Ultralow Noise, 48 V, Phantom Microphone Power Supply Using a Tiny DC-to-DC Boost Converter

[mediatechnology]

From this month's "Analog Dialog."

"Ultralow Noise, 48 V, Phantom Microphone Power Supply Using a Tiny DC-to-DC Boost Converter" https://www.analog.com/media/en/analog- ... ue-176.pdf


"Ultralow Noise, 48 V, Phantom Microphone Power Supply Using a Tiny DC-to-DC Boost Converter," Analog Devices, "Analog Dialog," April 2020.

[JR.]

I did a somewhat unusual phantom supply inside a DSP vocal processor design I consulted on, this century but almost two decades ago.

The design already had a switching supply and needed Phantom voltage for one mic input. Rather than add a switcher to a switcher, I used a variant cap doubler/tripler. Since I only needed mA of phantom power and had a robust high frequency square wave to tap inside the existing switching supply, I used tiny SMD caps and normal signal/switching diodes. After tripling up the lower voltage to well above 48V I used a simple zener regulated pass transistor, looking a little like that cap multiplier..

I never measured noise, but since it pulled from the existing square wave there was no opportunity for beat frequencies, worked like a charm.

JR

[mediatechnology]

I did a somewhat unusual phantom supply inside a DSP vocal processor design I consulted on, this century but almost two decades ago.
...
I never measured noise, but since it pulled from the existing square wave there was no opportunity for beat frequencies, worked like a charm.

JR

Sync to Fs is one of the things I liked about this design when I saw it.

In the century before this one I had the opportunity to work with one of the first "production" Otari Mitsubishi-format digital recorders that was being delivered to Masterfonics in Nashville. I was with SSL at the time and was commissioning the console as the Otari was being delivered. When we fired it up and monitored the return we could hear "birdies" of the supply switching frequency beating with sample frequency. To identify that it was Fs as one of the carriers I suggested that we put the machine into varispeed, which changed Fs, to see if the beats changed. They did.

The Sony PCM-3324 digital machine had custom supplies that were locked to Fs. I knew this factoid because I had already been forced into repairing one after a SMT tantalum cap shorted and burnt a 1" charred hole in a >$1K PC board. The supplies were locked to Fs AND you could weld with them or, in this case, start a fire.

Otari used off-the-shelf switchers and didn't have that option. If I recall they were able to fix it by changing the grounding scheme which I think involved mounting the enclosed frame switchers on plexi-glass spacers.

Older RF-based condensor mics sometimes beat with converter Fs. I have a friend that does location recording and we've heard it with his field recorders. So mic RF, switcher RF, and Fs can make for a rich in-band beat frequency environment.

BTW the burst and pulse skipping modes in the above circuit will probably be noisy.

Meanwell took a really quiet brick switcher and "Greened" it to meet current regulatory environments which used a burst or pulse skipping mode. The new model became unusable.

[JR.]

I did a somewhat unusual phantom supply inside a DSP vocal processor design I consulted on, this century but almost two decades ago.
...
I never measured noise, but since it pulled from the existing square wave there was no opportunity for beat frequencies, worked like a charm.

JR

Sync to Fs is one of the things I liked about this design when I saw it.

I recall experimenting with syncing that switching supply to the DSP clock.... Since the PS is performing PWM with it's switching waveform you can only sync one side, (rising or falling edge). The variable width duration will still generate random HF content. I did not find the sync useful for silencing the noise floor with that application, but that was years ago, and as usual I was teaching myself as I worked.

Trash in that DSP noise floor seemed more affected by PCB layout tweaks around the CODEC (4 layer PCB).

JR

In the century before this one I had the opportunity to work with one of the first "production" Otari Mitsubishi-format digital recorders that was being delivered to Masterfonics in Nashville. I was with SSL at the time and was commissioning the console as the Otari was being delivered. When we fired it up and monitored the return we could hear "birdies" of the supply switching frequency beating with sample frequency. To identify that it was Fs as one of the carriers I suggested that we put the machine into varispeed, which changed Fs, to see if the beats changed. They did.

The Sony PCM-3324 digital machine had custom supplies that were locked to Fs. I knew this factoid because I had already been forced into repairing one after a SMT tantalum cap shorted and burnt a 1" charred hole in a >$1K PC board. The supplies were locked to Fs AND you could weld with them or, in this case, start a fire.

Otari used off-the-shelf switchers and didn't have that option. If I recall they were able to fix it by changing the grounding scheme which I think involved mounting the enclosed frame switchers on plexi-glass spacers.

Older RF-based condensor mics sometimes beat with converter Fs. I have a friend that does location recording and we've heard it with his field recorders. So mic RF, switcher RF, and Fs can make for a rich in-band beat frequency environment.

BTW the burst and pulse skipping modes in the above circuit will probably be noisy.

Meanwell took a really quiet brick switcher and "Greened" it to meet current regulatory environments which used a burst or pulse skipping mode. The new model became unusable.