I need some design help please. I have built a headphone amp that needs a +/-15Vdc supply (minimum 400mA) . The amp circuit and board are the work of someone else on DiyAudio. I am currently using a LM7815/7915 power supply. A recurring comment made on this forum is to use an LT1963a/LT3015 supply. According to the amp designer a supply capable of providing 12 to 15 volts dc at a minimum of 400mA is required.

The current supply and amp are working great. So one may ask "why bother"? For the fun of it and to learn. I will also be putting this design to a "DesignSpark" board design. So I may have some layout questions later. I am not asking for hand holding guidance. I am willing to dig for the answers with some pushing in the right direction.

I can find a lot of online information on the LM78xx/79xx regulators. Plenty to draw from. There is very little online help/examples using the LT1963a/LT3015 LDO regulators.

I have reviewed the datasheets for both LT1963a/LT3015 LDO regulators. There are some design considerations that I am confused or at least unsure about with regard to component requirements and values.

1 - Please look over the schematic for gross errors and then any recommendations.

2 - I am not sure how to determine if the 15VA Transformer will supply the required 400mA.

3 - The LT1963a has a maximum of 20Vdc input. Should I use a pre-regulator to insure I do not exceed this maximum. From my research it looks like a 15Vac transformer output > rectified > with a 3300uF smoothing cap will yield close to the maximum. Am I OK?

4 - I really don't know how to size the required protection fuse.

That is a good start.

 

1 - Please look over the schematic for gross errors and then any recommendations.

Your graphic is blocked.

2 - I am not sure how to determine if the 15VA Transformer will supply the required 400mA.

What is the output winding?
Is is 30Vac with a center tap?
Your circuit requires 0.8A total DC minimum at 15V.
Typically you should derate a transformer's current about 60% when driving a rectifier-capacitor supply, so that means the transformer should be rated at 0.8 / 0.6 = 1.33A.
You 15VA transformer has a 1A total RMS output so is thus a little marginal and may overheat.

3 - The LT1963a has a maximum of 20Vdc input. Should I use a pre-regulator to insure I do not exceed this maximum. From my research it looks like a 15Vac transformer output > rectified > with a 3300uF smoothing cap will yield close to the maximum. Am I OK?

Not quite.
The peak AC voltage is 1.4 × 15 = 21V.
The peak DC voltage will then be about one diode drop less or 20.3V which is marginal for a 20V maximum.
You could add a couple more diodes in series to reduce the peak DC voltage to about 18.9V which should be okay.

All in all, if your circuit works okay it seems foolish to mess with it.
My motto is always "If it ain't broke, don't fix it".

 

Sorry, I did not know the schematic did not post.

 

Thanks for your answers!

What is the output winding? Is is 30Vac with a center tap?".

Yes.

Your circuit requires 0.8A total DC minimum at 15V.

How did you determine 0.8A? Are you doubling the recommended 400mA?
The rest of your answer to question #2 I understand. Thanks

Not quite.
The peak AC voltage is 1.4 × 15 = 21V.
The peak DC voltage will then be about one diode drop less or 20.3V which is marginal for a 20V maximum.
You could add a couple more diodes in series to reduce the peak DC voltage to about 18.9V which should be okay.

Thanks. I thought it was close.

All in all, if your circuit works okay it seems foolish to mess with it.
My motto is always "If it ain't broke, don't fix it".

Good motto. I just wanted try this low noise regulator and maybe learn something along the way.

 

How did you determine 0.8A? Are you doubling the recommended 400mA?

Yes.
I assumed the plus and minus currents were the same, so he total transformer average output current is 800mA.