[sdiy] Matrix 1000 grounding question

[Ingo Debus]

Tom May wrote:
> 
> So I've got this Matrix 1000, and they seem to be noted for their hum.
> As it turns out, the hum doesn't vary with the volume setting which
> makes it look like a grounding problem.
> 
> Indeed, here is the output section of the M-1000.  I'm not really sure
> about the size of the inductor but I don't think it's important for
> this discussion:
> 
>              +
> TL081 out ----||-----/\/\/\/--+---@@@@@@---+--------------------- output
>              4.7uF   100ohm   |    20mH?   |         +-------+--- jack
>                               |            |         |       |
>                          1nF ===      1nF ===      signal   === 1nF
>                               |            |        gnd      |
>                             earth        earth             earth
>                              gnd          gnd               gnd
> 
> The 1nF cap at the output seems to be the only connection between
> earth and signal grounds.  Earth ground is the chassis, connected to
> the green wire in the plug.

The pi filter is most probably for EMC. Perhaps it was added afterwards,
when they found out that the unit didn't pass the EMC tests :->
The ground of an effective EMC filter has to be chassis ground, as close
to the output jack as possible. If you connect the caps of the filter to
some other ground, the filter might become ineffective. But if you don't
encounter any EMC problems then, why not do it this way? You can even
try and throw out the filter completely.

> It turns out that if I use a 3-to-2 adapter on the Matrix's power
> cord, the hum goes away completely.  But I would prefer to leave the
> safety ground in place.

Yes, I wouldn't do that either.

Ingo