Home > DIY HiFi, POWER > Resonant SMPS for Audio

Resonant SMPS for Audio

February 26, 2011 Leave a comment Go to comments

SMPS have gotten a bad rap for audio because they emit large amounts of EMI. No more! The quest for higher and higher efficiencies especially for digital TVs Home Theater Systems and PCs have made the “resonant” topology more popular and more affordable. Currently available resonant SMPS power supplies provide the highest efficiency and lowest EMI radiation attainable. [You can check the reviews of PC power supplies on Hardware Secrets and you’ll find that the latest, high end supplies are based on resonant topologies]

I purchased the SMPS500R power supply shown above from Connexelectronic to upgrade the current unregulated linear supplies I use  in my UCD180HG amps.

The SMPS500R is a +/- 45V, 500-Watt REGULATED supply. I guess the appeal for SMPS is the “regulated” aspect. Linear supplies are already low noise but for the voltage and power of interest, they are unregulated. Additional advantages of the Connexelectronics supply are (more is explained in the user’s manual):

  • High efficiency, up to 95.6%
  • Short-term peak power of 800W
  • More suitable for class D amplifiers as compared with class AB because of class D amps have comparatively lower power supply rejection ratio, thus regulation provides greater benefits
  • Low cost and yet uses the latest power supply technologies
  • Simple design because there is no need of PFC. This also allows for a more compact design, making traces and interconnections shorter
  • Custom voltages and adjustable +/- 10%. I ordered +/- 45V
  • Auxiliary bipolar power supply (500mA). I ordered +/- 18V DC

Noise Measurement: (According to Connexelectronic):

“The SMPS500R both single and dual output has line and load regulation below 1%. this means that the output voltage variation from zero load to 100% load will not drop or increase with more than 1%. typical values which i measured are ~220mV for the +-54V single one, ~190mV for 48V single one, ~120mV for 24V single one. output ripple is also below 200mV for >35V versions and below 100-150mV for lower output voltage ones.”


According to a Fairchild design document:

“The 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.”

“Meanwhile,  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.”

“Among   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.”

This device, the ST L6699A, a second generation chip from ST Microelectronics is the “heart” of the power supply


I installed two 330 ohm 10-watt power resistors at the outputs to simulate a load. 330 ohm at 45V is 6.1 watts of power dissipation. This is probably more power draw than the power used in normal listening levels. The measurements are “out of the box” without any adjustment

Noise measurement with my el cheapo scope: ~80 mV peak to peak at 6 watt power dissipation. This is within the numbers quoted by Connexelectronics. If we look at the period of the ripple, they are about 1o usec which is about 100KHz and this (I think) is related to the switching frequency of the power supply.

  1. Gregg
    February 27, 2011 at 21:04

    Just curious… why did you select the Connexelectronic SMPS instead of the ones offered by Hypex? Wouldn’t the Hypex SMPS be a better match for your UCS180HG amp modules?

    I have a multichannel setup with 5 UCD modules. Three UCD400HGs power the front left, front center and front right channels; and two UCD180HGs power the rear left and rear right channels. I have linear supplies for all of these modules. Since I have different voltage requirements for the UCD180 and UCD400 modules, I used 2 Hypex UcDSupplyHG power supplies and a couple of toroids. I also have one of their softstart modules.

    If I were to build a new amp with a similar variety of amp modules, but instead with SMPS, would the softstart be necessary? Do these SMPS provide the same push button control and protection?

  2. BlogGeanDo
    February 28, 2011 at 01:06

    Basically two reasons: one is that Connexelectronics was more descriptive about their power supply technology, which allowed to to read up about it from different industry sources convincing myself that indeed it is the latest, lowest noise SMPS technology. The second reason is price.

    According to the manual, there is “on board sofstart”, and “complete protection set”. However, there is no “on-off” capability.

    I have not hooked it up yet. Just fired it up.

  3. Florin
    February 28, 2011 at 15:15


    Speaking of psu’s, I am doing a DAC board around the AK4396 (and maybe AD1955) and I want to place some low noise regulators near the DAC chip for the digital and analog power lines. I like the way twisted pear managed to squeez the regulators near the ESS part for example.

