Hi-End Audio DC Power Supply with Multiple Voltage Outputs Architecture and Design
Last few years we have seen the rise of hi-fi audio equipment using external DC power supplies. With CAS (computer as source) becoming more popular, it is not uncommon for users to require multiple DC power supplies of different voltages and current ratings to power different equipment. A high end audio desktop system case would be as below:
- DC power supply for Mac Mini as source (with Mac Mini linear power supply kit)
- DC power supply for the external HDD used to store the audio files (Flac/Wav etc)
- DC power supply for a USB re-clocking/clean up device
- DC power supply for a desktop digital to analog converter (DAC)
- DC power supply for a headphone amp.
There is quite a number of DC power supplies needed for a system.
Traditionally, these DC power supplies are SMPS based (switched mode power supply). Equipment manufacturers include these due to cost reasons (SMPS are cheaper, lighter to ship and wide mains AC voltage). There are now manufacturers of aftermarket linear power supply to replace these SMPS for improved performance.
Unfortunately these linear power supply are also much bigger in size than their SMPS counterpart. Coupled with the fact that you may need a number of these to power the hifi system, it is an issue of space and cost for the end user.
To solve this issue, we are now seeing manufacturers coming out with linear power supply with 2 or more DC voltage outputs.
This article focuses on the system issues of a design that is most commonly adopted for a linear power supply with two or more DC output voltage, in a simple and graphic manner.
Typical DC Power Supply with 2 DC Output Voltage
The above shows the most common architecture on how a 2 voltage output DC power supply is designed. I will reckoned this is representative of more than 90% of 2 (or more) output DC power supply.
There is 1 power transformer and a capacitor bank, which forms the main power supply. This is fed to 2 different regulator circuits, so that each can be individually adjusted for different voltage output (in this case, 12Vdc for Mac Mini and 15Vdc for a DAC). The ground is connected to all circuits for proper operation. Take note of the ground connection between the Mac Mini and DAC (thru the USB cable connection). Its significance will be described later.
As long as the total DC RMS current of the 2 equipment does not exceed the capacity of the main power supply and the respective individual regulator circuit, this circuit will work.
Let us take a deeper look into how the power supply noise will affect the whole system. In particular, we are interested in how the noise generated on the power supply from one equipment (Mac Mini) will affect the other equipment (DAC). For simplicity sake, the ground connection is removed. See below:
The first thing we need to establish here is that regardless of claims, what all regulator design does is to attenuate noise (consist of many forms). It reduces noise as much as its circuit allows. It tries to make its output as stable as possible. It does not eliminate noise. Better designs attenuates noise more and under extreme external conditions. That is it.
In our example with our typical architecture, the Mac Mini, as the primary noise source, generates noise on the DC power supply on the 1st DC regulator. The 1st DC regulator tries to attenuate this noise (as it is supposed to do) and in turn draws current of different frequency and magnitude in an attempt to “quash” this noise. This actually induces another noise on the mains power supply, albeit usually at a lower magnitude. It can be reduced by the proper use of capacitor banks on the main power supply. The speed and bulk value of capacitor bank also affects the magnitude of this induced noise. Nevertheless this noise does exist.
Now the 2nd regulator, which shares the same main power supply, also gets this induced noise at its input. It tries to attenuate this noise. A portion of it will be leaked to the DC voltage supplied to the DAC.
This action is vice versa and the DAC will induce another noise in a similar manner onto the main power supply. A portion of it will be leaked to the Mac Mini as well.
Thus it can been seen that in such a design with a shared main power supply, noise from one equipment does affect another equipment.
Now we take a look at a more disturbing issue with such a power supply design, the grounding. As previously done, we now remove the power supply lines in the picture 1 and shows the grounding scheme.
Typical DC Power Supply with 2 DC Output Voltage
If there is a physical ground connection between the equipment that uses a shared main power supply and ground, a ground loop is formed. This connection could be a USB cable between Mac Mini and a DAC, USB data cable between Mac Mini and an external HDD/SDD, or even a Mac Mini and a USB clean up device.
The detrimental effects of ground loop is very well documented and many information can be found over the internet. Among the problems it introduces to the Hifi system includes digital noise pollution to the analog ground, creating noise in the analog circuits. EMI / RFI noise emission to and noise pickup from (antenna effect) the surrounding pollutes the whole system as well.
There are ways to improve this ground loop issue with the use of galvanic isolators (note not all galvanic isolators treat the ground loop issues) or simply to add a resistor to the USB ground connection between the devices (to increase ground impedance). These remedies will not be 100% effective in my experience, and once a ground loop path is formed, it will be difficult to eliminate.
Thus it can been seen that in such a design with a shared main power supply, a ground loop is usually formed when the components using the same power supply are interconnected.
What then should be the best way to build a DC power supply with multiple outputs?
Ideal DC Power Supply with 2 DC Output Voltage
The above should be how a DC power supply with 2 DC output voltage should be designed and built for the best performance. An independent main power supply with no shared ground, for each DC regulator output. This will mean a separate transformer and capacitor bank for each output, or at the very least, a separate diode bridge and capacitor bank, with a share transformer.
In this way, there is maximum isolation of noise between equipment. Noise from the Mac Mini do not have a clear path to pollute the DAC from the power supply and ground. The system grounding scheme is a star ground at the equipment side, which is preferred.
Incidentally, this power supply design scheme is also common in high – end audio amplifiers. It is known as the mono design. In the above case, it will be known as dual mono (audio enthusiast will know what this means).
Note that at Sound Affairs, our PLiXiR range of multiple DC output and custom power supplies are all built with the mono architecture, right down to separate balanced transformer for each DC output. Such will ensure minimal noise interference between devices and no ground loop is formed with equipment connected to our DC power supplies.