Bridged LM 3886
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 8th January 2013, 11:53 AM #1 slungu   diyAudio Member   Join Date: Nov 2010 Location: Germany Bridged LM 3886 Hello everybody, I am playing with the idea of using a bridged configuration of two LM 3886 chips for at least 100W/8Ohm mono's. The idea was to implement Peters schematics but with two LM3886 instead of one LM4780. So I started to calculate my power supply using Po=50W and Rl=4Ohm as this is how each of the two amplifiers will be working. As described in the LM3886 datasheet I get Vopeak=20V Iopeak=5A and after that I get Vmax=30V. Now that should make it 30V and 5A, or and my question was : do I need a transformer that has 21Vac in the secondaries and over 150W or 200 or over 300W ? I always was under the impression that 300W for one 100W mono would be a little overkill, I would have taken one 160W for 100W or 200W for 100W but not 300. Where am I wrong in my thinking ? Regards, Stefan
 8th January 2013, 06:19 PM #2 djoffe   diyAudio Member   Join Date: Oct 2008 100 Watts into 8 Ohms translates to sqrt(800)=28.28 Volts RMS, or 80 Volts peak-to-peak. If you bridge, each amp needs to put out swings of 40 volts peak to peak. If you allow 5 volts of head-room from each rail, that says each amp would need to run from a 50 Volt supply. As a split supply, this would be plus and minus 25 VDC. The current peaks are 28.28*1.414/8=5 Amps from each rail. However, the average, or DC current is 2/pi times that from each supply, or 3.183 Amps. Thus the power supply must deliver 25*3.183+25*3.183=159 Watts. That a bit less than the theoretical Class B efficiency of 78%, which says the power supply would need to deliver 128 Watts. That difference is due to the headroom we postulated earlier. There are some white papers on these kind of calculations here: Update My Dynaco You might also find some interesting stuff on bridging...BTW, in the GT-101, I have pushed it to 200 Watts into 8 Ohms...but don't try that at home, as the fuse isn't really set up to do that long term. Akitika GT-101
 8th January 2013, 08:15 PM #3 slungu   diyAudio Member   Join Date: Nov 2010 Location: Germany Ok, so far so good, but what is that about the Vmax National is talking about in the lm3886 pdf ? I got at 25V but they state that I should multiply that by 1.15 because of 15% regulation and 10% high line conditions. That brings me to those 31V. Up until now I would have assumed that I would take those 25V divide that to 1.4 and get the data for the secondaries of the transformator but now I am not sure. Is this only to be used as max voltage for the caps or is this the value from which should calculate the tranny ? The power supply part was never my best part during college, I always liked to stay within DC and bits and bytes
 9th January 2013, 07:24 AM #5 slungu   diyAudio Member   Join Date: Nov 2010 Location: Germany The huge difference is what I was afraid of. Going with 37V rails into 4Ohm that I would have since I am in a bridged configuration is something that is out of the spec as far as I can see. Ok, it is not a continuous state, and the LM is protected and would probably get shut down if this happens over a longer period of time, but it is still pushing it a little.
 9th January 2013, 07:32 AM #6 madtecchy   diyAudio Member   Join Date: Dec 2010 Location: wigan Lm3886 can sound real good untill pushed even just a little bit then things change for the worse very quickley
 12th January 2013, 02:39 AM #7 Yurka   diyAudio Member   Join Date: Jan 2013 Bridging LM 3886 make sense if you want to have a load of 16-32 ohms. In your case better solution is to parallel 2 chips. As a benefit- better heat dissipation. Last edited by Yurka; 12th January 2013 at 02:51 AM.
 13th January 2013, 09:35 AM #8 slungu   diyAudio Member   Join Date: Nov 2010 Location: Germany I understood that a parallel configuration is of no use for a 8 Ohm load while it is recommended for smaller loads, i.e. 4 Ohm in order to supply the needed current . Since my speakers are the Start Air Kit 1R I am safe over 8 Ohm, I was thinking that this would be the easiest way to get some more power while dissipating the heat trough two devices instead of one as it would be the case with a LM 4780.
 13th January 2013, 11:04 AM #9 djoffe   diyAudio Member   Join Date: Oct 2008 Perhaps with the amount of power you are looking for, you are planning for a future set of speakers? The link that you sent shows that the speakers can take at most 20 or 30 watts. Please clarify...
 13th January 2013, 11:06 AM #10 djoffe   diyAudio Member   Join Date: Oct 2008 Oops...my mistake...I see now, you are doing the 1R...rated at 70 to 100 watts...

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