SRM Smart Rails Management

In any amplifier output stage, the efficiency is a function of the difference in the output voltage delivered to the load and the rails voltage delivered from the power supply. This is true for any topology among Class B, C, D, H, and G, despite each of these providing different absolute efficiency performances and load-dependent efficiency performances versus output power.

The Powersoft Smart Rails Management technology (SRM) implements real-time voltage tracking in the power supply that minimizes the differences between the output voltage rails voltage in order to improve the overall efficiency. SRM system feeds back the output signal to the power supply and modulates the rails voltage in order to reduce heat dissipation and improve efficiency.

The advantages of SRM technology focus on:

Lower idle consumption
Due to the fact that the output stage idle dissipation depends exponentially on the rails voltage, a consistent reduction in energy sensitive designs can be achieved by lowering the quiescent rails voltage. For example, if an output stage wastes 12.5W at idle at full nominal rails (let’s say at +/-150Vdc power supply), at half the voltage (+/- 75Vdc) will reduce its own idle dissipation to 3.125W, i.e. a net saving of 9.375W per channel.

Lower audible noise floor
Basically, a switch-mode amplifier works as a multiplier and provides an output voltage as a result of the rails voltage-time the “on” conditions of the output switches, hence a reduction on the rails voltage accomplish a reduction of the open-loop noise floor. For example, just as in the previous one, the reduction from +/-150Vdc of the rails to +/-75Vdc will provide a 6 dB noise floor reduction on the output stage.

Reduced EMC issues
Unintentional radio-frequency radiations depend on the exponent 2 of the switching voltage and current values. Unless other phenomena income, still halving the rails voltages reduces to ¼ the radiated undesired RF power.

Improved efficiency under load conditions
As stated above, a closer matching of the rails voltages with the output voltage yields to improved efficiency. Nowadays power devices suffer much more on the switching losses than conduction losses, thus the efficiency is driven by the amount of energy that has to be switched to achieve a certain output voltage and current. Thanks to the fact that the rails voltage closely follows the amplitude modulation of the signal, switching losses will be reduced.

Rails voltage on an amplifier without SRM. The efficiency is a function of the difference in the output voltage delivered to the load and the rails from the power supply.

Rails voltage on an amplifier with SRM. The output voltage is routed to the power supply and compared to the rails voltage in order to dynamically set the rails voltage.


The hold time Thold account for fast dynamic VOUT modulation. For sake of clarity, the picture has been simplified; i.e. real rising Tstep is faster than falling Tstep.

VRail > VOUT
Voff = offset voltage
Vth = threshold voltage
In A: Vth < VA < Voff ? VRail = VRail Min + Vstep
In C: VC < Vth ? VRail = VRail Max