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Old 7th July 2007, 08:42 PM   #21
x-pro is offline x-pro  United Kingdom
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Quote:
Originally posted by Dxvideo
I suspicied that MOSFET sinks a huge current from gate.
It looks like you did damage the MOSFET - it is easy if you do not observe antistatic precautions carefully. If the gate is broken you would have a big leakage current through the gate. That is what the zener for - to protect the gate. Do not remove it. Replace the MOSFET and this time be careful - at the very least put a strip of foil or a copper wire around the pins of the MOSFET before handling and soldering it in to the circuit and do not forget to remove it after you've soldered it in.

Cheers

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Old 7th July 2007, 09:33 PM   #22
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Quote:
Originally posted by Dxvideo
Dear Alex,

I dont know the reason but I cannot adjust the bias voltage!
I've tried 150K+20K Trimpot+56K, 100K+20K Tr.+10K even 100K+20K+1K and without zener. However I couldnt get over 1,6v offset at gate.
At 100K-1K combination I can get only 1,6 v not more!
I am really confused. Whats the G-S resistance of that MOSFET? I suspicied that MOSFET sinks a huge current from gate. Then I cannot stabilise the mid point of voltage..
What do you say?
Thx again.
A mosfet's gate is isolated - it CANNOT conduct current in any circumstances! (unless it is punctured and destroyed.
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Old 7th July 2007, 09:38 PM   #23
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what about voltage drop over R1?
regards
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Old 8th July 2007, 08:39 AM   #24
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How about this?
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Old 8th July 2007, 10:09 AM   #25
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it is still missing the coupling cap, without that the bias wouldn't work
@dxvideo:
did you tried the adjustments with or without the suggested cap?
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Old 8th July 2007, 10:46 AM   #26
AndrewT is offline AndrewT  Scotland
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Hi Christer,
re post20 circuit.
Will the opamp automatically bias the FET gate to zero the DC offset at the output?

What happens to voltage swing from the opamp? Will it cause asymmetric clipping done this way?
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Old 8th July 2007, 02:20 PM   #27
Dxvideo is offline Dxvideo  Turkey
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Dear friends,

I found my mistake finally. I havent tried yet but, when I am adding the capacitor between op-amps output and MOSFET s gate, the op-amps feedback resistors was in the front of that cap. So the bias voltage always at 0 volts because of the feedback resistor goes to the gnd via just one 1K as you can see in the schematic. And my pull up and pull down resistos are about 100K levels. So I couldnt adjust +4v point. Tomorrow I will fix the problem. And report you the results.
However I am still confused, how it was working under that situation. The sound was really incredible!
This is the mistaken connection;
Click the image to open in full size.
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Old 8th July 2007, 03:30 PM   #28
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Quote:
Originally posted by AndrewT
Hi Christer,
re post20 circuit.
Will the opamp automatically bias the FET gate to zero the DC offset at the output?
That was the intention yes. Did you spot any errors?

Quote:

What happens to voltage swing from the opamp? Will it cause asymmetric clipping done this way?
Yes, but I think all variants so far will clip assymetrically anyway. The MOSFET gate must be biased at least 3 or 4 Volts above ground. Maybe one could get rid of that problem in some cases if keeping the output cap, but the idea was it could be omitted in my version although I kept it in the schematic.
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Old 8th July 2007, 04:04 PM   #29
AndrewT is offline AndrewT  Scotland
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Hi Christer,
I see no errors in post20.
But what about a couple of changes?
Take the speaker/headphone return to Vcc instead of to ground.
Replace R2 with a constant current sink (somewhere between 100mA and 200mA).

Maybe add a 2mA sink from pin6 to Vee to convert the opamp to single ended output stage.

Now, we have a truly constant current amplifier where the load current in the supply rails does not vary with signal level, provided the single ended stages do not clip.

Adding one DC blocking capacitor to pin 3 and a couple of resistors converts the whole amp to single polarity power supply. Just connect the grounds to Vee, in the correct order.
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Old 8th July 2007, 04:10 PM   #30
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Andrew, there is a lot of things one could do. My suggestion wasn't intended to be optimal in any sense, but just one type of solution with minimal changes from the original.
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