How many chips do I need ?

[Cro maniac]

I am building active subwoofer and i have four 12" 8ohm 120W woofers. I need amplifier for driving them (480 undistorted watts minimum) and don't know how many LM3886 chips to put in my amp.
If I connect all the woofers in parallel they will give me 2 ohms.

Will BPA200 with 6 chips be enough, or I must use 8 chips ?
Or is maybe paralleling 8 chips the best solution ?
Which configuration would give me the lowest output impedance ?

Thanks !

[AudioFreak]

If you are going active, I'd suggest you run each of the drivers from their own amplifier. 4 chips as per BPA200 style per driver should do the job nicely. You could also run a pair of drivers in parallel from 6 chips as per BPA200 style.

[Cro maniac]

If you are going active, I'd suggest you run each of the drivers from their own amplifier.

I think that would not be a good idea because each pair of woofers will be working in push-pull mode so they need to be driven from the same amplifier to minimize nonlinearities.
Or maybe I'm wrong ?

4 chips as per BPA200 style per driver should do the job nicely. You could also run a pair of drivers in parallel from 6 chips as per BPA200 style.

OK, so I guess I need to build 1 amp for each pair of woofers
Do you think it is relly necessary to put 6 chips in amp to get 240W to 4ohms ? From that what You have said I need to buy 12 chips to power up my sub. Also, I have to put that amplifier in the same box with woofers so I think amp with 12 chips would be too big.

[Rick NL]

Hi, Cro Maniac,

1. For 120 Watts per woofer you need bridging.
2. With 2x22 V AC you get 40W single and 160 W Bridge.
3. For 2x22V Bridge you need 2x2 3875 for 8 Ohm
4. For 2x22V Brigge you need 2x2 3886 for 6 Ohm.
5. For 4 woofers of 8 Ohm you need 16x 3875's or 12x 3886.

You will get around 640 Watts, with 40 resp. 53 Watts per chip and enough current capability.
This is on the safe side but you will need reserve on a sub.

I would not go for less with this power and current.
When you reduce the power by going to 2x18V AC you will find 12x 3875 or 8x 3886 will be plenty, but a maximum of about 400 Watts are all you get.
BTW, this is only 2 dB down in Soundlevel compared to 640 Watts.

Rick

[Cro maniac]

Rick, thank you for answering

So 12 chips it will be. Damn, the heatsink would have to be really big

Do you have any experience with BPA200 in active subwoofers ? It looks to me that those 0,1ohm output current sharing resistors will ruin amplifier's output impedance which is very important when driving large woofers.

[dhenryp]

If it were me, I would build four bridge mode amplifiers each with 2 lm3886s and use 1 amp per speaker. See figure 3 here:

http://www.national.com/an/AN/AN-1192.pdf#page=4

Per National, each will give you 105 watts of continuous power at very low distortion using +- 25.5 volts. Four would give you 420 watts of continuous power (< 1db less than you 480 watt requirement). You could never detect the difference. In the real world, continuous power is only important for playing test tones. Real sounds have peaks and you are really interested in not clipping on these peaks.

I would increase the power supply up to ~ +- 28-30v. Make sure you have adequate sized transformer(s) and filter capacitors. If you look at the 4 ohm Output Power vs Load Resistance diagram in the lm3886 data sheet, it looks like this will give you a peak power of ~75 watts per lm3886. Each bridged amp will give you ~150 watts peak power at low distortion. 4 * 150 watts gives you 600 watts of clean power.

Another advantage is that with bridge mode amps you don't need to worry about balancing the load among multiple parallel lm3886s so you don't need the .1 ohm resistors.

The bottom line you would need eight lm3886 amps and you would be building simple and "non-tweaky" amps.

[Rick NL]

Hi Cro Maniac and Dhenryp,

As long as you stay within ±25.5 volt DC (is ±18 V AC) You will have plenty safety with 4x2 bridge.
Yes the solution with one bridge per Woofer, avoids the need for 0.1 Ohm's and has not any drawback. This is the most prefereed option to my judgement.
Any higher voltage stresses the part with ±30 Volt (is ±22 V AC) being the absolute max for powerdissipation and output current when using it in an 8 Ohm bridge or 4 Ohm single.
When you can get a ±20 V AC trannie I would use it, otherwise I would opt for ±18 V AC and the 4 x 2 Amp bridge without 0.1 Ohm as DhenryP describes.
Rick

[Cro maniac]

Thanks guys, that was very helpful !

But now I have few more questions.

With more feedback I get less output impedance, yes ?
So, can I set the gain of each chip to less than 11 as suggested in the AN-1192 ?

For heatsinking I will use a big copper plate on which the chips will be mounted. On the back side of the copper plate there would be 2 big aluminium heatsinks with computer fan attached to them. So' I guess I could go by with 2x20V AC.

I haven't bought transformer yet. Near the place I live there is a company which makes transformers, so I can order whatever power and voltage I need. Should one transformer of 1kVA be enough for 4 bridges ?

Here at home I have few TL072's and OP37. Are they good enough for input crossover and buffer, or must I use some better opamps ? If so, what opamps would you recommend (not expensive ones) ?

[mrlots2do]

Originally posted by Rick NL


1. For 120 Watts per woofer you need bridging.
2. With 2x22 V AC you get 40W single and 160 W Bridge.
3. For 2x22V Bridge you need 2x2 3875 for 8 Ohm
4. For 2x22V Brigge you need 2x2 3886 for 6 Ohm.
5. For 4 woofers of 8 Ohm you need 16x 3875's or 12x 3886.

Rick

I have a similar question. I have two parallel 8 ohm woofers @ 4 ohm load. I'm using LM3875 chips. Is bridged amp (2chips) enough or do I need parallel-bridged amp (4chips). What will the output wattages be?

I was planning on using dual secondary tranny 2x25v. How many VA for the bridged-parallel amp?

Thanks

[jackinnj]

suggest that you pay careful attention to the heat sink suggestions on the last page of the application note