I came across an interesting phenomenon today. I was measuring an amplifier distortion at 5W, 1kHz into a dummy load and I noticed that the amplifier itself was emitting a 1kHz tone. My ears aren't great, so if I could hear it, then it must be quite loud. After a bit of experimentation, I determined the sound was actually coming from the dual 2W emitter resistors. If I put my fingers on them to apply some damping, then the sound level dropped considerably. I noted that they are on 'long legs' to give them plenty of air. The long legs will obviously help them to vibrate quite nicely. I was just wondering if anybody else has encountered this phenomenon? I'm wondering even if I replace them, will it change anything?
The high harmonic content in the emitter resistor waveforms is making the sound especially audible.
Ed
Ed
Yep, I have heard singing resistors and plenty of singing capacitors. Even a Wifi router PCB that pulsed and buzzed with changing Wifi traffic.
Have you tried wrapping the resistors in solder? Does that kill the buzz?
Have you tried wrapping the resistors in solder? Does that kill the buzz?
No problem to be solved.
Pretty common phenomenon, you hear it only because you are driving a silent dummy load, are paying attention to it (you are testing) and probably in a silent room.
IF you drive speakers, at any level, it becomes absolutely unnoticeable.
Pretty common phenomenon, you hear it only because you are driving a silent dummy load, are paying attention to it (you are testing) and probably in a silent room.
IF you drive speakers, at any level, it becomes absolutely unnoticeable.
Yes they are the usual ceramic encased wire wound double pair of emitter resistors with 3 legs.
I understand this noise would be masked at normal volume levels, but I have tested plenty of other amplifiers that don't do this. So I was wondering if resistors of this type of construction normally exhibit this effect?
I understand this noise would be masked at normal volume levels, but I have tested plenty of other amplifiers that don't do this. So I was wondering if resistors of this type of construction normally exhibit this effect?
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It may be that you've hit a resonant frequency of the package, and having long leads may help it be free to vibrate more - changing the frequency of the test tone will help show if resonance is involved. With half-cycles in them any multiple of the test tone may be the one to excite a resonance.
I think you're probably right Mark. These packages are smaller than the usual 5W versions and probably resonate more easily. I forgot to fiddle with the frequency to say for sure, but I did replace them with 5W versions and the sound was greatly reduced. Just very faintly audible thereafter.
Maybe the title of this thread should be altered from 'Microphonic' to 'Resonant'.
Maybe the title of this thread should be altered from 'Microphonic' to 'Resonant'.