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Sybex23 15th November 2004 07:55 AM

Help With DC Offset
 
I have just completed the LM4780 kit from BrianGT. It works well, and sound great, but I am measuring about 108mV of DC at the output. The LM1875 I previously built had only 10mV. Is there a preferred way to adjust this down with this kit?

Thanks,
Sybex23

carlmart 15th November 2004 10:23 AM

Re: Help With DC Offset
 
Quote:

Originally posted by Sybex23
I have just completed the LM4780 kit from BrianGT. It works well, and sound great, but I am measuring about 108mV of DC at the output. The LM1875 I previously built had only 10mV. Is there a preferred way to adjust this down with this kit?


Either you add blocking capacitors on the feedback to ground or you add a DC servo.


Carlos

Sybex23 15th November 2004 04:14 PM

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?

carlmart 15th November 2004 05:18 PM

Quote:

Originally posted by Sybex23
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

Sheldon 15th November 2004 06:12 PM

If you don't want a cap in the output and don't want to go the full servo route, you can try this. Easy to implement.

http://www.diyaudio.com/forums/showt...317#post449317

Sheldon

jackinnj 15th November 2004 07:30 PM

the input bias current is ~1uA -- some of the input resistor values are "nominal" and may need to be changed to reduce the DC offset (AN1192 refers to this.)

Sybex23 15th November 2004 07:45 PM

Quote:

Originally posted by carlmart


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

maxw 15th November 2004 07:52 PM

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.

Details here if it helps any:
http://www.diyaudio.com/forums/showt...threadid=41133

carlmart 15th November 2004 08:11 PM

Quote:

Originally posted by Sybex23


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.


Quote:

Originally posted by maxw
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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