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5th March 2005, 08:08 PM
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#1 |
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diyAudio Member
Join Date: Dec 2002
Location: Calgary, Alberta
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Balanced -> oscillation?
I just finished building my LM3875 GC. It is built similarly to the following schematic, except using BrianGT boards:
http://www.diyaudio.com/forums/showt...907#post263907 It works just fine (GREAT!) with an SE source (grounding the -), heatsinks never feeling hot. Yet if I just turn it on with input disconnected completely, it will just heat up on it own until I have to shut it down. It is not a short, this is all using a smallish fuse which never blows. Is this just runaway oscillation? I did not think that open loop gain would be such a problem with just 100K/10K resistors. No, I cannot test it with a balanced input because my preamp isn't finished.
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Aerodynamics are for people who can't build engines. Enzo Ferrari |
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5th March 2005, 08:43 PM
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#2 |
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diyAudio Moderator
Join Date: Apr 2002
Location: Chatham, England
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This is very common with GCs, I would guess that approx. 1/3 of the ones I have built oscillate with no input.
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Al I conceive of nothing, in religion, science or philosophy, that is more than the proper thing to wear, for a while. Charles Fort |
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5th March 2005, 08:48 PM
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#3 |
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diyAudio Member
Join Date: Dec 2002
Location: Calgary, Alberta
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Thanks pinkmouse!
I see upon further searching that the design I chose to build has been heartily slammed. I will look around some more and plug in the desoldering iron 
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Aerodynamics are for people who can't build engines. Enzo Ferrari |
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5th March 2005, 11:53 PM
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#4 | |
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diyAudio Member
Join Date: Jan 2004
Location: Montreal
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Quote:
If i want to implement a mute setting on my source select for my gainclone, what should i connect to it to avoid oscillation? i was planning on just leaving it open...
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Most people wouldn't know good music if it came up and bit them in the ***. - Frank Zappa |
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6th March 2005, 12:24 AM
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#5 | ||
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Banned
Join Date: Mar 2003
Location: Lisbon, Portugal
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Re: Balanced -> oscillation?
Quote:
 Quote:
 You need a resistor from each input to ground. 100k will do. |
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6th March 2005, 12:36 AM
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#6 | |
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diyAudio Member
Join Date: Dec 2002
Location: Calgary, Alberta
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Re: Re: Balanced -> oscillation?
Quote:
http://www.diyaudio.com/forums/showt...017#post262017
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Aerodynamics are for people who can't build engines. Enzo Ferrari |
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6th March 2005, 12:57 AM
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#7 | |
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Banned
Join Date: Mar 2003
Location: Lisbon, Portugal
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Re: Re: Re: Balanced -> oscillation?
Quote:
You will your speakers if you forget to connect a source.
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6th March 2005, 06:16 AM
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#8 | |
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diyAudio Moderator
Join Date: Apr 2002
Location: Chatham, England
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Quote:
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Al I conceive of nothing, in religion, science or philosophy, that is more than the proper thing to wear, for a while. Charles Fort |
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6th March 2005, 07:35 AM
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#9 |
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diyAudio Member
Join Date: Dec 2001
Location: SIUE, Illinois, USA
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well, that circuit listed on the site does look like it will have issues with ossilations.
not sure what effect adding a 100k resistor from inverting to ground would have on the balencing or if it even matters at this point. on a side note thank you. i think i found out what may have happened to my last 4780 amp!
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