From: mweissX294@earthlink.net   
      
   I built this system 32 years ago, and have regularly maintained and upgraded   
   it over that span of time. The System consists of multiple amplifiers in a   
   rack fed with 240vac, split 120vac for the low power equipment, which   
   included preamps and electronic crossover networks.   
      
   I recently started using a spare Hafler 500 amp (one of two spares) for   
   Surround Sound amplification, driven directly off the outputs of my DVD   
   player's discreet 5.1 channel outputs. Speakers are floating and the amp   
   outputs are not connected to any AC ground other than the speakers   
   themselves. It has worked fine since last summer.   
      
   System was working normally the night of 12/31. Problem occurred 20 minutes   
   into a movie, shortly before midnight.   
      
   Problem manifested as a loud popping noise, dominant in right surround   
   channel, but heard in both surround channels, at about 6 cps-it sounded like   
   a square wave. This blew the B+ fuse and the N-channel MOSFETs in the left   
   channel of the surround sound amp, a Hafler 500, even though the popping   
   issued from the right channel with greater amplitude.   
      
   After repair, amp works perfectly on bench. No oscillation, perfectly   
   stable.   
      
   Reinstalled in rack, oscillation starts when left input is connected-even   
   with connected equipment turned off. If the DVD player surround output is   
   connected to the left input on the amp, oscillation occurs. However, if the   
   power to the entire video equipment is disconnected, the DVD player can be   
   connected to the amp without adverse effects. This suggests some sort of   
   loop between AC power line and amp audio in/out.   
      
   Amplifier inputs are at same ground potential and both connected to amp   
   chassis.   
      
   When left channel is connected and popping occurs, a large voltage   
   difference between chassis and rack appears. This was discovered because   
   arcing was observed between chassis and rack when amp was loosely sitting in   
   rack. This was repeated several times to verify that it was not imagination.   
      
   When amp switched on and off (before speaker relay has a chance to close),   
   4-5 seconds later, a loud popping sound is heard in the right and possibly   
   left speakers, and chassis voltage difference appears. (Note, speakers are   
   theoretically NOT connected to amp as the relays are supposed to be open.)   
      
   Problem persists no matter which branch circuit amp is connected to.   
      
   Speakers are not connected to building ground; they are floating, as their   
   only connection is the amp's output terminals. The only persistent   
   connection to the speakers is the chassis ground (the relay only disconnects   
   the 'hot' side). If one side is disconnected and the ground side is moved to   
   a different potential, how can substantial current flow in the voice coil?   
      
   Even stranger is that when a battery-powered WAV player is connected to the   
   amp inputs, the amp plays fine. If the portable player is connected to the   
   left input and the DVD or the mixing board is connected to the right input,   
   the portable player's signal can be heard in BOTH outputs.   
      
   The suspect amplifier was replaced with an identical Hafler 500, known to be   
   working, and it too exhibits the same motorboating problem when some   
   AC-powered signal source is connected to its LEFT channel input. Connecting   
   to the RIGHT channel input, it behaves normally.   
      
   The offending motorboating noise appears at the INPUT jack of the amplifier   
   WHEN THE SPEAKER RELAY CONNECTS THE SPEAKERS. The output of the amplifier is   
   a replica of the input signal, only larger.   
      
   If a tone signal larger than 100mV is applied to the amp input from the   
   mixing board, the motoboating stops-however the sine signal becomes an   
   overshooting (rising edge) square wave with a rounded trailing edge.   
      
   Thought: it seems as if somehow the AC power circuits are acting as a   
   positive feedback loop for the audio circuit. There seems to be common mode   
   conduction going on, but cannot figure out why the common mode path appeared   
   without any user intervention, during a movie. No one touched the racks when   
   the oscillation started for the first time.   
      
   Neither can I explain how two independent amplifier channels behave like   
   they are connected together as mono when one input is connected to the   
   mixing board or the DVD player, while the other is connected to the portable   
   player. The Hafler 500 employs no means of switching to mono with one or two   
   inputs connected. The spare Hafler 500 behaves the same way, so I'm inclined   
   to believe the problem lies elsewhere.   
      
   When I run the amp in the shop, it works normally. When I feed either of the   
   suspect audio sources to the amp WHILE IN THE SHOP it works normally. The   
   common denominator seems to be that the amp misbehaves only when in the   
   rack. Even when electrically isolated from the rack rails with rubber and   
   cardboard strips.   
      
      
      
   I have been working on this problem all night and all day today, but am no   
   closer to a solution. I even checked the main breaker panel and tightened   
   all the ground screws. No other electrical problems in the house. The rest   
   of the sound system works normally. It is just the one amplifier used for   
   the surround that suddenly doesn't like external sources connected to its   
   LEFT channel input.   
      
      
      
   None of this makes sense, but I tell you that I could not make this up if I   
   thought about it all night! It's just too incredible. All the standard   
   troubleshooting methods were used, but the common aspect is anything   
   connected to AC power, WHEN THE AMP IS IN THE RACK. It's happy on the shop   
   bench with any source. But not when sitting in the rack, even isolated from   
   the rack, or even with extention cords bringing AC power from another branch   
   circuit. The proximity to the rack seems to be a factor.   
      
      
      
   In fifty years of working in electronics, I've not run into anything like   
   this in the field of broadcast engineering. But here it is in my own home   
   studio and all the classic remedies are ineffective.   
      
      
      
   Anyone got any ideas on this mystery?   
      
      
   --   
      
      
   Take care,   
      
      
      
   Mark & Mary Ann Weiss   
      
      
      
   VIDEO PRODUCTION . FILM SCANNING . DVD MASTERING . AUDIO RESTORATION   
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   www.adventuresinanimemusic.com   
   -   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)