a pair of 9vs to a wall wart....

[toomanyslurpees]

I have a little outboard preamp for my bass rig, it runs at 18V, I'd like to get rid of the batteries and run it off a wall wart so I don't have to change batteries (aka forget to change batteries) I've mounted a jack and just used a pair of 9v connectors to hook up to the existing battery connections but I'm getting a fairly decent hum. I'm assuming the adapter I have isn't filtered enough to replace the batteries, I was wondering if someone doesn't mind showing how I would further filter the power? The preamp is an aguilar DB924 which is one of their onboard preamps stuck in a metal box.

[Tiny_Daddy]

Is your wall wart 3-prong grounded? If yes, this could cause a ground loop hum.
Is the wall wart unregulated? If so, maybe you just need more filter capacitance, like 2200uF @ 25V.

[stingray_65]

Hi Slurpee!

I'm not all that hip on pedals so take this with the grain of salt ok?

recently I've been looking at pedal designs and thought I might try and home brew one just for funzies.

What I've seen is the ones that have 18V power supplies are actually -9vdc - 0 - +9vdc.

So if you're just running 18vdc to a couple of battery leads without a voltage divider to give you a 0V reference you may want to give that a try.

HMMM
now that I'm thinking more about this you may need 2 separate 9 V supplies. I'm not imagining in my head how you would connect that 0v referance.

Ray

[stingray_65]

OK!
I've thought about this.

Do you have a meter and know how to use it to check for continuity?

If so, one of the red leads (in your pedal) should have continuity with one of the black lead (on the battery clips)

Once you find the continuity, call the clip with the red lead that has continuity the -9V clip
The other will be your +9v clip.

Label them with a sharpie or tape.

Go to radio shack and get 2 9V batter leads, a pack of 470 ohm 1/2 watt resistors and a pack of heat shrink to cover your connections.

Connect your clips with a Voltage divider like in my illustration.

NOTE. on the +9vClip on the wall wart the black wire on the lead is going to the +18vdc wire. this is because common battery clips aren't designed to be used as power supplies. look at a 9V battery. the + terminal on a battery looks like the - terminal on the lead. So hook it up the way I have it drawn and double check for yourself looking at a 9 V battery.

All that is left is to connect the -9v clip from the wall wart to the -9v clip in the pedal and both positives.

Ray

[stingray_65]

Edited

[SLW]

As you have discovered most wall warts are not filtered enough for audio applications.  The simplest thing to do is buy a wall wart from BOSS etc. that is designed for audo and meets your needs (jack size, power rating). 

If you want to try to make the one you have work, another filter cap Like Tiny Daddy suggessted is a good starting point.  100uF or larger is a good place to start. The best way to do it would be to use a voltage regulator. The ripple in the output will be very low.  Any good powersupply for DC is basically built transformer-rectifier-filter cap-regulator-filter cap - output.  In the example of your case use a 24VDC wall wart.  The output of the wall wart will feed the cap-regulator-cap setup.  The regulator would be a L7818 18VDC @ 1A.  If you need nore than 1A get a 3A regulator.   That should be nice and quiet.

Link to L78XX data sheet.  http://www.fairchildsemi.com/ds/LM%2FLM7818.pdf

SLW