GND Scheme questions

[Davebassman]

Hello,

I've some questions about a good GND scheme. I read a few articles and I didn't see any consent about it. So, I draw two schemes. Wich one is better?

Scheme 1.

each XLR cable has its shielded cable. And chassis to pin 1. IEC earth anywhere bolt on the chassis, near the IEC connector.

Scheme 2.

Only first input have the shield cable to the pin 1. Pin 1 distribute shield to the others connectors, rest of the XLR cable only have shield at one end. Chassi and IEC earth soldered to pin 1 of input connector.

I read that the earth connection of the IEC socket need to be near to input power, bolt on chassis. Some others says that the earth connection need to be soldered at pin 1 of the XLR input connector.

Any suggests?

[dai h.]

hi, (I am no expert, but 1 (but with each XLR connector pin 1 having a short connection to chassis) looks more correct to me).

(My understanding) pin 1 should have a very short connection to chassis, otherwise you give opportunity for RF to come in and spray around (example: very recent Schoeps microphones have a sort of metal piece with fingers around it (pin 1 connects to this piece and 2 and 3 are isolated) that connects to the mic body (I think it's safe to presume it's metal). So the idea seems to be to make the pin 1 connection to chassis as low impedance as practical (I think this is *part of* their measures to deal with RF and not the only one). Other mics with a long wire from pin 1 to chassis inside the mic can apparently have RF problems.

For diagram 2, (my (again) non-expert understanding) the incoming safety ground should have a short and individual (not shared) connection (based on observation) to chassis. I think if was run really long as in the diagram, it could be an avenue for RF ingress? With the shared connection maybe a concern with human error (forgetting to put it back on if shared with something else) and maybe the possibility of it coming loose?

[pdf64]

2 seems a fundamental misconception, and a dangerous one.
Why make a direct, hard connection from pin1 to the chassis anyway?
It opens the door to earth loop hum if the unit is connected to other items of mains powered, earthed equipment.
See http://www.valvewizard.co.uk/Grounding.pdf

[dai h.]

(I'm presuming balanced ins and outs--which should take care of hum in general.) The shield is supposed to be a continuation of the chassis. Maybe google "pin 1 problem"? There are documents such as this:

https://www.hypex.nl/img/upload/doc/...1_problems.pdf

There was one long paper (and if I understood it correctly) making more ground loops was actually better since you make more giant conductive mass brute force equalizing of the chassis grounds. With unbalanced I thought one of the common problems, where 2-prong to 3-prong (added safety ground) converted amps had hum problems with other 3-prong safety ground gear was the grounding schemes not being arranged to be consistent (the low level stuff sees a path it shouldn't with the new safety ground at the power input end where there might be heavy power tube cathode currents and lots of ripple esp. from the power supply input cap with the high current charging pulses (as I understood)). But I could be wrong and those isolating networks (as in the Fender reissue Reverb) are necessary to deal with hum.

[Mick Bailey]

The rest of the circuit and also the intended application (including connected devices) have to be considered when looking at a grounding scheme for a piece of equipment, as the signal ground may be separated from chassis ground and connected to a single chassis point elsewhere, sometimes using an elevated ground (which is what dai h. refers to). If this is the case, any other connection to chassis can result in hum. Another issue is the XLR connection of equipment that has both digital and signal grounds (such as Kemper amps) where common chassis grounding to connected equipment can cause digital hash to appear on signal lines - special consideration has to be given to designing a workable scheme in this instance.

[Greg Robinson]

Check out this article by Rane.

[dai h.]

(Again, might be wrong here, but) I just keep thinking there must be some way to get safety-grounded gear and unbalanced connections to work together. (I just imagine a studio with (balanced, yes but) also many unbalanced inputs and outputs (mixers, effects, etc.).)

I mentioned in another thread that when I examined the grounds in the mixer section of my old Tascam 488mkII (as a learning exercise) I found a lot of ground loops, so it seems possible for them to be (relatively?) benign. Maybe sometimes even used to try to force points to be closer in potential?

[Mick Bailey]

Studios usually have DI boxes available to deal with ground loops, plus if necessary use pseudo-balanced cables. You don't necessarily know beforehand if interconnected (usually mains powered) equipment is going to be a problem, but it's handy to have the means to resolve a situation in a studio setting where random pieces of equipment may be required to work together.

[g1]

Agree with the others that IEC safety ground should be close to the receptacle and have it's own single connection. That is the law in many places.
Aside from that, lots of pro gear has a ground strap or switch where you can choose whether you want pin1 connected to chassis or not.

[Davebassman]

Thanks.

I need to wiring this way, and it's a bit confusing for me. It's correct the scheme that I draw?

I attach the schematic that I need and the drawing that I made.

