Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Chip Amps Amplifiers based on integrated circuits

Troubleshooting LM1875
Troubleshooting LM1875
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
Reply
 
Thread Tools Search this Thread
Old 25th August 2018, 04:43 PM   #1
leRooie is offline leRooie  Netherlands
diyAudio Member
 
Join Date: Dec 2015
Location: Haarlem
Default Troubleshooting LM1875

Hi guys,

I recently finished building my first chipamp, with LM1875 boards from chipamp.com from when they were still around (must be almost 3 years ago now). I'm very pleased with the result, but I'm running into some problems after a first test. Both channels worked before, but after re-soldering some internal wiring and starting everything up again, the left channel seems to be bad.

The symptoms are: there's audio coming out, but very regularly, the signal drops. When playing on low volumes, it's less often, but at higher volumes, the drop out frequency increases and becomes a stutter. The right channel works fine, all inputs behave the same.

Some details: single bridge rectifier, stereo setup, I included a phono amp (the VSPS) on one of the inputs. I sourced the parts myself.

My knowledge of circuits is limited and I'm hoping one of you recognises the problem and can maybe point me in the right direction. Thanks a million!

Sietse
  Reply With Quote
Old 25th August 2018, 04:47 PM   #2
rayma is offline rayma  United States
diyAudio Member
 
Join Date: Apr 2011
Quote:
Originally Posted by leRooie View Post
The symptoms are: there's audio coming out, but very regularly, the signal drops.
When playing on low volumes, it's less often, but at higher volumes, the drop out
frequency increases and becomes a stutter.
Can you post the complete schematic? Sounds like either an instability, or a lack
of DC offset voltage (if it's a single supply circuit).
  Reply With Quote
Old 25th August 2018, 05:46 PM   #3
asuslover is offline asuslover  Romania
diyAudio Member
 
Join Date: Aug 2016
Location: USA
Troubleshooting LM1875
check for cold joints.....
  Reply With Quote
Old 25th August 2018, 06:26 PM   #4
FauxFrench is offline FauxFrench  France
diyAudio Member
 
Join Date: Dec 2017
Would you happen to have an oscilloscope? My experience with LM1875, apart from that it sounds very good when working correctly, is that it has a bad habit of self-oscillating (around 1MHz) above a certain output signal level. You won't hear the self-oscillation as such only the side effects. It can still provide an output signal but it tends to get very warm. Does your LM1875 become hot when it stutters?
  Reply With Quote
Old 25th August 2018, 06:46 PM   #5
leRooie is offline leRooie  Netherlands
diyAudio Member
 
Join Date: Dec 2015
Location: Haarlem
Quote:
Originally Posted by rayma View Post
Can you post the complete schematic? Sounds like either an instability, or a lack
of DC offset voltage (if it's a single supply circuit).
Sure, but I used the standard schematic for the boards, same values etc. The right channel works fine.

Click the image to open in full size.

Quote:
Originally Posted by FauxFrench View Post
Would you happen to have an oscilloscope? My experience with LM1875, apart from that it sounds very good when working correctly, is that it has a bad habit of self-oscillating (around 1MHz) above a certain output signal level. You won't hear the self-oscillation as such only the side effects. It can still provide an output signal but it tends to get very warm. Does your LM1875 become hot when it stutters?
Unfortunately, I don't. The left channel, which has the problem, does run significantly hotter than the right channel, which sounds fine.

Thanks for the reply's guys!
  Reply With Quote
Old 25th August 2018, 07:28 PM   #6
FauxFrench is offline FauxFrench  France
diyAudio Member
 
Join Date: Dec 2017
As it runs hotter than the other channel it is likely to be such high frequency oscillation. It is not something trivial to get rid of.

What are your supply voltages?
What is your speaker impedance?
On that faulty channel, do you have a DC voltage on the output when the LM1875 becomes hot? (you can measure with a voltmeter in DC setting).

Last edited by FauxFrench; 25th August 2018 at 07:30 PM.
  Reply With Quote
Old 26th August 2018, 07:00 AM   #7
CB89 is offline CB89  France
diyAudio Member
 
Join Date: Nov 2013
Location: Burgundy
Do you really have c3 = c4 = 100nF ?
  Reply With Quote
Old 26th August 2018, 03:05 PM   #8
leRooie is offline leRooie  Netherlands
diyAudio Member
 
Join Date: Dec 2015
Location: Haarlem
Quote:
Originally Posted by FauxFrench View Post
What are your supply voltages?
What is your speaker impedance?
On that faulty channel, do you have a DC voltage on the output when the LM1875 becomes hot? (you can measure with a voltmeter in DC setting).
Supply is 25V, speaker impedance is 4 ohm. I'm back in my studio tomorrow and will check the DC offset.

Quote:
Originally Posted by CB89 View Post
Do you really have c3 = c4 = 100nF ?
Yes!
  Reply With Quote
Old 26th August 2018, 07:11 PM   #9
FauxFrench is offline FauxFrench  France
diyAudio Member
 
Join Date: Dec 2017
For a start, replace the 100nF for C4 with at least 47uF.

The problems I observed are described here in posting #66: LM1875 Need help
I got a couple of very kind responses from someone more experienced than I. The responses were in short "don't use the LM1875 with more than 25V and if you use 4 ohm loads reduce the supply voltage".

The self-oscillation I observed was worse the more I increased the supply voltage and decreased the load impedance. I was careful with my power line decoupling but with 8 Ohm load and good 24V supply the self-oscillation would start. With a 4 Ohm load it would start around 18V.

When you are in your studio tomorrow, try to put a 10 Ohm power resistor in series with the 4 Ohm speaker and see if it still stutters. You can disconnect the speaker for the good channel meanwhile.

Last edited by FauxFrench; 26th August 2018 at 07:17 PM.
  Reply With Quote
Old 27th August 2018, 06:42 AM   #10
Bruce@bluff is offline Bruce@bluff  South Africa
diyAudio Member
 
Join Date: Sep 2005
R5 also seems a bit low, imho it should be closer to 10ohms
  Reply With Quote

Reply


Troubleshooting LM1875Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
LM1875 PCB, Which To Use Paswa Chip Amps 281 7th February 2018 08:06 AM
Help troubleshooting LM1875 kit TequiaKez Chip Amps 15 17th December 2016 09:46 AM
Lm1875 THD subarusti999 Chip Amps 11 13th September 2016 05:35 AM
LM1875...how low can you go? Gcollier Chip Amps 7 6th May 2010 03:43 AM
How hot does an LM1875 get? Dominique Chip Amps 15 23rd January 2007 02:13 AM


New To Site? Need Help?

All times are GMT. The time now is 01:02 PM.


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 15.00%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Copyright ©1999-2018 diyAudio
Wiki