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Connex SMPS300RE

Purchased pair of power supplies from Connex. This is their their mid-power, heatsink-less power supply. When I ordered, Cristi of Connexelectronic was kind enough to inform me that a new version was in the works and asked me if I wanted to wait for the new model instead of getting the current model at that time. Obviously, I waited for the new model ūüôā

The latest edition of this supply, the “RE” model, incorporates a CLC output filter to further clean up the output waveform. Factory measurements show 7-11 mV peak-to-peak @ +/-45V at 2A [link]. In addition, I like the following features:

  • ‘Perfect” companion to the Hypex UCD-180 amplifier module in terms of size and power for a dual-mono configuration
  • The input high-voltage capacitors have been increased in size (as compared with the photo of the older model)
  • Based on the latest ST L6599A resonant controller
  • $uper deal!

Here is a closer look at the output CLC filter. I we use the reported ripple value of 7-11 mV peak-to-peak for an output voltage of +/-45V, we get .011V/90V=0.012% or .009% RMS for the high value and .005 % RMS for the low value. These values are in the same  ballpark as the (0.003%) number for a LM7805 regulator.

The input capacitors have been increased in size as compared with the photo in the Connex website (or this is the size for the 120V model. The 400V input caps are probably for the 200V world)

The auxiliary supply filter caps.

Here is the previous model without the CLC output configuration

The ST L6599A [link] is the resonant controller. This “resonant LLC” topology is widely used for the 80-Plus Gold and Platinum PS’s for PCs because it allows for highest efficiency( [example 1], [example 2]). There are also indications that Hypex is also using this topology for their SMPS: [link]

The use of resonant LLC also results in lower EMI emissions due to the soft switcingh of the FETs and sinusoidal current [link]. In hard-switching topologies, fast changes in voltage and current generally results in higher amounts of EMI.


According to a Fairchild design document:

‚ÄúThe Conventional PWM technique processes power by controlling the duty cycle and interrupting the power flow.¬† All¬† the¬† switching¬† devices¬† are¬† hard-switched¬† with¬† abrupt changes¬† of¬† currents¬† and voltages, which¬† results¬† in¬† severe switching¬† losses¬† and¬† noises.‚ÄĚ

‚ÄúMeanwhile,¬† the¬† resonant technique¬† process¬† power¬† in¬† a¬† sinusoidal¬† form¬† and¬† the switching¬† devices¬† are¬† softly¬† commutated. Therefore,¬† the switching¬† losses¬† and¬† noises¬† can¬† be¬† dramatically¬† reduced.‚ÄĚ

‚ÄúAmong¬†¬† many resonant¬† converters,¬† the¬† half-bridge¬† LLC-type¬† resonant converter¬† has¬† been¬† the¬† most¬† popular¬† topology¬† for¬† many applications since this topology has many advantages over other topologies; it can regulate the output over wide line and¬† load¬† variations¬† with¬† a¬† relatively¬† small¬† variation¬† of switching frequency, it can achieve zero voltage switching (ZVS)¬† over¬† the¬† entire¬† operating¬† range,¬† and¬† all¬† essential parasitic¬† elements,¬† including¬† junction¬† capacitances¬† of¬† all semi-conductor devices and the leakage inductance of the transformer, are utilized to achieve soft-switching.‚ÄĚ

Soft switching is the main reason for much reduced EMI

The transformer has a new look -a cap- (don’t know if it is functional or just esthetics)

The output FETs are ST W20NM50. Specified at 500V / .22ohm /20amp. It is also rated at 14 amp continuous at 100 degrees C. Ample current carrying headroom mitigates the need for heat-sinks. These two parts alone are worth near 25% of the entire cost of the supply


Selecting mains voltage: 230V or 120V.

At first I was getting 0V. at the outputs. It turned out that the supplies are shipped for 230V operation. For 120V operation, the “120V jumper” must be installed (right photo).

Output voltage adjustment.

The nominal voltage of the supply is +/- 45V. There is a pot for voltage adjustment as shown in the photo below. I also replaced the output metal tabs for screw connectors for easier hookup.

These are the high and low values for the output voltage (previously measured with 330 ohm 10W power resistors loading the outputs)


I gutted an old discarded MUZAK PA amplifier and salvaged the case.

Reused the power cord and the lighted power switch. I added a ferrite bead to the power cord to further filter any potential EMI coming in our out through the power cable. The power supplies already have built-in EMI filters (as most SMTPs do).

The two green LEDs are connected to the auxiliary supplies and indicate that the power supplies are ON. I will later expose the LEDs from the UCD amplifier modules. Thus there will be light indicators for the AC in, the power supplies and the amplifier modules.

For testing and voltage adjustment, I use 1.5 Kohm 5W power resistors connected at the outputs.

The output voltage is very stable. The starting voltage is shown (44.96v). Eventually, it stabilizes at 45.00V.

In addition, with or without loading the outputs with the resistors, the power supplies are dead quiet (at least with my ears a few inches away from the supplies avoiding being zapped with high voltage :-)).


What is PFC? [link], [link]

Some thoughts on PFC in Audio: [link], [link]

  1. BlogGeanDo
    May 17, 2012 at 16:06

    I’ve had to remove all the comments in this thread. Please do not attack anyone or any product. I use this blog to share information.

  2. cobranets
    May 23, 2012 at 11:24

    Thank you for turst

  3. Anonymous
    June 3, 2012 at 14:20

    It seem’s that the design of the SMPS300RE has changed again as you can see on this page : http://diyclassd.blogspot.fr/2012/06/reception-des-alimentations.html
    What do you think of that stange wire coming out of the transformer ?

    • BlogGeanDo
      June 3, 2012 at 22:16

      The wire seems half a turn (to set the voltage) of the low voltage aux supply. Take a look at the other photo that shows the wire coming down from the side of the transformer. There is an additional device on the low voltage side. I don’t know what is that for.

  4. Axel
    July 19, 2012 at 06:07

    Ausgezeichnetes Produkt, verwende ich zwei von ihnen f√ľr meine UCD Monobl√∂cke.

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