DC offset problem from noninverted GC - diyAudio
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Old 15th August 2006, 03:11 PM   #1
5150ed is offline 5150ed  Thailand
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Default DC offset problem from noninverted GC

I just finished my LM3875 hardwired noninverted gainclone but I have DC offset problem.

I got very very high like 1-2 V dc offset on both chennel which I still can not solve it.

However, I try disconnect input cap [left the pin 7 with only 22k resistor to ground] , the dc offset went down to 50mv. So I try it many time, the dc offset go very high even I connect only RCA input and 22k R to pin 7.

But one thing very stange, I connect a wire to pin 7 with the other side float [ still 22k resistor on pin7] the DC went down to 20mv. I recheck my RCA input many time it is not shot and also totally isolate from the case. So I am really blank.

Can anyone please kindly give me some advise???
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Old 15th August 2006, 03:33 PM   #2
AndrewT is offline AndrewT  Scotland
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Hi,
the output offset can be minimised by balancing the DC impedances on the inverting and non-inverting input pins.

Once you have done this, you can fine tune one or other impedance to zero out the output offset.
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Old 16th August 2006, 12:49 AM   #3
5150ed is offline 5150ed  Thailand
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Hi Andrew,

Can you explain more in detail how can I balanc the DC impedances on the inverting and non-inverting input pins?

I already put 1 K resistor on pin 7 after that Step and the 1-2V dc offset go down to 200mv, a lot better but still have unwanting radio on my speaker.
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Old 16th August 2006, 08:15 AM   #4
AndrewT is offline AndrewT  Scotland
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Hi 5150,
I often talk through solutions but this one would be much easier if you posted a schematic with component values and identifiers.

Can you post?
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Old 16th August 2006, 08:56 AM   #5
5150ed is offline 5150ed  Thailand
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I am using classic noninverted gainclone.

Righnow I can reduce the DC offset to be aroun 1-2mv however, the radio still exist on both chennel.

Can you suggest how to get rid of the radio?
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Old 16th August 2006, 09:05 AM   #6
AndrewT is offline AndrewT  Scotland
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Hi5150,
can you find and link to a valid schematic?
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Old 16th August 2006, 09:25 AM   #7
5150ed is offline 5150ed  Thailand
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This is the schematic
Attached Images
File Type: gif non inverted gc.gif (13.7 KB, 203 views)
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Old 16th August 2006, 10:46 AM   #8
AndrewT is offline AndrewT  Scotland
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Hi 5150,
there is no low pass filter on the input to the amp. Fit one.

The recommended range of filter is 0.5uS to 2uS (about 320kHz to 80kHz).

A single pole filter (6db/octave) is formed by a series resistor and capacitor to ground.
You need to intercept the trace between r2 and pin7.
Insert a series resistor of about 1k0 to feed pin 7.
Connect pin7 to ground with a 1nF capacitor.

This gives a 1uS filter, -3db @ 160kHz and virtually no effect on signals below 16kHz and probably not audible for an CD type source. It may be audible with vinyl, SACD or DVDa which all have extended high frequency content.

Balancing the DC resistance seen by the non-inverting and inverting inputs to the amplifier.

the inverting input sees the 680r (r3) in parallel to 22k(r4) going to ground (pin3 is effective ground due to the low output impedance of the amplifier). this parallel resistance is 22000*680/[22000+680]=660r.
The non-inverting input sees 22k (r2) to ground.
660r does not equal 22k, this is no good for a DC coupled amplifier.
The problem is caused by the DC block on the input but no complementary block on the NFB loop.
You have two choices.
Delete the DC block on the input, risking losing your speakers if the source develops a DC fault.

or

Add a DC blocking capacitor between ground and r3.

I suspect all these changes will make a bit of a mess of your PCB. If you're lucky the designer will have allowed for these options in his layout. If he was less than diligent he should get the sack.

The input pot has a cap separating the wiper from ground. This is bad practice. DC leakage through the cap will put a constant DC potential on the wiper. Instead the wiper should have a high value resistor to ground to take the leakage to ground. The pot will last longer and probably sound better particularly in the longer term.

ps. the low pass filter could be fitted to the input RCA rather than on the PCB if it helps your layout. It will perform, as a filter, just as well although it will have a small effect on the audio signal.

pps. omitting the input DC blocker will only work as a DC balancing tweak if the source has no DC blocker of it's own. Then the source impedance must be added to the series resistance seen by the non-inverting input.
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Old 16th August 2006, 04:39 PM   #9
5150ed is offline 5150ed  Thailand
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Thank u so much Andrew.

I will try them.

PS. I do hardwire not PCB.
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