LM4780 - How should I make it?

[Cyberzim]

Hi there,
I have a 2x2 ohm car subwoofer, and I have just recieved 6 LM4780 power opamps.
Now my question is, how should I drive the subwoofer?
I have a 25-26volt supply from 2 carbatterys, and I was thinking of driving each 2 ohm with 2 LM4780 in a bridged mode, where each of them is paralleled.
But can it handle 2ohm?
According to the datasheet I would get about 100watt in 8ohm brigded, so will that mean 400 watt in 2ohm?
But ofc. I could just lower the gain of the amps to get lower output.
But when they say each channel can handle 60watt, is it peak or rms?
Hope you can help me with solving this problem.


Regards,

Simon H.A.

[Redshift187]

Bridged and parallel are not the same, and you can definitely not use bridged with 2 ohm speakers. If the chips can handle 4 ohms in "single" mode, you double the impedance to minimum 8 ohms in bridged mode. You want parallel mode in order to drive low impedances. With 2 channels parallelled the chips can handle 2 ohm. Each LM4780 is basically 2 LM3886 amps in one chip. If you use one LM4780 (in parallel configuration) per subwoofer voice coil, you should have no problem driving it.

Each chip will put out a bit more than 100W at 25V. So using 1 chip in parallel mode for each voice coil would give you a total of a little more than 200W. Which is RMS, not peak, and a lot more power than people who are used to typical retail "marketing ratings" realize.

Look up the "Overture Design Guide" and app note AN-1192.

[Cyberzim]

Thanks for your fast response!

I'm glad that it's 200 watt rms, because that should be plenty of power to my subwoofer. I'm going to use it for a danish festival called "Roskilde festival" if you know it.
Do you think 200 watt rms is enough for a 8 x 8 meter area?

Thank you,

Simon H.A.

[Redshift187]

As far as loud enough... that depends on A) the efficiency of the subwoofer and B) how loud you want it to be. The only ways to know would be to A) do a lot of math or B) try it I would say it's likely to be loud enough. Is the subwoofer in a sealed or ported enclosure?

[Cyberzim]

It's in a ported enclosure, about 30liter with a 10.8" long port that is 2" in diameter. The woofer itself is 10" big.
That was what I got recommended in another thread I made.
And where should I ground the speaker? between 2 capacitors or between the 2 carbatterys?

Regards,

Simon H.A.

[AndrewT]

60 watts into 4r0 is the limit from +-26Vdc.

in parallel mode you can get ~120W into 2r0
in bridge mode you can get ~120W into 8r0.

If you parallel three sets of bridged you can get ~360W into 2r7.

But you cannot drive a 2.7ohm speaker to this level. The 6chipamps in bridged parallel can only drive >=2r7 resistor and will not be able to produce any sound.

If you fit 4ohm speakers to a 6chip bridged parallel amplifier you should get ~ 240W into 4ohm.

According to National these 6chips (3 off 4780) requires 2.4C/W for each single chip. That's ~0.4C/W for one channel, if Ta<=25degC.

[Cyberzim]

I think I'll take 120watt each coil in a parralel mode.
But again, how do I ground the speaker?
Between 2 caps or the batteries since they are serial connected?
And if it's the caps, how much capacity should they have?

[AndrewT]

go and read the datasheet and the application notes that describe this bridged parallel mode in detail.

[Cyberzim]

I'll do that.

Thanks for your help,

[Redshift187]

BTW, using two batteries, you will need a "virtual ground" setup, which is usually not simple. It's not a good idea to just take the ground from between the batteries, because they probably won't discharge at the same rate, and you'll end up with a higher voltage on one than the other.