I posted this on GL's AX100 thread and I am getting ready to listen to the newly constructed AX100 mono amps but I have trouble. I have to travel again so therefore my urgency.
In doing final testing for the Aleph 1.2 conversion to the AX100 using 6 output devices per quad and after carefully adjusting the abosolute dc offset to .25mv and doing sine wave testing etc I thought things were ok.
However my relative dc offset is too high as it is also .25mv. My amps are dc coupled in balanced mode. I tested in single ended mode.
Of course I first suspected the input differential pair (I am using GL's schematic of Grey's design). So I measured in circuit with the input shorted (my rails are about +/-27 volts under load).
I measured the Gate to Source voltage of each half of the diff pair and got these results (from in front of the 221 ohm gate resistor directly to the source pin of Q6&Q7):
3.805Volts
3.799Volts
diff is 6mv-seems ok
I then measured across each of the diff input pair 392 ohm drain resistors R22 and R26 on GL's schematic and got these results:
4.86 volts
4.77 volts
diff is 90mv
I then measured absolute dc offset for each half of the amp and got
+50mv--positive leg
-200mv--negative leg
My conclusion is that the matching in the 24 output devices-12 per half and 6 per quad is not good enough and I should measure each output source resistor and then swap from side to side to get better balance?
Or
Change the input diff pair, I have one set of IR9610's whose gate to source measurement at 10ma is 3.766 or 0 difference?
Or
In a past thread NP mentioned that this amp should be ac coupled to prevent dc offset problems if the inputs have any dc.
I don't think this will matter in my case but I can try it.
All the other values are the same as on GL's schematic. I have not yet tested the 50% gain since I used his value.
The PM wants her xmas lights up, the dogs walked etc and I have only time for one experiement-any advise? Thanks for your time.
I am nearing the finish line for this project and I am trying to listen to it before xmas.
dave
In doing final testing for the Aleph 1.2 conversion to the AX100 using 6 output devices per quad and after carefully adjusting the abosolute dc offset to .25mv and doing sine wave testing etc I thought things were ok.
However my relative dc offset is too high as it is also .25mv. My amps are dc coupled in balanced mode. I tested in single ended mode.
Of course I first suspected the input differential pair (I am using GL's schematic of Grey's design). So I measured in circuit with the input shorted (my rails are about +/-27 volts under load).
I measured the Gate to Source voltage of each half of the diff pair and got these results (from in front of the 221 ohm gate resistor directly to the source pin of Q6&Q7):
3.805Volts
3.799Volts
diff is 6mv-seems ok
I then measured across each of the diff input pair 392 ohm drain resistors R22 and R26 on GL's schematic and got these results:
4.86 volts
4.77 volts
diff is 90mv
I then measured absolute dc offset for each half of the amp and got
+50mv--positive leg
-200mv--negative leg
My conclusion is that the matching in the 24 output devices-12 per half and 6 per quad is not good enough and I should measure each output source resistor and then swap from side to side to get better balance?
Or
Change the input diff pair, I have one set of IR9610's whose gate to source measurement at 10ma is 3.766 or 0 difference?
Or
In a past thread NP mentioned that this amp should be ac coupled to prevent dc offset problems if the inputs have any dc.
I don't think this will matter in my case but I can try it.
All the other values are the same as on GL's schematic. I have not yet tested the 50% gain since I used his value.
The PM wants her xmas lights up, the dogs walked etc and I have only time for one experiement-any advise? Thanks for your time.
I am nearing the finish line for this project and I am trying to listen to it before xmas.
dave