    Do you have any recommendation for good, low drop, low noise regulator IC’s in a small package?


  4. BlogGeanDo
    February 28, 2011 at 18:31


    Do a search for “ultra low noise 3.3v”. The cellphone industry seem to have pushed the IC industry to develop regulators in the ~10uV rms noise range. The OPUS board uses the LP2985 at 30 uV; the AMB Y2 DAC board uses TPS79333 at 32uV.

    PS: I don’t have first hand experience with those regulators, but they are an improvement to what is currently being used in terms of noise level.

  5. avr300
    February 28, 2011 at 19:32

    Looking forward to read your judgement – being an owner of a 5ch. UcD180 box.

  6. BlogGeanDo
    March 1, 2011 at 03:04

    Yeah, me too ;-)…

  7. Florin
    March 1, 2011 at 08:19

    Thanks for advice! Take a look here: http://pdf1.alldatasheet.com/datasheet-pdf/view/337603/AD/ADP151AUJZ-3.3-R7.html

    Is 9 uV RMS possible?

  8. BlogGeanDo
    March 1, 2011 at 16:53

    Yeah, that is pretty nice and 200mA-capable. How about this: http://www.national.com/ds/LP/LP5900.pdf
    6.5uV rms 10Hz to 100KHz and guaranteed by design. And neither requires a noise bypass capacitor. the AD part has a little better PSRR. Keep us informed of your project…

  9. Oreo
    April 17, 2011 at 01:07

    Any update?
    Would be very interested to hear your comments regarding this PSU comparison! 🙂

  10. BlogGeanDo
    April 19, 2011 at 00:07

    Hello Oreo. Unfortunately, I killed the supply by accidentally shorting the aux supply. (The aux and main supply shares the transformer) I am in the process of getting it repaired. Connex has been very good with their support and offered to repair it by sending it back…

  11. Alex
    October 3, 2011 at 19:50

    I got my SMPS500R about 2 months ago and since then is working great with my 2 UCD180 modules. What i noticed is that the aux. output has fuses, perhaps to protect the transformers or diodes in case of a short circuit. Did yours had fuses ? I spoke with Cristi, and he told me that was added on the new revision, as a new level of protection, just in case.

    • BlogGeanDo
      October 4, 2011 at 17:43

      That is a great addition to the PS. Mine did not have the fuses so when I accidentally shorted the aux PS, it burned the transformer.

  12. avr300
    November 6, 2011 at 07:48

    Alex, any comment of the sound ? Did you come from traditional transformer/rectifier/caps to SMPS?

    The AUX output, do you use that supply the input stage stage of the UcD ?

    • Alex
      January 26, 2012 at 19:38

      Hello guys, Thx for asking. The sound difference when using SMPS500R compared with mains transformer and large elco’s is at least as much as the diference between UcD180ST’s and UcD180HG’s, yet by using UcD180ST with SMPS500R i can say that it sounds like a UcD180HG.
      I’m not a geek, not a beginner either, i practice electronics since i was a kid, now i’m my 50’s. I used to build class A and class AB amps, though i messed up with tube and guitar amps back in college, in the 70’s. Thus i have some background and i can clearly make a difference, not based on my old rusty ears.
      The build quality of this power supply is much better than others, i have a Coldamp supply, thrown somewhere in the garage, after i got pissed off by the noise which this crap made at low load, and i could not fix even by adding dummy resistors as some folks suggested on the forum. I ended up blowing the transistors while i tried to do what the manufacturer didn’t: Make-it work, not yell like a horny cat.
      Hypex supplies look good, but are damn expensive, for what they claim to do, i could just barely afford the UcD’s, the 180’s and the new 400’s. B&O looks better, but isn’t affordable and as friendly to use as the SMPS500R
      I ordered 2 UCD400 modules, and i tested them with SMPS500R, and surprisingly the power supply was able to drive them without sag close to maximum power rating. Just in case, i’m gonna order two more SMPS500R next month, to give some juice to these ucd babies.

