Power Supply Problem

[davent]

Hello,

I'm working on a little DC power supply to have on the workbench for breadboarding and troubleshooting, there's never a fresh battery when you need one. So I tried it out tonight and when powering pedals there is a noticeable increase in the background noise a shhhhhhhhhhhhhh sound. Switch back to battery and the noise subsides. I also tried it powering a headphone amp and with that i get a hum where with a battery there's silence.

I've tried it with the RF filtering and with that filtering bypassed and it makes no difference.

Tried powering from the 12v node or the 9v node and again no difference in the results.

The only differences from the schematic are the secondaries of the transformer i used are paralleled and there is a resistor/led between the rails before the 12v regulator.

Voltages are 19v before 7812, 12.19v after 7812 and 9.01v after 7809.

Any ideas? Faulty diodes, caps or regulator, bad layout?

Thanks for the help folks!
dave

[Sir H C]

Keep the supply as far away from the guitar and FX (especially if one is a wah pedal).  It could just be coupling like that.  I would also reduce the cap off the diodes, 1000uF is just too big, you don't need super high amounts of cap there, the regulators clean that ripple up.  I don't see anything wrong with the design that should have hum issues.

[davent]

Hi Chris,
Thanks for the info. I thought it may have been induced noise as that has been a problem, there's a couple power circuits/light dimmers my guitars really don't like. Turns out the problem was me and not the PS after all. I went back at it today trying a bunch of pedals with it and only one exhibited the increased background noise, the one I chose to test the PS with last night... of course.

So then what's up with the headphone amp? Again me, i had nothing plugged into the amp's input so the inputs were floating. I accidentily brushed a finger against the input jack while trying to figure out what was going on and there was silence, take my finger off the jack and the hum and chatter returned, finger back on the jack = silence.  Hooked the amp up to a discman, put it on pause and with the volume all the way up silence, just like with a battery! Yeah!!!  Instead of a PS to troubleshoot looks like I've got a pedal that needs a second look. It's a clean boost that seems to work fine except for this newly discovered little quirk. I am my own worst enemy.

Take care
dave

[Sir H C]

Some pedals don't like no input, they hum.  Look at Fender amps, with no input they switch the inputs to ground so that there is no hum.