Post by speakertom on May 26, 2021 19:11:13 GMT -5
A lot of people have been commenting on the necessity for replacing the switched mode power supplies supplied with many music servers with a sometimes large and expensive linear supply. I thought completely replacing the power supply would be unnecessary if a suitable filter could be made and inserted between the switch mode supply and the music server. I wanted to see what would happen if I retained the standard power supply and added a passive filter between it and the NUC. I decided to test that by making a filter for the output of the switch mode power supply that feeds the Intel NUC that acts as my music server. I have now progressed in that process and here is where it stands to date.
The concept is to build a filter that would effectively decouple the NUC from any noise generated by the power supply. To achieve that, there is an initial stage of high frequency filtration, followed by a large choke, and then several stages of output filtration including a very large storage pool of capacitance in stages. To ensure that the power supply would start into that very large capacitive load, a P channel enhancement Mosfet was used in series with the large filter bank on the output to provide a soft start function.
Another consideration since the filter was passive was to minimize any voltage loss across the filter. To achieve that, a choke with very low DC resistance was chosen with a value of 0.05 ohms. It is rated at 2.5 mh at 10 A. The Mosfet used also had an on resistance of about 0.02 ohms. The following is a circuit description:
Input filter capacitors C1 - C10 are 0.1 uf monolithic ceramic which are very good at bypassing high frequency signals. L1 is the choke mentioned. C24, C25, and R1 form a timing circuit to slowly turn on the series Mosfet. Q1 is the Mosfet that starts out at a very high resistance but as the capacitor C25 charges, lowering the gate voltage on Q1, it turns on slowly, eventually getting tho a very low resistance saturated condition. C11 is a large 10,000 uf electrolytic capacitor that provides the bulk of the filtration on the output and in conjunction with the choke forms a very effective low pass filter. C12 is a 1,000 uf electrolytic that has a series 0.5 ohm resistor to provide a step function to the filtration bank and also some damping. C13 is a 1 uf film cap that further provides a continuation of the filtration function of the output circuit. Output capacitors C14 - C23 are 0.1 uf monolithic ceramics that not only further reduce any high frequency noise that may get through the filter but also act as a noise sink to any noise present on the power input from the NUC itself.
As far as voltage drop across the filter, when connected between the power supply and the NUC with the NUC running and playing music, the input voltage to the filter from the power supply measured 19.65 volts, and the output of the filter going to the NUC was 19.60. Certainly, with the current being drawn by the NUC under actual operating conditions, the voltage drop is only 50 mv which is totally inconsequential.
I took measurements of the noise going to the NUC without the filter and then with the filter, both under actual operation conditions. In both cases the time base of the oscilloscope was set to 500 us per division and the vertical sensitivity was set to 50 mv per division. You can see that there are many high frequency spikes in the power going to the NUC with peaks reaching just under 200 mv. After running through the filter, with the same settings on the scope, the trace is basically a straight line with what looks like a very low level of white noise. These traces were made on my work bench with an open circuit with no benefit of shielding from a metal chassis.
At this point, I have not done any listening tests with the filter in circuit as that will have to wait until I get it in an enclosure. My next post will have some construction photos and the results of the listening test. I'll also include parts costs. Looking at the measured results I would say the project so far is a success in eliminating the noise coming from the switcher, making it look to the NUC as a linear supply. You might want to consider this as an option to purchasing or building an expensive linear supply.
Tom
The concept is to build a filter that would effectively decouple the NUC from any noise generated by the power supply. To achieve that, there is an initial stage of high frequency filtration, followed by a large choke, and then several stages of output filtration including a very large storage pool of capacitance in stages. To ensure that the power supply would start into that very large capacitive load, a P channel enhancement Mosfet was used in series with the large filter bank on the output to provide a soft start function.
Another consideration since the filter was passive was to minimize any voltage loss across the filter. To achieve that, a choke with very low DC resistance was chosen with a value of 0.05 ohms. It is rated at 2.5 mh at 10 A. The Mosfet used also had an on resistance of about 0.02 ohms. The following is a circuit description:
Input filter capacitors C1 - C10 are 0.1 uf monolithic ceramic which are very good at bypassing high frequency signals. L1 is the choke mentioned. C24, C25, and R1 form a timing circuit to slowly turn on the series Mosfet. Q1 is the Mosfet that starts out at a very high resistance but as the capacitor C25 charges, lowering the gate voltage on Q1, it turns on slowly, eventually getting tho a very low resistance saturated condition. C11 is a large 10,000 uf electrolytic capacitor that provides the bulk of the filtration on the output and in conjunction with the choke forms a very effective low pass filter. C12 is a 1,000 uf electrolytic that has a series 0.5 ohm resistor to provide a step function to the filtration bank and also some damping. C13 is a 1 uf film cap that further provides a continuation of the filtration function of the output circuit. Output capacitors C14 - C23 are 0.1 uf monolithic ceramics that not only further reduce any high frequency noise that may get through the filter but also act as a noise sink to any noise present on the power input from the NUC itself.
As far as voltage drop across the filter, when connected between the power supply and the NUC with the NUC running and playing music, the input voltage to the filter from the power supply measured 19.65 volts, and the output of the filter going to the NUC was 19.60. Certainly, with the current being drawn by the NUC under actual operating conditions, the voltage drop is only 50 mv which is totally inconsequential.
I took measurements of the noise going to the NUC without the filter and then with the filter, both under actual operation conditions. In both cases the time base of the oscilloscope was set to 500 us per division and the vertical sensitivity was set to 50 mv per division. You can see that there are many high frequency spikes in the power going to the NUC with peaks reaching just under 200 mv. After running through the filter, with the same settings on the scope, the trace is basically a straight line with what looks like a very low level of white noise. These traces were made on my work bench with an open circuit with no benefit of shielding from a metal chassis.
At this point, I have not done any listening tests with the filter in circuit as that will have to wait until I get it in an enclosure. My next post will have some construction photos and the results of the listening test. I'll also include parts costs. Looking at the measured results I would say the project so far is a success in eliminating the noise coming from the switcher, making it look to the NUC as a linear supply. You might want to consider this as an option to purchasing or building an expensive linear supply.
Tom
