What's this resistor for?

[JonnyP]

This is from the Capi VP26 500 series preamp. The top side of the board has a copper pour connected to the signal ground and power ground pins, and the bottom side has a copper pour connected to the chassis pin. This resistor is connected between them. What's it for?

Sorry if this is obvious to you guys. I'm still learning.

 

[emrr]

Offsets chassis reference from ground, gives some isolation, potentially good for noise.


Prob wrong….

 

[JonnyP]

Thanks for the reply.

On the schematic the resistor is drawn along side the ground pin of the 48v LED. I know that LEDs can be noisy sometimes.

Is it common practice for indicator LEDs to be powered off the 48v and then ground to chassis? even for other indicators like pad, phase etc. ?

 

[ccaudle]

Offsets chassis reference from ground

In the ideal case of one connection between the circuit reference 0V node (aka "circuit ground) and chassis, there should not be any current flow other than parasitic leakage between the circuit and chassis, and so there should be no voltage offset across the resistor (other than several microvolts of power line noise which may be flowing through parasitic capacitances in the power supply).

It would be instructive to check the DC and AC voltage across that resistor with a sensitive voltmeter, or better yet an o'scope, to see if there is actually any current flowing there.

 

[Matador]

If the 48V is used for phantom power, and ultimately connects to an XLR plug, that may explain some of the weirdness. Phantom power is strange in that the return current for the out-bound device flows through the XLR shield, which is tied to the chassis as it enters whatever enclosure it is connected with. So some consideration needs to be made between analog 0V and chassis, which ideally should just be tied at a single point with no intermingling currents.

I think in an ideal world, there would be 4 conductors flowing down an XLR cable, the two differential audio signals, and a power and ground rail. Pin 1 should only ever be used to extend the chassis along the cable, and never used for anything related to the audio signal. In such a system, the 500 series PCB wouldn't need to consider anything to do with the chassis or it's potential, as it would be all handled by the rack enclosure itself (and the PSU would contain the single analog 0V to chassis connection).

Also in an ideal world, that resistor can be removed and the system should still fully function, with the exception of the weird shield/chassis/PSU 0V mixing that happens in any phantom powered system.

 

[ccaudle]

Also in an ideal world, that resistor can be removed and the system should still fully function

The problem with removing the tie between circuit reference and chassis is that unless the power supply reference node is tied to chassis somewhere else, the common mode voltage of the circuit with respect to chassis is only determined by parasitic leakage currents. Depending on what the transformer capacitance values are, and whether you have power line filter capacitors or not, that could be anything between a few mV and the full power line voltage (at very high impedance), with half power line voltage being relatively common because of the common filter capacitor configurations. In Europe that could be 120V AC common mode voltage, which is pretty difficult for the other equipment connected.

 

[emrr]

Jeff has gone into great detail about this in CAPI threads.

 

[JonnyP]

Found it. Thanks. I had searched the thread previously but didn't find anything. This time I searched "10r"

"All that [R14] does is connect the pin-1 card edge gold finger to pin-5 card edge gold finger. Its a safety feature in case a rack has the pin-1 card edge floating in the rack."

 

[Somkereki]

Eltolja az alváz referenciaértékét a talajtól, némi szigetelést biztosít, potenciálisan jó a zaj szempontjából.


Valószínűleg rossz….

egyetértek! Részletesebben kifejtve megakadályozza, hogy nagyobb áramérték alakuljon ki az alváz és a jelföldelés között. Ez bevett gyakorlat a hangtechnikában.

 

[gyraf]

..sorry, in english?

 

[warpie]

I'm not an expert by any means but I think connecting audio ground (0V) to chassis via a resistor (and sometimes a cap in parallel) can be dangerous and if I'm not mistaken it's not allowed on commercial products..

EDIT: I also found this

https://groupdiy.com/threads/grounding-through-a-cap.18880/#post-220865

 

[emrr]

See post #8

 

[warpie]

What about post 8?

 

[JonnyP]

Audio ground is not connected though the resistor. The resistor is just incase pin 1 has no connection.

 

[emrr]

In fact I had a rack with this problem

 

[warpie]

Sorry but I don't get it. Effectively 0V is connected to chassis (and PE) via a resistor, no?


EDIT: OK, after a closer look it seems that 0v and PE are connected on the right-hand side (i.e. external power supply) directly with no resistor in-between, right?