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Old 26th July 2008, 08:54 PM   #201
mattjk is offline mattjk  United States
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Quote:
Originally posted by Peter Daniel


What offset do you measure on each channel? Does the offset change when adjusting volume?

Hi Peter. I have a 25k nobel pot.

Left channel is -5.0mv with volume all the way down
-32.8mv volume all the way up

Right channel is +9.8mv all the way down
-20.9 all the way up

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Old 26th July 2008, 08:59 PM   #202
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Your readings are not reversed, the offset is just lower. I used 50K pot you are using 25K that one reason it's lower. The offset is almost always lower with volume down.

Also, those values are quite typical and that the amp reacts for volume change may indicate that both chips work fine. It's just connection of the signal is somehow lost, check also power supply connetion.

BTW, check this post for offset measured for a batch of chips: http://www.diyaudio.com/forums/showt...77#post1524877
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Old 26th July 2008, 09:07 PM   #203
mattjk is offline mattjk  United States
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Quote:
Originally posted by Peter Daniel
Your readings are not reversed, the offset is just lower. I used 50K pot you are using 25K that one reason it's lower. The offset is almost always lower with volume down.

Also, those values are quite typical and that the amp reacts for volume change may indicate that both chips work fine. It's just connection of the signal is somehow lost, check also power supply connetion.

BTW, check this post for offset measured for a batch of chips: http://www.diyaudio.com/forums/showt...77#post1524877

Thanks for your reply.

That's what I figured after I posted, so I edited it out to not look stupid. haha.

I will check everything again, thanks!
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Old 26th July 2008, 10:01 PM   #204
mattjk is offline mattjk  United States
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found the problem!

seems I put too much heat on the right channel #2 pin on the nobel pot, and melted it inside.

edit: I was able to fix it by mashing some solder wick on the plate, but I'm getting 1.8 ohms of resistance.

I guess I need a new pot. What would you recommend? 50k with a shunt? I have a spare 100k noble, can I use that?
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Old 27th July 2008, 01:48 PM   #205
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100k will work, but it may not be the best choice.

I would go with either 25k or 50k.
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“Do something really well. See how much time it takes. It might be a product, a work of art, who knows? Then give it away cheaply, just because you feel that it should not cost so much, even if it took a lot of time and expensive materials to make it.” - JC
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Old 1st August 2008, 04:39 PM   #206
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Just finished making a LM1875 amp following Peter's instructions.
DC offset is about 10mV each channel, no hum and is sounding pretty good.. Great enjoyment .Just need to find a nice case.
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File Type: jpg lm1875new.jpg (98.8 KB, 3871 views)
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Old 2nd August 2008, 01:18 AM   #207
mattjk is offline mattjk  United States
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Sure took a while for my chip amp to break-in. It's amazing how much the sound has opened up. No longer rolled off at all. Leaving the unit on for a day improves the sound also.
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Old 8th August 2008, 02:38 AM   #208
Pocoyo is offline Pocoyo  Indonesia
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Default Master Piece

Superb Tutorial Peter !
A lot Of new things found here

This is out of my expectation
All you give to forum is your commercial amp
Wonderful !

Maybe you will invention a new Class A Amp ?
Who was take the picture when you soldering ?

A great respect from your friend

regards, jeffry
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Old 8th August 2008, 02:46 AM   #209
Pocoyo is offline Pocoyo  Indonesia
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Default Dear Casiomax

Quote:
Originally posted by casiomax
Torroidal transformer are not available in my country, have search but come to no avail.

A standard transformer with center-tap would be the next choice of mine, how much amp does it need to supply just a mono amp? i wanted to build for my guitar amp. would a 5A with 22v-0v-22v transformer adequate?
You can find standart transformer in Glodok Harco
But hear that hum in transformer

For Better option, order from our neighbour country
http://singapore.rs-online.com/web/s...5v+sec&x=0&y=0

email me if you need more information
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Old 12th August 2008, 10:01 PM   #210
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Peter,

I’ve had a quick search, and can’t find an answer to my query. I have built an Audiosector LM3875 kit in a dual mono configuration with a separate passive pre amp. I have used 2 identical 120VA, 18-0-18v torroidal transformers. The amp is used to drive 8ohm speakers.

Due to space limitations (new furniture), I have no room for the pre. I now need to turn the amp into a stereo integrated amp. To save me buying another transformer, could I get away with either using 1x 120VA transformer (low power I know, but I don’t listen loud anyway), or could I safely parallel the 2 transformers to give more power? What are the grounding options?
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