Designing a Low-Noise System
Often an audio component may measure well on a test bench, but when connected to real systems found in typical homes, it may generate much more noise than that reflected in the bench-top measurement. At Anedio, we are acutely aware of this discrepancy, and we strive to design our equipment to perform well in real audio systems.
In a typical home audio system, there are usually three or more components, e.g., a CD player, Blu-ray player,TV, computer, music server, preamp, and power amplifiers. When these component are connected together, the total noise of the system is not merely a sum of individual noise sources. It may increase far more drastically because of synergistic interactions among components' grounds.
The main culprit is the ground loop, formed when two or more components are connected together. The presence of ground loops is evident in a noise spectrum that is in multiples of power-line frequency. Shown here is a poorly-designed system comprised of a computer sound card, a preamp, and a power amplifier. In the FFT measurement, you can see multiples of 60Hz, e.g., 120, 180, 240, 300, etc. What is alarming is that the ground-loop noise reach well into the critical midrange and even upper midrange, where the human ear is highly sensitive. Since these frequencies are not harmonically related to the input signal (1 KHz), they are more detrimental to our perception of music than the harmonic distortion at 2KHz, 3KHz, etc.
What then causes ground loops to be formed? After all, the grounds are shorted together with wires, so how can they be the cause of noise?
Perhaps, the most erroneous assumption in audio circuit design is to consider all the grounds to be equal or simply at 0 volt. But the grounds are never at exactly the same voltage. Ground is merely a local reference point, connected to other reference points called "ground" through a low-impedance path. The voltage differences may be ever so slight, but are sufficient to cause a drastic rise in the noise. Ground loops are a reality to be faced whenever two or more components are connected together.
The reasons for the differences in ground voltages are too numerous to cover adequately here, but among the many are stray magnetic and electric fields, leakage currents from the AC mains and other sources, and ground conductor resistance. These slight differences in ground voltage may not cause any harm as long as the components are left unconnected, but as soon as they are connected together through interconnects, loops are created, and unwanted currents circulate through the loops.
The simplified diagram shows the ground layout of a three-component system (e.g., a CD player, preamp, and power amp). For the sake of illustration, only the grounds are shown without the signal paths. Inside each component are chassis ground and circuit ground -- each at a slightly different voltage. In reality, there can be multiple circuit grounds within a single component: digital ground, USB ground, analog ground, etc. But for simplicity, we've lumped all these into the "circuit ground." But even with such simplification, there are already six grounds and six different ground voltages.
Once the three components are connected together through interconnects, ground loops are formed via the shield (ground) conductor of the interconnects, and unwanted currents circulate, as indicated by the pink lines. If we were to add another component to the preamp, we would add three additional loops, and the whole thing becomes rather complicated quickly.
Minimizing the Effect of Ground Loop
How then do we minimize the adverse effects of ground loops? In our design process, we believe it's important to keep the whole audio system in view and try to optimize the whole, not just individual components in isolation. We pay attention to where ground currents flow and optimize the circuit topology and PCB layout, we try to break the ground loop wherever possible, we galvanically isolate all digital inputs, and we carefully shield magnetic fields. There is not much innovation here, but simply old principles implemented with meticulous care.
The result of such optimization is shown here in the FFT spectrum of a complete system -- a computer playing a 1KHz tone through its USB port, driving the USB input of the D1 DAC, which in turn driving the A1 Power Amplifier. The noise due to the power line frequency and its multiples is below -118 dB, on the order of one part per million. Note that this is the actual noise levels you will get in a complete system when playing music through the Anedio D1 DAC and A1 Amp. It does not matter which digital source you use since all the digital inputs are galvanically isolated.
Steps You Can Take
To ensure the lowest possible ground loop noise in your audio system, we recommend the following:
Use One Power Strip for All Components: We highly recommended that the power cords of all components be connected to a single point — usually a single surge protector with multiple outlets. This keeps all components referenced to a single point, as far as it is practically possible, thus minimizing the differences in ground potentials among the components.
Isolate Cable TV Ground: If you have a cable TV connected to your audio system, ensure that the cable ground is galvanically isolated from that of the audio ground. The cable TV ground, tied to the earth outside your home, is at a significantly different potential and can cause audible hum if it is not isolated properly. If you suspect this is the case, insert a cable TV ground isolator inside your home, just before your cable set-top box or TV RF input.
Bypass the Preamp If Possible: If you listen to music only from digital sources, we highly recommend bypassing the preamp completely. The built-in volume control in the Anedio D1 DAC allows power amplifiers to be connected directly to the D1 DAC. Moreover, all digital inputs of the D1 DAC are galvanically isolated using high-performance digital transformers, cutting off potential ground loops right at the source. In most systems, you will find that bypassing the preamp is one of the most significant steps toward a higher level of transparency. With the ground loop noise eliminated, you can hear more easily reverberations of the hall against blacker background, and you can experience another layer of sonic space opening up.