| savenger |
Hello all!
I got almost finished my first "real" discrete amp http://www.ampslab.com/c200.htm, but ran into some problems, but first, i got some incredible broken part, LED. Currensources red LED got 3v treshold voltage though it should be 1,6v, neves happened to me ever (Q4 and Q5 got little hot:hot: ).
But now to the real problem, it has some DC in output (330mV), it is a bit high imho. How can i get it near 0v? Q1,Q2,R3 and R4 are matched, other parts aren't, still i assumed no DC offset would exist.
except DC in output it seems to work fine, i don't have oscilloscope, so can't be 100% sure, but nothing gets too warm with 100mA bias current (and nothing broked yet :clown: )
Here some pictures: |
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| savenger |
Okay i got DC to 0v by making R3 adjustable. 0v was obtained at 1,8k position. Is that right way to deal with this or does it effect sound quality. I got the idea from this site.
My amp has some different parts, Q1, Q2 are MPSA42 and TIPs are BD243 and BD 244. Rail voltage is also only +-35c DC.
Haven't got the time to proper test the sound quality yet, but maybe tomorrow. However it might sound, the bass is the most important because it will drive my sub. |
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| burnedfingers |
| I would say your diff pair wasn't matched close enough. I have matched sets and had DC offset less the 5mv. Changing the resistor value will work fine as it makes up for the missmatch. |
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| savenger |
Hi, i did try 2 other MPSA42 in Q2 and chanced the Q1 too, but same offset voltage remained, i think it's not the Q1 and Q2 matching thing. After those attempts i added the pot to R3.
Any other suggestions? Maybe lower rail voltages?
By the way, all TP measuring points gives supposed voltages. |
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| burnedfingers |
| I would use it as is providing the DC offset was 25mv or lower. It should be fine. Enjoy |
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