Help With DC Offset

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Are the feedback caps the 47uF caps national shows on the typical app circuit? They indicate they are used to ensure unity gain at dc which sounds right. Also, could you tailor frequency response here because of the RC filter that is formed with the resistor?
 
Sybex23 said:
Are the feedback caps the 47uF caps national shows on the typical app circuit? They indicate they are used to ensure unity gain at dc which sounds right. Also, could you tailor frequency response here because of the RC filter that is formed with the resistor?

Yes to first and yes to second.

My advice is to wire two 100uF in series, + terminals as new ends, instead of a polarized one.

You can certainly tailor the frequency, along with the input cap, to do a high pass.


Carlos
 
carlmart said:


My advice is to wire two 100uF in series, + terminals as new ends, instead of a polarized one.


Carlos


I am not sure I follow you here. 2 100uF in series would be 50uF total? and are you saying to connect the negative ends together and let the positive ends be the connection points? Since this is a parallel circuit I will have to do this twice, once for each side or can I parallel both feedback resistors into the same set of caps? So, if I understand right, I would disconnect the 1k feedback resistor from ground and connect it to the first leg of the cap, and then connect the other end of the cap to ground. Am I on track here at all?

Thanks,
Sybex23
 
Sybex23 said:


2 100uF in series would be 50uF total? and are you saying to connect the negative ends together and let the positive ends be the connection points? Since this is a parallel circuit I will have to do this twice, once for each side or can I parallel both feedback resistors into the same set of caps? So, if I understand right, I would disconnect the 1k feedback resistor from ground and connect it to the first leg of the cap, and then connect the other end of the cap to ground. Am I on track here at all?


Yes, 50uF as final result for cap series. What you end up, by joining the - ends is a non-polar cap, which may probably sound better than a polarized one. But don't take my word for that: first disconnect the 1K resistor from ground and add the 47uF cap as specified on datasheet. That should cut the offset.

Then, if you want to, do the cap replacement for the non-polar combo and listen for any improvements.


maxw said:
I had a problem with DC-offset on my LM3875 amp. I am not familiar with the LM4780 schematic and whether it has the Ci cap or not but I added this cap to my amp and DC-offset turned to 0mv.


Yes, it's the same problem and the same solution.

The matter now is that this cap may have some effect on how the amp sounds. Besides using no cap, the next best thing would be to use a film cap, which would be quite difficult because of cap size.

If this amp is just for bass, then it would be fine using good quality 'lytic cap. But if it is for mid/treble amping it might be better to replace that cap with a film type, probably 100n or 47n, according to your high-pass filter.

As you see, it wouldn't be so difficult to do an active system using the feedback to build your filter.

The bipolar mod, for input and feedback caps, was suggested some time on article on caps in "Electronic World & Wireles World".


Carlos
 
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