Do-it-yourself audio enthusiasts love power supplies. Large capacitor banks, complex voltage regulators, shunt regulators – the bigger, the better. You will find also some power supplies of this type in our shop. But do you really need them? Many modern circuits have a relatively good power supply rejection ratio (PSSR). The better the PSSR, the smaller the impact of a noisy power supply to the output. Note that PSSR is not a single value, but depends on the frequency. For audio circuits the most imports frequency range is the audible range from 20Hz to 20kHz. However, even frequencies outside this range can have an impact on the audible performance.
Do you know what is the impact of power supply noise to your circuit? Often you do not know this. While SPICE simulations might help, the reality is often different? Why? Because a real-world PCB is different from an ideal Spice simulation. There are lots of parasitic capacitance and inductance that you do not model in a Spice simulation. Therefore it is always a good idea to test effects of the power supply on the real circuit. But how do you do this? Injecting noise into the power supply can be done with an modulated power supply.
What does a modulated power supply look like? Basically it is a simple summing amplifier. You could even use an audio power amplifier as a modulated power supply. However these devices are not designed to provide DC. Often you will find DC blocking capacitors inside. Even without these, Hi Fi amplifiers are not designed to provide a continuous DC current. This could result in damages of the amplifier. Also Hi Fi amps are not good in amplification of higher frequencies.
Therefore we will build our own symmetrical modulated power supply. We found a nice design at the Defense Electronics website. This design includes even a function generator. In the first step we will not include a function generator, but add the AC overlay from an external input.