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Old 9th July 2007, 03:06 AM   #31
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Quote:
Originally posted by juergenk
it is still missing the coupling cap, without that the bias wouldn't work
I think my edited schematic doesn't need a coupling capacitor.
I don't see why it doesn't work.
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Old 9th July 2007, 06:48 AM   #32
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Dear Zing,

Your "edited" drawing is same as my first circuit exactly. If you examine my first drawing, you'll see.
Anyway, It already works fine with that situation (and I cannot understand how?) , yes I feel some treble weakness but I hear no distortion or noise.
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Old 9th July 2007, 01:50 PM   #33
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Hi,
Quote:
Originally posted by zing

I think my edited schematic doesn't need a coupling capacitor.
I don't see why it doesn't work.
because of the feedback, opamps output is a zero impedance dc-source, the 100R resistor does not help
only a cap could isolate the bias circuit from the opamp
Quote:
Anyway, It already works fine with that situation (and I cannot understand how?)
it works for ac, but it does not work for dc
which you have recognized yourself
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Old 9th July 2007, 02:16 PM   #34
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Hi,
it might help understand what is happening if you use the standard rule for opamps:
the output tries to settle at a voltage so that the two input pins are at the same voltage.

Now working back from that statement.
If the two input pins are both at zero volts, then you can work forward through the resistors to calculate what DC voltages need to be at the various points to get the circuit to be DC stable.
This will result in the output cap sitting at -Vgs.

If instead you work backwards from the output and assume zero volts at the source pin, then what voltage will appear on opamp pin2. Now using that same rule what voltage must pin3 be biassed to? This can be done with three resistors and a DC blocking cap.

Once you understand what the circuit is supposed to do then it becomes easier to design it to make it do what you need.
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Old 9th July 2007, 02:35 PM   #35
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Quote:
Originally posted by juergenk
Hi,

because of the feedback, opamps output is a zero impedance dc-source, the 100R resistor does not help
only a cap could isolate the bias circuit from the opamp
Thank you, juergenk!

I misread R1 as 100k without thinking of such a high resistance would cause HF roll-off back then. What a fool! :0)
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Old 10th July 2007, 06:30 AM   #36
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Hi all,

I am fixed the unadjustable bias problem. Its now +4v and the offset on output is about 0v.
But now I have the unstability problem. DC offset voltage always swinging. I wait until the MOSFET comes to 55 C for bias adjusting. However while the temperature was stable output DC offset was still decreasing. And I had to put the 1500uF on output again.
But the sound is beatiful. No noise or distortion. The mids and trebles are very detailed. And the basses are impressive!
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Old 20th July 2007, 03:02 PM   #37
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This is the last situation of the circuit;
Click the image to open in full size.
It works fine and HOT! But there is -2mV offset on each channel after warming up.
Do you think that make problem?
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Old 20th July 2007, 04:13 PM   #38
AndrewT is offline AndrewT  Scotland
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Hi,
2mV is an excellent result. Well done.
Does it hold steady as you alter the volume pot?
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Old 21st July 2007, 06:44 AM   #39
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Hmmm... Good question. I havent tried that! I dont hear any noise while volume adjusting via pot. However technically, I am changing the input load and I have DC coupling so it must be changed.
I will test that.
Thx Andrew
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Old 21st July 2007, 06:50 AM   #40
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Sorry for double posting.
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