Need help with LM317 voltage regulator

[newbiediy]

I was building this preamp: http://www.redcircuits.com/Page120.htm. Nice result indeed with battery pack. I even got good result with 9V battery (instead of 18V which is recommended). <3)

The problem started when I used a cheap wallwart adaptor. I got annoying hum which also distorts my lower notes. So I decided to make some kind of voltage regulator using LM317.
BUT I couldn't get rid of the hum. Thinking that I got a defective part, I replaced the LM317. Still same result. xP

Here are the measurements with analog voltmeter (intended output was 9V):
With 18V input, the output was around 15-16V.
With 12V input, the output was around 10V.

I guess there is no regulation at all. :duh

Could anyone point me any mistake I made? My plan is to make an internal voltage regulator in the preamp. I made it switchable between 9V and 12V. The schematic was taken from the LM317 datasheet.

[phatt]

My best guess,
                     You are dropping down a fair amount of voltage and if they run hot they may stop regulating. I think?
You may want to heat sink it to dissipate the heat created.

Other thoughts,
The 470uF on the output only needs to be small,, maybe 10uF but you may need to put a bigger Cap on the input side to get rid of hum. 
I'm assuming a normal Transformer plug pak is in use?
IF it is a *switchmode type Plugpak* then you will be better served by throwing it in the bin as some are horrendously noisy and are not designed for use with sensitive audio circuitry.
Phil.

[J M Fahey]

The schematic looks fine, yet you have no regulation.
Please post actual voltage at each of the LM317 pins, on both switch positions.
You probably have a wiring error you didn't notice.

[Loudthud]

The LM317 is connected wrong. You have Vin and Vout terminals swapped.

Also, the voltage adjust switch should be connected differently. The way you have it connected inbetween selections when you are moving the switch, the output will go to max voltage. Leave the resistors connected and simply short them out with the switch.

[Enzo]

And just a thought.   Many wall warts are poorly filtered.  You might try adding some filtration.  Where the wall wart connects to your project, just put a hefty cap across it.  1000uf for example.


An example of this is Boss pedals.  The official Boss wall warts are better filtered than acerage wall warts.  SO if you use a generic wall wart instead of a Boss one, the pedal stil works, but now it hums.

Any regulator needs some headroom, so by regulating the wall wart input, you will lose a couple volts.  Yet another reason to try a simple cap.

[newbiediy]

Sorry for late reply. Been rather busy.

... The way you have it connected inbetween selections when you are moving the switch, the output will go to max voltage. Leave the resistors connected and simply short them out with the switch.

Good point. I was thinking part/cost reduction but didn't think about safety.

... The 470uF on the output only needs to be small,, maybe 10uF but you may need to put a bigger Cap on the input side to get rid of hum. 
I'm assuming a normal Transformer plug pak is in use?
IF it is a *switchmode type Plugpak* then you will be better served by throwing it in the bin as some are horrendously noisy and are not designed for use with sensitive audio circuitry.
Phil.

I had opened the adaptor and found a transformator, some diodes, resistors, and capacitor. I guess it's not the switchmode one. I'll try the cap thing.

I'll try all of your suggestions and post voltage measurements later. Thanks for the quick replies. :tu:

[phatt]

Good ,,
         Switchmodes are easy to pick ,,, they weigh nothing as there is no iron and far less copper.
Yes as enzo noted, go with that extra cap first.
You can always add a 10~20 Ohm resistor before the cap as they do in some pedals.
Phil.