LM3886 problems - diyAudio
Go Back   Home > Forums > Amplifiers > Chip Amps

Chip Amps Amplifiers based on integrated circuits

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 18th August 2012, 10:32 PM   #1
Rnickl is online now Rnickl  United States
diyAudio Member
 
Join Date: Dec 2005
Location: San Jose, CA
Default LM3886 problems

I bought some of the BrianGT boards and stuffed 2 mono blocks. At first listen the highs were piercing and the lower end was bloated and overwhelming, very bad! After a bit of thought I figured I might have an impedance mismatch because I'm not using a volume pot so I put a 25k R on the input to ground.

Results in the high/mid frequencies were better but still not good, can't seem to get much "imaging" from them. An oddity, they drive my old Mirage 280i's (test speakers) with sloppy, bloated bass. When driving my Ariel's there are no lower frequencies to speak of, no measurements but I'm guessing the Ariel's are rolling off at 50-60Hz.

It's has been near 20 years since I've used my electronics training but, this problem is still making me feel stupid! I'm guessing I'm overlooking something obvious.

Quick specs:
BrianGT boards stuffed to specs.
Triad 48VCT 100VA per channel.
Temporary test chassis with no signal input filtering.
Tried several source components, CD, TT(VSPS preamp) and an MP3 player

Any ideas what might be going on here or how to troubleshoot with minimal equipment?
  Reply With Quote
Old 19th August 2012, 12:54 AM   #2
diyAudio Member
 
Join Date: Sep 2004
Location: Minneapolis, Minnesota, USA
Where did you buy the chips? There are problems with fake chips being sold on flea-bay and such. Also, a schematic of what you built would be helpful.

Mike
  Reply With Quote
Old 19th August 2012, 02:53 AM   #3
Rnickl is online now Rnickl  United States
diyAudio Member
 
Join Date: Dec 2005
Location: San Jose, CA
I hadn't considered the chips might be fake. I did buy through a US supplier but they are a discount/second hand type place.

Here's a link to the assembly manual for the amp. The schematic is on the 3rd page
http://chipamp.com/docs/lm3886-manual.pdf

Thanks for the reply,
Rick
  Reply With Quote
Old 19th August 2012, 05:14 AM   #4
BWRX is offline BWRX  United States
diyAudio Moderator Emeritus
 
Join Date: Jan 2005
Location: Pennsylvania
What kind of amp were you previously using with your speakers?
__________________
Brian
  Reply With Quote
Old 19th August 2012, 06:52 AM   #5
Rnickl is online now Rnickl  United States
diyAudio Member
 
Join Date: Dec 2005
Location: San Jose, CA
Quote:
Originally Posted by BWRX View Post
What kind of amp were you previously using with your speakers?
To put things in perspective, my $30 off the shelf Labtec PC amp (using LM1875's) sounds much better.

Rick
  Reply With Quote
Old 19th August 2012, 11:28 AM   #6
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
Quote:
Originally Posted by Rnickl View Post
................. with no signal input filtering...............
That's your first mistake.
Fit both a Low Pass Filter and a High Pass Filter to turn your amplifier into an Audio Signal Amplifier.

Once these passive filters are in place, you can alter component values to change the turn over frequency/ies of these filters (independently) to hear the effects and you may be able to come to some conclusions that tell you something about your room/ears/speaker interactions.

Have you fitted all the stability component options?
A badly behaving amplifier can/will sound horrible !!!!!!!!!!!!!

The p3 schematic shows 6 stability components, Cs has 4 caps, Rz & Cz on the output. There is R//L missing on the output. Are Cs in the correct location? In the wrong location they can be effectively useless.
As noted earlier the High Pass Filter will also give you a DC blocking cap on the input. That cap must be in the correct location, after the volume pot.
__________________
regards Andrew T.

Last edited by AndrewT; 19th August 2012 at 11:37 AM.
  Reply With Quote
Old 19th August 2012, 07:46 PM   #7
bcmbob is offline bcmbob  United States
diyAudio Member
 
Join Date: Dec 2009
Location: Bay City, Michigan
Rnickl,

Please take a look at this thread. The RR is a little gem.

http://www.diyaudio.com/forums/chip-...ml#post2924833
__________________
Bob M.
"Arrange Whatever Pieces Come Your Way."
  Reply With Quote
Old 19th August 2012, 09:07 PM   #8
Rnickl is online now Rnickl  United States
diyAudio Member
 
Join Date: Dec 2005
Location: San Jose, CA
Thanks for the input Andrew. Yes, all the stability components are in place but I did omit RL. Perhaps that is my problem here. How many watts does RL need to be? I'm assuming it needs to be fairly high power, is that correct?

"Fit both a Low Pass Filter and a High Pass Filter to turn your amplifier into an Audio Signal Amplifier."
I'm a bit rusty, OK way rusty, on this stuff. Can someone push me in the proper direction on how to calculate the values for this?


Thanks to you also Bob. I ran into that thread a few days ago and it might be something I play with down the road a bit. I need to get something that sounds reasonable first.

Rick
  Reply With Quote
Old 20th August 2012, 09:19 AM   #9
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
the R of the R//L passes significant signal only when the L impedance approaches as high as the R value. That will not happen in the Audio passband. R should never heat up unless you test with high frequency signal, say around 300kHz to 1MHz.
I use a parallel pair of 10r 600mW for an effective 5r, 1.2W

Similarly the R of the R+C should only get hot on HF test signals. I use a quad set of 10r for an effective 10r, 2.4W
Finally I add a R+C at the speaker terminals. This R is just a pr of 10r for a total of 5r. The amp output sees all these R values at HF as a single 10r load, when the speaker cables are not fitted.

The input filter can be
Low Pass RC: 1.5us to 0.3us
High Pass RC: 10ms to 200ms

RC time constant = R value times C value, i.e 1ko times 47uF = 47ms
F-3dB turn over (roll-off) frequency = 1 / Pi / 2 / RC
__________________
regards Andrew T.

Last edited by AndrewT; 20th August 2012 at 09:23 AM.
  Reply With Quote
Old 20th August 2012, 11:54 PM   #10
Rnickl is online now Rnickl  United States
diyAudio Member
 
Join Date: Dec 2005
Location: San Jose, CA
Thanks much for the input Andrew. I didn't have proper values but I did fit a couple caps that seem to have put me on the proper path. I'll need to pick up some components for fine tuning.

Rick
  Reply With Quote

Reply


Hide 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
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Problems with Chipamp (LM3886) BrianGT Puffin Chip Amps 46 25th June 2014 09:22 AM
LM3886 Chipamp problems... FenderBender11 Chip Amps 30 18th July 2012 03:09 AM
Chinese LM3886 Board having problems mattthegamer463 Chip Amps 20 13th November 2009 04:46 AM
LM3886 Problems ooops! Chip Amps 2 9th February 2009 08:09 AM
LM3886-problems joensd Solid State 5 24th June 2002 06:43 PM


New To Site? Need Help?

All times are GMT. The time now is 01:24 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2