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 Post subject: Shielding Issue
PostPosted: 11 Aug 2014, 16:31 
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Joined: 04 Jun 2008, 20:59
Posts: 3987
Location: Arizona, USA
Hi Everyone, Just when you think you have things figured out something comes along that while obvious in retrospect is not obvious when first encountered. This has to do with grounding and shielding in sensitive audio circuits. Picture a switch box for selecting two low output moving coil cartridges (0.3mv). Simple right, no not really if you want a S/N of over 90db. The shielded wires come in and transition to single conductor wires to the switch. The box is cast metal. The input and output jack grounds are all tied together. The box is grounded to the system ground point (I use a sort of star point for all chassis). The signal ground in the box is not tied to the chassis. Looks like it should work, it doesn’t. The box if moved near anything with a transformer it will pick up hum. Not a lot, but enough to upset the -90db S/N. What appears to happen is the shielded cables are interrupted by a section (the wire in the box) that is not shielded at the same exact potential as the shields themselves. It is a small difference for sure, but sufficient to allow hum pick up. The solution is to shield the wires in the box, or at least use a twisted pair with a ground arrangement. Hum is gone.

Good listening

Some of my DIY Tube Amplifier Projects:

 Post subject: Re: Shielding Issue
PostPosted: 12 Aug 2014, 03:11 
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Joined: 08 Aug 2009, 03:11
Posts: 2229
Location: Chilliwack, BC
Great advice, Bruce! :D

When I use a cast box, I ground it to the left channel only and use a twisted pair of telecom wire (most people don't know the stuff is OFC!).


-= Gregg =-
* Ratings are for transistors - tubes have guidelines*
Home: GeeK ZonE
Work: Classic Valve Design

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