25vdc regulated power supply

Hi Manek,

Without getting into purist Math justification, let me answer as follows:

A. You are correct, the No LOAD DC from the 30 VAC Transformer + Bridge Rectifier will be 1.4 x 30 VAC = 42 VDC which is too high for the LM 317.

B. When there is a SUFFICIENT DC load ( similar to the Transformer's output current capacity ), the DC Output voltage will fall to 0.9 VAC = 27 VDC, which is fine for the LM317.

To elaborate on what is a 'sufficient' load for the transformer, if you have a 1 Ampere, 30VAC transformer, then the current drawn by your Pre amp ( usually 100 mA to 250 mA ) is NOT likely to load the DC down sufficiently.

You have the following work arounds:

1. Load the output of the bridge rectifier with a Resistor so that it continuously draws current and drops the DC voltage to around 30 VDC. The resistor value will depend on the Transformer's output current capacity. A 47 or 50 Ohm resistor 20Watt could be a good starting guess, if your transformer is rated at 1 Ampere output current.

To minimise trips to Lamington Road, buy 3 resistors, each of 100 Ohms, 20 Watts is recommended. ( Rs 10 per resistor, approx)

Start with only 1 resistor as a load. If the voltage is above 32 VDC, add a second and if necessary a third resistor. All resistors must be placed in parallel.

Please ENSURE that the resistors are firmly soldered in place so that they are PERMANENTLY connected BEFORE the LM317

OR

2. Place 10 ( yes, TEN ) or more Diodes in SERIES between the Bridge rectifier output and the LM317 Input. these will drop the voltage by 10 x 0.7 Volts = 7 Volts from the peak value of 40 VDC. Use 1N007 diodes... easily available and cost about Re 1 each...

yes, its a standard lamington road 30vac 1 ampere transformer bought sometime back.

now since i need to go to lamington road to get the resistors i might as well also get a 24avc transformer, that should be the right transformer rating ? But i will definately try and do what you say as this will help me in the future.

this is for a dac and media convertor(which are two separate boxes), not the preamp. I got a reasonable looking chasis so I thought I would make one power supply device and power both through it. What would be better ? two power supply circuits fed by one transformero or just a single transformer and power supply curcuit for both ? I am thinking two transformers and two power supply boards would be a definate overkill.

both together may not pull more than 200-300ma I should think. Want to get rid of the two wall warts.

once I see good results, i guess i will slip in a good transformer.

I have been told that people are also starting to use laptop power supplies to power audio devices which need 15-20 volts dc, wonder whats the deal there ?

Manek

Hi Manek,

A few thoughts ...

1. Why not consider a Shut Voltage regulator, using yr existing 30 V AC transformer ?

Shunt regulators ( not just a simple Zener) are usually raved about for audiophile applications !

2. It has long been an established fact that For The Same Money 1 transformer will provide more grunt than 2 separate transformers.

However, in your case, i would ideally suggest :

One Transformer and PSU for the digital Circuits + a separate Transformer & PSU for the analog circuit portion...


I too have heard good things being said for Lasptop PSUs feeding audio projects. I remain skeptical. Actual experience, Anyone ?

So 2 transfotrmers and 2 psu it is.

Both take 25vdc so two transformers with 24vac on the secondaries is the way ?

I was thinking of stuffing in a single emi/rfi filter before the two tranny's

Most of the laptops I have seen have a 19v psu nowadays. Are they not smps type ?

Manek

Manek, Selecting a transformer for a 3 Pin regulator ( &8xx, 79XX or 317/319) is VERY Tricky, a Tight rope walk at best, when the DC output voltage is 24 VDC

Consider these:

The regulator needs ATLEAST 3 VDC accross it to function. 5VDC is nice.

That makes the Worst Case / full load voltage at the regulator input to 29 VDC.

The DC voltage from the rectifier will range from :
1.4 times (Square root of 2) the AC voltage on no lead
to
approx 0.9 times the AC Voltage on full load.

Ofcourse the transformer output voltage regulation will further drop the voltage on full load ..

..... and Lamington Road Transformers are not specified for Load regulation, though, in general, SMALLER transformers (for the same Voltage and current output) will have BETTER regulation than larger transformers, even though the smaller transformer will run hotter..

Most of the laptops I have seen have a 19v psu nowadays. Are they not smps type ?

Yes, they are, and I remain skeptical ... despite some reports to the contrary.....

Only one way to know , I guess ....

They are typically priced not too expensive @ Rs 500 each, ...

Ampnut

The lam road ones arejust to make the circuit work

Not going to use em Long term.

I get my long term transformers from a very nice guy in pune who custom winds em for me.he only does torroids but never had an issue till date. Any specific details you think I should be giving him ?
Would apprecite any inputs.

Upto now I just tell him voltage, current rating and the device its going to go into and he does the job.


Manek

This power supply never got built though I have everything at hand to build it. Lazy me.

So my Dac wall wart got busted and I found myself a hunting for another wall wart at lamington road. Luckily I happen to glance at a few lab power supplies and instinctively asked for a 24vdc regulated one for my dac. The sales man got one out for me rated at 2 amps. It sure was huge and heavy for a 2 amp supply with the power transistor mounted externally on a huge heatsink. Impulse got the better of me and I bought it.

Its from Testronix, a small company in bhandup with a lot of spelling mistakes in the 5 page manual :-).

Made a cable for my dac to connect into the output jacks of the power supply and fired her up.

I have been using it for 3 days now and it seems to sound better as I use it.

For about 3k and the difference it has made sonically, its worth every penny. Comes with a years warranty.

For guys who are looking for external dc power supplies for their equipment, very recommended.

Oh yes, it comes with a very shabby AC mains cable which I will replace after a while :-). Right now enjoying every bit of it.


Manek