Linear PSU circuit
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 20th July 2011, 01:11 PM #1 gabanyayaya   diyAudio Member   Join Date: Aug 2008 Linear PSU circuit Does anyone know how a 12vdc linear psu circuit look like for a T amps similar to this one - Trends PW-10
 21st July 2011, 02:30 AM #2 !   diyAudio Member   Join Date: Jun 2005 Location: Midwest I don't know for certain that this is exactly what it uses but this is the basic topology and should work fine. Transformer needs be rated for at least 12VAC, 2A output. Better if it is rated for closer to 14VAC, > 2A output. You can use the LM317 datasheet to calculate the resistor values you need, but the amp chip will tolerate at least 13.x voltage so a couple values that would work are R1 = 240 ohm, R2 = 2.2K ohm fixed resistance instead of the potentiometer in the schematic. Make C1 at least 2,000 uF, bridge rectifier at least 2A, D2 and D3 3 can be 1N4001 diodes. C2 can be 10uF tantalum, 100uF electrolytic, or equivalent. C3 is not necessary, it can be omitted but can be 100uF or a little higher electrolytic capacitor. C4 can be 0.1uF to 1.0uF. You can get very elaborate building up a linear supply circuit but it is not needed for the class-T digital amps. You could use an LM7812 instead of LM317 and omit a couple of components. LM7812 has noisier output but it won't matter powering a T-amp.
 21st July 2011, 02:51 AM #3 gootee   diyAudio Member   Join Date: Nov 2006 Location: Indiana Unregulated Power Supply Design If you add a regulator, use your max load current to calculate the smoothing capacitance value needed so that the lowest part of the ripple waveform's voltage can never make Vin minus Vout of the regulator be less than the regulator's dropout voltage spec. The simple equations are at the link above. Also look at: Elliott Sound Products - Linear Power Supply Design And do some searches here for "star grounding". P.S. Don't use any tantalum caps. C2 above should probably be about a 22 uF electrolytic. Much larger doesn't do much good and sometimes causes startup problems. Last edited by gootee; 21st July 2011 at 02:54 AM.
 21st July 2011, 05:49 AM #4 !   diyAudio Member   Join Date: Jun 2005 Location: Midwest ^ There is no reason not to use tantalum caps. They are decades proven, today there is no quality control issue and they excel at low voltage differentials. 22uf electrolytic is ok, I mean better than nothing, but impedance is poor, it would be better to use 1uF film than 22uF electrolytic there. Granted, it's just about lowering the noise floor which with a T-amp, isn't really important.
!
diyAudio Member

Join Date: Jun 2005
Location: Midwest
Quote:
 Originally Posted by gootee Unregulated Power Supply Design If you add a regulator, use your max load current to calculate the smoothing capacitance value needed so that the lowest part of the ripple waveform's voltage can never make Vin minus Vout of the regulator be less than the regulator's dropout voltage spec.
That's not necessarily needed, it's the purpose of capacitors that the lowest voltage input is smoothed so attain a higher output. In fact it is always the case with an unregulated stage in the PSU that the ripple goes to zero volts after the regulator then the capacitors buffer that.. every 100 or 120 Hz with a full bridge rectifier.

 21st July 2011, 06:40 AM #6 sofaspud   diyAudio Member     Join Date: Nov 2005 Location: San Antonio The Trends is rated for 12V, 3.3A and you can't get anything close to that from the basic LM317 circuit. For a small T-amp it should work fine though. The LM350 is the 3 amp version of the 317. Don't go higher than the suggested 12-14AC transformer and mount the regulator on a good heatsink. __________________ It is error only, and not truth, that shrinks from enquiry. - Thomas Paine
gabanyayaya
diyAudio Member

Join Date: Aug 2008
Quote:
 Originally Posted by ! I don't know for certain that this is exactly what it uses but this is the basic topology and should work fine. Transformer needs be rated for at least 12VAC, 2A output. Better if it is rated for closer to 14VAC, > 2A output. You can use the LM317 datasheet to calculate the resistor values you need, but the amp chip will tolerate at least 13.x voltage so a couple values that would work are R1 = 240 ohm, R2 = 2.2K ohm fixed resistance instead of the potentiometer in the schematic. Make C1 at least 2,000 uF, bridge rectifier at least 2A, D2 and D3 3 can be 1N4001 diodes. C2 can be 10uF tantalum, 100uF electrolytic, or equivalent. C3 is not necessary, it can be omitted but can be 100uF or a little higher electrolytic capacitor. C4 can be 0.1uF to 1.0uF. You can get very elaborate building up a linear supply circuit but it is not needed for the class-T digital amps. You could use an LM7812 instead of LM317 and omit a couple of components. LM7812 has noisier output but it won't matter powering a T-amp.
LM317 can supply up to 1.5A max. Is is safe to feed it with a 2A 12VAC without blowing anything? I got a 13.7VAC 2A traffo with me now. Thinking this might works since it will rectify to 19VDC after rectifier (and generates more heat... )

One more thing do thing this circuit is 'quiet' enough for a digital devices....??

Thanks for the suggestion everyone....

DF96
diyAudio Member

Join Date: May 2007
Quote:
 Originally Posted by ! That's not necessarily needed, it's the purpose of capacitors that the lowest voltage input is smoothed so attain a higher output.
I think gootee is referring to C1, the reservoir capacitor. This needs to be big enough to keep the regulator happy at max load current.

Any caps after a regulator are not there for smoothing, but stability or noise reduction.

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