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Old 30th December 2012, 08:09 PM   #21
Boscoe is offline Boscoe  United Kingdom
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They claim 150W into 8 buts its no where near possible on the 36V rails they recommend and not possible with the LM3886 anyway.
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Old 30th December 2012, 08:15 PM   #22
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Quote:
Originally Posted by Fast Eddie D View Post
Of course, but this thread is about more current, not more voltage.
But twice voltage through same resistance gives twice the current.
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Old 30th December 2012, 09:36 PM   #23
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Default DC Offset Resistors.

Ok, from what I understand, the problem with the DC offset can be corrected matching the feedback resistors, and that means using the same high precision resistors in all locations.
I have few Mouser part numbers for those resistors, can you please check them and tell me if these are OK to use:

200K 0.5% 400V Mfr. #: HV732ATTD2003D

47K 0.1% Mfr. #: ERA-6YEB473V

10K 0.1% Mfr. #: PNM0805E1002BST5

5.1K 0.1% Mfr. #: W0805R035101B-R1

Again, this can be found at www.mouser.com .

These boards have the same issue as the XY boards, the input signal ground and main ground need to be separated. Can you guys tell me where the cuts need to be made in the PCB to separate these grounds?.

I have more questions, but I'll post more later.

Thank you.
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Old 30th December 2012, 09:43 PM   #24
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Quote:
Originally Posted by nigelwright7557 View Post
But twice voltage through same resistance gives twice the current.
Of course, but are we going to get twice the voltage swing into even 8 ohms with a simple bridge amp? No we are not. A look at the data sheet claims

- 68 watts into 4 ohms with +/- 28V
- 38 watts into 8 ohms with +/- 28V
- 50 watts into 8 ohms with +/- 35V

What does that mean for our bridge amp? That we can get 136 watts into 8 ohms with +/- 28V. Forget +/- 35V with a bridge amp. Forget about 4 ohms.

Now if we parallel 2 chips in each leg of the bridge (total of 4 chips/channel), we can up the supply to +/- 35V and get 200 watts into 8 ohms. We can run it off of +/- 28V and get around 250 watts into 4 ohms.

I home my math is correct but more importantly I hope you're getting the picture of what I'm talking about.

Running these chips parallel into a lighter load can theoretically avoid tripping the protective circuitry. It should result in a lower distortion amplifier capable of handling cranky loads better. Theoretically, of course.
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Old 31st December 2012, 12:46 AM   #25
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answers:

LM3886 Parallel Use

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Old 31st December 2012, 04:50 PM   #26
Marc Vi is offline Marc Vi  Netherlands
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Hello,
Each LM 3886 chip will be different in the way that the offset voltages will never be the same, even if you use the same feedback resistors with 0,00000000001 % tolerance. So you have to tune out the offset voltage of each chip manually by a adjustable resistor. After tuning and several warming ups, you replace the adjustable resistor by a fixed one.
See my thread: "LM3886 without electrolytic feedback capacitor and (hardly) no DC offset voltage", how to do this. With a feedback capacitor it's even easier.
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Old 31st December 2012, 05:01 PM   #27
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Agreed - I suspected the link I posted hadn't cleared up that question. Those adjustments are interactive and each effects the other two. I did a lot of insertions and removals as I didn't have adjustables. With the components I used ~ 11 mv (vendor docs suggests 5 mv as good) was the best 3-way match I could get. It's a bit of a pain but well worth the effort.
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Old 1st January 2013, 03:16 PM   #28
Marc Vi is offline Marc Vi  Netherlands
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If you do it the way I described in my thread (please read it), the adjustments are not interactive. Just build three of those amplifiers, tune out the offset voltage of each amplifier and then connect them together.
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Old 2nd January 2013, 05:37 PM   #29
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Marc Vi., where is your thread about the DC Offset?. Can you please post a link to it. I am getting some adjustable resistors as required by the PCB's, I already have a part number selected from Mouser, I don't have it handy right now but I'll post it soon so you guys can tell me if these are right.
Thanks.
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Old 2nd January 2013, 05:46 PM   #30
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I also want to know about these additional boards I've seen being used by other builders using the 3 chip parallel, I found two at the auction site, which one is better, and do I really need to use this or I can build my amp without it?.

Unbalanced]Unbalanced to Balanced Stereo Preamplifier for BTL | eBay to Balanced Stereo Preamplifier for BTL | eBay[/url]

BTL]BTL Bridge Board XLR to Balanced Board DIY Kit | eBay Bridge Board XLR to Balanced Board DIY Kit | eBay[/url]

Thanks again.
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