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DC Offset on Class A headphone amp.

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6th July 2007, 01:03 PM

#1

Dxvideo

diyAudio Member

Join Date: Nov 2003

Location: Turkey

DC Offset on Class A headphone amp.

Hi all,

I made a Class A headphone amp
last week. Its based from originally headwize forum "A HeadWize Design
Series Paper" article. "Class A MOSFET Follower" design.

I use
OPA134 op-amp for the buffer and it has 2 gain.
When first try, I
faced there was -4 v offset on outputs (there was no output capacitor) And
I couldnt fix that problem with adjusting the offset trimmer. Then I
decided to put a 1500uF cap on output. However its a bit disturbing.
Because the original design has no output capacitor. (There is no offset
on op-amps output)
Anybody has any idea?
Thanks a lot for replies
in advance.
Best regards..

__________________
Best regards,
Ozgur

6th July 2007, 01:13 PM

#2

Juergen Knoop

diyAudio Member

Join Date: Jan 2007

Blog Entries: 3

thats Vgs!

6th July 2007, 01:30 PM

#3

x-pro

diyAudio Member

Join Date: Apr 2003

Location: London

Could you provide the link to the original
schematics? The circuit you've posted here should produce about -4V on the
output as the o/p voltage of the opamp is near 0V and a MOSFET needs 4V to
open. I suspect that in the original circuit there was a capacitor between
the opamp and the MOSFET. On top of this the polarity of the output
electrolytics on your diagram is
wrong.

Cheers

Alex

P.S. - I've attached a modded
picture to show where the cap should go. I suggest about 1uF film would
do.

Attached Images

modded.gif
(40.3 KB, 792 views)

6th July 2007, 01:42 PM

#4

Dxvideo

diyAudio Member

Join Date: Nov 2003

Location: Turkey

This is the original designs link, pls find
it at the Class A MOSFET Follower title.
http://headwize.com/projects/showfile.php?file=opamp_prj.htm
And
the original circuit is here;

However
I couldnt see any cap between op-amp and MOS!

__________________
Best regards,
Ozgur

6th July 2007, 01:49 PM

#5

x-pro

diyAudio Member

Join Date: Apr 2003

Location: London

Quote:

Originally
posted by Dxvideo
However I couldnt see any cap between
op-amp and MOS!

It is not a complete
circuit and so it is confusing and wrong on that particular picture. If
there is no capacitor, the whole network of resistors to set the gate
voltage of the MOSFET is completely
useless.

Cheers

Alex

P.S. I have to add that that
particular article is misleading in many places and some of the circuits
shown are plain wrong and some are even dangerous to implement - i.e.
"High Voltage Regulator"on fig. 20 .

6th July 2007, 01:56 PM

#6

Dxvideo

diyAudio Member

Join Date: Nov 2003

Location: Turkey

Ok then.
You say, I MUST put a series 1uF
capacitor between op-amp output and MOSFET s gate. The output capacitor
(1500uF) will be ommited in this case. And as I can understand it must be
before 100ohm and trimpot node.
Am I right?
THX.

__________________
Best regards,
Ozgur

6th July 2007, 02:01 PM

#7

x-pro

diyAudio Member

Join Date: Apr 2003

Location: London

Quote:

Originally
posted by Dxvideo
Ok then.
You say, I MUST put a
series 1uF capacitor between op-amp output and MOSFET s gate. The
output capacitor (1500uF) will be ommited in this case. And as I can
understand it must be before 100ohm and trimpot node.
Am I
right?
THX.

Yes. You also may need
to change the value of R3 - as the DC voltage on the gate should be about
+4 +5V. Value would depend on the power supply voltage you are using. It
would be essential to use a stabilized supply for this circuit - otherwise
the output voltage would vary too much following the supply variations

Cheers

Alex

6th July 2007, 02:11 PM

#8

Dxvideo

diyAudio Member

Join Date: Nov 2003

Location: Turkey

I see.
I can adjust the gate voltage via
R5 (theorically) after putting the DC blocking cap.
My power supply is
a regulated one. I use 7812 and 7912 for regulating.
But thats
interesting; in THIS situation, circuit works fine, except I feel some
treble weakness.
And a final question;
May my op-amp have any
defect from this happen?
Thz again.

__________________
Best regards,
Ozgur

6th July 2007, 02:18 PM

#9

x-pro

diyAudio Member

Join Date: Apr 2003

Location: London

Quote:

Originally
posted by Dxvideo
And a final question;
May my op-amp
have any defect from this happen?

No, I don't think that opamp would
suffer any damage.

Cheers

Alex

6th July 2007, 02:25 PM

#10

ilimzn

diyAudio Member

Join Date: Feb 2005

Location: Zagreb

The schematic is wrong on many levels (some
more serious than others). Perhaps the best solution would be to make a
separate DC feedback from the output.

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