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PSU noise pollution: iFi, Meanwell, Teddy Pardo, UpTone JS-2 and others


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Hi Adam:

 

Since you brought the topic over there, I just replied to you in the UltraCap Listening Impressions thread.  But since your report here seemed lonely, I'll cross-post my thoughts here as this really is the more relevant thread for the conversation.

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I read your report a week ago, but have been too busy to comment.  While your effort is interesting, I caution against drawing much conclusion from it.  Here is why I say that:

 

1) Your $130 meter is distilling all the noise it measures (with unknown weighting across the band) down to one number, and as such is not an entirely useful result for qualitative comparison. For example: a supply with a bunch of of large nasty spikes at the lower harmonics of its switching frequency (typically 80KHz-100KHz) might read out on your meter a lower number than an SMPS which spreads most of its noise across a very broad band ("spread spectrum") at much higher frequencies--in the MegaHertz range and at lower levels. Thus, the unit with a higher reading on your summing meter might well be the more "line benign" back to the wall.

 

2) Your meter "measures" noise beginning at 10KHz and is therefore inappropriate for comparing the harmonics kinked back into the mains by linear power supplies.  The harmonics from LPS units will always be at multiple of the line frequency (50 or 60Hz), and your meter will miss most of those.  The diode switching (especially from cheap LPS units) and transformer ringing (on units that don't use an RC snubber on the secondary) will result in quite a bit of harmonics back to the wall. [As an aside, the filtering effect of the inductance and capacitance of AC cables--of what a component's PS kicks back in harmonics--is the primary reason people hear differences between AC cords; assuming decent wire gauge for current draw, it is not at all about the "quality" of the power delivered TO the component.]

Traditional LPS units (transformer>diodes>caps>regulator) draw current over only about 50% of the AC wave cycle--hence their harmonic distortion of the mains, even if relatively benign.  That effect is part of what has driven the decades long push for "power factor corrected" power supplies worldwide (most easily accomplished with SMPS designs.  While one rarely sees a power factor corrected linear power supply, our own JS-2 is an example of one.  Its large DC choke results both in a smooth half-cycle wave delivered to the regulators (making their job easy), and in a power-factor of about 0.97--as it draws current over the entire AC wave cycle, thus not generating harmonics back into the line.

 

If you really wish to measure what various supplies kick back into the wall, then a more elaborate scope or spectrum analyzer should be used.  But what I expect you will find is that what is coming out of your wall in the first place (the spectrum of that residual 23mV you started with) is far nastier (i.e. at undesirable frequencies) than what is being added by the SMPS units you are testing.

 

None of the above in any way is meant to minimize the problems or differences in:

a) the quality of the DC output from various SMPS and LPS units;

b) the inherent "evil" of ALL SMPS units--very high leakage currents (due to their required use of Y-capacitors) (AC>DC linear supplies also have leakage, and while it is MUCH lower, it still will form "loop" with leakage from other connected components.)

 

Of course, the whole point of the UltraCap LPS-1 is to render both a) and b) above irrelevant! :D

 

Adam, please do not take this post as any indictment or dismissal of your fine efforts to shed light on the subject of what SMPS units put back into the mains.  This is just my attempt to show how the matter is more complicated than it might seem (and wait until John corrects me and tells of even more complex factors!).

 

By the way, short of having a 100X scope probe and needing to do other things to safely look at noise on wall voltages with a O-scope, I did find this neat (if somewhat pricey at $389) adapter that plugs into the wall (there is a version with leads too) and provides a BNC to go directly to a scope or other analyzer. http://www.onfilter.com/products.html?s=MSN01

 

Best,

--Alex C.

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3 hours ago, Elberoth said:

You may have noticed that he places much greater importance on the freq in the 100+kHz bands as more important to the performance of audio equipment. In other words - he doesn't seem to think 10kHz meter limitation is a problem.

 

Great post Adam.

 

Even speaking only of their "pollution" back to the wall (and without even getting into discussion if/why it matters and how that can be dealt with), I think that SMPS units for small devices should be measure and considered separately from linear power supplies--especially large LPS units.

I say that for two reasons:

1) The spectra both kicked back noise and distortion of the waveform due to design is far different for an LPS, but even for those an undamped transformer will pass back some 100KHz+ ringing.  And the unregulated typical LPS of a big power amp can have a huge impact on the line.

2) The criteria (and performance measures) for a high current supply are quite different from those for a small wall wart.

 

Really, in the end I think we can agree that what comes out the DC side is much more important than the few tens of millivolts of noise that gets put back into the mains.  The latter is not so hard to filter out (though of course large leakage current from SMPS still persist).  Yet heavy truncation/distortion of the AC wave (from a shocking number of components as well as some household items) is something serious to guard against.

 

Cheers,

--Alex C.

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