LM3886 Current needs.

[svbear]

Hi Everyone.

A real junior question for all the seniors out there.

What is the normal to maximum current draw for the LM3886 at maximum music load?

I have a pair of 6ohm line arrays I built and want to bi-amp them with a pair of lm3886 per side. Active 4th order crossover will be external for now, until I built some from scratch to include in the amplifier unit.

I am looking into regulating the power supply and wonder what is the current I will need to provide to each chip for maximum potential.

Also any recommendations on max psu voltage for 6 ohms?

I will be using Brian GT boards (great stuff!).

Many Thanks,

Adrian

[chipco3434]

Start with the data sheet.

http://www.national.com/pf/LM/LM3886.html

[svbear]

From the sheets,

assuming constant 6ohm load and 50watts of continous power,

Vopeak = 24.5
Rails = +/- 28.5 VDC @ 4.1 amps

But I am assuming 50watts is good. I am not sure max output for 6 ohms but I am extrapulating say to 60 watts of power, which gives,

Vopeak = 26.8
Rails = +/- 30.8 VDC @ 4.5 amps

The question though is the Iopeak value represent max current when the chip is at 60watts continous, or when it could hit higher during dynamic power outputs?

[Nordic]

Check the table on page 3.
Io (output current limit) at +/-20VDC is between 7 and 11.5A

Instantaneous Peak Output Power is 135W

[svbear]

That shows how much of a novice I am.

I thought that value was the output current going to the speaker.

So if 11 amps is possible, I would have to think that is the a result of the supply capacitors kicing in the extra juice.
So therefore, you invite a lot of problems if you use a smaller Rectifier than the mur860. correct?

[Nordic]

the diodes are only conducting for a fraction of each second, so the actual pulses they pass will be MUCH higher... however you just need to concern yourself with the average output demands... I.e. an 8A bridge will work just fine....

the 7 to 11A is the range of variation you get among diffirent chips... so you would design around the bottom figure in case one of your chips is a weak one.

I would imagine this peak would be into 4 ohms off a +28V -28V supply, and huge heatsink. Remember the chip is a limiting factor and will burn out if you exceed a certain threshold... so the input rails and the output current (volts out / load) would determine the amount of heat the chip would need to get rid of.

[svbear]

Many Thanks so far guys.

So I just the 3amp adjustable volt regulator won't suffice.

Can I get away with a LT1070 - 5A High Efficiency Switching Regulator?

And to avoid crosstalk, it better to go with independent bridge recifiers for each lm3886, correct?


Also, can seperate identical transformers be used then combined with linked regulators? I saw a sale of large 24vac radio shack transformers at liquidation world.

And secondly, can a virtual ground + regulators be used with these chips? I have a large transformer with 39vac output and was woundering if I split it with a virtual ground then regulators to ensure even voltage split if it could work.

I hope this is not too off topic.

Thanks again.

[AndrewT]

Hi,
a normal transformer + rectifier + smoothing capacitance PSU does four functions met by the two halves of the PSU.
The transformer + rectifier provides the DC AND the average current requirement of the downstream circuit.
The smoothing caps store the rectified DC pulses AND supply the PEAK demand of the downstream circuit.

If you substitute a regulating PSU for the traditional version you must address all four functions listed above.
DC pulses.
Smoothing.
Average current.
Peak current.

If your PSU can tolerate high value capacitance on it's output, then the caps supply the peak demands of the circuit. Then it is simple to build a PSU that meets the maximum average current demand and if you require it add current limiting.

If your PSU will not tolerate high value caps on the output then the SMPS must meet the peak current demand and this could be {Ipk = Vrail / driver Re} and maybe slightly worse.

[Nordic]

Here is a discrete regulator for you, I have only build Pedjas preamp, and it is stellar (my current pre)

http://www.pedjarogic.com/gc/supplies.htm

[svbear]

Very good info.

Nordic, can that design you linked to be subbed with dual single output transformers for the single dual windings transformer in the diagram.
Theory would say yes, but I just want to confrim before I go and pickup all they have of those transformers. I know they are not toroidals, but I figured I can house them seperately from the amplifier main unit to minimize emf interference from them.

AndrewT, I agree with you totally. I figured large caps on both sides on the regulator should help. The reason why I was thinking of regulating the voltage was because if the voltage fluctuates to much, that the volume being deliveried to the speaker will also fluctuate and be heard as the volume level goes up and down.

FYI, The transformers were ~24vac @ 3amps (i think) for like 3 or 4 dollars canadian.