Parallelling two LM 3876 for sub

[Vidi]

Hi!
I want to build a simple amplifier to pull my 4ohm subwoofer
(ie I want to build an active sub, the crossover is allready made).
The power of the amp should be at least 100w in 4ohm, and I were wondering if it is possible to parallell two LM 3876 to archeive 112w in 4ohms? And if so, is it just to build two identical amps, and connect them together with two current sharing resistors like the datasheet for LM 4781 describes?


PS: If someone else are looking for an active crossover for subwoofer, take a look on this: http://lcaudio.com/index.php?page=27


Regards Vidar

[AndrewT]

Hi,
yes, just follow the datasheet.
Do ensure adequate heatsinking. Try for almost doubling the minimum recommended for the voltage and load. Keep in mind you have to cool two chipamps.

[navin]

I have built a 3876 stereo amp PCB as per the design posted on ESP's project 19 (see link) Single Chip 50W Stereo Amplifier.

Is there anyway I can bridge the outputs to produce 60-80W rms @ 8 ohms (the power supply I intend to use is +/- 25VDC to ensure that the chip is protected).

[AndrewT]

to get 80W into 8ohms from a bridged arrangement you need two amplifiers capable of 40W into 4ohms.

[navin]

Originally Posted by AndrewT

to get 80W into 8ohms from a bridged arrangement you need two amplifiers capable of 40W into 4ohms.

Yes I know that. As per the Overture design guide with 24VDC I should get 40W rms per channel @ 4 ohms (stereo) from a pair of 3876. What I need to know is how do I SIMPLY bridge the ESP Project 19 circuit to produce 80W (mono @ 8 ohms) from a pair of 40W (stereo @ 4ohms) amplifiers.

http://www.national.com/vcm/national...gn_Guide15.xls

http://www.national.com/vcm/national...sign_Guide.pdf

[AndrewT]

Do you know?

40W into 4r0 (a resistor) requires the amplifier to output ~4.5Apk to the load.
Which National chipamps can guarantee that performance when operating at Tc>40degC?

40W into a 4ohm speaker requires an output current of at least double that 4.5A value.
Expect speaker demand on transients to be <=9Apk to <=13Apk.
How many paralleled chipamps do you need to guarantee meeting that performance?

I don't think you do know.

[navin]

Originally Posted by AndrewT

Expect speaker demand on transients to be <=9Apk to <=13Apk.
How many paralleled chipamps do you need to guarantee meeting that performance?

I figured since the PS will be 24VDC the current will be limited to safe limits.

according to this link the 3875 (and I assume 3876) can deliver 4A
LM4780 / 3886 / 3875 / 1875 Comparison

LM3876 parallel into 6ohms. Possible?

[navin]

Originally Posted by AndrewT

Do you know?
40W into 4r0 (a resistor) requires the amplifier to output ~4.5Apk to the load.

I was thinking of something like Figure 5.2.1 in the PDF below
http://www.national.com/an/AN/AN-1192.pdf

The heatsink I am using has 50% more area than of the one shown in figure 9.1.

[AndrewT]

Originally Posted by navin

I figured since the PS will be 24VDC the current will be limited to safe limits.

that is the problem that you do not seem to appreciate.
Your speaker demands current. Your amplifier tries to meet that current demand.
If your amplifier cannot meet that demand then it clips. It can be voltage clipping or current clipping, the effect is the same. As you turn up the volume the amplifier sends a distorted signal to the speaker.

All National chipamps are severely current limited. The highest output I am aware of is the 3886 which has 7Apk available when Tj=Tc=25degC.
As soon as the chip temperature rises above this, the protection triggering levels are reduced to prevent damage to the chip.

National sell millions of chipamps to drive 8ohm speakers.
National use BPA style topologies to sell tens of millions more to drive unsuitable loads.
An lme498xx with 1pair of MJL4281/4301 will at least match and probably better the performance of a 4chip BPA assembly.

Chipamps suit higher impedance loads and suit higher efficiency loads.
They do not perform well into low impedance, low efficiency loads.

[navin]

Originally Posted by AndrewT

If your amplifier cannot meet that demand then it clips. It can be voltage clipping or current clipping, the effect is the same. As you turn up the volume the amplifier sends a distorted signal to the speaker.

All National chipamps are severely current limited. The highest output I am aware of is the 3886 which has 7Apk available when Tj=Tc=25degC.
As soon as the chip temperature rises above this, the protection triggering levels are reduced to prevent damage to the chip.

An lme498xx with 1pair of MJL4281/4301 will at least match and probably better the performance of a 4chip BPA assembly.

Do you have a schematic for the 498xx/MJL4281/4301? BTW I was not looking at a 4 chip BPA but just bridiging 2 chips. My "subwoofer" is a small 7" woofer from Wavecor SW182BD01_02 so I do not need more than say 50-60W rms. So I figured 60W/8 ohms = 8 amps and 2 3876 can deliver 4 amps each for 8 amps. Maybe I was naive.

Maybe a safer option is to use the 3886 instead. Unfortunately I cant find the 3886 in Mumbai, India.