[sdiy] Who apart from me things the whole MIDI board needs replacing ?

rburnett at richieburnett.co.uk rburnett at richieburnett.co.uk
Wed Jan 26 16:30:52 CET 2022


> I have solved a similar problem in the past with a custom jack cable
> with the braid connected with a 470 ohm resistor at each end.
> 
> Makes bugger all difference to shielding (because 470 ohms or 235 ohms
> if both ends are grounded is peanuts compared to the infinite
> impedance of the braid in free space), drastically reduces ground loop
> currents to the point where they don't have an effect.

That's a good point.

Often you can solve problems like this by connecting the shield to 
ground via a 100nF capacitor and 1Meg resistor in parallel (at one or 
both ends.)  The impedance of the parallel combination is low for 
high-frequency signals (and most noise that has come through the air 
before coupling onto your cable is high-frequency in nature,) and 
conversely the impedance is nice and high for low-frequency signals 
(like 50Hz "hum" currents due to earth loops,) so not much current flows 
at these low frequencies.  The 1Meg resistor prevents static build up 
across the 100nF capacitor, so it at least tries to keep them with the 
same DC potential if one of the bits of kit is "floating" relative to 
the other.

-Richie,



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