JohnSwenson;685571 Wrote: > OK, here is the in depth reply to Mr Pepper's inquiries. > > I am an electronics engineer working for a semiconductor company (I'm > not allowed to say which one), I layout integrated circuits, mostly > large high speed digital with sensitive analog circuitry as well. My > specialty is power distribution networks in these chips, providing low > noise power and ground networks to provide very low jitter on the > interface between the digital and analog sections. This jitter needs to > be kept in the single digit ps range, which is not easy. In order to > meet this we need on chip regulators, these regulators are always > linear regulators, switching regulators simply produce too much noise. > > > At the system level we usually have a switching regulator on the board, > followed by a linear regulator on the board and the linear regulator on > chip. This is required in order to keep noise levels low enough so we > can meet the jitter requirements. Believe me, if we could get away with > a switching regulator on the chip we would do so. > > As to my personal experience, I have designed many switching and linear > regulators, I have only designed a few switching AC mains to DC > supplies, but I have worked closely with those that do. I have measured > many switching supplies, I have a fairly good good idea of what they can > and cannot do. I have designed many linear AC Mains to DC supplies and > measured those as well. > > As to how this relates to audio, that is a long saga. Many years ago I > was building audio DACs for myself and found out that what power supply > I used had a significant impact on the sound. I set out to find out what > differences in these supplies caused the difference. I ran many hundreds > of tests with different supplies, listening and measuring the supplies > and trying to come up with correlations, it was not easy and took a > couple of years. One thing I learned from these tests was that most > supplies feed a fair amount of noise back into the AC mains as well as > the noise feed to the audio circuits. This noise injected back into the > mains turned out to be one of the most important aspect of the tests, > and one which is frequently completely ignored in power supply tests. > > It turned out that frequency range that caused the most impact on sound > quality was the 40KHz to 200KHz range. MHz and up was usually well > controlled and filtered, and frequencies in the audio range were > usually handled very well by the regulators in the circuits, but the > intermediate frequencies were not well dealt with. And it was the > amount of this noise injected back into the mains that had the most > impact on sound quality, presumably by being carried to other > components in the "stereo system" such as power amps and preamps. I > tried this with many different audio systems, from inexpensive consumer > systems, expensive audiophile systems, pro audio systems and all of them > seemed to be susceptible to this incoming noise on their AC. > > Many of these components contained "line filters" designed to block > incoming noise, but most of those only seemed to be effective in the > MHz and up range, having very little affect in the 40KHz to 200KHz > range. I probed around inside these other components and watched what > happened to this noise coming in over the AC mains and found many of > these components actually had resonances in this range which > significantly magnified incoming noise. In quite a few cases even if > their own power supply was not generating noise in this region, the > resonances wound up feeding significant noise into the circuitry when > noise in the right range was on the AC. > > So the summary of all these tests was that the biggest requirement for > a PS was to not send noise in the 40KHz to 200KHz range back down the > AC line, keeping noise out of the DC fed to the DAC also made an > impact, but it actually was not as big a concern as what went to the > mains. It also had to not be susceptible to noise in that range coming > from OTHER components. > > So I set out to design a PS that met these goals. Since this was going > to be a onesie for my own use it was a lot easier to build a linear > supply. I did a LOT of spice simulations trying out different > topologies and components which eventually evolved to what I have > today. I built several and proved that the real world did in fact > closely match the spice results so I was quite confident that doing the > exploration with spice was a reasonable approach. > > The result is very different than most linear supplies. Most linear low > voltage supplies employ a single stage very large value cap after a > bridge regulator. This of course only conducts over a very narrow > portion of the cycle, giving rise to very high current pulses in the > transformer. Common rectifiers produce switching noise in the bad > frequency range, with in conjunction with the current pulses cause the > power transformer to ring like a bell right in the bad range. This > noise goes right through the power transformer and into the mains. Even > if you have a good regulator which blocks this noise from the circuit > beeing fed from this supply, the noise still goes out to the mains. > > My design attempts to alleviate these issues in several ways, first it > uses Schottky rectifiers, second it uses a choke based asymetrical PI > filter after the rectifiers. The first cap is much smaller than the > second cap. The result is a supply which has cunduction over almost the > entire cycle, thus no large current spikes. Having the small first cap > allows the large conduction angle, but it also allows the filter to not > need nearly as much minimum current as a true choke input filter. What > comes out of the filter is almost pure sine wave which is very easy for > the regulator to deal with. In a tgraditional linear supply the sawtooth > output has a lot of high frequencies which are much harder for simple > regulators do deal with. The third part is a damping network across the > power transformer which damps the winding resonance so it will not > resonante from noise coming in from the outside world. > > The result of all this is a design which has extremely low noise > injected back into the AC line and very low noise sent to the audio > circuitry. It's simple, does not contain any custom components, no > special "audiophile" parts, is easy for inexperienced DIYers to build. > Is it the only design possible? of course not. Can a switching supply > be made which can match this performance, almost certainly. But its not > going to be nearly as simple, will probably need custom magnetics and > could very well cost more money to make and will be way out of the > range of a DIYer to build. > > I have tested many commercially available switching supplies (certainly > not all), designed for many different applications, including many that > come with audio devices including some very expensive pro audio > equipment and NONE of them bettered the noise levels of this simple > linear supply. So while it is certainly possible that a switching > supply good do as well or better, it does not seem that this is common > in the market place. It is also true that most linear supplies are also > very bad as well, and many linear supplies are worse than the switching > supplies that people are replacing. I have NEVER said that a linear > supply is always better than a switching supply. > > If you would like to test these claims go ahead and build this design > (its easy and not very expensive) and compare it to any of your > switching designs, paying attention to what is injected back into the > AC mains as well as what gets sent to the device, I'd be very > interested to see how it stacks up with your switching desings. Oh > yeah, and while you're at it try hooking them up to a Touch and see if > you hear any differences. > > Here is the schematic of the design mentioned above: > > http://johnswenson1.home.comcast.net/stereo/SB_5V.GIF > > John S.
John Thankyou for the prompt and in depth analysis of your circuit and your background. I have looked at the circuit and can find no reason why it would not work. A simple but highly effective well thought out design. Linear Tech devices take some real beating for quality comps. I had made some meaurements on the 'Touch' supply as a 'Buck' convertor and found emissions to be well below any level that would cause problems. Maybe because of the low current drawn. Not quite sure I would want the same design topology for a power amp (which are normally not switchers). >From my findings, I am still not convinced that a linear PSU can have so much acoustic effect than a switcher....but that's the nature of Audiophiles who can hear the difference by changing their mains plug to one which costs >£500 regardless of what crap wiring is hanging behind the wall. Best Regards WTP -- Waldo Pepper ------------------------------------------------------------------------ Waldo Pepper's Profile: http://forums.slimdevices.com/member.php?userid=39029 View this thread: http://forums.slimdevices.com/showthread.php?t=82648
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