Low-noise Power Supply

Here is a quick article describing a low-noise power supply for low current applications. It’s simple, and easy to build. All parts are widely available and cheap. I’m using this power supply to power-up my RIAA preamplifier. It’s more that capable of delivering excellent results. Build one and try it for yourself.

First of all lets see the basic functional diagram of the regulator.

Positive voltage regulator:

The schematic above shows the basic implementation of a positive regulator. A zener diode and a constant current source define the voltage reference. The resulting voltage (Vref) is then applied to the non-inverting input of a low-noise operational amplifier /error amplifier/. The output drives the base on a series pass transistor. In that case a medium to high power NPN transistor. The following equation defines the output voltage of the regulator:

   

Negative voltage regulator:

The schematic above shows the basic implementation of a negative regulator. For this application I’ve decided on using the same voltage reference for both positive and negative rails. In this scenario the operational amplifier is in inverting mode. The following equation defines the output voltage of the negative regulator:

   

A practical implementation of the low-noise power supply

All the above brings us to the  actual schematic diagram of the power supply as a result. I need ±15V rails for my application. Different values are possible. Use the above equations to calculate the voltages for your project.

Because I want to keep thing simple I chose the voltage reference to be the well-known TL431. You can find more info on TL431’s noise performance here at TNT-Audio. It’s readily available, cheap and gives excellent results when used properly. Q200, R200, R202 and D200 form the current source. In this particular situation the reference is biased at around 3mA which is more than enough. The following equation describes the current :

   

Use a standard green or red LED here with forward voltage of 2.1V. If you need to use different type of LED with different Vf use that value into your calculations.

Complete schematics for this project is available for download >>> LOW-NOISE-POWER-SUPPLY-AWS

The PCB

Finally we arrive to the PCB part. The board itself is a two layer compact design. I’ve decided to mount the power transformer directly on the PCB. As a result I’ve got a rather elegant solution.

And here is the actual PCB ready for some testing.

So how does this low-noise supply actually perform?

I have to warn you. This is a quick test of the board. Because I had no time for extensive testing I had to test with just simple resistor loads and an oscilloscope. More testing is in order for sure. I wired the load and hooked up the scope. The quick setup led me to satisfaction. While there is more to be desired I’m quite happy with the result. In summary the noise performance is excellent and I call that a low-noise power supply. Here are some measurements:

The graph shows a total voltage noise of about 113 Vrms. We can use the result and calculate to noise density over range of 100kHz. Now we could go and do this with absolute precision by integration. However in practice that’s not really necessary. There is much simpler solution that gives just about enough precision. I calculate the voltage noise density by assuming a 1st order equivalent noise bandwidth which is equivalent to 1.57 times 100kHz. The equation for calculating noise density is as follows:

   

Not too bad at all 🙂