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 slungu 8th January 2013 11:53 AM

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

 djoffe 8th January 2013 06:19 PM

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

 slungu 8th January 2013 08:15 PM

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 :)

 djoffe 8th January 2013 11:52 PM

We agree on the +/-25VDC Rails...that's a good first step. So, if regulation were perfect, then that would be the end of the story....But...

Some things make life not so ideal:
1. power supply ripple
2. loss in the diodes
3. IR voltage losses in the transformer
4. line voltage variations

Approximate Ripple from i=Cdv/dt, so dv=dt*I/c. On a 60 Hz line, dt=1/120.
We'll approximate I as Idc, at around 3 amps...let C=10000 uF...dV=2.5 Volts...so that's 2.5 Volts of droop due to ripple.

The diodes have almost no loss (say 0.4 V) at light loads, and maybe 1 volt at heavy loads, so the bridge rectifier takes away about 1 volt from the 25.

The IR losses in the transformer are usually hardest to get a handle on...not because it's that hard, but the mfrs. don't usually give very good models. As one example, a toroid that I use rates the outputs as 35-0-35. So the total voltage should be about 70*1.414-1=98 volts when lightly loaded. It puts out about 106...we conclude then that the rated voltage is the under load voltage. We'll say that is roughly 10% regulation.

Line voltage could be plus/minus 10%.

So...if you wanted to end up with +/-25 under max load, you'd need the half winding voltage, Vhw, (assumed to be a loaded voltage) to satisfy:

Vdc>=Vhw*0.9*1.414-1-Vripple, so If Vdc=25, then the half winding voltage would be spec'd at 22.4 Volts. Now, under light load, and at high line, you'd have the half voltage of 22.4*1.414*1.1*1.1-0.4=37.92 Volts.

That's ok, since you'd have about 38-0-38. Still, that's a huge difference!
76 Volts (high line, light load) versus 50 volts (low line, heavy load). That's still within an acceptable window on the LM3886. But, if you're trying to get the most out of the LM3886, a regulated supply helps (though there are certainly other ways to attack the problem...everything has its pluses and minuses).

That's why in the GT-101 I use a regulated power supply.

Akitika GT-101

As the EPA (United States Environmental Protection Agency) says about all such numbers, "Your mileage may vary".

But I hope that gives you a rough idea of what you're up against.

 slungu 9th January 2013 07:24 AM

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.

 madtecchy 9th January 2013 07:32 AM

Lm3886 can sound real good untill pushed even just a little bit then things change for the worse very quickley

 Yurka 12th January 2013 02:39 AM

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.

 slungu 13th January 2013 09:35 AM

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.

 djoffe 13th January 2013 11:04 AM

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.

 djoffe 13th January 2013 11:06 AM

Oops...my mistake...I see now, you are doing the 1R...rated at 70 to 100 watts...

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