Tis so. I was refering to not connecting sheild on either end being a bad idea. While you could probably get away with it in many applications a high RF environment soon ferrets out what works and what doesn't. Didn't mean to step on any toes... I quite like it here.
Doc
In that case you are using one wire as send, the second as return and the sheild/ground as ground refference on the pot. The sheild would then be connected to local ground on board end but only to low end of pot on volume control end. A stereo pot can be wired using two 8451 cables. Best is to run one cable per channel with send-return leads inside sheild/ground ref. Rather than using one cable for send and one for return. That might contribute to crosstalk between channels.
Doc
No... The ground at the pot end doesn't normally connect to anything else, such as chassis. The refference point (source connection) is at the amp board end.
Don't know what configuration for balancing you are using, but if it is a board that has common ground reference connections the balance board itself should not connect to chassis ground except through feed cable. The sheild connections should not create a full loop.
====GC_______________GC======= creates a ground loop through chassis (where GC=ground connection)
====NC__________________GC=== or =====GC_____________NC===== is correct. (Where NC=No connection) (=== is twin wires inside)
Doc
I wasn't talking about "chassis ground". I just meant the board's ground plane. By ghetto balance thing, I mean just two potentiometers, each controlling a channel. I'm just asking if the braided shield gets hooked up to the ground wire for the pot. I'll have to make a diagram so I'm understood.
I see that, but if I'm using a 3wire coax...?
Also what happens if I don't hook up the shield to anything?
I mean I could just use the extra 8451 that I have instead of the coax, jw.
probably nothing
but depends on your amp, and layout
I have very short wire connection, and only use ordinary very thin wire witout any shield
for long and complex run of wire, maybe you will need to connect the shield
depends
for the RCA inputs any time you run a signal thru a really "hostile" EMI environment you are usually only able to keep the coupling to common mode - which then requires a differential receiver to reject the (hopefully well balanced) common mode coupled noise
boxing up the PS in (perforated for ventilation) cheap iron sheetmetal contains the mag field somewhat
I'd put the inputs where the internal wiring doesn’t go near the PS
if your source is floating - battery power or 2 wire Class II (2 wire IEC plug, “double”, reinforced insulation) then the common mode could be rejected at that end
then it would be critical to keep the xfmr mag field induced V exactly equal in both channels, signal gnd – again shielded twisted pair could be better than 2 coax runs
a Class II source may “acquire” a earth gnd when say a DVD player is connected to Class I display
or you may have safety gnded source like a desktop PC or many receivers
then you could have gnd loop issues
avoiding the potential mag field coupling in the 1st place is the better choice
boxing up the PS in (perforated for ventilation) cheap iron sheetmetal contains the mag field somewhat
I'd put the inputs where the internal wiring doesn’t go near the PS
if your source is floating - battery power or 2 wire Class II (2 wire IEC plug, “double”, reinforced insulation) then the common mode could be rejected at that end
then it would be critical to keep the xfmr mag field induced V exactly equal in both channels, signal gnd – again shielded twisted pair could be better than 2 coax runs
a Class II source may “acquire” a earth gnd when say a DVD player is connected to Class I display
or you may have safety gnded source like a desktop PC or many receivers
then you could have gnd loop issues
avoiding the potential mag field coupling in the 1st place is the better choice
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So the input to the preamp (tube grid?) is what you are labelling as 'Return" ?
And you are using the shield as the 'ground' conductor?
I've been trying to follow this thread, and this points out my confusion - I keep wondering what is "the" (and yes, there of course should be one and only one) path to ground?
There are several terms for things which might or might not do more than one function: (signal) return, shield, ground, chassis (the last one should be obvious, but it also operates as a shield to outside interference). In recent posts, it's obvious that the "low side" of a volume pot goes to "a ground" somewhere somehow, but there are questions whether it's through a wire covered by a shield, or through the shield itself, and then there are confusing questions about whether "both ends" of the shield connect somewhere. If one end connects to the pot's low side, the other end certainly needs to connect somewhere.
"The Ground Article" is a good attempt at covering this, but it seems there's still a need for more. As far as diagrams, when it comes to shielding and grounding I'd rather see a physical picture or drawn diagram of the physical layout showing the wires and connections rather than a schematic. Schematics have a convention that "all wire connections are at the exact same voltage level and all things so connected have zero impedance between them." Attempts to show physical grounding connection in schematics are usually done with attempts to represent physical connections, which goes against schematic convention. It's no wonder it can get confusing.
schematics are meant to convey information - sometimes different levels of detail are useful
basic circuit topology may well ignore gnd and pwr routing with just the symbols dropped into the schematic to avoid cross overs, long non-signal runs
but schematics can be drawn with gnd, pwr lines, nodes indicating order of connections to be made, annotated with trace width/resistance requirements, components grouped with dotted boxes indicating gnd/pwr sub regions - maybe having "moated" off planes...
I have used all of the above in designs meeting Intrinsic Safety requirements to convey PCB build requirements both for the layout people and so the Intrinsic Safety elements of the final PCB assembly can be audited by the reviewing agency
it is not unusual to have 3 "gnd" symbols - "clean" analog gnd, "dirty" (often digital) gnd and Safety/chassis gnd all on one schematic, Star or Hierarchical gnd/pwr routing schemes can be implied by the schematic lines interconnecting these
basic circuit topology may well ignore gnd and pwr routing with just the symbols dropped into the schematic to avoid cross overs, long non-signal runs
but schematics can be drawn with gnd, pwr lines, nodes indicating order of connections to be made, annotated with trace width/resistance requirements, components grouped with dotted boxes indicating gnd/pwr sub regions - maybe having "moated" off planes...
I have used all of the above in designs meeting Intrinsic Safety requirements to convey PCB build requirements both for the layout people and so the Intrinsic Safety elements of the final PCB assembly can be audited by the reviewing agency
it is not unusual to have 3 "gnd" symbols - "clean" analog gnd, "dirty" (often digital) gnd and Safety/chassis gnd all on one schematic, Star or Hierarchical gnd/pwr routing schemes can be implied by the schematic lines interconnecting these
BENB,
the following sites (and any books by the two gentlemen, especially Electromagnetic Compatability Engineering, by H. Ott) should provide all the information you require on grounding, shielding etc.
Funny though. both say that for RF shielding the shield has to be connected at BOTH ends.
Tech Tips
EMC Information Centre
Class two eqipement often aquires a indirect ground connection via the units interconnects, and the mixture of class 1 and class 2 equipement can often cause poise problems that are even harder to solve. I am trying to find an article that explained it in detail.
the following sites (and any books by the two gentlemen, especially Electromagnetic Compatability Engineering, by H. Ott) should provide all the information you require on grounding, shielding etc.
Funny though. both say that for RF shielding the shield has to be connected at BOTH ends.
Tech Tips
EMC Information Centre
Class two eqipement often aquires a indirect ground connection via the units interconnects, and the mixture of class 1 and class 2 equipement can often cause poise problems that are even harder to solve. I am trying to find an article that explained it in detail.
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