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Tweakers' Asylum Tweaks for systems, rooms and Do It Yourself (DIY) help. FAQ. |
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Tweakers' Asylum Tweaks for systems, rooms and Do It Yourself (DIY) help. FAQ. |
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In Reply to: What to do with DC bias current?? posted by EBPaul on May 2, 2002 at 17:13:34:
Wow - a moment of understanding has just been visited upon me.( It seems a big deal to me because my dumb a$$ has been trying to undestand this for the past week!! )I took the spec sheet into an electronic repair shop in town and asked how to get rid of this DC bias thing. Decent old guy realy knows his biz and knows tubes. I didn't fully understand all about using a DC blocking cap untill I was thinking about what he said on the drive home. He was talking about how the value of the cap would depend on the impedance load of the pre amp I would use, capacitive reactance being a combination of capacitance and resistance. Then it hits me - The cap is like a high pass filter between the chip and the pre amp. DC if you can imagine, is the infinitly lowest freq of all, staying stationary at the wave peak, and there for is damped out infinitly. Have I got it this time??
So the signal comming form the chip looks like a regular sound wave except the peaks and dips would all be raised up together so that what used to be the 0 point is now the value of the DC bias. And buy putting the cap in there the wave is put back on centre because the DC can't flow thru the cap. Right??
The problem I see now is that I don't know the optimal value for this cap. If it behaves like I describe then there will be a corner freq below wich the bass will roll off at -6dB per oct. Using a passive filter calc and putting in the speaker at 100k (the input impedance of my pre amp) and the tweeter cut off (high pass, block DC) at 10Hz (If I should use a different freq tell me, but I'd like good bass to extend low as possible, is this reasonable? ) I get a value for the cap at 0.159 uF Is this right? and what voltage value should this cap have? should I just get one that covers the highest voltage that is likely to flow thru that point or is this all so an important factor?
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Follow Ups
- I think I got it this time - ??? - EBPaul 14:21:30 05/03/02 (0)
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