Push pull BPA

[warriorcookie]

I'm looking to build an amp (preferably a gainclone) with power up in the 400w (4 ohm) range. This will be used to drive my subs (currently 2 10" 300w rms each in a horn loaded 4th order bandpass aka "Wicked One" design by Decware) I've been driving this box for 3 years now with an audiosource amp one/a a 4ohms. I love the box, but the amp just doesn't cut the mustard for my theater room.

I've done searching and found a couple of designs that use Bridge parallel setups but not with the output I'm looking for.

I was wondering about useing 2 banks of either (2) 4780's (4 total) or 4 3886's (8 total) in a push pull. I know it's alot of work, but I'm sure someone here has done it and has a schematic.

I prefer the simpler the better and with as little in the signal path as possible, but beggars can't be choosers

I had seen one site with a BPA300 using 6 3886's, but he had them in inverting configuration because it was easier to adjust individual DC offset. Does someone have a solid design that uses non-inverting configuration but still able to control individual DC offset?

Thanks for all the help, hopefully I can contribute something back in the future.

[CJ900RR]

Why do you want to control the offset of each LM? My design, a mixture of BPA200 and Shine7's BPA300 uses 3 LM3886/board, made for easy bridging of two boards, and it uses op-amps as recommended by National to control the dc-offset. Works like a charm.

But as I said it uses only 3 LM/board, bridging would give you somewhat like 325W into 4R, depending on powersupply.

PA150 PCBs, is there any interest

You could use the schematic and make something own out of it. To hook up another LM3886 to get 4/board is no problem.

[AndrewT]

if you want my opinion, maybe you don't, then give up the idea of 400W from a BPA and use one of National's new driver chips.
http://www.diyaudio.com/forums/showt...91#post1562791

[CJ900RR]

As AndrewT says, for 400W LME498** would probably fit your demands better. LM3886 has it's limitation when it comes to high power demands.

[warriorcookie]

Shine7's BPA 300:

I used the LM3886 in the inverted configuration, the reason is that I can use a trim pot to adjust the DC offset of each 3886. National's application note AN1192 suggested to use servo to set the DC offset to zero, but I prefer this simpler solution. After some adjustment, I can get the DC offset of each 3886 to about 9mV, because this is a push pull configuration, the DC offset at speaker terminal would be very close to 0mV.

I thought the only opamp he was used to invert the signal going to one of the pa150's? I understand that since it is push pull the offset can cancell, but it has to be the same (or close) on both sides in the first place doesn't it?
I see you have a noninverted design that does use servo to control offset... interesting, that might be an option.


AndrewT, I always value other people's opinions, thank you very much for chiming in. Are you telling me not to do this style because it is more complex/expensive? The new chips sound very promising, I could definatly manage to do a basic design, but I think I would be better off using one that is already tried tested and true. My current demands are pretty high, I understand the 3886 and the 4780 enough to take someone else's design and add to it to suit my needs. But I don't know much about the newer chips and the output transistors needed to go with them. Also, at about 50ma drive current, I'd have to add a driver, which is more territory i'm unfamiliar with. Does anyone know of any schematics/project logs using these chips for the power range I'm looking at?

[AndrewT]

download the National datasheets for the 498x0 series.

[CJ900RR]

Originally posted by warriorcookie
Shine7's BPA 300:


I thought the only opamp he was used to invert the signal going to one of the pa150's? I understand that since it is push pull the offset can cancell, but it has to be the same (or close) on both sides in the first place doesn't it?
I see you have a noninverted design that does use servo to control offset... interesting, that might be an option.

He is using a balanced linedriver, DRV134, to create the inverted signal. That's also what Im doing. The DRV134 is internally made of op-amps and resistors. It can be done by using op-amps but it's more space-saving with DRV134. The disadvantage is that you cant configure it the way you want it as you can with op-amps.

The dc-servo works very good. I can recommend it! When paralleling its important to get the dc-offset equal on every LM3886, or else they will fight each other making one or more chip to oscillate. That will lead to meltdown of the circuit. I have experienced that when using pots and not so well match resistors