Editor’s Note: In the development of his highly respected digital audio components, Scott Berry of Computer Audio Design discovered the impact of High Frequency ground noise. Later, he found this signal ground noise also impacted on the performance of analogue components in an audio system. The result was the Ground Control range of high frequency noise reduction devices, including the mighty GC-R reference model tested in issue 185.
We spoke to Scott about ground noise, why it’s a problem, how the signal ground differs from the mains earth, and how this issue can best be resolved. While, of course, Scott’s focus is on his own GC and GC-R devices, it’s clear the resolution of high-frequency noise through the signal ground is becoming more of a universal concern among audiophiles and a plethora of products from different brands address the same concerns.
The CAD Ground Controls are aimed at reducing High Frequency “noise” on ground in an audio system. Can you start by explaining what ‘ground’ is?
‘Ground’ is the general name electrical engineers give to the reference against which a voltage is measured. In audio components, we call it ‘signal ground.’ Signal ground is the negative side of RCA, XLR, BNC etc. connectors on the back of audio components.
Signal ground is often confused with Mains Earth – on the mains power connection – that ‘third pin’ on a 13A UK or Type B USA plug.
In some audio component designs, signal ground and earth may be directly connected – but in most, they are not. For example, in many streamers and servers, and certainly in non-audio quality computer devices, the signal ground is typically directly connected to Mains Earth. However, in most higher-end audio components, there is typically either no connection, or a simple filter connection between Mains Earth and signal ground.
In a classic linear powered audio component design, mains power comes into the device and the power line and neutral go to a transformer, whereas Earth is typically directly connected to the chassis (case). The signal ground comes from the DC circuitry after the transformer – at which point, it has nothing to do with Mains Earth.
What is the purpose of Mains Earth in an audio product?
Mains Earth is there primarily for safety. Any electrical device that plugs into Mains power, and which has a conductive case (metal), must by law have that case connected to Mains Earth. And, the connection must be what is called ‘low impedance,’ meaning, easy for current to flow through.
This is to mitigate the risk of electrical shock if the device develops a fault. For example, if an internal wire becomes disconnected during shipping, and by chance touches the metal case, then if you plug that device in at the wall, the metal case will be live – if you touch it, mains current is going to try to go through you! By connecting a low impedance route to earth from the case, current will go through that route rather than going through you.
Mains Earth is also used by electrical designers to reduce high frequency noise within components. Attaching a ‘noisy’ part of circuitry directly to Mains Earth can reduce the noise within the product.
Computers are a good example of this. Computers produce a significant level of high frequency noise and the common method to reduce this noise (so that they will pass legal requirements) is to attach the entire signal ground of the device directly to Mains Earth. There are many other electronic components throughout our homes that are designed in a similar manner. The number of these devices has increased dramatically in the last fifteen years.
Unhelpfully, because of this, Mains Earth in many homes now contains a substantial amount of high frequency noise. This is a real problem for higher quality audio components! Attaching any part of an audio component to Mains Earth can actually increase the noise within the audio component instead of reducing it.