DC/DC converter to go from 12VDC to say ~250V. How to avoid switching noise?

[777funk]

I've done this twice now and it seems difficult to avoid the very high frequency noise. It's much different than the usual 50/60 Hz problem we face with traditional transformers and FWBR diodes with caps, resistors, and chokes.

How can I use a DC/DC without this high frequency noise?

 

[ccaudle]

Buying a pre-made DC-DC converter, or designing and building your own?

 

[Khron]

How can I use a DC/DC without this high frequency noise?

Additional (LC / RC) filtering. Especially since, presumably, it won't be a particularly high-current supply..?

That, and proper ground connections.

 

[Matador]

This is hard to answer with a blanket recommendation, since each system is different based on topology.

"High frequency noise" implies "noise I can hear"...is that true? When you power equipment with that SMPS you hear an audible whine?

 

[777funk]

This is going to be 12ax7s in between a THAT1510 chip preamp and AKM converters in an interface. I did it as a TRS insert on my M Audio 610 and will be doing the same on my 2626 on 2 or 4 channels.

The first thing I tried is sharing the 12VDC supply (also switch mode) by splitting off to two 2.1mm connectors. One for the interface and one for the tube (heater and DC/DC for high voltage). This was very noisy due to ground issues. The THAT chip has a dual rail power supply. Maybe that was part of the issue. I ran a separate 12VDC supply for just the tube and that helped considerably. But... I'm still getting some almost white noise plus some whine. My RC filtering has helped but it's still there. I can especially hear it (loud) if I use any compressors or distortion on the tracks (i.e. electric bass or guitar).

 

[Khron]

Interference can also be radiated, not only conducted...

 

[Matador]

If you FFT the whine, what is the frequency?

 

[ruffrecords]

This is going to be 12ax7s in between a THAT1510 chip preamp and AKM converters in an interface. I did it as a TRS insert on my M Audio 610 and will be doing the same on my 2626 on 2 or 4 channels.

The first thing I tried is sharing the 12VDC supply (also switch mode) by splitting off to two 2.1mm connectors. One for the interface and one for the tube (heater and DC/DC for high voltage). This was very noisy due to ground issues. The THAT chip has a dual rail power supply. Maybe that was part of the issue. I ran a separate 12VDC supply for just the tube and that helped considerably. But... I'm still getting some almost white noise plus some whine. My RC filtering has helped but it's still there. I can especially hear it (loud) if I use any compressors or distortion on the tracks (i.e. electric bass or guitar).

You need to be very careful when running a SMPS from a dc supply. The SMPS will draw significant pules of current at high frequency, which is something the preceding dc supply is probably not designed to cope with. So you need to help it out by providing a local energy store in the form of an adequately sized capacitor fitted very close to the input of the SMPS.

Cheers

Ian

 

[777funk]

If you FFT the whine, what is the frequency?

I'm sure I have a track with the noise. I'll have to take a look.

 

[MaxxDM]

If it was me, I'd either place the PSU outside the main enclosure, or, I'd get a sealed Hammond aluminum enclosure just for the PSU, and build it inside, taking special care to avoid ground loops, and filtering everything in the enclosure.

Maybe also worth trying, if you can tolerate a diode loss, is, placing a diode between the output and the filtering network. This might also help, as the noise would tend to charge the filter caps, as opposed to modulating them up and down in voltage.

 

[Newmarket]

I've done this twice now and it seems difficult to avoid the very high frequency noise. It's much different than the usual 50/60 Hz problem we face with traditional transformers and FWBR diodes with caps, resistors, and chokes.

How can I use a DC/DC without this high frequency noise?

If low noise is a priority you may want to investigate a sinewave based method - Baxandall, aka "Resonant Royer"
Wiki link below BUT note that you need to scroll down to the Sinewave Oscillator section. It's quite different to the "Original" Royer converter.

https://en.wikipedia.org/wiki/Royer_oscillator

 

[tinn]

Yes, SMPS can be integrated with proper filtering of in and output and some shielding.
I used the transformer of a 12V 5 watt powersupply in reverse.
The magnetic field was much less than a toroid mains transformer (25 Watt)

The sinewave approach is maybe more easy to implement but has worse efficiency
I used a sinewave converter in the 80s to run Tube Neumanns KM254 in field recordings.
(cause I did not have expensive KM84s)
This converter delivered 120 V and 7 mA for 4 KM254. The pot core had a diameter of 21mm x14mm
Frequency is about 100Khz. you might synchronize it with the 96 Khz of your AD.
The 2.35 Ohm resistor on the left (schematic) is limiting some currenspikes.
An inductor might have been better here, but it worked and still runs perfectly.

 

[trobbins]

The first thing I tried is sharing the 12VDC supply (also switch mode) by splitting off to two 2.1mm connectors. One for the interface and one for the tube (heater and DC/DC for high voltage). This was very noisy due to ground issues. The THAT chip has a dual rail power supply. Maybe that was part of the issue. I ran a separate 12VDC supply for just the tube and that helped considerably.

Perhaps worth confirming if you have an ok 12Vdc supply for starters. Did you use a separate 12Vdc supply for the THAT, and was it the same supply model, and did that show any sign of noise ingress?

Did you confirm the tube related noise was from the heater, the HV, or both. For example, use a 12V battery to power the heater. Was the HV dc/dc isolated or use a common 0V with the heater, or ??

 

[777funk]

Thanks for the feedback. I need to do a little more research on if it's heater related or supply related. It's been a couple years since I worked on this project, but I've been using it since and like the results (good sound to my ears anyways). I tried again today sharing the 12VDC by splitting it at the 2.1mm barrel connector from the power supply to the interface and it does not work for whatever reason. Requires its own 12VDC supply.

 

[ccaudle]

sharing the 12VDC by splitting it at the 2.1mm barrel connector from the power supply

Are you sure the 12v supply has enough current capacity? Could you be overloading the existing supply?
Is the existing supply linear or a switcher? You sometimes need extra filtering to prevent one switching supply from interfering with another.

 

[TomKelly]

Heikki's Vari-Mu uses an SMPS power supply (running from a 36VDC wall-wart supply, IIRC). Schematics available here: https://ghr.fi/varimu.html

I can vouch for this design, as I've built the compressor, and use it daily. No audible noise in the signal (there is an audible startup whine, but that disappears once the SMPS has started)