X-fi and DC offset
Apr 14, 2006 at 2:35 AM Thread Starter Post #1 of 5

silvalis

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I just recently bought an X-fi extreme music to replace my revo 7.1. I normally run a PPAv2 off the front L/R output but I've noticed that it's giving me a 30mV DC offset which my PPA amps up to something around the 120mV mark.

Has anyone else with an x-fi (of any sort) noticed something like this? The other channels don't have anywhere near as large a DC offset - more along the lines of 1mV!
mad.gif


I decided to pop out & jumper the opamps and remove the class A bias resistors and the offset fell to 50 or 60mV. Putting the ground channel opamp back in made it 80mV. I'm using the standard AD8610s and the offset without being connected to the source is something alonge the lines of 2mV.

I think I'm going to have to add in a DC blocking cap/highpass filter on the input stage, though I'd prefer not to. Are there any disadvantages of placing the filter on the input of the amp, before the volume pot? Would it be better to insert this between the pot and R1/R2? Are there any other methods for removing this dc offset? What about introducing a +ve DC bias into the ground channel?
 
Apr 14, 2006 at 4:13 AM Post #2 of 5

cetoole

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You could either capacitively couple the input of the amp; it probably doesnt matter whether you have the cap between the pot and r1/r2 or on the input, or you could try building a DC servo for the amp, though those are rather large offset values. A servo is kind of like what you were talking about with raising the ground to an equil DC offset, but instead you use an opamp to inject a voltage exactly opposite the offset into the signal, to null it, I believe. It seems odd that a commercial card like that wouldnt be AC coupled from the factory; my personal tendancy would be to correct that with a large pair of polypropylene film caps on the input of the amp.
 
Apr 15, 2006 at 2:03 AM Post #3 of 5

Garbz

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Honestly put a polypropelyne film cap in and be done with it. They cost <$1 each and you won't hear and audible difference. The aversion against capacitors in general on this forum is blowing insanely out of proportion.
 
Apr 15, 2006 at 7:01 AM Post #4 of 5

silvalis

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Yep, already done it. Back to <1.5mV offset.
I only had metallised .22uF polyester greencaps though, was thinking of putting in something a bit higher quality...
 
Apr 15, 2006 at 9:45 PM Post #5 of 5

cetoole

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Quote:

Originally Posted by silvalis
Yep, already done it. Back to <1.5mV offset.
I only had metallised .22uF polyester greencaps though, was thinking of putting in something a bit higher quality...



Should be just fine, I would leave it like that.
 

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