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Old 30th April 2012, 03:11 PM   #1
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Default AudioSector LM 3875 help

I have searched through all the threads and not found an answer. - must admit it is hard to filter through the thousands of posts - so please bear with me if this is repetitive.

I completed the AudioSector premimum LM3875 last week.

Dual mono with avel lindberg 22 volt transformers
No pot
r1 omitted
CHG on amp boards not connected
no leds (may add to front panel later)
12au7 preamp with dc offset approaching 0 v

The amp sounds very good. My concern is that the DC offset varies between ~80 mv and ~110 mv.

I have it connected to a pair of fisher XP 55B speakers that I got at a thrift store for $5

I am just a little leery of connection to my FE126En horns, don't suppose the dc would really harm them, but think it will affect the sound.

My question is how to reduce this offset.

Any help will be appreciated.

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Click the image to open in full size.

I plan to neaten up the mains wiring, forgot to order fuse holders.....and they are not available locally. - small town.

Thanks,
Ralph in Taos
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Old 30th April 2012, 05:04 PM   #2
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I just took a few minutes to verify the dc offset.
on the gainclone, with music playing at a low level, offset varies from about 100mv to around 200 mv

if I crank it up, the dc offset will approach 1 volt


dc output from the preamp is 0

what can I do?

Ralph
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Old 2nd May 2012, 07:09 PM   #3
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What is the offset on the amp without source/preamp connected?
I assume it's less than 100mV.

Now, with music playing and offset levels approaching 1V, it can only lead to conclusion that the offset is coming from a preamp. Check the offset on a preamp with music playing and volume cranked up. If the offset indeed comes from a preamp, install coupling cap in place of R1 (2.2uF or more)
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Old 2nd May 2012, 07:12 PM   #4
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Here some results for search on offset: diyAudio - Search Results
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Old 26th June 2012, 08:05 PM   #5
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Hello.

I have the LM4870 kit, and with no input or output I get -235mV and -220mV:
http://felisberto.net/1paZ
http://felisberto.net/NiqL

Transformer is 25V and I get 34.8 and 34.5 after the rectifier:

http://felisberto.net/9fQh
http://felisberto.net/YlLL

Is anything wrong with my assembly?

Last edited by humpback; 26th June 2012 at 08:09 PM.
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Old 27th June 2012, 02:09 PM   #6
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Whatever is connected to the input of the amp will affect the offset, so check the amp with input connected.
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Old 8th July 2012, 07:48 PM   #7
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Hello Peter. I connected and powered the pre-amp (HK 725) and followed the instructions on audiokarma: Amplifier Distortion, DC-Offset, and You! - AudioKarma.org Home Audio Stereo Discussion Forums

I now get -79mV.
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