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Old 10th October 2013, 09:14 PM   #1
muttled is offline muttled  United States
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Default LM3886 Input Resistor Position

I'm using a 220r input resistor (Rb) on an LM3886 amp and currently have it wired BEFORE Rin instead of after it like it is on the datasheet. I've already etched the pcb/soldered all the parts in and the amp sounds fine, maybe a little quiet, but otherwise fine. The DC offset is fine.

Am I screwing anything up wiring that way? I can probably re-wire using jumpers and scratching out traces if need be to fix the problem.
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Old 11th October 2013, 01:30 AM   #2
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Will you please show the schematic?.... and pcb too!
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Old 11th October 2013, 09:13 AM   #3
AndrewT is offline AndrewT  Scotland
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Sounds from your description that the layout will be OK.

The series resistor needs to be before the RF attenuating capacoitor.

Similarly the Series capacitor needs to be before the DC attenuating resistor going to signal ground.

The order of these two pair of components can be moved around. Just keep the individuals of the pairs in the correct order.
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Old 11th October 2013, 09:27 PM   #4
muttled is offline muttled  United States
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A very quick edit to show what I did (except Rb is 220r):

Click the image to open in full size.
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Old 12th October 2013, 10:46 AM   #5
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by muttled View Post
A very quick edit to show what I did (except Rb is 220r):

Click the image to open in full size.
why are you still using that stripped down schematic?
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Old 13th October 2013, 04:06 PM   #6
muttled is offline muttled  United States
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Quote:
Originally Posted by AndrewT View Post
why are you still using that stripped down schematic?
Laziness and complacency since it worked when I tried it and had already made the PCBs, largely

In all reality, though, the PCB layout as designed could pretty easily accommodate a Zobel retrofitted. I just kind of wanted to make sure I wasn't screwing up the essentials like when I wired the gain resistors wrong on accident and ended up with 27 volts of DC offset. Had to say goodbye to that $1.99 RCA 20 watt speaker I found at a closeout place that day!

On a (semi) related note: if I am using a potentiometer on the input, would it make sense to delete the input resistor?
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Old 14th October 2013, 07:45 AM   #7
AndrewT is offline AndrewT  Scotland
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Quote:
if I am using a potentiometer on the input, would it make sense to delete the input resistor?
That is another component you would be omitting.
Place a DC blocking capacitor on the input.
That DC blocking capacitor isolates the input from the variable resistance of the pot.

Variable resistance affects the input bias !
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Old 16th October 2013, 03:34 AM   #8
muttled is offline muttled  United States
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Quote:
Originally Posted by AndrewT View Post
That is another component you would be omitting.
Place a DC blocking capacitor on the input.
That DC blocking capacitor isolates the input from the variable resistance of the pot.

Variable resistance affects the input bias !
Isn't that what I have there? I have a tiny electrolytic capacitor in series before the resistor. Or do I need another one?
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Old 16th October 2013, 07:33 AM   #9
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How small is that cap?
Because with the 10k pot it will be a high pass filter. If the cap is small, you will lose some low freq.
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Old 16th October 2013, 09:00 AM   #10
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by muttled View Post
A very quick edit to show what I did (except Rb is 220r):

Click the image to open in full size.
I wish you would attach your schematic.
You don't know how much work it is for me to bypass my computer virus protection to view a file stored in a site that has known virus issues.
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