Lm3886 Bpa

[Billy0810]

Finally,I understand the mistakes of my schematic,and I`ve given up using this schematic and am planing to build the amp using your schematic,but I have some question about it.Is U1 DRV134?
Do I need to put a buffer before the DRV134?Where do I put the decoupling cap(I am not sure if it is called decoupling cap,I mean the cap to block the DC )?before or after the DRV134?
Thank you very much!

 

[jaudio]

No buffer needed.

The caps go bewteen drv134 output and input resistor of each lm3886. Be sure to match all resistor as close as possible

 

[AR2]

I have been doing similar concept but it is based on non inverting amp. Here is the schematic that I am using. It is Brians GT amp. I am connecting two of this boards LM4780 paralleled per one side of the bridge. I am not using DRV 134 because I am supplying balanced signal.
My question for jaudio is: on this schematic, where I could put trimpot for DC offset adjustment. Would that be R4 - 22.1 K? I am having hard time putting these boards together paralleled. Any time I try to parallel two boards that have on them 0.1 ohm on each side of the LM4780 chip - they oscillate. Than I started adding extra resistors between boards, and I ended up with 0.5 ohm between. I do not understand why is this happening. Each board because of some reason produces about 150mV DC, but in bridged configuration that gets canceled.
I posted this question in this link, but I never got full understanding of the problem.http://www.diyaudio.com/forums/showthread.php?s=&threadid=54575&perpage=10&highlight=&pagenumber=1
post #7

 

[jaudio]

In inverted mode the input resistor sets impedance. In non-inverted mod impedance is set in the chip, normally it is very high.
Put a trimpot on the (-) signal input and you will get noise.

Here is something Thomasfw posted that might help
I didnt try it,so I dont know if it will work.

 

[jaudio]

AR2

Is that a total of 4 lm4780?

 

[AR2]

Yes, it is 4 LM4780 per channel. Two and two paralleled in bridged connection.
Thank you

 

[jaudio]

AR2


On the application note,each powersupply rail has a .1uf and a 10uf cap in addition to a 1500uf cap. I dont see the .1 and 10uf caps on the schematic you posted.

 

[AR2]

I do have it. The schematic posted is original Brian's schematic, and I didn't modify. I have PS board for each LM board, and each one is snubberized (all together 4 PS boards per channel) I bypassed all caps with .1uF and I have 6300uF per rail per board.

 

[jaudio]

Are there any problem with the .5ohm connected.

Believe Jeff Rowland uses .5ohms or .47ohm.

 

[AR2]

No there are no problems, but it is kind of wierd. On the board I have .1ohm on each side on the chip, and between two boards .5ohm. I tried various values but it oscilate on anything below .5ohms. I was expecting that all resistors should be same.

 

[jaudio]

Are all the resistors 1% or .1%?. I didnt bother with the .1ohm output resistors. I used .22ohm.

The only time I got 150mv is when I didnt ground the input properly.

You should email Jackinnj. He has more experience with the lm4780 chip?

 

[thomasfw]

Dear All,

Please see the attachment !

Can I reduce the cost by don't put the "Input Cap" for each LM3886 ?

Thanks !

Thomas

 

[Upupa Epops]

Don't make it with DRV 134 - it's not quite good sounding circuit .

 

[jaudio]

The lm3886 needs a gain of aleast 10. If you want to use a gain of four use the OPA chips (549 or 541). The OPA chips cost 3 times as much as lm3886. You can leave out the input caps,but you might have a problem with dc offset.

 

[macboy]

The lm3886 needs a gain of aleast 10.

Nice catch. It's no wonder that the amp is oscillating. Do people even bother to read datasheets anymore?

 

[troystg]

Don't make it with DRV 134 - it's not quite good sounding circuit .

Pavel-

So what do you suggest as a replacement?

 

[mateo88]

Is it even neccessary to use the DRV134 or a similar chip at the input?

i.e., can you just take two of image 5.2.1 in the AN-1192 datasheet and parallel them?

 

[jaudio]

i.e., can you just take two of image 5.2.1 in the AN-1192 datasheet and parallel them?

Two bridged amp in parallel would still require output resistors. Image 7.2.4 would be better.

 

[mateo88]

Yes, I know it would still require output resistors. I guess my real question is in figure 7.2.4, why is U5 needed?

 

[jaudio]

U1 is used to isolate the source from the chips low impedance input. The input without the buffer, would be less than or around 500ohms. Input impedance plays a role in frequecy response.