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Old 17th June 2003, 03:51 PM   #11
Join Date: Mar 2003
Location: Lisbon, Portugal
Originally posted by carlmart

Two things:
1) Could you see this noise on a scope? Could it be an oscillation?
2) Did you try bypassing the "offset" resistor as on the original Thorsten IGC?
I don't have a scope.
Anyway, I built a GC with LM3875 and no problems at all.
And no, no cap bypassing the offset resistor.

Originally posted by tiroth
If we aren't talking about the substrate then I don't see where the loop is.

GND pin ------\
+in -- 220k --\
Cap1 ---------- GND
Cap2 ---------/
Supply -------/

Every ground node has its own line to ground. ???

On the GC I am talking about there is 220k to ground at the noninverting input. With external power supply there is no noise.
Well, that was in the original Thorsten schematic.
He now recommends a 18k resistor in place of the 220k and 0.1uf cap.
Anyway, or I did something wrong with the LM3886 circuit, or I don't know what's happening.
Anyway, it's playing beautiful sounds for two weeks now, without a resistor in the non-inverting input, and I don't see why I would have oscillations.
I have the 1000uf caps and the 0.1uf bypass caps very near the chip's pins.
What I can tell you is that with the LM3875 I tested with 18k, 50k multi-turn pots and no problem at all.
It stayed with the pots in place, and I've got 0.0mv on both channels.
When I tried to do the same with the LM3886, there was noise.
What bugs me is that misterious ground pin that mucks it all.
The LM3875 behaves exactly as a normal op-amp.
The LM3886 doesn't, because an op-amp doesn't have a ground pin.
Make no mistake, I'm not talking big noise.
It's that I'm used to the silence of my LM3875 GC.
I take the volume to max and absolutely nothing, no noise, even with my ear on the speaker.
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Old 17th June 2003, 04:48 PM   #12
rickpt is offline rickpt  Portugal
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Join Date: Jan 2002
Location: portugal
If you put a resistor on the non-inverting input, instantly you have a ground loop.
It isnít a ground loop... It is noise generated by the non inverting input current (only fets and tubes dont generate current noise) the higher the resistor, higher the noise!
Think why there is a cap in parallel with that resistor

The LM3875 behaves exactly as a normal op-amp.
The LM3886 doesn't, because an op-amp doesn't have a ground pin.
Obviously this is completely wrong... The Lm3886 is an opamp

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Old 17th June 2003, 04:51 PM   #13
Join Date: Mar 2003
Location: Lisbon, Portugal
Originally posted by rickpt

Obviously this is completely wrong... The Lm3886 is an opamp

And who said it isn't?
I said "behaves".
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Old 27th June 2003, 09:43 AM   #14
Join Date: Mar 2003
Location: Lisbon, Portugal
Talking Sometimes I sit down and read...

...and I found this on the AN-1192 appnote from National, on page 5:

"The non-inverting input resistance, Rb, is used to create a
voltage drop at the non-inverting terminal to offset the
voltage at the inverting input terminal due to the input
bias current flowing from the output to the inverting input.
Generally, the value of this resistor equals the value of
the feedback resistor so that the output offset voltage will
be minimized close to zero. However, if this value is too
large, noise can easily be picked up which will be amplified
and seriously affect the THD+N performance. If the
resistor is eliminated and the terminal is grounded, the
THD+N performance will be much better, but it will not
necessarily be optimized. By connecting the
non-inverting input directly to a ground reference, any
noise on that ground will be directly injected into the
amplifier, amplified and thus will also affect the THD+N
performance. The best solution is to use a value of
resistance not too large to pick up stray noise and not too
small as to be affected by ground noise fluctuations. The
value used in the previous plots was a 3.32k resistor. It
should be noted that this is not necessarily the optimized
value and can change with varying circuit layouts."

It seams that my conclusions are correct.
If you can't remove DC offset with a low value resistor (say... until 50k), then don't bother with it.
You'll have noise with a high value resistor.
Or use a low value resistor or a wire (if DC is low).
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