DC output from amp with no input - diyAudio
Go Back   Home > Forums > Amplifiers > Solid State

Solid State Talk all about solid state amplification.

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 13th June 2004, 04:10 PM   #1
diyAudio Member
 
Join Date: Jun 2004
Location: Wellington, FL
Default DC output from amp with no input

I woke up one morning last week to the smell of burning voice coils in the woofers of my left channel speaker. The amplifier was on (I typically leave it on), but there was no program input into the amp. In fact, the system was quiet.

I immediately turned off the amp, but the woofers (2) in the left speaker were toast. I deduced that it was a DC signal applied to the speaker because both woofers were stuck in the same position, at maximum deflection. Also, the midrange and tweeter are just fine. I am assuming a voltage potential somehow developed on the bases of the output transistors (I believe they are bipolar).

The amp is an Adcom GFA-585. Has anyone seen this happen before? Does anyone know what the cause is?
  Reply With Quote
Old 13th June 2004, 04:54 PM   #2
diyAudio Member
 
jan.didden's Avatar
 
Join Date: May 2002
Location: Great City of Turnhout, Belgium
Blog Entries: 7
Hi,

There may be many reasons why the output suddenly (or slowly, we don't know that) goes to one supply line. It can vary from a shorted output device, via a shorted cap, an out-of-spec small-signal transistor or anything. What I want to say is that you really would need a schematic and at least a multimeter to chase this up. Undoubtedly there will be advise from people to change all electrolytics or to change all transistors. Although it is possible that it restores the amp, it may also lead to exactly the same result a week from now. There are no easy answers I'm afraid. Can you read a schematic?

Jan Didden
__________________
If you don't change your beliefs, your life will be like this forever. Is that good news? - W. S. Maugham
Check out Linear Audio!
  Reply With Quote
Old 13th June 2004, 06:04 PM   #3
diyAudio Member
 
Join Date: Jun 2004
Location: Wellington, FL
Thank you Jan!

I don't have a schematic for this amp. I'll try to get one from Adcom.

I was hoping that there might be one type of component failure that is "typically" responsible for the scenario I described so that I could focus my trouble shooting efforts.
  Reply With Quote
Old 13th June 2004, 06:43 PM   #4
sam9 is offline sam9  United States
diyAudio Member
 
sam9's Avatar
 
Join Date: Jun 2002
Location: Left Coast
Unfortunately almost any component failre that results in an open or short tends to dive many/most test points to the rail voltage. You have one big advantage over someone who encounters this when building an amp -- you know all component were correct and funtioning once. Rsistors almost never change value and are hard to damage- thus narrowing possabilities.

You have a second advantage - the other channels is fine. This gives you a ready reference for correct values. I suggest to start with the unit unplugges and check the resistance across the pins on the transistors on the good channel. Then on bad if they are not approximately the same, you have found a problem. This can be ambigous, however, because something else may be in parrallel to what you are measuring which mean the problem could be either. Also rember not to stop when you find the first bad component - you can't always (usually?) tell the victim from the perptrator!
  Reply With Quote
Old 13th June 2004, 06:46 PM   #5
diyAudio Member
 
jan.didden's Avatar
 
Join Date: May 2002
Location: Great City of Turnhout, Belgium
Blog Entries: 7
Good idea.
And beware of burned components - they surely are broken, but you may be looking at the effect rather than the cause!

Jan Didden
__________________
If you don't change your beliefs, your life will be like this forever. Is that good news? - W. S. Maugham
Check out Linear Audio!
  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
Input/output switcher Pyramideon Instruments and Amps 1 24th April 2009 11:33 PM
balanced input/output q's kanifee Chip Amps 6 14th September 2008 07:29 PM
New input/output Selector Russ White Chip Amps 96 3rd July 2006 06:23 PM
Need amp for DC input --> AC output chipco3434 Solid State 12 31st July 2005 09:51 PM
Zen V4 PCB input and output vsr123 Pass Labs 2 25th April 2003 04:22 AM


New To Site? Need Help?

All times are GMT. The time now is 04:17 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