[sdiy] [AH] Tech info request on pwr supply polarity & case grounding

[cheater cheater]

Your mixer is most possibly using floating ground. This means the
power supply is 0-9Volt but e.g. it uses 0, 4.5 and 9V as -4.5, 0 and
+4.5 respectively. The new 0 is "generated" by averaging out the real
0 and 9.

The simplest way to fix the problem is often to use balanced
connections. I assume your mixer doesn't allow that.

One way to get around this, but it doesn't always work, is to get a
DIY wall plug with a ground prong, and a single-wire cable. This cable
you then connect to the prong and, when you stick the plug into the
socket, only the ground is connected. The other end of the cable
connects electrically to the chassis of your mixer.

Another way is to use a ground-bypass cable. This means you connect
the chassis of your mixer to the chassis of whatever is connected to
it, using a copper wire. The hum comes from the current flowing
between grounds, and it finds its way through the shield of the cable
connecting the two grounds. So you use a thick wire to connect both
grounds and so give it a more convenient path. Sometimes you can
augument that by putting a resistor in series with the shield of your
cable, to make sure the current really has a "hard time going there".
You would add a small value resistor between the plug and the shield
on one end of the cable.

Hope this helps
D.

On Mon, Aug 23, 2010 at 04:31, Logan Mitchell Sr
<prowlerraven32 at hotmail.com> wrote:
> 1. I have a Nady MM-242 mini mixer that can use a 9 volt battery or an
> external power supply (wall wart).
>
> 2. The mixer's external power supply jack uses a negative electrical tip.
>
> 3. The 1/4" audio input & ouput jacks are held in place against the mixer's
> metal chassis by plastic mounting nuts.
>
> 4. When I have one of my synths connected to the mixer then have its output
> connected to an amp, I get a humming sound until i place one of my fingers
> against the mixer's chassis & another finger of the same hand against the
> metal sleeve of the connecting audio cable that's either going to the
> mixer's input or output jacks.
>
> 5. Since the power supply uses a negative center tip, if I were to solder a
> grounding wire from the audio jacks common ground pc trace inside the mixer
> then connect the other end of that wire to the mixer's metal chassis, would
> that cause an electrical short ?
>
> Logan
>