Resistor, M-T pot and DC offset - diyAudio
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Old 17th June 2003, 09:19 AM   #1
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Lightbulb Resistor, M-T pot and DC offset

A little R&D from myself.
I made two modules with LM3886, which have been playing on my bench.
I did it to test these chips, and because this will end up as an amp for my father.
First, I didn't use any resistor in the non-inverting input (direct wire).
I had 28mv on one channel and around 8 mv on the other.
Everything else being equal (I even hand-select and measure the 1% resistors), it's a fact that these chips have a huge production drift.
They are so alike as Michael Jackson is to Ben Harper.

Some days before, and playing with no problems, I decided to mount some 50k multi-turn pots to see if I could lower the DC offset to around 0mv, as I do with the LM3875.
I selected the channel with more DC offset (28mv) to test this.
Unfortunately, with the 50k pot, I couldn't get lower than around 24mv.
I removed the pot and tested with resistors.
As I increased resistance, DC lowered.
With the resistors I've got to the conclusion that a 250k multi-turn pot would do it.
Next day I bought some 500k multi-turn pots.
They didn't have anything near 200k.
Well, I mounted them, adjusted, and... 0.0mv!
Then I measured the pot and... 216k!
Hey Nuuk, are you there?
Then I turned it on and there was hiss on this channel.
Not hum, hiss.
I tried to connect the ground pin on the chip to the signal star ground formed by the pot, and I heard a loud noise.

Then I removed everything and back to the original form, just a wire.

The conclusions I get from this are:

1 - Forget the resistor or pot on the LM3886, because the ground pin on the chip doesn't let you do it. Use a direct connection, as Peter Daniel does, and hope for the best.
You won't have this problem with the LM3875.

2 - The pot you will use to null the DC-offset is related to the DC offset you originally have.
I'm sure that on the other channel (8mv, same circuit) the 50k pot would do fine.
Some guys, like master Kuei, will instantly calculate the pot you need for a given circuit, but that may not be sufficient because of the production drift of these chips.

3 - This applyes only to the LM3875: a 250~300k multi-turn pot seams to be the ideal for any situation.

4 - I wonder how could we hand-select two chips before we use them, so that we don't have huge differences in DC-offset from one channel to the other...

I hope this helps.
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Old 17th June 2003, 10:19 AM   #2
Nuuk is offline Nuuk  United Kingdom
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Quote:
I wonder how could we hand-select two chips before we use them, so that we don't have huge differences in DC-offset from one channel to the other
Good morning Carlos. Well we could all build the minimal inverted GC with the wire from non-inverting input to ground and measure the DC offset. Then mark the LM3875 that we used with that offset figure and post the results here. Then we could look for somebody else's chip with exactly the same offset and try a swap!

When I built my GC's with the wire to ground, both my amps measured 28mV so I must have been lucky! Or was I? Because when I used the 18K/22K combination, I got 0mV on one and 4mV on the other!

With the 216K from non-inverting to ground I get 0mV on one amp, and 0.3mV on the other. So it seems to depend on which method that you use to reduce the offset as to how much difference there is between channels, ie there is more of a difference with both inputs connected to ground.
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Old 17th June 2003, 10:50 AM   #3
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Quote:
Originally posted by Nuuk

When I built my GC's with the wire to ground, both my amps measured 28mV so I must have been lucky! Or was I? Because when I used the 18K/22K combination, I got 0mV on one and 4mV on the other!

You were lucky, because you had two channels with the same DC offset.
But a high value, unfortunately.
You could have had better luck still.
When I built my first GC, with 18k resistor, I had 10mv on one channel and 10.5 mv on the other.
A 50k multi-turn pot managed to cure that.
The point here is that a multi-turn pot of 250~300k will do for any situation.
And the point here too is that this doesn't work with the LM3886.
And when I said "hand-select" a chip I mean a way to measure with a multimeter on the pins, before mounting the chips.
Maby with some tests we can get to a conclusion, but well... it's not that important.
It's much more easier to test an op-amp on a socket.
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Old 17th June 2003, 12:17 PM   #4
tiroth is offline tiroth  United States
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I built an amp using LM3886 with resistor to ground on the non-inverting input. There was no audible noise on the output. This resistor is going to raise the noise floor itself, but the big problem tends to come from induced noise due to the high input impedance. This is going to be determined by layout, shielding, and your environment.