The 1/4" jacks are isolated.

The output doesn't worry me, because is straight to XLR outs.

Someone suggest:

"INPUTS:
All XLR pin-1 inputs should always go immediately to chassis, either a wire to the XLR fixing screw or a PCB track from pin-1 to the XLR connector shell if you're using board-mounted connectors. No extra wire from pin-1 to circuit ground is required, circuit ground will be connected to chassis at the place which suits the amplifier circuit best, often a star in the power supply.
IEC earth should go to chassis close to IEC connector.

OUTPUTS:
If you're using transformer-balanced outputs, all XLR pin-1 outputs should go immediately to chassis at the nerest screw, just like the inputs.
If you're using opamp-based output drivers, run the XLR pin-1 outputs back to circuit ground near the driver stage."


I see no consensus on this issue.

[Mick Bailey]

Do you have a full schematic? There's no indication of what circuit is being powered and how this relates to the inputs/outputs.

FWIW, the last box I built for a pro guitarist combined mixed balanced/unbalanced connections and op-amp drivers and just used regular metal chassis-mount Amphenol connectors with pin 1 internally grounded to the shell. This box connected an unbalanced guitar input, 2 FX loops, dedicated active volume pedal loop, balanced XLR input, balanced XLR output to desk, Balanced XLR input from desk to in-ear monitor feed, and output to tube amp, plus some switching. Mains input grounded immediately adjacent to IEC socket. Circuit (signal) grounds star connected to Hammond enclosure. Connecting XLR cables grounded at one end only. This works hum-free, but the caveat is I had a clearly defined connection diagram from my customer and so knew what was being connected.

[Davebassman]

Do you have a full schematic? There's no indication of what circuit is being powered and how this relates to the inputs/outputs.

FWIW, the last box I built for a pro guitarist combined mixed balanced/unbalanced connections and op-amp drivers and just used regular metal chassis-mount Amphenol connectors with pin 1 internally grounded to the shell. This box connected an unbalanced guitar input, 2 FX loops, dedicated active volume pedal loop, balanced XLR input, balanced XLR output to desk, Balanced XLR input from desk to in-ear monitor feed, and output to tube amp, plus some switching. Mains input grounded immediately adjacent to IEC socket. Circuit (signal) grounds star connected to Hammond enclosure. Connecting XLR cables grounded at one end only. This works hum-free, but the caveat is I had a clearly defined connection diagram from my customer and so knew what was being connected.

This is the build.

http://jhaible.com/legacy/subtle_cho...le_chorus.html

If you see the schematics in this page:

"Main board schematics, page 2"

http://jhaible.com/legacy/subtle_cho...d_sch_dwg2.pdf

You see the "interface" connection that is confused for me.

I want to build it with XLR and unbalanced connections.

http://jhaible.com/legacy/subtle_cho...o_balanced.pdf

If you see the unbalanced scheme and compare to "only balanced connections", you see that one pin of "interface" are tied to gnd if you use balanced, and unconnected if you use unbalanced.

http://jhaible.com/legacy/subtle_cho...unbalanced.pdf

http://jhaible.com/legacy/subtle_cho...anced_only.pdf

[Davebassman]

I found this in an old forum. It was written by the designer himself.

The right pin of the Interf connector is a control pin that changes the internal gain of the circuit, allowing the same (unity) gain for both balanced and unbalanced connection.

See: http://jhaible.com/legacy/subtle_cho...d_sch_dwg2.pdf

Doesn't have anything to do with plugging an unbalanced output into a balanced input, which *is* ok. Just don't plug the balanced output of the Dimension D into an unbalanced input!

[Davebassman]

From the same old forum.

electro-music.com :: View topic - Dim D


The output stage of the "vintage" version is just that: a faithful reproduction of the vintage circuit, for those who want it exactly like the original. Personally, I would not design an output stage like this, powerful output stages with just signal inversion (and no auto-balancing). I'd go for short-circuit protected opamp outputs (my unexpensive version), or a transformer output, or an electronic transformer.

[Mick Bailey]

The silk screen overlay shows that the signal grounds are already connected via the PCB. Wiring the XLR/unbalanced connections exactly as per the diagram is how I'd wire it. I took a look at the original rack mount Dimension D, and pin 1 of the XLRs are all connected to signal ground, whilst the XLR shells are connected directly to chassis ground. Roland perhaps expected the use of cables where pin 1 is not connected to the shell. The pictures of the prototype unit you're building show the boards isolated on nylon standoffs, though that unit is wall-wart powered. It may be worth considering connecting the PCB signal ground to chassis ground via a single connection using 'loop breaker' as per Figure 5 in the link. I've used this method with good success (though using antiparallel diodes rather than a bridge).

https://sound-au.com/earthing.htm#s6