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13th September 2010, 01:43 PM
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#1 |
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diyAudio Member
Join Date: Jun 2010
Location: Italy
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Problem:the amplifier oscillate with the preamplifier only on subwoofer applications
Hi!..First excuse my English, i'm Italian..I have a problem with my homemade car power amplifier. I have made this amplifier:7100-FT15M.pdf , i supply it with this SMPS p89-f9.gif and protect the speaker whit this speaker protectionp1000232.jpg. I connect this power amplifier into the car but without the preamp volume was too low. So I did a preamplifier like this AMPOP.GIF without other capacitors.
Then i riconnected the amplifier with the preamplifier on the subwoofer RCA out of my car radio but at low volume the subwoofer works and when i increase it the subwoofer oscillate. With an mp3 connected to the preamplifier the amplifier work very well but with car sub out no. Are there any problems in coupling between amplifier and preamp? Thanks leo |
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13th September 2010, 08:58 PM
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#2 |
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diyAudio Member
Join Date: Apr 2008
Location: Carlisle, England
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Your diagrams are very blurred so its hard to see what is going on.
Have you got a coupling capacitor between preamd and amp ? You need to block DC.
__________________
http://www.murtonpikesystems.co.uk PCBCAD50 pcb design software. |
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14th September 2010, 02:14 PM
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#3 |
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diyAudio Member
Join Date: Jun 2010
Location: Italy
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I'm sorry you're right. I am attaching a PDF with the pattern of all the configurations I tested. If necessary I can show the video of the problem. Thank you!
problem.pdf |
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14th September 2010, 06:49 PM
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#4 |
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diyAudio Member
Join Date: Jun 2010
Location: Italy
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Update!When I supply the amplifier with an external source (battery or another power supply) works very well but when I connect it at the battery of the car the subwoofer oscillate. Then i think that is the SMPS the problem. I think there is a problem with ground loops, right?..The ground of my smps is the same for the 12 and +/-50V. may be going into conflict with the ground of the RCA and create problems, although I have a differential amplifier input?...Using another type of SMPS with a opsoisolator in the feedback to isolate the ground could be a solution? Thanks!
My SMPS 
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14th September 2010, 07:34 PM
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#5 |
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Did it Himself
diyAudio Member
Join Date: Nov 2003
Location: Gloucestershire, England, UK
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From what you have posted so far you don't have a differential (balanced) input. Try the unit with no source connected and see if the problem is still there.
__________________
www.readresearch.co.uk my website for UK diy audio people - designs, PCBs, kits and more |
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15th September 2010, 03:29 PM
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#6 |
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diyAudio Member
Join Date: Jun 2010
Location: Italy
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I just did more testing. Then whith no source connected it don't oscillate. If I connect the sub out to the gain variable preamplifier (formed by a differential amplifier and a noninverting amplifier with gain variable) with a gain approximately equal to 1 does not oscillate and the music is heard but a very low volume. While only increasing the gain begins to oscillate. It works only with an external source (not with the car battery) or with the car battery but with a car speaker outputs or an MP3....
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15th September 2010, 10:15 PM
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#8 |
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diyAudio Member
Join Date: Jun 2010
Location: Italy
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Sure! This is the complete schematic (I tried to put together on a single sheet) is a little messed up I hope you understand. The ground is the same for 12V and +/-Vcc (it is not opsoisolate)
schematic.pdf many thanks to all!! 
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