What I do? Simple LM3886, DC on output - 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 25th June 2012, 04:04 AM   #1
diyAudio Member
 
Join Date: Oct 2009
Location: Anchorage, AK
Default What I do? Simple LM3886, DC on output

A few months back, I built a successful LM3886-based amp. I copied the design almost verbatim to new (fab'd) PCBs, and assembled eight of them. Now I'm finding negative DC on the output. I tested two with identical results.

Ugh... I know I must be missing something simple, so would you mind taking a look?

Attached is the reference schematic -- based on ESP Project 19 -- and the PCB design.

On the schematic, the mute circuitry (circled in red) has been moved off-board. There will be five of these modules in each speaker cabinet (4 woofers, 1 tweeter bank) so the PSU, input buffer, crossover, and mute controller will be on a separate PCB.

I'm using a high-power wall-wart with individual +DC, -DC, Ground, and +5v (not used) for testing. This is plugged into a Variac for now. I tested with about +7/-7 on the PSU rails. Probing the pins of the chip showed this (DC volts), re: ground:

V-: -7v
V+: +6.6v
Out: -4.5v
In-: -4.5v
In+: 0v
Mute: -2.2v

No input (but there is a resistor to ground on the PCB). No load. The two test subjects are mounted to a temporary heatsink. No thermal paste, but I did use one of those uber-thin thermal pads. It's bolted to the sink, with no attempt to isolate the V- supply.

Any clues? Again, the older PCB on which this is based works perfectly.
Attached Images
File Type: png mini-lm3886-sch.png (34.0 KB, 248 views)
File Type: png mini-lm3886-pcb.png (10.1 KB, 246 views)
  Reply With Quote
Old 25th June 2012, 07:05 AM   #2
diyAudio Member
 
aparatusonitus's Avatar
 
Join Date: Jan 2004
Location: Split, Croatia
Your PSU voltage are to low for proper operation of LM3886...read the datasheet
__________________
Non mai abbastanza...
  Reply With Quote
Old 25th June 2012, 09:11 AM   #3
diyAudio Member
 
Join Date: Oct 2009
Location: Anchorage, AK
Yes, I know, designed for 20v minimum, and under 12v (which I've exceeded) the undervoltage protection should kick in and mute the output. Nowhere in the datasheet does it say that nearly full-rail negative DC is the protection applied, and I began testing at the supply's minimum output at ~+/-3v and worked up from there.

I will try full +/-15v tomorrow, but I suspect this particular sign of trouble is not the result of underpowering. If I'm wrong, I worry about the safety of a DC-coupled load. :-/ Power supply issues could cause some real trouble if this is expected behavior.

For everyone's sanity, the final PSU is a 250va toroid, 36vct. The wall wart is JUST for initial smoke testing.
  Reply With Quote
Old 25th June 2012, 09:33 AM   #4
diyAudio Member
 
Join Date: Feb 2012
Check if you R-G 22k is properly grounded. If your R-G isnt grounded you will have DC - 2-3v on the output...
  Reply With Quote
Old 25th June 2012, 10:23 AM   #5
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
throw away the wall wort.
Build a mains bulb tester (BT). Use your BT to check the wiring of your mains transformer. Then check the wiring of the PSU via the BT. Now add on the amplifier and again check via the BT.

When you have completed ALL changes and additions and modifications, only then do you power up direct off the mains.

If the bulb lights up you know you have to "find the fault" before you can proceed.
__________________
regards Andrew T.
  Reply With Quote
Old 25th June 2012, 06:36 PM   #6
diyAudio Member
 
Join Date: Oct 2009
Location: Anchorage, AK
Quote:
Originally Posted by McDaisy View Post
Check if you R-G 22k is properly grounded. If your R-G isnt grounded you will have DC - 2-3v on the output...
Will do.

Quote:
Originally Posted by AndrewT View Post
throw away the wall wort.
Build a mains bulb tester (BT). Use your BT to check the wiring of your mains transformer. Then check the wiring of the PSU via the BT. Now add on the amplifier and again check via the BT.
Isn't that more or less the same as testing through a Variac? The bulb tester is effectively a current limiter, right? That's kind of what I'm achieving through limiting the voltage supply, I thought. If I'm missing something, please clarify. I appreciate the advice, just trying to understand why the bulb approach is inherently better/safer.

FWIW, the full-fledged power supply is known-good -- complete with fuse(!), transformer, rectifier, and 5x2200uF per rail. I only favor the wall wart PSU at this stage because it has handy banana plug leads that mate well with alligator clips for quick connections while building and testing. I have the full Molex plug setup on the PCB, so the trafo PSU is perfectly doable, just not as convenient. Also kludgy without a case.

You guys think this is potentially a PSU issue? Admittedly, I never got to low-voltage test the older (working) design because it has a transistor-controlled relay to connect the rails. At low voltages, the transistor wasn't saturated and wouldn't close the relay, so there was no possibility of powering up at less than about +/-12vdc. Maybe I'm just seeing this behavior because it's only just now possible to underpower it. Still, I don't like the implications of DC on the output while ramping up to (or down from) full power.

Thanks for the replies -- I'll test further and get back to you.
  Reply With Quote
Old 4th July 2012, 08:04 PM   #7
diyAudio Member
 
Join Date: Oct 2009
Location: Anchorage, AK
You guys were absolutely right. Once I brought the voltage up higher, the offset went to 0.00v. I thought for sure the under-voltage protection would prevent DC offset on the output at less than minimum supply. Does that ever cause any problems, or is there no current drive under those conditions? Anyone know? I'd hate to lose a driver if the supply ever drops unexpectedly.
  Reply With Quote
Old 5th July 2012, 07:06 AM   #8
diyAudio Member
 
Join Date: Feb 2011
some simple relay setup can detach the speakers if any of the supply rails fall under a certain point.
use your imagination
  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
lm3886 - dc on output Wizardmaxx Chip Amps 4 24th October 2009 06:54 AM
DC on speaker output - LM3886 Stuey Chip Amps 7 2nd October 2006 08:09 AM
My Lm3886, Dead or not? DC at OUTPUT! mathieujohnson Chip Amps 8 29th June 2006 09:19 PM
lm3886-dc on the output xplod1236 Chip Amps 70 29th January 2004 08:48 AM
Simple amp with DC on the output... SkinnyBoy Chip Amps 87 4th November 2003 01:07 PM


New To Site? Need Help?

All times are GMT. The time now is 02:57 PM.


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