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Boost/Buck ICs with current limiting are pretty nifty to drive indirectly heated tubes. They really do extend the life of the heaters with the current limiting on switch on, at least emotionally. The boards I have been using recently have a voltage and current display, which impresses the impressionable.They are quiet in the 0 - 20kHz region. However, they produce pretty substantial noise at +/- 200kHz. I should not care about this, but it does bother me as a principal.
Is there a good tried and true circuit to quash this? I have some 4A / 10mH common mode inductors. I don't want to set up any mHz oscillations.
Below is a "silly" preamp I just built for my daughter using this circuit (5Z3 6C5 with MOSFET source follower output, NIXIE HB 106 "historic" tube). She drew a picture I based it on.
Edits: 09/12/24Follow Ups:
Consider the Amy Alice DC filter for SMPS.
ray
I'd suggest you need to show a block diagram of the converter module, and what/how it connects to inputs and outputs, and provide details on the converter module (at the least a spec/description and photos of top and bottom), and on the nominal operating dc voltages/currents. Google check if someone has reverse engineered a schematic of the board, or if the switching control IC has been identified, along with any diodes that are rectifying the smps pulses.If practical, some retrofit items can be made to the pcb module, as that is where the switching originates. Any externally added filtering would need to be at the pcb ports, and physically small, and appropriate for the dc currents (as you indicate) and suit the switching frequency.
200kHz indicates smt style parts may need to be retrofitted to restrain current loops to the pcb, or at worst to its close environs. Often it is practical to add smt ceramic bypass caps directly across e-cap pads and connector terminal pads on the pcb (both for input and output dc connections).
Edits: 09/12/24 09/12/24 09/12/24 09/12/24 09/12/24
Thanks. I will add more filtering. Given the nature of these units, absolute quiet was probably not on the design list.
LC filtering can help, but frequencies that high usually migrate around the chassis. This type of noise can also conduct back into the AC line and into other areas from there. Don't know if it will help, but my SETs use a square wave switcher that heats the DHT filaments with an 80kHz square wave. I don't hear any odd effects, and distortion measurements don't indicate anything untoward happening in the audio band. I think a small amount of residual noise from a buck/boost isn't likely to create any issues.
The 40mA AC on the outputs at 200kHz is probably nothing I should care about.
I guess it is just the price of using high frequency switchers.
Thanks.
"The 40mA AC on the outputs at 200kHz "
Did you mean millivolts? And that's at the output jacks? If so, seems a bit too high. The level can probably be attenuated with a simple bypass cap across each output jack, maybe 100pF or so. The highest value you can use will probably be determined in part by the output impedance of the preamp and its ability to drive across the additional capacitance. It might also be worthwhile to add an LC filter right at the output of the buck/boost. It's one of those things, you won't know until you try.
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