But you can see from this graph how the amplifier's ultrasonic output rolls off rapidly, presumably due to the low-pass filter necessary to minimize the HF noise produced by the switching output stage—except that CIA claim that there is no output filter!
Yet, the
paper written by Bruno Putzeys, designer of the UcD module used in the D-100, and available from the
CI Audio website, describes the amplifier as having an R/C filter on the output. The other detailed
detailed design paper from the
Hypex website talks quite a bit about the output filter of the design.
So does the D-100 have an output filter, or not? And if not, how does that fit with all the design documents on the UcD modules?
Furthermore, I have been reading up on class D amplifiers and found
this informative article, containing this passage:
The high switching frequency used in class-D amplifiers is a potential source of rf interference with other electronic equipment. The amplifiers must be properly shielded and grounded to prevent radiation of the switching harmonics. In addition, low-pass filters must be used on all input and output leads, including the power supply leads.
A variation of the class-D amplifier is called a filterless class-D amplifier. In the absence of an input signal, its output signal is zero rather than a symmetrical square wave. This eliminates the need of a low-pass filter to prevent application of the square wave to the loudspeaker. When the input voltage goes positive, the output voltage is a train of pulse width modulated pulses which switch between 0 and +VOP . When the input voltage goes negative, the output voltage is a train of pulse width modulated pulses which switch between 0 and −VOP. This is illustrated in Fig. 9. The loudspeaker responds to the average value of the signal, which is the audio signal. Because there is no filter, rf interference problems require the amplifier to be mounted as close to the loudspeaker as possible. The spectrum of the output signal is shown in Fig. 10. One problem with the filterless class-D amplifier is crossover distortion. To eliminate this, the FET control logic must be designed so that the amplifier puts out very narrow alternating positive and negative pulses in the absence of an input signal. In effect, this biases the amplifier in the class-AB mode.
Dusty, would you mind commenting on the output filter and the resulting issues this article raises about potential interference due to radiation of the switching frequency? Should the user of a D-100 take special care to avoid this?
Thank you,
Darren
