Lm3886 in BTL mode with 4ohms load and Smps. - diyAudio
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Old 4th November 2010, 07:01 PM   #1
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Default Lm3886 in BTL mode with 4ohms load and Smps.

I am looking for amplifier that would go with my subwoofer.
for this I have 4Ohms woofer. I have finished designing Enclosure.
wish to use Smps supply ( 20V 4A) and Lm3886 in BTL mode.

Now I request you all to suggest me if the approach is correct.

I want to use 20V 4A SMPS because
1)This is easily available.
2)This has comparatively Smaller form factor to transformer supply.
3)Filter capacitors goes smaller in size.
4)Switching noise should not matter as the whole amp would be tuned for 20Hz to 120 Hz.

Use of LM3886 in BTL mode.
1) Both the Supplies ( +ve and -ve ) contribute simaltenuesly so lesser voltage at -ve and +ve supply terminal is needed.
2) I very much doubt if I can connect load of 4E to LM3886.

If the above approach is not recommended then please suggest me any other quick chipamp that can replace LM3886 for 70W to 100W,and can be used with the above mentioned supply.
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Old 4th November 2010, 07:20 PM   #2
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Hi,

I wouldn't recommend bridging an LM3886 with a 4 ohm load as this reperesents a load of 2 ohms to each chip which is lower than the minimum load recommended for the
LM3886. The power dissipation in the chip would be too high and the heat would not be able to transfer to the heatsink quick enough to cool the device properly. This would lead to the die temperature increaing and possibly causing the protection circuits to operate (which you really do not want).

One solution, short from buying a higher impedance subwoofer, would be to parallel two LM3886's to increase the current handling capability of the devices. This would still only get you 68W (as the datasheet says) output but the amp would be more stable on low impedance loads.

On the other hand, you could try using one of the TDA729* series chips which have higher output power ratings and are able to use higher supply voltages. You would then not need to bridge two chips and would therefore avoid the issue of the load impedance halving.

Regarding the power supply, high current, 24V fully enclosed power supplies that are designed for CCTV are readily available and would be able to supply the necessary current. The only problem with these supplies is that they do not output minus voltages. I am not too sure how you would overcome this problem.
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Old 4th November 2010, 08:21 PM   #3
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Thanks Jack,

I know If the 4Ohms load is used in BTL mode then the load seen by each Chip would be 2ohms. which would lead the problems mentioned by you. Thats the reason I wanted to know if any one has tried this or any solution is available for this.

Can you tell me what is the current rating of the CCTV supplies?

I would use two supplies to generate +ve and -ve rail supply. To my knowledge secondary is isolated. So should not give problem if configured in dual supply mode.
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Old 4th November 2010, 08:34 PM   #4
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As I said before, the possible solution for the impedance issue would be to parallel two LM3886's or to use one, higher powered chip from the TDA729* series. With regards to paralleling the 3886, I'm pretty sure that National Semi have written an application note on it somewhere.

The CCTV supplies have current ranges that vary but finding one with the appropriate output (around 3-4A) should not be difficult at all. They are also very cheap too and can be found from many different sellers on eBay. Some of the sellers are international too, so they should ship to your country.

If you used two completely independent supplies, I would not expect a problem and as you thought, the secondary IS isolated so there shouldn't be any issues there.
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Old 4th November 2010, 10:35 PM   #5
asbjbo is offline asbjbo  Norway
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If you look at the datasheet, especially the graph "Output Power vs Load Resistance" on page 13, you will see that the chip runs out of steam at approx 4 ohms. As others have said, bridging it across a 4 ohm load gives each half of the amp 2 ohms to work with. That will give you a lot less power than you expect, and the chip will probably activate its protection circuitry quite often. http://www.national.com/ds/LM/LM3886.pdf

I'd suggest parallelling two LM3886 or one LM4780, which is two LM3886 in a single package. Kits for doing exactly that are available, e.g. from Audiosector (Peter Daniel).
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Old 5th November 2010, 01:44 AM   #6
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Dear Jack And asbjbo,
I am looking for some BTL solution with 4Ohms load. I wish to use SMPS readily available for laptops.These are available easy and cheap. BTL uses power supply most efficiently, ie. during any half cycle of input, both the rail supplies would contribute to deliver the required wattage at load.This reduces the need of using higher rated supply like that used in non-BTL mode with dual supply. And so (20V 4A) x 2 can be enough for delivering 100W easily.

So, I need Amp that could deliver 70W to 100W using (20V 4A )x 2 supply.

Hope I have properly put my things here.
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Old 5th November 2010, 07:49 AM   #7
asbjbo is offline asbjbo  Norway
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Have you looked at the bridged/parallel configuration? Essentially, you place two LM3886 in parallel on each side of the bridge, in total four LM3886 (or two LM4780). That would fit quite well with your specification. You will need to match components very carefully, or use opamps as servos and input buffers. See section 7 in the application note, especially figures 13 and 17.

http://www.national.com/an/AN/AN-1192.pdf

Last edited by asbjbo; 5th November 2010 at 07:58 AM.
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Old 5th November 2010, 05:52 PM   #8
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Asbjbo,

Thanks, I read it and can keep it as an option. I found one more design but it is TDA2030 based. the link is from same Diyaudio. Thread and Ckt.

Somewhere on net I read that this is just a working design not a very good amp. pls comment.

And other members pls.... your contribution here, would be valuable for me.
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Old 5th November 2010, 06:44 PM   #9
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The TDA2030 isn't a bad little chip but in your first post, you stated that you wanted an amp that would output 70-100W into 4 ohms. The TDA2030 is only a 18W max device and even when bridged (which would not be possible with a 4 ohm load), would not provide the output power you require.

A non-bridged TDA729* series chip would provide plenty of power, do it into 4 ohms and, most significantly, work well off a 4A 20V supply. I understand your efficiency concerns, but to keep things simple, you are best simply using a single chip if one is available. Paralleling and bridging an amp, things start to get very complicated and can cause trouble because of the problems with matching the gain and getting the amps to share current properly.
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Old 5th November 2010, 08:04 PM   #10
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The output current of an SMPS is usually the peak current. With music being sine waves you only have 4 Apk / sqrt(2) = 2,83 Aeff available. That corresponds to ~32 W into 4 Ohm. Normal SMPS cannot deliver the same amount of overcurrent as conventional unregulated power supplies, so it is pretty likely that you won't get more than 32 W even for transients. Another issue is that not all SMPS are absolutely silent in the audio band.

The TDA729* has the same dimensions as the LM3886. It must therefore be just as limited when it comes to thermal transfer. The trick to get 100 W from it is to use it in class H, what the datasheet calls High Efficiency Application, but then the 6,5 A current limiter threshold may become an issue and the power supply becomes more complex, too.

For 100 W into 4 Ohms a parallel setup is recommendable. According to the Overture Design Guide that requires more or less 33 V at full load, which means 40 V or more idle from an unregulated supply. Two ICs in parallel could be easy enough to heatsink. Depending on the woofer and the way it is used a third IC may be necessary. You could google PA150 for a working example, but your SMPS won't do.
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