First One Build: Ground Connection

LC recommends the following ground wiring (basically a ground lift) for the amplifier modules [link]:

Improved schematic for stereo connection in a single chassis. GND potential of each channel is lifted from chassis-earth potential, meaning connection is done via 1 k resistor and anti-parallel diodes. In this way EARTH potential interference currents are isolated from GND potential. At the same time GND potential of each channel also isolated in between.

The purpose of the ground lift device (the diode-resistor-diode) is a compromise between best sound and safety [link]

Since I don’t want GND to be complete floating I tied GND from both channels to EARTH potential via a DRD (diode-resistor-diode) chain. It is a compromise needed for a safety reasons.

(In fact…)

Best sound is at complete GND to chassis-earth isolation, so no DRD present. My demo amp, which I’m just listening at the moment, is in complete GND isolation (to EARTH).

The user is encouraged to install a switch that can short the ground lifting the device (the 1 k resistor and anti-parallel diodes) and experiment with both options [link]

You can even install GND lift switch on the back panel, shorting the DRD. So one position for GND (direct GND to EARTH connection) and another for GND lift connection (GND to DRD to EARTH connection)

So basically it is a “lifted ground” connection.

EARTH CONNECTION?

In the recommended hookup diagram above, the GND terminal of the Supply is connected to the GND terminal of the amp module. This is the obvious normal connection for proper operation. Each module has a single return path to the supply.

It is also necessary to connect the earth wire to the chassis for safety. This is to prevent exposing any harmful voltage in case a failure happens. Only if you have a “double insulated” chassis, then you can dispense with the earth connection (and this is what is called “Class II” appliance).

If one looks at the Hypex power supply specifications [link], they are built as safety Class II devices. This means they are already isolated with the minimum 6 mm from all possible conducting parts (its own metal frame). And can in theory be installed in a chassis without EARTH connection if you follow the double-isolation approach (meaning among other things that the wire you use for mains wiring has to be double insulated and having the expertise to double insulate everything else).

But in the normal approach of having an EARTH connected chassis, then the power supply’s metal frame becomes also connected to EARTH and with the 2-wire mains terminal, the power supply is in effect connected to the 3 mains wires in compliance with safety standards.

So from a safety point of view, signal GND connection to EARTH is not really a safety requirement (since the chassis is already connected to EARTH)

Now the question is “what is the purpose to connect the components GND terminal to EARTH?”

The answer might be in this application note from Hypex [link] where it says:

I can’t recommend separating the audio ground from the chassis ground, because that’s a recipe for making a radio receiver

So the reason is to prevent picking up electromagnetic radiation in the environment. And the best thing to prevent this is to connect the signal GND to the chassis EARTH.

The paper gives the following options:

• If you want to use RCA inputs, disconnect the mains earth and employ double insulated construction techniques.
• Use balanced (XLR) inputs. This allows the whole thing to be earthed unless the ancillary equipment has problems.
• Make a “pseudo-differential” RCA input. I still haven’t figured out whether or not I should post a detailed description of how to do this, because unless I manage to explain with perfect clarity it’s almost certain to generate large volumes of mail.
• Anything else (e.g. floating the amps inside a grounded chassis), but then you’re on your own if you hear your mobile through the speakers.

Thus connecting signal ground to EARTH it is about noise immunity.

Indeed, according to this article from RANE, signal GND must be connected to Chassis GND (which in our case, it is connected to EARTH ground) [link]

It is easy to confuse chassis ground and signal ground since they are usually connected together — either directly or through one of several passive schemes. The key to keeping an audio device immune from external noise sources is knowing where and how to connect signal ground to the chassis.

First let’s examine why they must be tied together… There are at least two reasons why one should connect signal ground and chassis ground together in a unit.

One reason is to decrease the effects of coupling electrostatic charge on the chassis and the internal circuitry. External noise sources can induce noise currents and electrostatic charge on a unit’s chassis. Noise currents induced into the cable shields also flow through the chassis — since the shields terminate (or should terminate) on the chassis. Since there is also coupling between the chassis and the internal circuitry, noise on the chassis can couple into the internal audio. This noise coupling can be minimized by connecting the signal ground to the chassis. This allows the entire grounding system to fluctuate with the noise, surprisingly providing a quiet system. Further coupling reduction is gained when the chassis is solidly bonded to a good earth ground — either through the line cord, through the rack rails or with an independent technical or protective ground conductor. This provides a non-audio return path for any externally induced noise.

The second reason to connect signal ground to chassis is the necessity to keep the signal grounds of two interconnected units at very nearly the same voltage potential. Doing so prevents the loss of system dynamic range where the incoming peak voltage levels exceed the power supply rails of the receiving unit.

WHY USE GROUND-LIFT?

According to this document on audio grounding [link]

Some people believe that it is necessary to isolate the system star ground from the chassis and safety ground in order to have a hum‐free audio system. However, if all of the components in the system have their grounding implemented properly, there is absolutely no need for ground isolation,

Although isolating the grounds may eliminate a ground loop, it does come with two penalties:

• First, since the signal reference (signal GND) is not directly connected to the chassis, the chassis is not an effective shield for the electronics
• Second, since the power common is isolated from the safety ground and connected to the signal reference, any AC leakage current from the power supply may flow through the signal reference to get to the safety ground in another component.

If you must isolate the grounds; never, ever, for any reason, disconnect a safety ground (chassis connection to EARTH ground) or fail to provide a safety ground in any equipment that you build. First, it is unsafe and second, there are equally effective methods of isolating grounds that do not come with the safety hazard. The following figure shows two such methods.

First is to provide a “ground lift” switch between the two grounds to be isolated…

A better solution is to provide a Safety Loop Breaker Circuit (SLB). This circuit will allow the current from a fault to flow to the chassis and also provide ground isolation under normal, non‐fault conditions.

You can find a circuit for a ground loop breaking (or SLB) from Elliot Sound Products [link] which is in principle similar to what LC is proposing. Thus the recommended circuit provided by LC is the proper method to avoid ground loops and be able to interface with upstream components with less than ideal ground implementations.

In my diy builds I’ve never connected the signal GND to EARTH, but have always connected EARTH to chassis. I think it would be a good idea to try connecting the signal GND to EARTH with a break switch to compare.

DOES GROUND-LIFT COMPROMISES SAFETY?

Based on the discussion above, ground-lift (that is not connecting signal GND to EARTH) does not seem to compromise safety. The fact that you can purchase double-isolated appliance with a two-wire power plug also says that it is not required to have a current path to EARTH in case of a fault. But don’t take my word for it. I am not a safety expert, just using a little bit of common sense. In my projects, I never connect the signal ground to earth ground but ALWAYS connect EARTH to the chassis and when connecting EARTH to chassis is not possible (like using a wood plate) then I make sure there is plenty of air gap between the component and my fingers…