Great subject Doug and one very close to my heart (or head, whatever)!
As an EMC engineer for Bose Corporation, I get to see a lot of audio amplifiers with output levels from 10s of watts to thousands of watts. The extension to your article that I would like to make is that not all "Class D" amplifiers are created equal. Various chip manufacturers have different approaches to how the switched (and therefore Class D) signal is produced. One of the most troublesome is a vendor whose chip family produces equal and balanced square waves on both wires to the speaker driver when no audio is present. While differential mode current is minimal with no input, common mode di/dt charging currents down the wires to the driver can be quite high and have very high frequency components. On the order of 80% of our products that I have tested in the last 5 years have highest emissions profiles when the source is muted. For European standards this is mitigated by a standard test method in EN55013 of using IEC Pink Noise with a 6dB crest factor and 20Hz to 20kHz pass band at 1/8 of maximum power. Then we get to the US/Canada where it's "find the worst case". You can guess which is easier to pass! The Pink Noise dithers the pulses on either side of the driver leads and produces a very favorable spreading function in the higher harmonics of the amplifier "chopping" frequency, such that radiated emissions are significantly reduced. Conducted emissions also benefit. Bottom line is that I highly recommend that any EMC engineer first familiarize themselves with the characteristics of the amplifier chip vendor before launching into troubleshooting. It will save you time in the long run! With respect, Brent DeWitt, Senior EMC Engineer Bose Corporation, Framingham MA From: Doug Smith [mailto:[email protected]] Sent: Tuesday, August 19, 2014 7:13 PM To: [email protected] Subject: [PSES] Audio sources, class D digital amps, radiated emissions, and philosophy Hi All, Just finished and posted my August 2014 Technical Tidbit titled: Radiated Emissions Can be Strongly Affected by Driving Signals, A Problem for Emissions Testing (An Example Using a Class D Stereo Audio Amplifier) Abstract: Radiated emissions from electronic equipment can be a strong function of how the equipment is operated. In the case of class D audio amplifiers, the audio signal driving the amplifier can have a large effect on emissions. Test results and implications are discussed as they relate to the philosophy of radiated emissions testing. The link to the article is: http://www.emcesd.com/tt2014/tt081914.htm The link to the main page: http://www.emcesd.com/ (lots of new stuff there) Check to make sure your computer does not corrupt the links above. They should look just as above with no other symbols or numbers added as Windows sometimes does at the end of the link of the form "[1]". Doug -- University of Oxford Tutor Department for Continuing Education Oxford, Oxfordshire, United Kingdom -------------------------------------------------------------- ___ _ Doug Smith \ / ) P.O. 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