Paul Hynes Design

Supply Regulation

Many audio equipment manufacturers use industry standard solid state regulators in their products because they are readily available, cheap and easy to apply. They offer reduced power supply ripple breakthrough from the rectifier / energy storage capacitor. This allows a much smaller energy storage capacitor to be used, which in turn reduces component costs considerably, more than offsetting the cost of the regulator itself. Multiple regulator systems can also be applied cheaply. The main benefit of using these devices is essentially one of cost reduction. Whilst this is a laudable aim, most enthusiasts will generally prefer to look for performance improvement before cost considerations.

So let us look at regulator performance with a typical industry standard, the 317 / 337 type adjustable regulator. Starting with power supply rejection (PSRR) of these regulators with respect to frequency. At 100Hz this is 60-70dB (60dB is a ripple reduction of 1000 times). At 1KHz, the PSRR begins to drop due to the regulator's internal frequency compensation reducing gain at higher frequencies, leaving less loop gain available for error correction. At 10 KHz, the regulators manage 50dB (316 times ripple rejection). At 100KHz they only achieve 20dB of PSRR (10 times ripple rejection) and at 1MHz only 10dB (3.16 times ripple rejection). At low audio frequencies both devices offer usable PSRR, but this situation deteriorates rapidly above 1KHz, becoming relatively ineffective at frequencies above 100 KHz where radio frequency interference and digital clocking load current transients may have to be dealt with.

Output impedance shows the regulator's ability to control the load with respect to frequency. The 317/337 graphs show 10 milliohms from DC to 1KHz. At around 1KHz the frequency compensation capacitor comes into operation to aid regulator stability, reducing the loop gain and negative feedback with respect to frequency. The output impedance is a function of available negative feedback and as this feedback reduces with rising frequency, the output impedance rises. At about 10MHz the regulator runs out of gain and is no longer functional.
The internal frequency compensation capacitor used in these devices has another, more important, effect on their behaviour. This capacitor has to be charged and discharged by the internal circuitry before the feedback loop can apply error correction. Inadequate charging current causes Transient Inter-modulation Distortion (TID) and Slew Induced Distortion (SID) giving line and load transient settling times of up to 5 microseconds with these devices.