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In the Forum: Audio Discussions
In the Thread: It’s mad, mad, mad... electricity.

Post Subject: Smps ramblingsPosted by JOHNR on: 10/17/2015

I will address the low voltage DAC power supply first since it has multiple applications.

The front end comprises a commercial 48/12 volt 20 watt dc/dc converter. This basically is used to provide 1000 volt isolation input to output otherwise you cannot hook it up to a DHT.
I fit a number of common mode chokes plus high performance tantalum SMD capacitors to control the noise and deliver this to a POST shunt regulator chipset. The output of this package is also passed through 3 more common mode chokes plus more SMD caps. The footprint is 120 by 30 mm and with a height of 20mm. The output is floating.

Since I use a 48 volt bulk supply, feeding total of 3 regulators plus the B+ system, I rather obviously uses a 48/12 converter and , this particular package operates from 32 to 72 volts. However, in this footprint, there are a dozen or more combinations of inputs. A 12 volt to 12 volt combination is available if you want to use batteries.and the supplier is Meanwell out of Taiwan and supported by element12 aka Farnell. The supply code is jck2048s12 and the Farnell code is 173-8261.

for experimental purposes, I can supply you FOC and no obligation, one unit minus the converter because of its cost and weight. The front end is equipped with a capacitor generally 50 or 63 volts and yet another Cm-choke. Choose your input voltage, fit and go.

To cover the issue of the advantages of SMPS when optimized for low common mode noise, is that the source impedance is very low indeed. The operating frequency means that 1 uF caps are used for the holdover requirements. In this particular design, the shunt regulator operates with a forward mode inductor, just a few microhenries and it is turned on and off using high speed DMos typically at 500 kilohertz. When used for general purpose supplies, it does not require output filtering. In the case of a DAC or small signal tube, there is some common mode noise which needs to be controlled.

From a deeper technical standpoint, go and open any modern day SMPS and look for a CM-choke. You will find one on the ac supply side because the Regulators globally want to see one to keep the mosfet switching crud out of the public supply. Regrettably, the global specifications are based upon cheapness and not noise control. In simpler terms everybody fits the minimalist choke and nothing more.

With regards to the output side, you will not find such a choke because designers do not understand the subject and/or do not see the need.
To meet the regulations for EMC, the "high" side of the circuit is linked via a pair of "Y" capacitors to the zero volt rail of the output. Thus such noise is dumped without any attenuation whatsoever onto the ground rail of the following equipment. If you are confident enough, open up your DAC and locate the Y capacitors and cut them off. Surprise, your system is now much quieter and cleaner. These caps are typically Blue, marked "Y", rated at 2000 volt, 33 picofarad. The circuit board is always equipped with a broad white silk screen track that passes under the ferrite transformer. This is the high and low demarcation barrier and it sometimes has cutouts along the line to improve the isolation. The "Y" cap(s) will be seen to straddle this demarcation line.

Enough for now. I will share my solutions but please note the B+ supply is complex and requires technical skills to build. Anyway, we will wait and see if there is enough interest.

Regards
John

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