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Old 17th January 2016, 12:47 PM   #1
kcray is offline kcray  Indonesia
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Default My First LM1875 amp

Hi,
After lurking around in this forum for years, I'm tempted to build my own LM1875 amp.
The amp would be powered by 180VA 18-18V CT Transformer.

Here the schematic and the layout.
I adopted ESP and Daniel design, and AndrewT suggestions on others thread.

Suggestions and comments are welcome. before I etch the PCB.

Many thanks,

-- Peter
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File Type: png MyLM1875T-brd.png (18.7 KB, 383 views)
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Old 17th January 2016, 12:50 PM   #2
adason is offline adason  United States
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Just do it!
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Old 19th January 2016, 06:07 AM   #3
kcray is offline kcray  Indonesia
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so, everything ok then?
How about parts arrangement? and are all the parts' values already optimum?

thanks
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Old 19th January 2016, 07:06 AM   #4
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If you read what AndrewT says, you would have included all of the so-called "optional" components, I hope. As he says, they aren't optional.

Trace, re-trace and re-trace again. Is the any other way to get it right?
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Old 19th January 2016, 07:09 AM   #5
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What are you using for source? With my mobile phone, higher impedance loading on the output sounds better so now I aim for 200kohm minimum for my input impedance. You're currently at about 47k which you could increase.
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Old 19th January 2016, 03:49 PM   #6
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One issue I have is your connecting the small signal ground at the point where the output Zobel connects to ground and this goes through a trace to the main ground area where the bypass capacitors are. At high frequencies, heavy enough current can flow in the Zobel and cause distortion or oscillation of the amplifier. The input ground should be moved to have its own path direct to the main ground area.

Where is the output return pin?
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Old 19th January 2016, 04:19 PM   #7
kcray is offline kcray  Indonesia
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Quote:
Originally Posted by abraxalito View Post
What are you using for source? With my mobile phone, higher impedance loading on the output sounds better so now I aim for 200kohm minimum for my input impedance. You're currently at about 47k which you could increase.
Nice tips. It mostly will be connected to PC or laptop.
Does 200KOhm min. input impedance also apply to the PC or laptop source?

Johnr66:
Thank you for your analysis, but there is R7 to separate input ground and main ground, but if you think it's not enough, I will try to separate the ground path.

The output return will be directly connected to the Power supply star ground, Is that OK?

Regards,

-- Peter

Last edited by kcray; 19th January 2016 at 04:25 PM. Reason: adding info
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Old 19th January 2016, 06:26 PM   #8
johnr66 is offline johnr66  United States
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Quote:
Originally Posted by kcray View Post
Nice tips. It mostly will be connected to PC or laptop.
Does 200KOhm min. input impedance also apply to the PC or laptop source?

Johnr66:
Thank you for your analysis, but there is R7 to separate input ground and main ground, but if you think it's not enough, I will try to separate the ground path.

The output return will be directly connected to the Power supply star ground, Is that OK?

Regards,

-- Peter
A 1 ohm resistor will do very little to keep induced voltage in the ground out of the high impedance of the input.

OK on the output return.

Keep the input bias resistor as you have it or even better, at the datasheet value. Any proper line in signal will have no issues and less chance of noise if the input source goes high impedance such as its power is turned off or cable unplugged.
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Old 19th January 2016, 07:06 PM   #9
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Quote:
Originally Posted by kcray View Post
Hi,
After lurking around in this forum for years, I'm tempted to build my own LM1875 amp.
The amp would be powered by 180VA 18-18V CT Transformer.

Here the schematic and the layout.
I adopted ESP and Daniel design, and AndrewT suggestions on others thread.

Suggestions and comments are welcome. before I etch the PCB.

Many thanks,

-- Peter
I see a few things that I would do differently, but it's time for lunch and I'm hungry. In the mean time I recommend you take a look at this - Taming the LM3886 Chip Amplifier
Tom put a lot of good information about circuit layout and stability in there, and while it's based on the LM3886, the LM1875 is from the same chip lineage so everything still applies.

Mike
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Old 20th January 2016, 12:34 AM   #10
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Quote:
Originally Posted by kcray View Post
It mostly will be connected to PC or laptop.
Does 200KOhm min. input impedance also apply to the PC or laptop source?
No direct experience myself but another DIYer reported to me that his laptop sounded better after he'd done a mod I suggested which raised the input impedance of his amp.

Higher impedance inputs do have a downside and that's increased pickup of rubbish when the source isn't connected. Don't leave an input cable dangling with nothing at the far end with the amp powered and feeding a speaker, its asking for trouble when the amp input impedance is high.
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