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Old 12th July 2005, 10:03 AM   #1
Nicks is offline Nicks  Sweden
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Talking Massive GC? Suggestions please

Hi,

I have 2pcs 2x500VA (Total 2000VA) transformers with 24VAC secondaries. I have looked at the BPA 200 design but i think my transformers can handle a bit more. I read somewhere that Jeff Rowland used something like 12 LM3886 per channel.
Now that sounds really cool.
I think that would be overkill for me but what i want is some schematic thats bigger than the BPA 200.


My knowledge in this subject is limited. Thats why i humbly ask for this forums help.

BR

Nicks
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Old 12th July 2005, 11:54 PM   #2
Nicks is offline Nicks  Sweden
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ok, just to make things clear.

Im looking for something a little bit bigger than the BPA 200.

Would it be better to use 3 LM3886 in parallell instead of just 2 like in the BPA 200?

What would i gain?

I need some inspiration folks!

There must be something cool to build with 2kVa of transformer power!
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Old 13th July 2005, 12:25 AM   #3
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More current and the capability of driving low impedance loads.
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Old 13th July 2005, 01:58 AM   #4
paulmac is offline paulmac  Australia
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Howdy

I'm planning on making the attached BPA200 design with six 3886s.

I wanted the extra capacity to handle loads that weren't 8ohms all the time, or another way to look at it was to just give the amp a bit of extra overhead from the published design.

My layout was derived from the usual AN1192, and the front page of the 3886 design note, and from spending far too much time reading this forum, than actually doing it...

I chose the dimensions because 150mm x 150mm PCB board is cheap and easily available.

Obviously (I hope), this is only one "side" of the BPA200 design.

I laid out the board with the taller caps and components away from the chips, so that I could mount the chips to the heatsink using a spreader bar over the chips, to help squish them closer to the sink. I reckon it looks nice and symmetrical, but the purists amongst us won't like it because I put the power supply caps too far from the chips... Perhaps mounting the 0.1uF power rail caps on the trace side could help...

I have put in components for a zobel network on each chip also.

The "zen" (or is that chi?) was ruined a bit by adding two 10k resistors and a header pin for connection to a Rod Elliot SIM board from the middle chip.

My plan was to drive the boards from a balanced output from a mixing desk directly, so I hope I won't need a buffer. That could easily be built onto an input board if needed.

My power supply will be a 300VA toroid, which I had to unwind from 30 to 24v, so your 500VA transformers will be perfect for a heavy duty 200+ watt power amp.

So, forum members, what do you think? I wish I could say I've built it, but I'm having trouble with the toner transfer method and the paper I have at home. I keep getting 70-80% transfer. I've printed it out on overhead transparency film, so that's my next move to try.

Also, instead of the 0.1r 5w resistor, what about using the LR network as detailed on pg 20 of the 3886 notes? This would provide extra amp stability. Would it achieve the same thing for load sharing?

Paul
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Old 13th July 2005, 06:32 AM   #5
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I've been planning on doing a BPA design as well. Mine will be based off of BrianGT & Peter's LM4780 boards. I'll be using 2x4780's (essentially 4 3886's) in parallel, then bridge two of those together. I'll be doing thes as two monoblocks, with either a transformer to split the input into inverted/non-inverted, or using Digi01's DRV134 board (haven't decided yet...)

I think there's a big advantage on using the 4780, since you don't have to try to mounth 8-12 chips per amp (only 4-6)

Yes, it's crazy, but what the hell. It should be fun.....
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Old 13th July 2005, 10:15 AM   #6
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how do you get around the problem of the lm3886 producing less power at anything other than 4 ohms? (i think thats right...)
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Old 13th July 2005, 02:11 PM   #7
Nicks is offline Nicks  Sweden
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Correct me if i am wrong here...

But i think you get around that issue when you connect them in parallell.

Thanks for all the inspiration!

I think the way for me is to go with 3 chips in parallell on each side and then bridge the two sides.

That should be a potent recipe for an amp!

At least i hope so.

BR

Nicks
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Old 13th July 2005, 04:18 PM   #8
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Bridging also doubles the output voltage. More power into 8ohms.
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Old 13th July 2005, 10:33 PM   #9
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Check the National Semi doc.. i think it's AN-1192 (it's in other posts around here) on designed bridged-parallel amps with the 3886.

I humbly suggest going with the 4780 for ease of use if you have 4780 based boards laying around or if you're designing from scratch. You can build the simple BPA out of two chips with that, or you can build monster ones using half the chips that would normally be required (since it's two 3886's on the same package).

Regardless, it should turn out to be a fun monster amp. Just make sure you use adequate wiring to carry all the current those chips will be capable of pumping out.
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Old 13th July 2005, 11:10 PM   #10
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Does the lm4780 have twice the heat conducting area as 2 lm3886?
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