Digital Amplifier Questions

Guido & Noble100,

Why an FM radio? I would expect that an AM radio would provide a much more sensitive and potentially more meaningful test.

As you probably realize, FM is inherently vastly more immune to RF interference than AM, since the audio that ends up being reproduced corresponds to frequency variations in the airborne signal rather than to amplitude variations. Also, in contrast to the AM broadcast band the 88 to 108 MHz frequencies of the FM band (and even its 10.7 MHz IF frequency which follows down-conversion within the radio) are vastly higher than the bandwidths of any audio component, and are therefore much less likely to be relevant.

I suspect that if an FM radio audibly responds to airborne RFI generated by an audio component it is likely not what is picked up by its built-in antenna that results in the audible interference, but rather what is picked up by downstream circuitry within the radio that operates at much lower frequencies. That response probably being much less sensitive than an AM radio's response would be to interference picked up by its antenna.

FWIW, my guess is that you'll hear significant interference when an AM radio, preferably tuned near the low end of the band (e.g. 540 kHz), is placed close to the amp. But what will be of interest is how quickly the interference drops off as the distance increases.

Best regards,
-- Al

Thank you Al, I conducted the test with an ancient tuner a few years ago.... Device is a rather low end NAD from approx. 1979. The tuner has a button that causes it to reject interference in FM... If the switch is in the off position, the device picks up very significant statics whenever tuned to weaker stations, and in some days, FM is plainly unlistenable, regardless of station or switch setting... or amp connected to the system. Call it a hyperdistractable/ADD FM tuner... Or just a plainly crappy tuner.

But I might have conducted the test in AM... I just can't remember.

G

Al and Guido,

I'll test again using a station low on the AM dial and report back on the results. I Don't know why but I thought any RF leakage from class D amps would be closer to the FM band range.

Thanks,
Tim

I Don't know why but I thought any RF leakage from class D amps would be closer to the FM band range.

Thanks,
Tim
Noble100

Noble100, don't know why you were thinking this???
FM operates in the 88MHz - 108MHz range i.e. 88 million cycles per second --> 108 million cycles per second.
the class-D switching amp is probably running at 1MHz switching freq (if that high at all. more likely in the 500KHz region). I.E. 500,000 cycles per second --> 1 million cycles per second.
As you can see, 500,000 --> 1 million cyc/sec is far away from 88 million --> 108 cyc/sec.
Let's say that the class-D amp was switching at 1MHz, then the 89th harmonic of this 1MHz switching freq would be 89MHz but it's power would very, very low to the point that even if it interfered with the FM radio station tuned to 89MHz it would produce a very low amplitude distortion that would most likely be un-hearable.
(Just wanted to point out that the switching waveform of the class-D amp is most likely a square wave & square waves have only odd harmonics).

Like Almarg pointed out the AM freq would be right in the vicinity of the class-D switching frequency & much more likely to create interference.

My class D amp switches at 500kHz. This square wave is filtered by the Zobel network at about 60kHz. Harmonics are most likely filtered out and main problem is about 1% residue of 500kHz switching frequency present on the speaker cable. This 500kHz frequency represents wavelength of 600m. Antenna below 1/10 of the wavelength becomes very ineffective - and that is 60m. Typical 3m cable is practically non-radiating. It is still a good idea to keep cables apart to avoid capacitive coupling.