[sdiy] Jack to PCB "flying wires"

[Simon Brouwer]

David Ingebretsen schreef:
> I am undertaking several modules from various SDIY designers and I have
> studied various builders versions as well as the designer's documentation.
> However, I am still confused regarding wiring audio signals from jacks to
> the PCB.
>
> I have seen/built modules where the connection is:
>
> 1. just a piece of hook-up wire from the tip to PCB and ground to PCB
> 2. a twisted pair of hook-up sires from the jack to the PCB
> 3. Co-axial cable with:
>    a. shield connected at the jack
>    b. shield connected at the PCB
>    c. shield connected at both ends
>
> While I understand the general idea of ground-loop interference, I am
> confused by the variation in connections.
>
> Why would I do any one of these over another?

If the wire is short, is connected to a low impedance, carries high signal
levels, and is not close to sources of interference, the amount of
capacitively picked up interference may be low enough that a shield is not
needed. Otherwise you may want to use a shielded wire.

If the jack is already grounded via the panel you should connect the
shield only at one side, to prevent ground loops. If the jack is not
already connected to ground, connect the shield at both ends, so that the
shield in shielded patch cables is connected to ground as well. Except
when your patch cables have shield connected at both ends; in that case
only ground the jacks of outputs.

> Specifically, if I do the "one-sided" co-ax thing:
>
> 1. does it matter if I connect the shield to the jack rather than the PCB,
> and why?

Not really, assuming that the ground of the panel and the ground of the
PCB are essentially at the same potential.

> 2. does the shield really do anything if it isn't connected at both ends?

Yes. By tying the shield to ground, the potential that the center
conductor "sees around it" is ground, instead of a mixture of potentials
that includes the interfering signals on nearby wires.

> 3. if the co-ax really should only be connected at one end, why then, when
> using hook-up wire, do people run both a tip and ground wire between the
> jack and the PCB?

If the jack is already grounded otherwise: Out of misconception

> Last question, I've noticed a "star" type grounding on some modules where
> the ground of some jacks are connected together then run to the PCB with a
> one wire. How would I decide which jacks are to be "common"? EG, Mankato
> filter, there are three CV inputs and a 1v/Oct CV input which are summed
> on the board. If using hook-up wire on these jacks, would it make sense to
> tie the jack grounds together and just run one ground wire to the PCB?

Yes, if the jacks are not already grounded via the panel. Otherwise,
ground the panel using a single ground wire to one of the jacks.


-- 
Vriendelijke groet,

Simon Brouwer
-*- nl.openoffice.org -*- http://www.opentaal.org -*-