Go Back   Home > Forums > General Interest > Car Audio

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 12th November 2008, 11:44 PM   #1
diyAudio Member
 
Join Date: Sep 2008
Send a message via AIM to adelphia83
Default Audiobahn A16000V failed outputs

I had a mile long post a while back during a repair of an Audiobahn A16000V.

On my way home from work the other day, two outputs shorted (one PNP, one NPN) failed under only moderate load, only 5-10 minutes into operation, so it was cool to the touch.

It doesn't appear to have taken anything with it, other than a single emitter resistor which has has darkened from heat. This is what happened originally which caused me to repair the amp the first time.

Anything obvious that would cause this? I just can't see why it would fail under these conditions. I'm certain the speaker terminals did not short or anything of the sort.

Outputs are FJA4210 and FJA4310, which are not originals. I had no problems at all up until this point.
  Reply With Quote
Old 13th November 2008, 08:36 AM   #2
diyAudio Member
 
Join Date: Nov 2003
Location: Louisiana
It could have been a random failure but they're relatively rare with new components.

If you drove the amp really hard the last time you used it, one of the two transistors may have been damaged and failed at the time when the amp quit. I've seen a lot of amps that were driven hard and the next time they were powered up, the outputs failed.

If you have parts left from the last time you ordered, replace the defective outputs and emitter resistor. After you reassemble it, with a non-conductive probe, push on various points on the board (while monitoring idle current) to see if the idle current changes.
  Reply With Quote
Old 18th November 2008, 01:44 AM   #3
diyAudio Member
 
Join Date: Sep 2008
Send a message via AIM to adelphia83
Perry,

The amp is now on it's third life. I was real patient with the repair this time, and sourced all necessary parts ahead of time. I already had the outputs, but ordered a dozen emitter resistors, base resistors, and I replaced all rail caps with matching ones.

The emitter resistors I received were giant 0.1 ohm 10W monsters that looked like miniature cinder blocks. Replaced w/ these anyways. I guess they'll do the job?

Replaced all base(?) resistors (2.2ohm), which I only replaced the bad ones on the last go-around.

Two outputs were replaced that were shorted.

Amp fired up no problem, output is fine. I measured bias current while prodding all around the board, and measured 0.000-0.001v at idle. After playing the amp hard for 30+ minutes, the idle current was 0.001-0.002v. Pushing on the board in various places made no difference.

I guess it's okay for another round?
  Reply With Quote
Old 18th November 2008, 01:57 AM   #4
diyAudio Member
 
Join Date: Nov 2003
Location: Louisiana
It appears to be OK.

The larger resistors will work as long as they are supported well. The added mass will stress the solder connections when the amp is subjected to vibration.

Do you have a meter with a min/max function? If so, drive the amp until it's near thermal. Remove signal and place the meter on one of the emitter resistors. Set the meter to min/max and allow the amp to cool down. The voltage shouldn't change more than a few millivolts as the amp cools. I've seen a few amps that had a spike in the bias current as the temperature changed. I doubt that you're having this problem but it would be something that you could easily check.
  Reply With Quote
Old 18th November 2008, 11:28 PM   #5
diyAudio Member
 
Join Date: Sep 2008
Send a message via AIM to adelphia83
Add another output FET to the list of fatalities.

This time it was a single one, I probably wouldn't have noticed the problem if I hadn't had my headlights on-- These things must draw some serious current when they short.

I'm beginning to think the bias current is creeping up as the amp heats up. The shorted outputs only seem to occur after working the amp hard. I think perhaps the bias current is too high, and they're heating up too fast to dissipate the heat.

I'm going to drop the resistance of the bias circuit 200 ohms and see if that helps.
  Reply With Quote
Old 19th November 2008, 03:59 AM   #6
diyAudio Member
 
Join Date: Nov 2003
Location: Louisiana
Connect your meter across an emitter resistor and make a note of the voltage. Drive the amp really hard for 10-15 seconds then immediately reduce the volume to 0 (hit the pause button if your HU has one). At precisely the same time you set the volume to 0, check the voltage across the emitter resistor. This needs to be done VERY quickly. Does the voltage remain below 0.005v?

You should do this while the amp is cold and again when it's hot. Have the cover in place so everything (including the fan moving air over the board) is as it would be when the amp is operating in your vehicle.

The outputs are not FETs. They're BJTs.
  Reply With Quote
Old 14th November 2009, 10:04 PM   #7
diyAudio Member
 
Join Date: Sep 2008
Send a message via AIM to adelphia83
The amplifier made it a whole year driving an RE Audio SX12 in a ported box. Loud as hell and even caused one side of the box to split, therefore I'm building a new one. But today when I turned my truck on, I smelled the all-too-familiar voice coil burning, which tells me another output transistor has shorted. Luckily the power side of the amp is still okay. I suppose I'll replace the shorted outputs and hope the amp makes it another year!
  Reply With Quote
Old 15th November 2009, 02:08 PM   #8
Eva is offline Eva  Spain
diyAudio Member
 
Eva's Avatar
 
Join Date: Oct 2003
Location: Near the sea
Send a message via MSN to Eva
Be careful when replacing emitter resistors because their inductance matters. Some amplifiers that come with low inductance emitter resistors will oscillate and fail randomly when the bigger wirewounds are used. That kind of oscillation is hard to trace because it may become stronger or disappear depending on temperature, supply voltage and signal level (like working at the threshold of instability).

When I have to replace emitter resistors or transistors by something quite different I never feel comfortable until I have checked that the amplifier is stable with oscilloscope, from cold to hot and from 0V to clipping.
__________________
I use to feel like the small child in The Emperor's New Clothes tale
  Reply With Quote
Old 15th November 2009, 02:41 PM   #9
diyAudio Member
 
Join Date: Sep 2008
Send a message via AIM to adelphia83
Thanks for the tips. While taking the amp apart, I noticed that one of the rail caps had vented. There's four caps, two on each side and only one vented. I'll have to get a replacement before I can put the amp back together and test it.

Can a failed cap cause the output(s) to short?
  Reply With Quote
Old 15th November 2009, 03:33 PM   #10
Eva is offline Eva  Spain
diyAudio Member
 
Eva's Avatar
 
Join Date: Oct 2003
Location: Near the sea
Send a message via MSN to Eva
Think about it also in another way: Something could have caused the cap to vent and the outputs to fail, maybe excess voltage or temperature, or just excess ripple current through the cap.

On the other hand, when a cap shorts internally, very high current flows until something stops it. If the current stops because something inside the capacitor blows and opens the circuit, energy stored in parasitic inductances (like PCB tracks and capacitor connections) can produce a voltage spike on supply rails strong enough to blow output transistors. I have experienced that in one amplifier that I'm developing where supply bypass SMD capacitors from a defective batch were exploding sometimes and taking the pair of adjacent output MOSFET with them (this is 4KW class D fed from 170V DC, though).
__________________
I use to feel like the small child in The Emperor's New Clothes tale
  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
Audiobahn A16000V amp in Protection adelphia83 Car Audio 114 25th October 2008 09:10 PM
My 2+1 project; FAILED! Dxvideo Chip Amps 37 3rd March 2008 05:15 AM
another failed gainclone theChris Chip Amps 23 20th July 2004 06:23 PM


New To Site? Need Help?

All times are GMT. The time now is 08:22 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