[sdiy] Matrix 1000 grounding question

[harry]

Hi Tom:

My Matrix 1000 is noted for acoustic hum. The cheap-o power transformer
magnetic field that escapes rattles the top and bottom lid.

If that is possible... it might be that the magnetic field is causing
circulating currents
in YOUR chassis as well... and that for that reason the earth ground and
signal
geouns are not the same point as the designers intended...

Whatever wiring works for you, I'd do that.

I'm convinced that all (other) engineers are either INCREDIBLY stupid... or
geniuses so far beyond my ken I can't even dream of what they
accomplished...

The designers MIGHT have been thinking that if a voltage hit (static, etc)
came up
the output... they might shunt it to the most capable ground and away from
the
circuit board...

Who Knows ???

H^) harry

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.
>
> 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.
>
> So I tried connecting the three capacitors to signal ground instead of
> to earth ground.  This also solved the hum problem, but left the
> signal and earth grounds with no connection at all which doesn't seem
> like a good idea.  This is where my understanding gets fuzzy.  I
> believe the reason for connecting the grounds with various
> combinations of caps, diodes, and maybe even resistors have to do (at
> least) with using the chassis as an RF shield, and with possible
> faults between the primary and secondary windings of the power
> transformer.
>
> It turns out that connecting the two caps in the pi filter to signal
> ground and connecting the cap at the jack to earth ground also solves
> the problem.  In retrospect, this is perhaps obvious, since any signal
> on the chassis will be coupled into the output via the caps (right?),
> but I guess I was giving the designers the benefit of the doubt.
>
> Does this seem like a reasonable way to have things grounded?  How
> does the capacitor allow the chassis to function as an RF shield -- I
> don't see the complete circuit that allows current to flow in the cap?
> Maybe this some RF and/or ground loop voodoo.  If this were running
> off a wall wart would I just want a wire between signal ground and the
> (non-earthed) chassis, or nothing at all?  And what were the designers
> thinking when they connected the pi filter caps to earth ground?
>
> Tom.

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