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DC offeset - what produces it?

N

natzev_i

pfm Member
Hi, I am trying to finish my two mono HackerNAPs but one of them shows DC offset at around 60mV on its output while the other is 20mV.
I thought the offset is coming from different hfe's combinations between TR7&9 and TR8&10 and today I swapped the places of TR9 and TR10 leaving T7&8 as they were. Doing this I expected a little change in the output offset but to my surprise it didn't move at all.
So, what causes this offset and how I can reduce it a bit, please?
Thank you!
Ivo
 
IVO
There are a couple of things that affect DC offset

1. A leaking feed back cap C2, mine are at about 38mv and have been for 2 years

2 TR1 & TR2 relationship, TR1 should have a 10% higher HFE than TR2

Alan
 
Thank you both for the replies!
Alan, I changed the C2 100uF wet tant with 68uF MMK film cap with no results on the offset at all so I put back the tant.
Initially TR1 was with higher Hfe as suggested by Les and the offset was 70mV. Then I matched TR1&2 almost exactly and the offset went to 60mV and doesn't move from there. :-(
There should be something else I could do to lower it further even though 60mV might be fine!
Cheers,
Ivo
 
I must have been very lucky to get both boards measuring almost the same . I didn't match any parts at all .
 
keithsnaim said:
I must have been very lucky to get both boards measuring almost the same . I didn't match any parts at all .
Click to expand...

Yes, this is what concerns me because there might be something wrong with my build or some component(s) might be out of its parameters which may lead to future instability? But where to look for, please? :rolleyes:
 
Try matching the long tail pair resistors from tr1 & tr2

Measuring HFE can be tricky as I dfound during the Paradise phono build. Transistors are so sensetive to temperature of the room and you handling them so double check the transistors

Alan
 
337alant said:
Try matching the long tail pair resistors from tr1 & tr2
Alan
Click to expand...

Thank you, Alan!
Did you mean R3 & R5, please?
If TR1&2 are the source of the problem I have plenty of them and will test different combinations - will report later.
Cheers,
Ivo
 
OK, I did numerous testings with TRs from different manufacturers and different Hfe and the best combo so far which gave me DC output=40mV is TR1 with Hfe=200 and TR2 with Hfe=260.
It is just the other way round from what Les suggests but in my case it works.
Cheers,
Ivo

P.S. R3 & 5 are matched within 0.5%.
 
I read somewhere on this forum that adjusting R15 (620R) would adjust the DC offset, but I don't know what other effect it would have.
 
That resistor sets the constant current source for the long tailed pair. A very crude way of adjusting DC offset by altering the current they run at. They can be anything from 560 to 680R in Naim amps.
 
Thank you for the suggestions.
I tried more different combos of BC546 but couldn't achieve any better results so I left the pair which gave me DC=40mV which is within the margins.

Meanwhile I dropped the probe and burnt R27 & TR10 and I am waiting for parts now but this is another story! :(

Ivo
 
I changed TR10 and TR7&8 for precaution too. The new transistors had much different Hfe's and I thought the DC offset will drift a bit but it didn't move at all :rolleyes:
Anyway, the mono-blocks are burning-in nicely now before I put them in the main rig for evaluation.
Happy holidays!
Ivo
 
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