[LM3886] Best resistor value for mute ? - diyAudio
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Old 8th November 2005, 09:05 PM   #1
XELB is offline XELB  Portugal
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Default [LM3886] Best resistor value for mute ?

Hello guys.

I was looking at the LM3886 internal schematic and it came to my attention that the mute resistor interferes with the current inside the chip!

So, based in experience and knowledge, witch value do you guys think will suits best ?

In the schematic they use 1k resistor. (page 7 of the LM3886 schematic)






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Old 8th November 2005, 11:18 PM   #2
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What page 7?
No schematic on page 7...

Did you search the forum?
Anyway...
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Old 9th November 2005, 04:38 AM   #3
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Referring to the equivalent schematic on page 6 of National pdf documentation, I'm not sure if the 1k you're talking about is external or internal. Internal might be okay, if 1K were enough to turn the mute off. But if it were external, may be 1k is a safe value to ensure the muting function is off.

In case of carlos schematic, 10K is used. Higher R will draw less current but might not turn the mute off completely. I think, because the resistor draws current from the input bias (as is your concern), we will want to draw as small as possible current, but big enough (0.5mA from mute pin) to turn the mute off.

Now, who knows how this (drawing too much current than necessary) will affect the sound? I don't think it has so much effect on the sound so I don't think I need to calculate or experiment with other values. May be you want to calculate the exact current a 1K or a 10K draws from mute pin?
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Old 9th November 2005, 06:58 AM   #4
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Default Re: [LM3886] Best resistor value for mute ?

Quote:
Originally posted by XELB
I was looking at the LM3886 internal schematic and it came to my attention that the mute resistor interferes with the current inside the chip!
So.... can't see anything to be worried about.

Quote:
Originally posted by XELB
So, based in experience and knowledge, witch value do you guys think will suits best ?
You'll have to check the datasheet. It's about current and is dependend of the the used supply voltage.

10 k is OK but I have used a but more 33k becasue I wanted longer delay time with 100 uF.

As you can see from the simplified schema it might be an good idea to have this pin filtered. In other words: The softstart serves two purposes, Carlos. This cap is maybe a bigger improvement than the snubber I can't see any reason NOT to have it. A real smooth startup is not wrong. Check my Gainclone at startup och powerdown.

The pictures shows the supply voltage and the output signal. Can it be better?
Startup
http://web.comhem.se/~u33113170/gainclone/start.jpg
Power down
http://web.comhem.se/~u33113170/gainclone/shutdown.jpg
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Old 9th November 2005, 08:22 AM   #5
XELB is offline XELB  Portugal
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Quote:
Originally posted by carlosfm
What page 7?
No schematic on page 7...

Did you search the forum?
Anyway...

Attached, Page 7 of the LM3886 Schematic





As you can see, depending on the resistor value, the current inside the chip change! Or am I miss understanding the schematic ?



PS: I have search the forum and I could find any investigation about this particulary caracteristic.
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File Type: jpg mute.jpg (45.2 KB, 659 views)
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Old 9th November 2005, 08:39 AM   #6
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You miss only a bit . Remember that National shows only a simplified schematic and this is made only for understanding. I'm pretty sure that the bias voltage for this stage is in some way stabilized. One way to find out is to measure the current the IC consumes when you are changing this resistor. Since National doesn't mention this resistor especially much I wouldn't worry.

Read the datasheet carefully and go with thier recommendations. 10 kohms seems to be a value you can choose without thinking. I also recommend a cap, 100-470 uF.
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Old 9th November 2005, 01:37 PM   #7
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Per-Anders, have you ever tried different resistor values ?

I was thinking in something like a 10K or 100k pot and then look for the right resistor value.
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Old 9th November 2005, 02:57 PM   #8
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I did only take as much I could considering my supply voltage and desired delay with 100 uF.

The answer is no in other words but you can see from the measurements that 33 k is just about on the edge.
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Old 9th November 2005, 06:31 PM   #9
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Quote:
Originally posted by peranders
As you can see from the simplified schema it might be an good idea to have this pin filtered. In other words: The softstart serves two purposes, Carlos.
P-A, I'm using a cap too, on my main amp, and I know it filters the PSU noise after the resistor.
I didn't include the cap on the schematic because it was not practical, and it doesn't seem important to me (it doesn't make difference, I tried), and here's why:

1. With or without the cap, the amp is absolutely silent and power on and off (with my PSUs).
2. My PSUs are high capacitance CRC and regulated (much less noise than yours).
3. I don't need a big delay at turn-on, and as I said, there's no noise at all.

Quote:
Originally posted by peranders
This cap is maybe a bigger improvement than the snubber
Maybe?
You mean you are not sure?
Maybe on your amp, not mine.
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Old 9th November 2005, 06:35 PM   #10
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My implementation is based on the Carlosfm schematic.
I am using 10k but, I am willing to try different values and see how the LM3886 behaves.
I don’t know if Carlos have tried different values so, I will give it a try, measure it with AP and listen.


Something tells me that this resistor could influence the LM3886 performance
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