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Old 16th September 2008, 08:09 PM   #11
Jeb-D. is offline Jeb-D.  United States
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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, 09:46 PM   #12
cerrem is offline cerrem  United States
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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, 01:32 AM   #13
jkeny is offline jkeny  Ireland
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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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Old 5th March 2015, 12:42 PM   #14
ytliu is offline ytliu  Taiwan
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There is a more common design guide you can get from MAXIM. In principle, AC power is rectified to AC to the primary side of switching transformer mainly for safety to isolate wall power to device (it is also asked by regulation rule like UL). Also for this reason, an optical couple switch is used to control the on-off of switch transistor (BJT, or MOSFET).

The most simple and cheap approach is to use old AC adapter for mobile. Most of them is designed with 5V output. You can easily changed to 6.3V (or 6V) output by adjusting resistor values without problem.
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Old 5th March 2015, 02:32 PM   #15
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I have used switching for large transmitting tubes (gm100, gu81m, gu46, gmi90) and I've never heard thw high frequency switching. Ripple on the anode? YES. junk from the switching supply? NO.

However I have made it a habit to add a thermistor in the path of any large DHT's filament bypassed by 2 caps: a large 10,000uf + cap and a smaller 1uf cap. Works great
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Old 5th March 2015, 02:55 PM   #16
Celsius is offline Celsius  Spain
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In my case, I broke the tweeter by cheap switching PS to the 211 filament, you donīt listen but the tweeter, yes.

I donīt recommend the switching PS in direct heater , is the noisy silence, when I change to DC diode rectifier the sound was better.
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Old 5th March 2015, 05:13 PM   #17
ytliu is offline ytliu  Taiwan
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Default SPECIAL Careful using switching power supply work with direct header

Originally Posted by celsius235 View Post
In my case, I broke the tweeter by cheap switching PS to the 211 filament, you donīt listen but the tweeter, yes.

I donīt recommend the switching PS in direct heater , is the noisy silence, when I change to DC diode rectifier the sound was better.
celsius235 reminds us a very important thing on applying switching power supply on direct heater. Previous discussion is mainly for isolated heater since AC/DC on heater may not directly applied to the amplification circuit. However, it is totally another story when switching power supply goes to direct heater.

In principle, I may not suggest using switching power supply for direct heater as commented by celsius235. However, if you want to try, it could be OK with special care mainly on block switching frequencies (very high freq) get into the amplification circuit. Besides on adding appropriate filters, two things should be noted

1. Switching freq should be higher as possible, > 300KHz is suggested. If possible, > 500KHz should be better but it comes the MOSFET selection and reduce switch loss. Except you are professional here, otherwise get a matual power module is recommended.

2. Capacitor used for by-pass switching noise is different from the one used for audio. Special polymar cap like SP-Cap, or OS-con will be appropriate for their extremely high ripple current. However, most of them can only work at low voltage (but no de-rating issue).
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