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Old 22nd November 2008, 06:49 PM   #1
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Default looking for null circuit

Hi

Can anyone point me to the post that uses a trimmer and the supply voltage (Vcc and Vee) the null dc offset
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Old 23rd November 2008, 05:22 PM   #2
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Default Re: looking for null circuit

Quote:
Originally posted by jaudio
Hi

Can anyone point me to the post that uses a trimmer and the supply voltage (Vcc and Vee) the null dc offset

Apply a small voltage into one of the inputs, this can be derived from the power supply. If your output offset is 10 mV, you would introduce -10 mV into the feed-back circuit.
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Old 23rd November 2008, 05:29 PM   #3
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Default Re: looking for null circuit

Quote:
Originally posted by jaudio
Hi

Can anyone point me to the post that uses a trimmer and the supply voltage (Vcc and Vee) the null dc offset
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File Type: jpg offset_adj.jpg (27.1 KB, 414 views)
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Old 24th November 2008, 02:08 PM   #4
ide2003 is offline ide2003  Indonesia
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Hi Nico Ras,

Saw your schematic, thanks, I'm a noob, can I use this for LM3886 chip?. I did not like the sound with coupling capacitor, but do not want my speaker to suffer..

regards
TPS
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Old 24th November 2008, 02:18 PM   #5
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Quote:
Originally posted by ide2003
Hi Nico Ras,

Saw your schematic, thanks, I'm a noob, can I use this for LM3886 chip?. I did not like the sound with coupling capacitor, but do not want my speaker to suffer..

regards
TPS
Yes you can, it will work with any differential amplifier. Why do you use a coupling capacitor do you not have a split supply?

Nico
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Old 24th November 2008, 02:57 PM   #6
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Like Nico Ras posted, this is the same way used in Krell KSA-100
to provide some current to adjust the DC-offset.

If looking at attachment here you see two diodes around the 0V.
These gives a span of +- 0.7 Volt reference.
So can add both negative or postive adjust current.

The resistor values used are important.
Because these resistors:
R15, R16, R17(pot), R18 will give a resistance that is parallell to R9 - 475 Ohm.
475 Ohm resistor is a part of the feedback divider, which controls the gain of amp.

If offset resistance was 475R, than this would give the amplifier would have twice the gain!
( 475 parallelled with 475 is = 237.5 )

But as R15,R16,R17,R18 gives a total resistance of >25000 Ohm
which is >50 times larger than 475R
the offset adjust will only effect, increase the gain with (~1/50) ~2%.

If you can not get good adjustment with large enough resistors
then you have to increase the reference voltage.
For example use 2+2 diodes, giving +-1.4 Volt, instead of +-0.7 Volt.
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Old 24th November 2008, 03:02 PM   #7
lineup is offline lineup  Sweden
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And looking at Nico Ras schematic of the IC circuit
the offset adjust resistance should be a lot bigger than resistor R2
which is the one that corresponds to the Krell R9, 475 Ohm.
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Old 26th November 2008, 01:17 AM   #8
ide2003 is offline ide2003  Indonesia
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Quote:
Originally posted by Nico Ras

Yes you can, it will work with any differential amplifier. Why do you use a coupling capacitor do you not have a split supply?

Nico
Thanks Nico, I have first tried without coupling capacitor, sounds very transparent..then for safety sake's added a MKT 2.2uF cap, the magic is gone..I don't have a budget for expensive cap and I use split supply. Will try your scheme soon

TPS
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Old 27th November 2008, 12:18 AM   #9
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Quote:
Originally posted by ide2003
Hi Nico Ras,

Saw your schematic, thanks, I'm a noob, can I use this for LM3886 chip?. I did not like the sound with coupling capacitor, but do not want my speaker to suffer..

regards
TPS
This reply is regarding the input circuit, not the NFB. . .

DC offset with LM3886? Most of the documents indicate a 20k pot + a 20k resistor. That's a 10k input impedance and it lowers DC offset nicely. Sometimes, its like this: buffer, cap, 20k linear pot (load), 20k resistor (load), lm3886
So, you see, if you use an input filter cap, that you still to make sure your input impedance resistor is 10k.

Next. . .
Here's what doesn't work:
pot, then cap, then 20k load = big dc offset!
cap then 20k load = big dc offset!
So change that to 10k resistor.

Edit: Take a look at the very first page of LM3886.PDF from National Semiconductor.
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Old 30th November 2008, 01:01 PM   #10
ide2003 is offline ide2003  Indonesia
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Quote:
Originally posted by danielwritesbac


This reply is regarding the input circuit, not the NFB. . .

DC offset with LM3886? Most of the documents indicate a 20k pot + a 20k resistor. That's a 10k input impedance and it lowers DC offset nicely. Sometimes, its like this: buffer, cap, 20k linear pot (load), 20k resistor (load), lm3886
So, you see, if you use an input filter cap, that you still to make sure your input impedance resistor is 10k.

Next. . .
Here's what doesn't work:
pot, then cap, then 20k load = big dc offset!
cap then 20k load = big dc offset!
So change that to 10k resistor.

Edit: Take a look at the very first page of LM3886.PDF from National Semiconductor.
hi there danielwritesbac, appreciated your tips.

will check mine, schem. attached, I use 50k pot, C1=2.2u, R1=1k, R2=20k in that order.. is this the culprit of 'big' dc-offset?

thanks
TPS
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File Type: jpg lm3886-cplt.jpg (31.0 KB, 222 views)
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