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Old 5th January 2005, 07:17 PM   #1
nitin is offline nitin  India
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Default Zero DC Offset

I'm newbie in electronics(Doing Mechanical Engg. ) but gainclone seem to be so interesting that i decided to try some soldering .

I made a gainclone based on LM3886tf in non-inverting(bob's scm.)with 27db gain with only 6.78k after 1k to sgnd as against 19.8k.

The problem is i get DC Offset that range from +20 to -35mv on multimeter during obsv. is this caused by improper power bypassing or is source is to blame. so point is that how should DC Offset ibe made zero.

Dc offset(-20mv) is lower without speaker lead connected.Also no capacitor is connected after 1k resistance to SGND it is optional as in datasheet fig. or essential and is 47uf ok for bookshelf spkrs.

Thanks in advance.
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Old 6th January 2005, 05:51 AM   #2
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Less than 50 mV is not a problem.
/Per-Anders (my first name) or P-A as my friends call me
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Old 6th January 2005, 01:38 PM   #3
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Per-Anders is quite correct. If you think of it from the point of view of speaker dissipation, 50mV output offset represents a dissipation of about 313 micro-watts into 8 ohms. An offset of even 10 times this still only gives a dissipation of 31.35 mW (at current of 62.5 mA). Only a delicate tweeter is likely to be distressed by this, and wouldn't see it when decoupled by a capacitor, which is almost always the case.

IMHO, the importance of output DC offset is often overplayed, usually in the cause of specmanship.
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Old 6th January 2005, 05:28 PM   #4
nitin is offline nitin  India
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So the ~-30mV is not big to worry about,also upto what level in mV there is no harm to speakers as i'm not using crossover.

I have seen 47uf in many scms. after 1k and that of non-polar type possiblely to reduce DC offset but i am not using any capacitor after 1k so the 1k goes straight to SNGD w/o any capacitor in b/w and my speakers are sounding good w/o it. does i need it or it is optional comp. or what value is going to be fine 47uf or 33uf for bookshelfs.

i want to make buffer based on OPA627BP for my gainclone. Do you guys can pls point to some good scms.
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