low noise power supply - advice needed

[R.G.]

I mean, even if the DC supply does not touch the enclosure, if an AC effect is connected to a DC effect, the same will happen, since their grounds will be connected.

To avoid guessing, I need a diagram of the power inputs, signal grounding, and how power connects to signal ground in the effect. Basically, a schematic of power and ground, including power jacks to the case and signal jacks to the case.

Things like this can be caused by the misrouting of as little as one wire.

[blana]

Hello again,

Thank you very much for helping me!

I took some pictures of the actual source, hoping to be as clear as possible. Also i have made a very rough schematic of the insides and wiring. Sorry for my drawing skills.

Here they are:

https://www.dropbox.com/s/h7b4i2rp53wwf2v/source%20insides.png

https://www.dropbox.com/s/9mth14e30lhvayb/source-inside-pics.jpg


Thanks a lot!

[Jdansti]

I'll leave the analysis to the experts, but just wanted to note that when running wiring through holes in metal, you should use rubber grommets. This is especially true when you're dealing with the mains voltage.   Wire insulation and sharp metal edges are not compatible.  This is a very good way to start a fire. A small nick in the insulation could allow enough current to start a fire, yet not enough current to trip the breaker.  Radio Shack sells an inexpensive pack of grommets that would work. You just need to drill a hole and insert the grommet before running the wires.

[blana]

Hello,

Thanks for the idea! I filed the metal edges, and the plastic isulation of the wires is very resistant, but more protection could not hurt so i'll take care of that.

Thanks!

[R.G.]

That was what I needed.

You get the hum because the AC power output is not really isolated from the DC ground. It's connected to the DC ground by the two diodes on the minus side of the bridge rectifier on alternate polarities of the AC power cycle. A pedal plugged into the AC power side almost certainly rectifies this to DC to make some other voltage from, and this causes another offset of the minus side of its power supply from the DC ground that the power supply makes. Then if you connect an AC and a DC-powered pedal together, the DC ground and doubly-offset AC/rectified ground are tied together by the ground in the signal cables. And it hums.

[blana]

Hey! Thanks for your reply!

I understand!

Would a small cap placed in series with the ac wire act like a short between the dc ground and the ac supply?

Thanks!

[PRR]

> Would a small cap

No.

You need a *separate* winding. Either multi-winding transformer or another transformer.

[R.G.]

No.

You need a *separate* winding. Either multi-winding transformer or another transformer.

+1

[blana]

Thanks for your help guys!

I'll see what i can do.