Testing an LM3886

Ulew · 20th March 2009, 11:03 PM

Hi!
I'm building an LM3886 amplifier, and now I'm about to test it.
I got an psu with around 35V after the bridge (24V secondary).
But what do I do with the input on the amp while measuring the dc offset? In Peter Daniels guide he says "(input shunted to ground)". What does that mean?

wakibaki · 20th March 2009, 11:23 PM

Connected. Input connected to ground.

It's a fancy way of saying 'with a parallel bypass to ground', since the input appears as a resistance between 'the input' and ground.

Old school.

w

Got to keep these youngsters in line.

Ulew · 21st March 2009, 08:57 AM

Hmmm.. I'm not quite following here!
Should I connect the input signal on the amp to ground?

CJ900RR · 21st March 2009, 09:06 AM

Yes put a wire between input positive and input negative. Power the amp and measure the DC-offset on the output. It should be between 0mV and up to about 100mV. I think 100mV is high but acceptable.

Test your power supply first but I read on Faktiskt.se that you already have done that, so just connect the amp and go

Some people recommend that you use a "Light bulb" in series with the power. Search in this forum if you want to use it.

//CJ900RR aka Bullit på faktiskt.se

Ulew · 21st March 2009, 09:11 AM

Thanks! Now I understand!

I'll try the amps today, so I will come back with results both here and on Faktiskt!

Ulew · 21st March 2009, 11:00 AM

Well.. Something is wrong. One of the amps show 0V offset and the other show something like 35V..

CJ900RR · 21st March 2009, 12:55 PM

The one showing 0V, have you connected it to a source and if so, is there music?

The other one showing 35V must be something really wrong. Any pictures, close-up and good resolution would help. Also schematic.

Ulew · 21st March 2009, 04:08 PM

Quote:

Originally posted by CJ900RR
The one showing 0V, have you connected it to a source and if so, is there music?

Yes, there is music (and good music, Dropkick Murphys)!!

What does it mean that the offset is 0V? (What is the offset anyway?)

Schedule

Pics of the board

Can you see something thats wrong (except from my bad soldering..)?
Iv'e checked with a multimeter, and as far as I can see there is nowhere the signal is cut. The IC or the capacitators can be the problem, if there is nothing else wrong.

djoffe · 21st March 2009, 06:31 PM

The DC voltage with respect to ground, at the output of the amplifier, should ideally be zero volts.

Owing to the input coupling capacitor, whether or not you ground the input or leave it open, it will make no difference to the DC offset.

Question: Do the amps work? Do they play music and sound ok?

If they do, perhaps the issue is misinterpretation of the method of doing an offset voltage test. If you follow CJ900RR's method verbatim, "Yes put a wire between input positive and input negative. "
that is, a connection from pin 9 to 10, then you short out all the negative feedback that keeps the output voltage correct. Don't do it!

Just leave the signal source disconnected or turned down, and measure the DC voltage across the speaker.

Hope this helps

Ulew · 21st March 2009, 07:29 PM

I haven't tried both amps with music, but one of them works.
The amp I haven't tried is the one that measures 35V DC-offset. I will try with the input loose tomorrow!

20th March 2009, 11:03 PM	#1
Ulew diyAudio Member Join Date: Oct 2008	Testing an LM3886 Hi! I'm building an LM3886 amplifier, and now I'm about to test it. I got an psu with around 35V after the bridge (24V secondary). But what do I do with the input on the amp while measuring the dc offset? In Peter Daniels guide he says "(input shunted to ground)". What does that mean?

21st March 2009, 06:31 PM	#9
djoffe diyAudio Member Join Date: Oct 2008	Offset Clarification The DC voltage with respect to ground, at the output of the amplifier, should ideally be zero volts. Owing to the input coupling capacitor, whether or not you ground the input or leave it open, it will make no difference to the DC offset. Question: Do the amps work? Do they play music and sound ok? If they do, perhaps the issue is misinterpretation of the method of doing an offset voltage test. If you follow CJ900RR's method verbatim, "Yes put a wire between input positive and input negative. " that is, a connection from pin 9 to 10, then you short out all the negative feedback that keeps the output voltage correct. Don't do it! Just leave the signal source disconnected or turned down, and measure the DC voltage across the speaker. Hope this helps

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
amp testing	jackduckwort	Chip Amps	0	16th March 2006 12:39 PM
testing, testing 123	neil_kaye	Full Range	0	25th December 2005 08:21 PM
Are two separate lm3886 = 2*lm3886 in parallel or bridge mode	rs1026	Chip Amps	11	21st October 2004 10:24 AM

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

20th March 2009, 11:23 PM	#2
wakibaki Banned Join Date: Jan 2008 Blog Entries: 2	Connected. Input connected to ground. It's a fancy way of saying 'with a parallel bypass to ground', since the input appears as a resistance between 'the input' and ground. Old school. w Got to keep these youngsters in line.

21st March 2009, 08:57 AM	#3
Ulew diyAudio Member Join Date: Oct 2008	Hmmm.. I'm not quite following here! Should I connect the input signal on the amp to ground?

21st March 2009, 09:06 AM	#4
CJ900RR diyAudio Member Join Date: Jan 2006	Yes put a wire between input positive and input negative. Power the amp and measure the DC-offset on the output. It should be between 0mV and up to about 100mV. I think 100mV is high but acceptable. Test your power supply first but I read on Faktiskt.se that you already have done that, so just connect the amp and go Some people recommend that you use a "Light bulb" in series with the power. Search in this forum if you want to use it. //CJ900RR aka Bullit på faktiskt.se

21st March 2009, 09:11 AM	#5
Ulew diyAudio Member Join Date: Oct 2008	Thanks! Now I understand! I'll try the amps today, so I will come back with results both here and on Faktiskt!

21st March 2009, 11:00 AM	#6
Ulew diyAudio Member Join Date: Oct 2008	Well.. Something is wrong. One of the amps show 0V offset and the other show something like 35V..

21st March 2009, 12:55 PM	#7
CJ900RR diyAudio Member Join Date: Jan 2006	The one showing 0V, have you connected it to a source and if so, is there music? The other one showing 35V must be something really wrong. Any pictures, close-up and good resolution would help. Also schematic.

21st March 2009, 07:29 PM	#10
Ulew diyAudio Member Join Date: Oct 2008	I haven't tried both amps with music, but one of them works. The amp I haven't tried is the one that measures 35V DC-offset. I will try with the input loose tomorrow!

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