I don't generally recommend using this resistor due to the possibility of noise problems, but it IS possible to get LM3886 working in the same manner as LM3875. (i.e. no hum or hiss with input impedances matched)
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Old 17th June 2003, 01:01 PM   #5
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Quote:
Originally posted by tiroth

I don't generally recommend using this resistor due to the possibility of noise problems, but it IS possible to get LM3886 working in the same manner as LM3875. (i.e. no hum or hiss with input impedances matched)

How?
What about the ground pin on the LM3886?
If you put a resistor on the non-inverting input, instantly you have a ground loop.
The amount of noise is related to the value of the resistor, the bigger it is, more noise you get.
If you connect the ground pin to that resistor, you'll have noise too.
If you don't connect the ground pin you'd better stay away, because it makes a big noise.
The problem here IS the ground pin of the LM3886, which the LM3875 doesn't have.
What's the solution for this other than no resistor on the non-inverting input?
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Old 17th June 2003, 02:13 PM   #6
tiroth is offline tiroth  United States
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Are you saying there is a ground loop due to ground connections inside the substrate?

And yes, the GND pin obviously does need to be connected. I connected GND pin to small signal ground: no hum or noise.
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Old 17th June 2003, 02:54 PM   #7
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Quote:
Originally posted by tiroth
Are you saying there is a ground loop due to ground connections inside the substrate?

And yes, the GND pin obviously does need to be connected. I connected GND pin to small signal ground: no hum or noise.

No, it's because the ground pin is connected to the star ground and the non-inv. input has a resistor.
That gives problems. It's a ground loop.
If you connect the ground pin to the signal star ground (the resistor) you may have problems if you need around 200k, for instance, to minimize DC offset.
Maby with 18k or lower you may not notice.
On one channel I have 8mv DC without a resistor, and on this one I may not notice that problem if I put a pot, because a 50k pot will do, and the final resistance will be lower than 18k.
But on the other channel, with 28mv DC, I need 216kohms to eliminate DC offset, and then there's noise.
This is lottery, because the chips are not matched.
That's what I'm trying to say, don't bother with the resistor or pot with the LM3886 if you have around 20~30mv DC offset, because it will not work.
But if you have less than 10mv, you can try to optimize that, you'll use a small value resistor and you may not have noise.
The LM3875 is different, you won't have these problems.
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Old 17th June 2003, 03:06 PM   #8
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Talking There are no two equal things in the world

Just to exemplify how the chips are different, on a preamp I made I measured the DC offset on the OPA627s.
On one I had 1mv, on the other, 0.2mv.
It's easy to test, because I have sockets.
I picked another one and now I have 0.2mv on one channel and 0.3mv on the other.
Then I removed the input cap, because it was doing nothing, only mucking up the sound.
No cap is better than any cap.
So, the circuit is the same, only the chips are changed and I could easilly optimize and match the op-amps.
With the LM chips in your GC you may have very different results depending on the chip.
Believe me, if you do another GC you may have very different results.
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Old 17th June 2003, 03:08 PM   #9
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Quote:
Originally posted by carlosfm

No, it's because the ground pin is connected to the star ground and the non-inv. input has a resistor.
That gives problems. It's a ground loop.
If you connect the ground pin to the signal star ground (the resistor) you may have problems if you need around 200k, for instance, to minimize DC offset.
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?


Carlos
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Old 17th June 2003, 03:17 PM   #10
tiroth is offline tiroth  United States
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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.
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