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Old 16th September 2008, 09:09 PM   #11
Jeb-D. is offline Jeb-D.  United States
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Join Date: Nov 2005
Location: SoCal
I built a line stage for a car. Both B+ and filament were ran off a switching supply.

Sure, it works, but it is very difficult to get performance that nears a linear supply. It take a lot of effort to remove all ringing and overshoot from a switching supply. There is a lot of harmonic content in square waves that makes them much more difficult to filter/work with when compared to a single sine wave.

As mentioned, capacitive coupling between the cathode and heater is an issue at switching frequencies. If powering any stages where the cathode is not bypassed with a capacitor, that noise will show up on the cathode. And if it's a stage with gain, or followed by a stage with gain, that noise will be amplified. Sure the frequency is out of the audible range, but it can intermodulate with the audible range, which alters the sound. Also, you can end up with a dirty looking signal, which to me isn't acceptable even if the noise is outside of the audible range.

Most switching based devices are measured using filtering which is why they get similar S/N ratios as linear based equipment. Without this filter it is probably impossible(at least very difficult) to get a S/N ratio that rivals it's linear equivalent.

There are really only three reasons that would justify going the switching route.
1- automotive application, which is a must
2- desperate need to shrink the transformer + reduce weight.
3- The need for better efficiency in a regulated power supply.
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Old 16th September 2008, 10:46 PM   #12
cerrem is offline cerrem  United States
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Join Date: Jun 2005
Location: San Diego, CA
If your worried about noise, that should not be an issue... Most converters I build have switching fequency output ripple lower than .1% ...and that is over 200kHz....
If you are really nervous about the typical SMPS then use a resonant mode converter.... These use quasi-sine wave not square waves and the EMI emmisions are mucho lower lower than a standard converter.... I typically design them at or above 500kHz....so these would be very clean for filaments....

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Old 17th September 2008, 02:32 AM   #13
jkeny is offline jkeny  Ireland
Join Date: Feb 2007
Location: Dublin
I can't remember where I saw this but it's wise to use a grounded smps for lowest noise

It's in this thread: http://www.diyaudio.com/forums/showt...t=#post1384491

specifically: http://gboers.xs4all.nl/daisy/home/g...al_insult.html

So, switching power supplies are only really clean so long as they are properly grounded.
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