Help - DC offset question

mikebarney · 2nd April 2004, 02:55 PM

I've just built my first amp with Brian's boards (using his bonus resistors for this one). I get 50 or 60 mvDC on the outputs of each channel with no input. It drops to a few mv with the input shorted (or when I add my 50K volume pot and set it to minimum). Both channels exhibit the same offset performance, so I built a completely independant 3rd channel (with the high quality kit parts) using a different xformer, and again measure 70mv with no input and 15mv with the input shorted. These offsets are low with the volume set low, but climb to 50 - 60 mv as the volume is increased, weather there is an imput signal or not. the amps all amplify AC quite normally until clipping.
I'm used to amps with offsets < 10mv. Are these offsets normal for a NIGC? If not, any suggestions on resolving them would be welcome. The layout, grounding, etc. is exactly as Brian decribes in his PDF manual.
By the way, with a 225VA dual 18V xformer, I get 24 watts per side before clipping into 8 ohms.
Thanks,
Mike

theChris · 2nd April 2004, 03:20 PM

possibe that one resistor is in the wrong place? i'm not totally familiar with the design other then the nice looking PCBs and the extreme minimalism of the design.

IIRC there is no feedback cap, so the offset will be multplied by gain. so long as there is no turn on/off pop and the IC is not used for a tweeter, it should be fine. if you need to use a tweeter, then you may concider a feedback cap of high quality.

peranders · 2nd April 2004, 03:25 PM

60 mV is normal and not harmful.

GregGC · 2nd April 2004, 06:14 PM

What value pot are you using? It's most likely 50k or more. The higher the value of the pot the more DC offset you'll see at max volume. One way to limmit the DC offset at max olume would be to put a 22k res in paralel to the slider of the pot to gnd. That will limit your input impedance to 22k or less at max volume depending on the value of the pot you're using.

/Greg

mikebarney · 2nd April 2004, 10:20 PM

Greg-
Yes, I'm using a 50K volume pot.
I tried your solution and it cut the offset in half.
Thanks to all,
Mike

2nd April 2004, 02:55 PM	#1
mikebarney diyAudio Member Join Date: Nov 2003 Location: Connecticut	Help - DC offset question I've just built my first amp with Brian's boards (using his bonus resistors for this one). I get 50 or 60 mvDC on the outputs of each channel with no input. It drops to a few mv with the input shorted (or when I add my 50K volume pot and set it to minimum). Both channels exhibit the same offset performance, so I built a completely independant 3rd channel (with the high quality kit parts) using a different xformer, and again measure 70mv with no input and 15mv with the input shorted. These offsets are low with the volume set low, but climb to 50 - 60 mv as the volume is increased, weather there is an imput signal or not. the amps all amplify AC quite normally until clipping. I'm used to amps with offsets < 10mv. Are these offsets normal for a NIGC? If not, any suggestions on resolving them would be welcome. The layout, grounding, etc. is exactly as Brian decribes in his PDF manual. By the way, with a 225VA dual 18V xformer, I get 24 watts per side before clipping into 8 ohms. Thanks, Mike

2nd April 2004, 03:20 PM	#2
theChris diyAudio Member Join Date: Dec 2001 Location: SIUE, Illinois, USA	possibe that one resistor is in the wrong place? i'm not totally familiar with the design other then the nice looking PCBs and the extreme minimalism of the design. IIRC there is no feedback cap, so the offset will be multplied by gain. so long as there is no turn on/off pop and the IC is not used for a tweeter, it should be fine. if you need to use a tweeter, then you may concider a feedback cap of high quality. __________________ if only it could be used for good, not evil...

2nd April 2004, 03:25 PM	#3
peranders Electrons are yellow and more is better! diyAudio Member Join Date: Apr 2002 Location: Göteborg, Sweden Blog Entries: 4	60 mV is normal and not harmful. __________________ /Per-Anders (my first name) or P-A as my friends call me Tube Buffered Gainclone in work \|Thread

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2nd April 2004, 06:14 PM	#4
GregGC diyAudio Member Join Date: Oct 2003 Location: Toronto	What value pot are you using? It's most likely 50k or more. The higher the value of the pot the more DC offset you'll see at max volume. One way to limmit the DC offset at max olume would be to put a 22k res in paralel to the slider of the pot to gnd. That will limit your input impedance to 22k or less at max volume depending on the value of the pot you're using. /Greg

2nd April 2004, 10:20 PM	#5
mikebarney diyAudio Member Join Date: Nov 2003 Location: Connecticut	Greg- Yes, I'm using a 50K volume pot. I tried your solution and it cut the offset in half. Thanks to all, Mike

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