Class-D audio amplifier EMI can be cut dramatically, to the point where it is even compatible with AM radio, according to Silicon Labs.
The firm is to integrate its technology into a range of amplifiers for consumer portables.
"It is challenging to get a Class-D audio amplifier to work reliably well with radios," Rick Beale, director of audio amplifiers at Silicon Labs told Electronics Weekly. "The problem is predominantly with AM, but also with FM and DAB."
The firm has a demo board.
"With a ferrite bar AM aerial 10cm from the amplifier, there might be 1-2dB signal to noise deterioration at some frequency, with small signals," claimed Beale. "11-12cm away, there is no measurable interference at all."
Proprietary noise shaping suppresses broadband noise above audio (see diagrams above). "There is 25dB difference between the red peaks [conventional PWM] and the blue humps [Silicon Labs]."
What sort of noise shaping?
All Beale was prepared to say was that carrier suppression technology from communication had been applied, which to a first order did not affect the quality of audio.
"If you look at the output, differential switching frequency does not change," he said.
Processing comes from 160Mips DSP - the die is 110nm digital CMOS - which adds a further 35dB suppression through a tuneable notch filter. Controlled over a serial bus, it can be parked on top of the chosen AM station. "We allow virtually none of the energy to spread into the notch," said Beale. "This is what allows an antenna within 10cm of the amplifier."
Ferrite beads added to the outputs further cut noise above the AM bands as did output slew rate control, giving sinex/x roll-off for FM reception and GHz frequencies: "It makes it friendly for smartphone docks", said Beale. "In the FM band we get 40-45dB of natural peak suppression."
The first product is the 2x5W Si270x which runs from 4-7V and offers 0.05% THD at 1kHz 1W and 95dB dynamic range.
"An audio system based on the Si270x can provide up to 8.4 hours of play time using four AA alkaline batteries," said Silicon Labs.
A 3-4.5V version for Li-ion cells is in the pipeline.