Digital Amplifier Questions

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.

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.

I then moved the radio as close to my amp rack as the cord would allow (about 10ft away) and there was no interference; the station continued to be clearly audible without hash/buzzing. I was thinking the interference would increase as I moved the radio closer to my amp rack (from about 20 feet to about 10 feet way) but it did not.

These somewhat mixed results have me a bit confounded. I think further testing is called for, perhaps with a better AM radio, before conclusions are drawn. My current thought is that at least one of my amps is producing RF interference but I don't know for certain at this point.

I'm going to get a better AM radio, plug it in in my dining room using a 20ft extension chord, tune in a station on the lower AM band and retest by walking toward my amp rack in the adjoining living room. I'll post again with results.

If anyone has a better idea, please let me know.

Thanks,
Tim

Hi Tim,

If you are going to get another AM radio, consider purchasing a portable one that is battery powered. Also, the test will be most sensitive if you tune to a frequency at which no station is present. If a station is present the radio's AGC (automatic gain control) circuit will reduce its sensitivity, especially if the station's signal is strong.

Best regards,
-- Al

Hi Tim, I recommend that you repeat test walking around with the radio as follows....

Amp 1 on, 2 and 3 off;
Amp 1 off, 2 on, 3 off;
Amp 1 and 2 off, 3 on;
Amp 1 and 2 on, 3 off;
Amp 1 on, 2 off, 3 on;
All three amps on.

G.

Guido and Al,

Both good ideas; I'll get a battery powered portable radio (I've been needing to buy a new hair dryer, anyways) and set the dial to a low frequency on the AM band that lacks a strong station signal. Next I'll test with the various amps on/off according to the permutations Guido listed.

This amateur RF interference technician will report back on results soon.

Thanks,
Tim