Audible generator sound from the qa403 enclosure

Is it normal, that sometimes I can clearly hear the sound of the signal generated by the QA403?
The sound is coming from the inside of the enclosure of my device.

For example, when testing the frequency response at a generator level of -3dB, the ExpoChirp sound is heard from a distance of 1 meter (quiet but clear) when changing the level to -2dB, you can hear the relay operating and the sound disappears.

At first I thought the sound was coming from the DUT, but I eliminated that possibility.

Hi @yegolebi, yes, it’s normal and it will be exacerbated at high DAC amplitudes and/or load currents. If you mute the DACs or set the level lower, the sound should improve and then become imperceptible. The source of the sound is ceramic power supply ceramic caps and microphonics. Search on “singing capacitors” for a lot of good articles over the years. It’s a well known phenom with ceramic caps. You can tap on them and they generate a voltage. Or you can change the voltage across them quickly and they generate a sound. This two-way behavior is usually lumped under the umbrella term microphonics.

A very difficult problem in early digital cell phones was the TDMA burst. That had a radiated component where the burst could be coupled into earpiece/speaker. But there was also the issue of the additional transmit current demands on the power supply ceramic caps radiating acoustically and coupling out through the earpiece. And it was very hard to tell one from the other.

Thank you for your reply. I was aware that there is a microphonics phenomenon of ceramic capacitors, I just did not expect it to be so loud …

Matt, ceramic NP0 type are not microphonics, as far as I know. Wouldn’t using NP0 solve the microphonicity?

I have not heard any sound from the QA402/3 except for the attenuators clicking. When testing power amps, integrated amps and receivers, sounds are often heard from them, particularly at higher power output levels. It could be during a swept test or just a THD/SNR test. I have attributed it to power transformers. Sometimes the sounds are quite loud and a little disconcerting to be honest. Some of the Carver magnetic field power amps make strange “thuds” when tested at higher power levels.

Yes, true, and same with tantalum and aluminum. But C0G/NP0 sizes start to get very rare above 0.1uF or so. Tantalum have their own sets of problems, and aluminum tend to get easily knocked off during the handling of the board unless they are in a forest of taller parts. It’s very hard to beat the performance of ceramic. Great size, great price, great ESR and performance.

Matt have you seen the new “Gamechanger” Murata COG parts?
They now have 5% and 2% COG in a 1206 package up to 0.47uF at very low cost (about $0.13). super new parts, but been a game changer for us. I get 1206 is a bit big for bypass but maybe worth it in a few key places. I wonder if it would lower the noise floor at all.

The QA403 gets its +/- supply rails from a TPS7A39, which needs a minimum of 10uF cap on the outputs (assuming 80% rating for X5R). That’s a lot of C0G parts! Additionally, TI does the characterization with ceramics, meaning aluminums (and their higher ESR) is left an an exercise for the engineer.

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Matt, if space or cost were not an issue would ceramic cap on the TPS7A39 output be still the best option in your opinion?

Not sure on space/cost, but if time were not an issue maybe an aluminum poly cap. In section 8.1.2 of the spec TI says that the parts is designed for low-ESR ceramics, but as long as ESR < 75 mOhm an aluminum cap could work.

This cap HERE is low-profile (4.8mm) and might be a good candidate to try. ESR is 60mOhm, so perhaps two in parallel but probably a single would suffice. 25V rating is almost 2X safety factor.

If you are handy with a soldering iron, it’d be quick to try.

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