09 Mar Filter for Switching Power Supply
There are a number of brands on the market that use wall-mounted switching power supplies (SMPS) to power their audio equipment. Topping, Cambridge Audio, Pioneer, Pro-ject, Rega and even the big prestigious brands, do not hesitate to move the power supply out of the equipment, significantly reducing its weight and volume. It’s not hard to find DACs, streamers, headphone amps, or preamps configured this way.
However, there are concerns that the high frequency noise from the SMPS circuitry could compromise the sound quality of the audio equipment it is powering.
In a DAC, for example, intermodulation could be caused between the image frequency of the audio signal and the harmonics of the switching frequency.
Or if crosstalk occurs with the DAC reference voltage, the DAC will multiply the source ripple frequency with the sampled audio signal. The sampled audio signal has copies of the audio spectrum around all multiples of the sample rate (image frequencies). If these image frequencies are multiplied with the ripple frequency, it may be the case that they jump to the audio band.
To avoid these problems, we have designed a passive filter circuit to remove much of the high frequency noise from the SMPS source. After some experimental tinkering, the circuit consists of two LC filtering stages, built with SMD (coils and resistors) and through-hole (capacitors) components. Depending on the type of power supply, noise attenuations between 8 dB (2.5x) and 23 dB (14x) can be achieved. The DC resistance (DCR) is approximately 100 mΩ.
The filter circuit is arranged on a small 35mm by 40mm board on which the wires are directly soldered with the I / O connectors. The unit mounts inside an insulated miniature plastic box where hot glue is applied to hold the cables securely in place.
The maximum input voltage of the filter is 35V, supporting a maximum current of 3A. With these voltages and current value we are within the limits of most portable audio products on the market, from those that use the 5V 1A USB power supplies, to the 24V that Naim uses, through the 12V 2A of Pro-ject, Rega or Topping. The customer will only have to determine the type of connector of the power supply. Originally, it has the standard 5.5mm diameter connector with a 2.1mm internal pin, the internal pin being the positive. In case our audio equipment has another type of connector, we will need to add a suitable adapter.
Below, we show the results of various tests carried out on the filter using different types of wall-mounted SMPS power supplies on the market. In the graphs, the signal from the power supply entering the filter corresponds to the yellow trace. The filter output signal corresponds to the blue trace.
Example #1. Power supply 5V 2,5A (fc = 250 kHz)
Example #2. Power supply 5V 3A (fc = 112 kHz)
Example #3. Power supply 12V 1,7A (fc = 58 kHz)
Example #4. Power supply 12V 1A (fc = 41 kHz)
Sweep frequencies and attenuation.
Not all switching wall power supplies are the same. Some are better than others, presenting very low output ripple. But in general, due to the low cost they have, the quality of the DC they take out is very poor. A good filter between the source and our equipment will guarantee us a high quality supply, very stable and free from unwanted frequencies.