Simple L4780TA Amp with single supply - diyAudio
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Old 5th January 2012, 09:13 PM   #1
ZLyzen is offline ZLyzen  United States
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Default Simple L4780TA Amp with single supply

I just want to make a quick and non complicated chip-amp with a lm4780, so I'm thinking about just using a plug-in power supply ~24V ~3-5A

I notice on some class T amps they use switching power supplies and the back where the ac/dc adapter plugs in has -/+

Are they taking each lead to be the positive and negative rails and just defining a ground inside the case or are they taking the negative terminal to be ground?

I'm just wondering if I need to use the datasheet's single-rail example or if I can treat it as a dual rail circuit.

I guess it's only a few extra parts to make it single supply, but there's also the large cap. at the output to get rid of the dc offset.

Are there any benefits to not using single-supply schematic?
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Old 5th January 2012, 09:49 PM   #2
! is offline !  United States
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Those T-amps chips are designed to be single rail powered, they aren't doing anything tricky. Yes negative power rail is ground.

To use a single rail power supply on LM4780 you can put a coupling capacitor on the output and change the schematic a bit. An example can be found in figure 4 of the LM4780 datasheet. http://www.national.com/ds/LM/LM4780.pdf

This is true of most class A/B amp chips, single rail supply means the output is always a DC voltage so an output coupling cap is needed, as well as a bias or virtual ground point (something like a resistor voltage divider and capacitor to ground at their midpoint to create a reference fake ground).

On the datasheet figure 4, pg. 7, that capacitor is "Ca". They show 10uF but performance is generally better if a higher value is used like 100uF but I haven't built that particular single supply circuit myself.

What is the plug in power supply like? Before I went single supply I'd probably pop open the plug-in supply and turn it into a voltage doubler - http://www.sciencelobby.com/diodes/i...age-dubler.gif -OR- use a second of the same power supply with a floating (non-earthed) ground and connect their outputs in series with that series connection being the center voltage ground.

Last edited by !; 5th January 2012 at 09:51 PM.
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Old 6th January 2012, 01:29 AM   #3
ZLyzen is offline ZLyzen  United States
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Yeah I was planning on sticking to that single supply example for this project and only changing a few things, at not much of a cost difference I'll take your word and bump up Ca and what do you mean by the double power supply? Say I were to use this: Amazon.com: Universal AC Adapter 15V 16V 18V 18.5V 19V 19.5V 20V 22V 24V 70W: Electronics on the maximum (24V) setting

so basically If I had two in series I could define their connection as ground and have 24V rails?
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Old 6th January 2012, 05:19 AM   #4
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Those power supplies are likely to have very bad output ripple, if you used high quality SMPS and knew (Or measured) there is no AC line neutral or ground connection to the DC output negative then it would be viable, then yes I meant two of the same power supply and two 24V outputs in series would give you the +24/0/-24 rails and their connection would be the amp ground.

Instead of that generic one linked I would instead look on ebay for two identical OEM/surplus major brand laptop supplies, if you must use something cheap. They might not be as high as 24V but there is this saying that the first watt counts the most and to me it equates to it being more important to have cleaner power at normal listening levels than higher output with dirty power rails and the reliability issues that cheap generic brick SMPS have.
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Old 7th January 2012, 02:04 AM   #5
ZLyzen is offline ZLyzen  United States
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Hmm. I might end up going with a nicer SMPS but still trying the single supply scheme in the datasheet. One for "simplicity" (once it's built) and the other because I probably won't bother to make a single rail chip-amp after this..

As for the output cap. that is to null the DC offset, they recommend a 4700uF in series with the output. I'm assuming (due to the capacitance value) that this is electrolytic? (there might have also been a '+' on the diagram giving this away now that I think of it) even though for capacitors elec. are considered slow it should be fast enough to keep up the audio right?

ohh and just to clarify. since the two "prongs" on the plug that goes into the wall of the SMPS aren't really "ground" I could call each one a rail and treat it as a 12V supply with a ground I define in the chassis? I could totally be wrong..

Last edited by ZLyzen; 7th January 2012 at 02:17 AM. Reason: more questions
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Old 7th January 2012, 03:32 AM   #6
ZLyzen is offline ZLyzen  United States
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Huh.. Looks like I might have to increase my power supply voltage.. it says the minimum supply voltage for a L4780 is 20V that would be per rail? so using a 24V single supply is sort of like tricking the chip into thinking it has a 12V supply.. which won't cut it... orrrr i could go back to this 2 24VDC power supply idea....
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Old 7th January 2012, 08:08 PM   #7
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Hi Zlyzen.
IMHO the single rail is more complex than the more common dual rail version. plus the dual rail needs no DC blocking capacitor on its output that can spoil the audio quality of these great sounding Integrated circuits.

regards ian
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Old 8th January 2012, 11:34 PM   #8
ZLyzen is offline ZLyzen  United States
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So say I used 2 SMPS in series with the center as ground to create a dual rail supply. Would be advisable to add a 4700uF cap to each rail (inside the project box that I will plug these into)? would this benefit any aspect, I'm worried that these ps's won't be able to provide the power i'm looking for. I could be mistaken
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Old 8th January 2012, 11:44 PM   #9
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Hi you wont need the 4700uF caps in fact they could cause problems when powring up the smps as the initial charging current could cause the smps over current protection to fire. some small value caps on the amplifier will suffice say 100uF or so as the smpsu will already have its own smoothing capacitors. Yes you can use two smps i tried it with two 19 volt ones from old laptops i had in my junk box . It works fine but the audio quality was not as good as a more common linear psu. What sort of power are you looking for

Regards Ian
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Old 9th January 2012, 12:11 AM   #10
ZLyzen is offline ZLyzen  United States
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I'm going to go with a couple SMPS from laptops that I have as well to power the 4780 and then regulate it down with a 317 337 combo to ~15V for a couple filter amps for 2 tone knobs and a headphone out.
so smaller caps would be better? okay, I will get started on the build in the next week and hopefully have something before or at the beginning of february. Just a small stereo amp for practice I got a bunch of 4780's and I want to practice using one before I start my active monitor project
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