How well a system performs depends a lot on how well you feed it, this is true for living organisms , automobiles and electronics. When We started exploring the best way to power our Player, we had to choose between SMPS and Linear power supplies. Linear power supplies use large transformers and an elaborate rectification and filtering circuit which allows the pre and power amps to perform well. Noise in the power supply will eventually trickle into the audio signal and will become audible. For example if there is a 100mv power supply ripple noise in the power supply on a 1v Peak to Peak audio signal that is 10% of the signal and will get amplified by the power amplifier just as much as the audio signal.
The advantages of Switch mode power supplies is that they are small and efficient so for a given amp rating, the power supply is much smaller. The downside is that it is pretty noisy.
On the other hand linear power supplies can be very quiet but if the current requirement
is large then we need large heavy transformers and big filtering circuits. Large
transformers come with their own problems of noise and EMI interference in sensitive pre-amp circuits if they are wired too close.
We stared testing and measuring various power supplies to evaluate the performance and found that SMPS was pretty noisy , linear power supplies were quiet but even their noise level started increasing when we increased the current draw (power).
We needed about 5v 3 amp for the SBC , 5V 50ma for the DAC and about +/- 15 100ma for the preamp. We initially wanted to use a single linear power supply but found that whenever the SBC’s power consumption increased when WIFI was active or when a large file was being downloaded, it increased the noise floor of the Linear Power supply. This introduced the noise into our pre-amp circuits.
So we decided to use two power supplies one for SBC and one for the DAC and the Preamp, we ended up using two transformers one with a 5A rating and one with a 1A rating, this created additional headaches of EMI HUM noise from the transformers. So we decided we needed a simpler solution.
Solution:
We are using a 5v 10A SMPS for the SBC, MCU for Remote, Front panel,
protection circuits, Input and speaker Switcher Relays,where the audio signal never travels.
We have a separate Linear power supplies with a torroidal transformer to supply power for the DAC and the Preamp. Thiese power supplies have CRC filter ,CLC filters and large power caps to ensure extremely low noise (less than 5mv and full power draw).
We have taken extreme care to ensure the transformers are as far away as possible from our sensitive audio circuits.
This is the Power supply board for the DAC with +5V
This is the +12 / 0 / -12 Supply for the Preamp. We can adjust the voltage up to +/- 18 V based on the opamps used.
Here we can see the measurements , This is the Noise in SMPS, this about 216mV . The frequency displayed is of the switching frequency of the SMPS.
Closer Look at the Waveform (250nS).
This is using a Linear PSU where the Noise is reduced and is around 17.2mV. and you can see the frequency is more closer to the AC Supply Freq.
This is the Noise level after CRC and CLC Filters. at just 6.4mV.
