LM3875: Switch off thump

NealG · 10th December 2003, 11:10 AM

Calling all LM3875 experts!

When I turn off my NIGC a loud 'thump' is heard from my 'speakers. I am not too happy about this and would like some input on how I can cure it.

Circuits wise it’s very simple like the 'typical app' circuit in the data sheet but without the 22uF Ci. Rf1 and Ri changed to 22K/681R respectively. I have followed lots of advise on this BB for the build but can't work out why this is happening.

The chip is supposed to mute when both supply rails drop below 12v. Looking at the 'scope a large DC offset is produced about 2 seconds after power off.

The supply is dual bridge and single 24-0-24 300VA TX, 2000uF on each rail with a further 1000uF hardwired to the chip pins and bypassed with 1uF. Rb is at 1K as per the data sheet.

Any ideas?

Many thanks in advance.
.

bigparsnip · 10th December 2003, 11:51 AM

You could try putting some bleader resistors between your power supply rails and ground so that you PS caps are discharged through these to ground at swithch off. I have no real idea if this will help in this case or not, but I doubt it could hurt to give it a try if people don't come up with any other sujestions.

Jax · 10th December 2003, 12:21 PM

Try adding Ci.

I think the thump is due to offsets drifting around when the supply discharges and without Ci will be amplified causing the thump.

Peter Daniel · 10th December 2003, 03:06 PM

The thump is usually the offset problem.

NealG · 10th December 2003, 03:51 PM

Thanks Jax,

I have added a 10uF BG cap for now and the switchoff thump has gone, also DC offset is now .5mV R/H and 1.4mV L/H channel.

ronc · 10th December 2003, 04:14 PM

Thinking about it a MOM switch which would go thru a low value (2 ohm) but higher current ,ie 25 watts,would allow the offset current to go directly to ground?
Any ideas?
ron

10th December 2003, 11:10 AM	#1
NealG diyAudio Member Join Date: Dec 2003 Location: .	LM3875: Switch off thump Calling all LM3875 experts! When I turn off my NIGC a loud 'thump' is heard from my 'speakers. I am not too happy about this and would like some input on how I can cure it. Circuits wise it’s very simple like the 'typical app' circuit in the data sheet but without the 22uF Ci. Rf1 and Ri changed to 22K/681R respectively. I have followed lots of advise on this BB for the build but can't work out why this is happening. The chip is supposed to mute when both supply rails drop below 12v. Looking at the 'scope a large DC offset is produced about 2 seconds after power off. The supply is dual bridge and single 24-0-24 300VA TX, 2000uF on each rail with a further 1000uF hardwired to the chip pins and bypassed with 1uF. Rb is at 1K as per the data sheet. Any ideas? Many thanks in advance. .

10th December 2003, 03:06 PM	#4
Peter Daniel diyAudio Member Join Date: Jul 2002 Location: Toronto, Canada	The thump is usually the offset problem. __________________ www.audiosector.com “Do something really well. See how much time it takes. It might be a product, a work of art, who knows? Then give it away cheaply, just because you feel that it should not cost so much, even if it took a lot of time and expensive materials to make it.” - JC

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
switch on/off preamp thump	ljk11	Everything Else	0	6th January 2009 10:48 AM
What is that thump,thump,thump?	65blkbkgt	Car Audio	7	17th June 2007 01:35 AM
Never try to switch 450V DC at 3A with a conventional mains switch...	Eva	Power Supplies	28	1st November 2006 09:24 AM
BrianGT LM3875 Turn-Off Thump	cantskienuf	Chip Amps	20	29th November 2004 02:50 PM
Using a momentary switch for main power switch?	FullThrottleRic	Parts	9	25th June 2002 08:23 PM

New To Site?	Need Help?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read


	Please consider donating to help us continue to serve you. Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving

10th December 2003, 11:51 AM	#2
bigparsnip diyAudio Member Join Date: Apr 2003 Location: Cambridge	You could try putting some bleader resistors between your power supply rails and ground so that you PS caps are discharged through these to ground at swithch off. I have no real idea if this will help in this case or not, but I doubt it could hurt to give it a try if people don't come up with any other sujestions.

10th December 2003, 12:21 PM	#3
Jax diyAudio Member Join Date: Nov 2003 Location: South Sweden	Try adding Ci. I think the thump is due to offsets drifting around when the supply discharges and without Ci will be amplified causing the thump.

10th December 2003, 03:51 PM	#5
NealG diyAudio Member Join Date: Dec 2003 Location: .	Thanks Jax, I have added a 10uF BG cap for now and the switchoff thump has gone, also DC offset is now .5mV R/H and 1.4mV L/H channel.

10th December 2003, 04:14 PM	#6
ronc diyAudio Member Join Date: Mar 2003	Thinking about it a MOM switch which would go thru a low value (2 ohm) but higher current ,ie 25 watts,would allow the offset current to go directly to ground? Any ideas? ron

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)