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Old 30th March 2005, 03:28 PM   #1
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Smile Has anyone tried servo-less BPA 200?

Hi,

Has anyone tried the basic bridge/parallel circuit for LM3886 as shown in Figure 17 at http://www.national.com/an/AN/AN-1192.pdf (page 15). I need at least 175W RMS of clean power for a sub. I intend to use 4 ICs or maybe 6 for a 4 ohm 12" driver.
What should be the supply voltage?

Thanks in advance,
Goldy
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Old 30th March 2005, 03:35 PM   #2
Cobra2 is offline Cobra2  Norway
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(where I used Zobel & snubbers & plenty more tricks...)

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Old 30th March 2005, 05:59 PM   #3
macboy is offline macboy  Canada
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Quote:
I need at least 175W RMS of clean power for a sub. I intend to use 4 ICs or maybe 6 for a 4 ohm 12" driver.
What should be the supply voltage?
For a bridge-parallel design (with 4 amps) the equivalent load that each amp sees is equal to the real load impedance, or 4 ohms. For a 4 ohm load, the "sweet spot" in power supply voltage is about +/- 28V. Go any lower, and the power output decreases. Go any higher, and the power dissipation increases too high.
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Old 2nd April 2005, 05:28 AM   #4
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Smile How much offset is safe?

Hi,

I intend to use only a pot at non inverting input to reduce the offset. But as I read in other threads, it will not reduce it beyond a point. How much of offset voltage is safe if I use 0.22 ohm at the output of each LM3886? The number of chips per side of the speaker is 3 (total of 6). The supply is 28-0-28.

Thanks in advance,
Goldy
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Old 3rd April 2005, 12:35 PM   #5
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I built mine without servo, and just to share what i learned:

1. Resistor matching is important. Be friendly with the store owner and they will let you spend a day in the store selecting your resistors.

2. Chips has 'internal offset', so build a few to select the ones with the least offset.

My sub amp has dc offset of about 200mV (I didnt observe rule #2) But for a 200-watter, I dont really care. There is more than enough power to dispense and my sub can take that.
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Old 3rd April 2005, 08:10 PM   #6
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I agree with skyraider"s two points.

I used trim pot on the non inverted inputs.

I built 20 indepentent modules. I picked 16 modules with the closest dc offset,then I broke that group into four group all with dc matched as close as possible. I ended up with four nearly matched parallel groups. I used two parallel groups(8 modules in total) with a drv134,to make two monoblocks. The dc offset in .0003 on one and .0001 on the other.

One thing I would add make sure all connection are proper and solid or you will get hiss.
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