Need a 12.6 VDC power supply schematic

[Dmeister]

Hello everyone. I recently finished my hybrid 12AU7/IRF510 headphone amp, and it sounds wonderful The problem is, there's a bit of hum every once in a while, and I think it's the power supply that's the problem. Here's a schematic for it:

An externally hosted image should be here but it was not working when we last tested it.

Now I've tried building that schematic 2 times, but it outputs about 15 VDC I'm looking for a regulator schematic that inputs about 15-20 VDC and outputs 12.6 VDC. If anyone has any suggestions I would be really thankful

 

[TG]

Changing RR1 from 120 to 270 Ohm will be quite enough.

 

[Dmeister]

Changing RR1 from 120 to 270 Ohm will be quite enough.

Thanks, I'll do that and report back here

 

[Dmeister]

I changed RR1 but it still outputs about 15 VDC. I don't think it's the LM317, since I tried 3 different ones. Can anyone give me another schematic for a 12.6 VDC regulator ?

 

[Dmeister]

Can anyone please help me ? I need this quite soon...

 

[TG]

What voltage is at the LM317's IN terminal?

 

[tomchr]

The schematic is fine. The issue is with the resistor values. Try these values:

RR1 = 220 ohm
RR2 = 1500 ohm
RR3 (pot) = 1000 ohm

The output voltage is Vout = 1.25*(1+(RR2+RR3)/RR1) in your case. Hence, above values should yield an adjustment range of 9.8 ~ 15.5 V.

~Tom

 

[Andy5112405]

I think your Vin is too low, the LM317 needs about 3V to operate. As an experiment try increasing Vin, try about 18VDC that should prove the point.

 

[TG]

Initial resistor values (with RR1 = 270 Ohm) are fine too.
I have a suspicion that DR3 and/or DR2 are mistakenly reversed.

With too low Vin LM317 should output about Vin-1.2..2V depending on load, so I believe this is not the case.

 

[Dmeister]

It seems the trim potentiometer wasn't properly working. Well, actually, not one, but THREE of them ! I am that unlucky. I bought a proper trim potentiometer so now it works.

 

[woody]

This device may require a minimum load current to function properly. I believe that
minimum load is 10ma. If you are powering it up without a load then the current through rr2 and rr3 may be below this minimum value. You might try just shorting
rr2 or placing a load on the output to se if this is the problem.

 

[tomchr]

I think your Vin is too low, the LM317 needs about 3V to operate. As an experiment try increasing Vin, try about 18VDC that should prove the point.

Yeah... The National Semiconductor LM317 has a worst-case drop-out of about 2.5 V. I.e. the input voltage has to be at least 2.5 V higher than the output voltage. Allowing for a few volts of ripple, an input voltage of 18~20 V seems reasonable.

You also need a reasonably sized heat sink on the LM317. If the chip overheats, it'll enter thermal shutdown.

Three bad pots?! Actually, that sounds like you either didn't hook the components up according to the schematic or there's a cold solder joint somewhere.

~Tom

 

[tomchr]

It seems the trim potentiometer wasn't properly working. Well, actually, not one, but THREE of them ! I am that unlucky. I bought a proper trim potentiometer so now it works.

Do you get 12.6 V out now?

~Tom

 

[tomchr]

This device may require a minimum load current to function properly. I believe that
minimum load is 10ma.

Actually, it's a bit lower than 10 mA typically. It depends on (Vout-Vin). See the datasheet for details.

With 120 ohm from OUT to ADJ, you already draw 10 mA through the regulator. No additional load needed for regulation.

~Tom

 

[Dmeister]

Yes, I can now adjust the output voltage with the trim potentiometer. It could have been a cold joint, I guess. I'm still not a master at soldering . And yes, I have a small heatsink mounted on it. Thanks for the help everyone !