Eliminate turn-off thomp on LM3886 - How? - diyAudio
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Old 30th November 2010, 11:27 AM   #1
Hubba is offline Hubba  Sweden
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Default Eliminate turn-off thomp on LM3886 - How?

Hi,

Having browsed through the forum I am yet to find an easy way to eliminate thomp or pop at power off of my LM3886 amp. I am using the standard stereo kit from chipamp.com, and when I turn on the amp there's nothing, but when i cut power there's a (huge) thomp in both speakers. Looking at the bass/midrange speaker I feel like i never want to turn the amp off, the coil is moving quite a distance on turn off.

On a side note: My current setup is using a 2x24VAC 120VA transformer, as I am waiting for the correct one, but I cannot see that it would make any difference when I receive the correct transformer (220 VA)

Any ideas how to move forward?

Cheers,
Fredrik

Last edited by Hubba; 30th November 2010 at 11:27 AM. Reason: spelling
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Old 30th November 2010, 12:38 PM   #2
jamal is offline jamal  Singapore
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I built a delay ON relay. so the relay will connect the speaker after few seconds. also the speaker will be disconnected immediately when the power switch is OFF. thus eliminating the pop sound. email me if you want the circuit. I have to snap the picture before sending it to you.
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Old 30th November 2010, 02:30 PM   #3
AndrewT is offline AndrewT  Scotland
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the 3886 has "mute" pin.
Have you used it?
The "mute" can be instant or it can be delayed.
The 3886 also has low voltage cutout. Are your voltages getting down to that level before the mute has silenced the chip?

Have you read the datasheet?
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Last edited by AndrewT; 30th November 2010 at 02:57 PM.
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Old 30th November 2010, 02:49 PM   #4
jamal is offline jamal  Singapore
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fed, you mean LM3886? oops. then you have to add a capacitor on the mute pin. I was using CarlosFM LM3886 circuit, no pop problem. my relay circuit is for the LM1875.
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Old 30th November 2010, 05:39 PM   #5
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Add an RC circuit across the mains switch/relay.
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Old 30th November 2010, 06:19 PM   #6
LAJ is offline LAJ  United States
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Make sure you + and - rails are collapsing together. You may need some bleeder resistors across your filter caps to accomplish this.
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Old 1st December 2010, 08:41 AM   #7
Hubba is offline Hubba  Sweden
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Thank you all for your replies. I'm not an electrical engineer, so the technical discussion may be a bit over my head, so be gentle

I've got a 100 uF connected to pin 8 of the LM3886, so yes the mute function should work, and it looks like it does as the amp is dead silence on start-up.

AndrewT: As I am currently using a 2x24VAC 120 VA transformer (I'm waiting for the correct one), could this cause a low voltage cut out -> producing thomp?

LAJ: I do not really follow you; By collapsing together do you mean draining the PSU caps " at the same time" or? What would I typically use as bleeder resistor for the PSU caps, and how are they connected?

Thanks,
Fredrik
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Old 1st December 2010, 08:56 AM   #8
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by Hubba View Post
I've got a 100 uF connected to pin 8 of the LM3886, so yes the mute function should work, and it looks like it does as the amp is dead silence on start-up.

AndrewT: As I am currently using a 2x24VAC 120 VA transformer (I'm waiting for the correct one), could this cause a low voltage cut out -> producing thomp?
the 24Vac is not the cause of the OFF thump.

What resistor are you using with the 100uF on pin8? look at the schematic on page 1 of the datasheet. are your cap and resistor placed like that and connected to -ve supply?
look at the mute graph on p16. check your current against that graph.
You may be holding the mute ON for too long by using too much current and then the cap extends the time it takes before it allows the mute to go OFF.
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