bias a trace elliot ta100r

[Enzo]

Let us look at your bias. Now with the whole circuit, the bias adjust works. YOu can adjust the gate spread from zero volts to 0.500v. SO zero to a half a volt is your bias range at the gates. Note that the bias has nothing to do with voltage to ground. If instead of -1.1 and -0.6 (0.500v), you had +23 and +22.5, you would STILL have a 0.500v bias.

What you are expecting is maybe +0.500 and -0.500v, centered around zero. But what you have is skewed negative 0.600v. That is not a bias problem, that is an offset problem. NO speaker load, right? Is there anay DC on the output? MAybe about a volt negative?

[lowell]

The output has -200mv of offset.

[Enzo]

Time for a load. Does putting a load on it knock that 200mv to zero? Or doesw it stay there?

[lowell]

with 8ohm resistive load the offset on output is still -200mv. However, the bias is now shifted down more to -1.7v and -1.2v

[lowell]

Enzo any thoughts on this?

[Enzo]

How does the amp operate? With load attached, so you have 200mv offset. How much current is drawn from the mains? Does the amp heatsink get HOT? Does it provide full power without clipping? Does it make clean undistorted output? SOmetimes things work OK even when small details are a bit skewed.

[lowell]

No it came back after the first repair just like this, and it sounded like it has crossover distortion. Shall I still check on those other things knowing this?

[lowell]

Well, thanks for your suggestion Enzo the thing sounds great and I don't see any crossover distortion on the output. Guess somewhere along the way it got fixed. This is a lesson in always "checking in" - seeing if there IS a problem. Good to know that an amp can work with offset. I didn't know that was the case.

[MarkusBass]

I've just measured almost identical power amp (2SK1530 and 2SJ201) and it has 30mV offset on the output. I think that the offset is caused by not matched transistors on the input of the amp (differential pair). Did you change them? Also the output transistors should be matched but I don't know whether this may cause the offset.

Mark

[lowell]

Markus I think I only replaced ONE of those diff pair transistors. That makes sense thank for the input!