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Old 20th July 2011, 01:11 PM   #1
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Default 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
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Old 21st July 2011, 02:30 AM   #2
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Click the image to open in full size.

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.
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Old 21st July 2011, 02:51 AM   #3
gootee is offline gootee  United States
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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.
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Old 21st July 2011, 05:49 AM   #4
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^ 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.
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Old 21st July 2011, 06:01 AM   #5
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Quote:
Originally Posted by gootee View Post
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.
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Old 21st July 2011, 06:40 AM   #6
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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.
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Old 21st July 2011, 11:28 AM   #7
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Quote:
Originally Posted by ! View Post
Click the image to open in full size.

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....
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Old 21st July 2011, 12:00 PM   #8
DF96 is offline DF96  England
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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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