1.3).

As I said keep it simple to start with. Forget about steep learning curves. Small baby steps will reach nirvana, just takes a little longer.

mitwrong 16th September 2007 07:27 AM

Hi,
From the info above, it's better to alter the tranny to a lower voltages. This is the ideal , simple and econ. way of doing your project. The +-44V supply just lift it for the future project.
If you want a +-30V dc, just simply use a 42v ac with center tapped tranny using 2 diodes full wave rect. with 2x 6800uf for filter. most of the amps, today, employing this method, they are reliable and low cost. Using regs. are much higher price and complicated. you decide to choose the method.


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