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Old 18th October 2007, 01:21 PM   #1
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Default How to achieve very high damping factor with LM3886

can anybody tell me how to get very good damping factor for LM3886 amp like arround 500-1000

thanks,
Ken
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Old 18th October 2007, 01:52 PM   #2
BWRX is offline BWRX  United States
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http://en.wikipedia.org/wiki/Damping_factor

Ensure that the chip drives a high impedance load.
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Old 18th October 2007, 02:13 PM   #3
jcx is offline jcx  United States
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the (excess) loop gain reduces the open loop output impedance of a feedback amp

the loop gain falls as frequency increases

there may not be enough loop gain to reach damping factors of 1000 with 4-8 Ohm loads at KHz with most chip amps
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Old 18th October 2007, 02:37 PM   #4
wg_ski is offline wg_ski  United States
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Place the chip amp inside the feedback loop of a high quality op-amp. This increases the overall loop gain, reducing distortion to vanishing levels and should improve DF as well. The technique requires putting a phase lead network and some attenuation between the two amps before applying global feedback and the miniumum stable gain is increased. It described how to do this some 15 years ago (or was it 20?) in an old issue of Radio-Electronics using an LM1875.
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Old 18th October 2007, 02:43 PM   #5
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Interesting question but is it necessary really? Isn't better than 50-100 high enough? Maybe it's a better idea to put the amp very close to the speaker instead.
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Old 18th October 2007, 04:13 PM   #6
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I think that use of a good quality high cap regulated supply would help a lot as many amplifiers can't really take advantage of a low output impedance because supply voltage drops under load thus ruining DF.
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Old 19th October 2007, 04:26 PM   #7
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Quote:
can anybody tell me how to get very good damping factor for LM3886 amp like arround 500-1000
Quote:
the (excess) loop gain reduces the open loop output impedance of a feedback amp

the loop gain falls as frequency increases

there may not be enough loop gain to reach damping factors of 1000 with 4-8 Ohm loads at KHz with most chip amps
JCX gives a very good hint. DF cannot be made higher (at a certain frequency) unless there is excess OL gain to be burned at that frequency. If there is no gain, no possibility of highering DF/lowering output impedance.

Maybe if you measure DF at 100hz, you already get that 100's of DF with ordinary gainclone CCT?

You can see OL decreasing steeply with frequency
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Old 19th October 2007, 05:03 PM   #8
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What chip is that graph for, lum? 3886?

Poinz
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Old 19th October 2007, 05:04 PM   #9
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Yes, it's from LM3886 datasheet.
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Old 20th October 2007, 08:15 AM   #10
djk is offline djk
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Considering that the DC resistance of the driver is in series with the back EMF of said driver, what does it matter if the output impedance of the amplifier is 1 ohm, 0.1 ohm, 0.001 ohm, etc.

It doesn't matter!

As a practical matter, if the DF is below about 20 or so it affects the Qes of the driver, and thus the Qts. In general, box size is proportional to the square of Qts, so it begins to make a bit of a difference in this case.
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