  13. Anonymous
    December 12, 2011 at 08:04

    BlogGeanDo :
    Hello Oreo. Unfortunately, I killed the supply by accidentally shorting the aux supply. (The aux and main supply shares the transformer) I am in the process of getting it repaired. Connex has been very good with their support and offered to repair it by sending it back…

    You told the supply has all protection you need.

    And now you have just killed it with simple short.

    Connextronic, sells bad things

  14. January 9, 2012 at 13:20

    I have ordered four smps500r +-54V power supplies few months back, for my speaker cabinet amplifier project, and while searching with Google i found this page. My power supplies had passed the most cruel tests since i’m a freak when it comes to squeeze all the potential out of a new product. I noticed too that there are fuses on the secondary side and i still wonder how come yours did not had. Was it purchased earlier ? I asked the company which produced them what is the MTBF as i want to purchase more, i’m having an active speakers business and i want to use them as ready available power supplies to be integrated into my active speakers. They told me that the MTBF figure is over 80k hrs, and as to date, no power supply had died in the field while was used properly, just two incidents, one being this related on this page, and the other, of a customer from Brazil which ordered a 110V version and he had used in another place of the country, where there is 230V. the fuse popped-out and had to be replaced.

    @Anonymous, you might be some gellous competitor, otherwise, why anonymous?

    • BlogGeanDo
      January 10, 2012 at 17:04

      Hello, thanks for your comment. Yes, my PS was the earlier version without the fuses for the LV. I remain a big fan of the Connex products and the support is very good. If you look carefully, they use “proven” technologies and designs from the people that invent this stuff (for example International Rectifier, ST Micro, etc). Connex then makes the implementations targeted to the audio market in a very cost effective way that benefit us the consumers. Take for example PC power supplies: the technology is there, but he voltages are not for audio amps.

  15. Nigel
    January 31, 2012 at 02:51

    Me and a couple of folks are using this tiny beauties for Quad 405 DIY amplifiers which we built and i can only say that using the SMPS500R power supply, is the most suitable choice which we found after countless hours of trials and listening tests. The guys who are buildin these power supplies, listened closely to our requirements, do we got some very robust power supplies at less than half the value of anything similar.
    I surely recommend it to any of you.

    • BlogGeanDo
      January 31, 2012 at 18:06

      Thanks for stopping by my blog. Glad to hear good impressions from these very good and very economical supplies. I haven’t had the time to replace the supplies in my UCD amps, so I cannot offer my impression at this time.

      It seems that my comment about damaging the supply by shorting the AUX has been used by some people to undermine the quality of the supplies. I remain a big fan of Connex supplies and I fault myself for being some clumsy.

  16. December 17, 2014 at 13:27

    Nice review of the Connex SMPS.
    Could you kindly tell me the output variation if the mains is varied? In other words, is it capable of universal input (90V to 264V) or half of that (185V to 264V)?

    Thanks and warm regards,


    • BlgGear
      December 17, 2014 at 19:20

      I don’t know about “universal”, but it is designed to support 120 and 220 with a jumper. (or near those standard voltages). And of course I cannot vary my mains 🙂

  17. September 14, 2016 at 16:29

    Pornography SEO

  18. August 12, 2017 at 07:54

    Hi there,

    It has been a few years since your wrote this article and I was wondering what your long term experiences are with these SMPS’s. Do you use them often and how are they doing today?

    Thank you.

  19. patrick mossley
    April 9, 2018 at 03:21

    I own one SMPS500R although slightly different than the one in the picture. It must be an improved version since I bought it in 2012. It works flawlesly ever since. In the meantime I bought two more SMPS800RE and one SMPS2000RxE with output voltage built to specs. All working great, after couple of years of 10-14hours a day usage. Compared with any other dedicated audio SMPS on the market I found that the Connexelectronics SMPS are the best value for money. And all their specs are real and hold over time. Hypex SMPS have unregulated voltage, with huge voltage variations, low filter capacitance and no flexibility in chosing other voltage than what they offer. They are unsuitable for any of my projects, I wonder if they perform any better when paired with any other amplifier than their own offering.

  1. No trackbacks yet.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )


Connecting to %s