Digital Amplifier Questions

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.

03-29-15: Kijanki
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.

makes sense Kijanki - 500KHz is 3.05 octaves away from 60KHz. Zobel network/filter is most likely a 2nd order filter which is 12dB/octave implying 36.6dB of attenuation of the 500KHz. That works out to 1.48% of residual 500KHz on the speaker binding posts.

03-31-15: Noble100

Thanks for the info Bombaywalla and Kijanki.

I tried the test again using the AM radio. I couldn't even find an AM station that was tunable. There was too much buzzing on the whole range. I'm thinking this is caused by RF leakage from my 3 class D amps, all located within a 5ftx 2ft area.

I plugged the radio into an outlet in my adjacent dining room (less than 20ft away) and the buzzing (hash?) went away and I was able to tune in a station in the low AM band.

why do you need to get a new AM radio?
Why can't you repeat the test (where you cannot even tune into any AM station due to the supposed RF interference from one of your class-D amps) with the existing AM radio & simply switch off one class-D amp at a time until you can tune into a AM station using Almarg's suggestion for the AM freq?
isn't this equally valid (as Guidocorona's suggestion of switching on the class-D amps one/2 at a time)?
thanks.