Moved+changed preamp and now got hum and dc-offset whith chipampkits - diyAudio
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Old 3rd August 2006, 08:14 PM   #1
MiWi is offline MiWi  Germany
diyAudio Member
Join Date: Jul 2004
Location: Dortmund
Question Moved+changed preamp and now got hum and dc-offset whith chipampkits

some weeks ago I built myself a 2way active hornsystem (2 fl Horns fullrange+2 basssections with active x-over) with chipsamkits from peter Daniel (stereo lm3875 for the horns and 2 monoblocks each with one bridged lm4780). Everythings was running quite fine.
Last week I moved to a new flat and things got worse. The same time I made some changes to the system (passive pre changed to another active one and voltage splitter in the stereo lm 3875 to set down the volume of the horns). So itīs impossible to get back to the starting point, that is why I wonīt bore you with the whole story and only try to describe the state itīs now and do one step after the other with your help to get back to working system.
I got a active preamp with 4 outs (2 left/2right). One pair is connected to the lm3875 directly and the other pair got a active crossover and then the lm4780 mono for each channel.
All chipamps were running without the ground (chg) connected (had hum before with chg connected).
I set up the system in the new flat and got a light hum (cd-player and turntable connected to the pre). When disconnecting the cd-player, hum gets louder, to loud to ignore. In the old flat I had absolutely no hum.
So I decided to check the amps. First I disconnected everything and just connected one amp alone to the preamp and a cheap loudspeaker. Hum, that was clear. Next step was to connect safetyearth to chg in all amps. No hum with the cheap loudspeaker for each amp connected alone. So I measured dc offset in this setup.
Preamp disconnected: about 150mA
Preamp connected to amp, but not turned on: 25mA for the LM4780 (should be quite o.k.) and 40-50mA for the LM3875 (to much)
Preamp turned on: 50mA for the LM4780 and 60mA for the LM3875 ?!!?
Is this normal ?
When connecting the amps to the crossovers an the rest of the system, I still got hum, but this will be the next step. First off all I got to reduce the dc-offset. Any suggestions how ?
Thanks for your help.
Best regards, Michael

P.S. I read some hum and offset threads but i canīt the fault in the setup pf the amps. Everything now is connected as described on peters website.
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Old 4th August 2006, 04:40 PM   #2
MiWi is offline MiWi  Germany
diyAudio Member
Join Date: Jul 2004
Location: Dortmund
Found time to search the web and this forum longer an found out, that i shouldnīt worry about 25-60mv DC Offset. I guess one part comes from my preamp, cause dc offset rises with turning preamp on.

Another question is, are there probably general problems in puttin a stereo lm3875 an two further 4780 monoamps behind one preamp ?
I read about someone having problems with 4 channels of gc behind one pre.

Best regards, Michael
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