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Old 16th September 2009, 09:51 PM   #1
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Default Horiffic DC offset

Hello guys

I have made my own chip based power surroud amp for my pc and it's working great. The only problem I have is DC offset. Short of installing capacitors in the signal going into the amp, is there any way to change the DC offset via windows? I'm using an audigy 2zs, incidentally the onboard should has next to no DC offset

The amount of offset, after amplification is about .87 volts! It's not damaged any speakers and the sound is still ok but I really dont want it. If i have to stick 5 caps in the signal lines, which would you reccomend and what size should I use for maximum transparency?

Cheers

Dan
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Old 16th September 2009, 10:09 PM   #2
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Hi,

have you got a cap between ground and the second resistor in the -ve feedback section? I'd suggest putting in a 47uF at this point, offset should reduce to next to nothing. What is the offset with input shorted?

I also use an audigy II ZS and have no problems.

BTW this should really be in the chipamp forum, I'll report the post and ask the mods to move it

Tony.
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Old 16th September 2009, 10:10 PM   #3
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Please move thread to chipamps forum woops... that wasn't supposed to happen... seems that the report post is not supposed to be used for this anyway, just read the fine print Hopefully a kind mod will notice.
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Last edited by wintermute; 16th September 2009 at 10:13 PM.
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Old 16th September 2009, 10:18 PM   #4
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Quote:
Originally Posted by dannybeckett View Post
I'm using an audigy 2zs, incidentally the onboard should has next to no DC offset
Worth confirming with a meter. My Benchmark DAC isn't supposed to have DC offset either but the 40 mV measured was enough to put 1.5 Vdc on the speaker outputs of an Audiosector gainclone kit. On the plus side it created an opportunity to whip up a transformer-coupled tube pre with independent floating secondaries. This particular chipamp seems to really like a low DC source impedance.
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Old 16th September 2009, 10:35 PM   #5
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Thanks for the reply, yeah if the thread needs moving then no probs =]

The problem doesn't lie within the amp, its the output of the soundcard. When the soundcard isnt connected to the amp, i only get 70mv after amplification DC offset, which I can definately live with.

http://www.dogbreath.de/Chipamps/Thr...reeResAmp.html

Thats the amp I've made, adapted for 5 channels.

The reason why I know the onboard has next to no offset is because theres (nearly) no offset at the speakers when everything is powered up.

thanks for your replies
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Old 16th September 2009, 11:14 PM   #6
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The problem DOES lie with the amp. The cause is the lack of DC gain capacitor on Ri as Wintermute has already mentioned.

The soundcard is already AC coupled, adding more caps to the input of the amp will only make it worse.
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Old 16th September 2009, 11:17 PM   #7
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Ok. Could you explain to me a few things.

If the problem is with the amp, why does the problem not appear when the onboard soundcard is connected? What does a DC gain cap do, and how does it do it? Where exactly would I need to install these caps (in between rf and ri to ground)? What type of cap should I use and what capacitance will I need? Will it affect the sound quality much?

Sorry for the barrage of questions

Last edited by dannybeckett; 16th September 2009 at 11:23 PM.
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Old 16th September 2009, 11:26 PM   #8
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It reduces the gain of the amp to DC signals to unity so offets etc are not amplified. Put a 100uF or 220uF cap between the ground side of Ri (+ve on cap) and ground (-ve on cap), ensure you break the link that is currently there or you will achieve nothing as it will be shorted out.
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Old 16th September 2009, 11:36 PM   #9
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electrolytic? and just to clarify -

----[ ri ]-----(GND)

to


----[ ri ]-----|(----(GND)


What effect will different sizes of caps have? will this affect audio output at all?
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Old 17th September 2009, 12:20 AM   #10
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Quote:
Originally Posted by richie00boy View Post
The problem DOES lie with the amp. ...
A properly operating DC-coupled amp is not a problem, DC on the input is a problem. My offset is about 5 mV with no blocking caps. Examine the situation and pick your poison.
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