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LM3886 PA150 oscillates !!!
LM3886 PA150 oscillates !!!
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Old 28th February 2021, 09:35 PM   #1
guyome is offline guyome  France
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Default LM3886 PA150 oscillates !!!

Hello,

I bought 2 PCB to build 2 pa150 boards (see attached picture)

Every component comes from Mouser

Resistors for gain are 0,6W 0,1% 20kR and 1kR

47pF have been replaced by 8pF

Only one side is filled with 2 caps Panasonic 2200uF

22uF // 1uF are soldered directly at the rear pins of the 2 rails of the 3 LM3886

Thiele network have been added - soldered on the side of the speaker connector (not on PCB side)

The 3 pots has been well adjusted

10uf has been put for the input cap

PSU with toroid transformer 2x25V with secondaries decoupled with 100nF, 2 bridge rectifiers KBPC5010, 4x 22000uf caps, 2 5,6kR bleeding resistors.

I thought it was a pretty perfect setup, but no. When there is no source connected to one PA150, it starts to oscillate - and I cannot know why

Help !!!!

Many thanks,

Guillaume
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Old 28th February 2021, 09:40 PM   #2
rayma is offline rayma  United States
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Can you post the exact schematic?
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Old 28th February 2021, 10:10 PM   #3
PRR is offline PRR  United States
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LM3886 PA150 oscillates !!!
Quote:
Originally Posted by guyome View Post
....47pF have been replaced by 8pF....
Why??
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Old 28th February 2021, 10:12 PM   #4
guyome is offline guyome  France
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Schematic :
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Old 28th February 2021, 10:13 PM   #5
guyome is offline guyome  France
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Quote:
Originally Posted by PRR View Post
Why??
Because it is on the schematic
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Old 28th February 2021, 10:16 PM   #6
PRR is offline PRR  United States
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LM3886 PA150 oscillates !!!
I could not see that this is Inverting.

In this mode, nothing at input, gain is unity instead of 20. Nearly any power amp will oscillate that way.
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Old 28th February 2021, 10:28 PM   #7
Mark Tillotson is offline Mark Tillotson
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I think it needs an opamp buffer on the input to stabilize it and cancel the inversion?

You can then lower the feedback network resistor values to improve the noise performance too. The 10k/200k values are only there so the input impedance isn't lower than the accepted minimum recommended impedance for an audio power amp. However paralleling 3 of them makes the input impedance actually 3k3, which is very low. And there's a (cargo-cult ?) 47k load resistor on the input too to lower the impedance a little bit further?

All in all using an inverting topology is problematic in a power amp.

Last edited by Mark Tillotson; 28th February 2021 at 10:30 PM.
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Old 28th February 2021, 10:47 PM   #8
guyome is offline guyome  France
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I already replaced 10k/200k by 1k/20k to improve the noise performance.

Maybe I should have also lowered the 47k resistor to 4,7k ?
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Old 1st March 2021, 12:07 AM   #9
audiopro is offline audiopro  Canada
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So now your input impedance is somewhere around 330 ohms.
On top of that, the 50k pot is now just an open circuit as far as the amp is concerned when set higher than the feedback resistor.
Your output filter network is incorrect as well, and the 8pf cap is too small.
Basically the entire circuit is junk.
And Mark is right, making this an inverting topology is just wrong.
Get the LM3886 data sheet to see how it should be done.
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Old 1st March 2021, 12:20 AM   #10
Panelhead is offline Panelhead
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Paralleling three LM3886 takes skill. Using the inverted input and having unity gain complicate that. Is this the normal design for a PA150?
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