Nuforce Amplifier Startup and Noise Behavior
Startup Condition

The principle of Nuforce amplifier is based on self-oscillation with the speaker as part of the loop. The self-oscillation requires input signal around 20mV (+/- 2mV) to start. Due to the analog based design, speaker's impedance and natural variations in components' values, the self-oscillation might start with different input signal strength and delay. After the oscillator has started (within the initial 3 seconds), it will continue to oscillate even when input signal is not present. This design characteristic resulted in different startup input signal strength or delay time observed from amplifier to amplifier (it could be different between two channels within an integrated amplifier).

Static Noise

Unlike traditional Class-D amplifier that utilizes a constant PWM to modulate the audio signal, Nuforce amplifier employs a self-oscillating circuit that switches the MOSFET at 500 kHz, and utilizes an ultra-fast feedback operating at 1 Mhz to correct for the THD almost instantaneously.

The above system illustrates a simple amplifier with feedback. The feedback network takes the error at the output and inject to the summing junction at the input, in order to 'cancel' out the harmonic distortion.

However, when there is no music, Nuforce amplifier may have a higher background noise. It is adding noise to the system in an attempt to cancel out distortion, but since noise being the only signal present, it could not be 'cancelled' out. The result is higher noise floor when listening (putting the ear next to) close to the speakers.