Clone Class D “digital” power amplifiers – can we really trust them….

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Alan Shaw  writes:

Clone Class D "digital" power amplifiers - can we really trust them.... as high fidelity components?

I have on my test bench a very typical, inexpensive, so-called 'digital' amplifier. It is not positively promoted as being of audiophile or even high-fidelity grade, but I understand that it uses similar components and technology to all 'digital' amplifiers although perhaps cost reduced.

What is alarming is that when I connect an oscilloscope across the output terminals, even when there is no signal connected and the volume control is fully off, there is a strong signal at about 250kHz, ten times greater than the highest frequency reproducible from CD, and 10 to 20 times beyond the limit of normal human hearing. The frequency is so far beyond the audio band that it cannot be reproduced by a tweeter, and the argument is that it can be safely ignored.

The problem is that the 'scope can't ignore it, and even when a normal audio sine wave signal - say, 1kHz - is connected to the amp, the amp's output is displayed as the ever present drone 250kHz super-audio whistle modulated by the 1kHz tone. It looks horrible on the 'scope screen, and if the test signal is a square wave, it shows alarming ringing behaviour - pictures to follow. Consequently, because the on-screen display is so confused, I am unable to make any meaningful measurements on basic frequency response, other than to observe that when a speaker load is connected, there is a huge boost in output to the speaker load around 30kHz, which is not far beyond the end of the audio band.

I first encountered this super-audio whistling some years ago, and had assumed that improvements in filtering technology in the amplifier circuit would eliminate this. Indeed, I recall that I could not use an AM radio within some meters of the speaker leads which acted as RF radiating antenna. I will try this with this new module.

I've written to the makers asking if there is some recommended external filter pre-loudspeaker (although that may make a mess of the speaker input drive conditions). Is anyone successfully using 'high-end' digital amps (is that an oxymoron?) and have they tried bringing an AM, medium wave radio near the speaker cables?

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