Trying to fix an AH400 - diyAudio
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Old 17th May 2015, 06:22 PM   #1
benc is offline benc  United Kingdom
diyAudio Member
Join Date: Apr 2010
Location: UK
Default Trying to fix an AH400


I'm helping someone to fix a duff channel on an AudioHead AH400 stereo PA (1U/125W channel).

It has a topology I'm not familiar with (alot of stuff appears tied to the output, such as the +/-15v zener regulated PSU for the NE5532 opamp).

All the components seem OK, I've replaced the opamp and all the transistors have been tested OK.

I suspect that AudioHead didn't design this and it's a private labelled chinese unit. However I can't find schematics (???) - and don't really want to reverse engineer the unit.

Any help/pointers appreciated.
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Old 18th May 2015, 01:25 PM   #2
wg_ski is offline wg_ski  United States
diyAudio Member
Join Date: Oct 2007
Does the 5532 op amp appear to directly drive a set of drivers and outputs, with nothing in between? If so, what appears to be the "output" is really ground - which makes sense for the +/-15V. That topology is very common, among both cheap and some not so cheap power amps. It's as if somebody took the "output" and center tap of the power supply and simply switched them. They are a little harder to diagnose than "standard" topology amps, but don't suffer as many disastrous failure modes when something does go wrong. There are guys around here who have a lot of experience fixing them - what kind of symptoms does the patient exhibit?
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Old 19th May 2015, 10:47 PM   #3
benc is offline benc  United Kingdom
diyAudio Member
Join Date: Apr 2010
Location: UK
Thanks wg_ski.

Yes - you've described the amp topology.

The problem was that the faulty channel's 'FAULT' LED was lit, and the output (before the protection relay) was reading about -50V. There was very little differential voltage across the dual opamp supplies.

Although all the semi's measured OK, we fixed it yesterday by replacing the zeners that made up the +/-15V regulation for the dual opamp.

Many thanks,
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