LM1875 PCB, Which To Use


2007-08-10 9:24 am
LM1875, you know, I know, it's a great IC. I'm building a 5 channel gainclone with 5 LM1875. There are many PCBs. But what to use?

PCB #1


PCB #2


PCB #3


PCB #4


Now tell me, which is the best PCB among these PCBs. You can add more PCBs to the list.
The 1875 schematic shows inverting topology with 100k to 200k on the inverting input and zero impedance on the non-inverting input. This will give an output offset. If you add 100k to the non-inverting input connection, to reduce the offset, it may be too noisy.
The input impedance is only 10k and that 1uF gives a high pass filter of just 10mS. Again a poor component choice.
Finally, why do a rail splitter when it's far easier to use single rail supply and capacitor couple the speaker?
Yes, LM1875 is a great chip, but the biggest implementation problem is that there are almost NO boards available. Lots of people make their own boards or hardwire the amp.

One of the few I could find for sale is from Quality Kits, and, even at just $13 each, it's hardly worth it. The components included in the kit are the very cheapest available, the PCB layout is not a good design, and the board itself is not very high quality. The amp built from the kit sounds awful. I have been experimenting with this kit, trying to turn it into something better, and I'm very close now. I threw away almost all the supplied parts and carved up the PCB.

It sure would be a lot easier if someone made a board that had a good design to begin with instead of having to buy and mangle a board to get a decent layout.

ESP sells boards, but I'm not crazy about his layout, either.

The nicest layout of a board I've seen yet is the one Nordic posted a long time ago in another LM1875 thread. I don't know if it's available for sale. Nordic, you never answered my e-mail to you, asking about that board. I would pay good money to get my hands on a couple of them.

Tom E
Haha I have such a horribly short memory (result of a car accident), I have forgotten about that board again....

Here is a later version, the one I posted was just to illustrate the concept...
If size is an issue we can always leave out the fuses and make them offboard, it will also reduce the cost of PCB significantly...


P.S. please forgive the silkscreen, I see it is not correct yet...

Tom, My appologies, I must have missed your e-mail... (I do searchengine submissions part time), which brings me a deluge of spam every day...


2007-08-10 9:24 am
nordic, your PCB is very very good. Can you please share the silkscreen and the bottom copper layer's high resolution image? That I (or anyone who wanna build 1875 chipampi baby) can directly make PCB. I'm just building my DIY audio amp and ain't any commercial use.
Please nordic, please gimme the silkscreen and the bottom copper layer's high resolution image.

I made a PCB of 1875 toooo...see it


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2007-08-10 9:24 am
thanks nordic, thank you very much ;) , please try to do it a little quickly,,I'm just can't wait anymore to build the 1875 chipamp and to enjoy my favourite musics. I know you'll do for me.

And, what about my PCB? is there any mistake? that you found?


2005-01-17 5:29 am
Paswa said:
There are many PCBs. But what to use?

What are your soldering skills like? Do you plan on making your own boards or having a board house make them? The answers to those questions will narrow down your choices...

Paswa said:
I made a PCB of 1875 toooo...see it

So why not just use your own design and see how it works? That will probably sound better to your ears than any other design ;)
Absolutely Brian, I think your SMD boards are realy cute


What do you guys think about this layout then?

W=60.6225mm x H=51.744mm (2.4"x2.1")

I am tempted to add the PSU to the boards, it makes life so much easier, less wires, less drilling etc.. and a cleaner look when installed, only requires a transformer ad the 2 boards in the case.
What would this one:
look like without the fuses?

I like it best because it has significant room between power and signal components, and because it can connect a wide variety of input filter cap sizes.
See, on the picture, if a poly input cap is wanted, then the input terminal can be omitted. That option makes room a variety of sizes, so it would be nice to have that option.

Speaking of SMD, howabout make the feedback resistor SMD? The back two pins of LM1875 are extra long, and I think that an SMD resistor just fits there. My only problem with going entirely SMD is (|00...0>^6 + |11...1>^6)*2, which will have stolen some of them.
Perhaps I would do this instead.

Center pin of LM1875 is -v

And, I'd move that 1m slightly farther from the power caps.

Terminal blocks can have the wires come loose, so I'd just omit it.

EDIT: Oops, however, I wouldn't increase the distance on the NFB resistor's leads like that picture. Sorry about that.

EDIT2: Um, those cap values.
Okay, the 22uF at NFB would be a 100v model, and then the 2.2uF at Input, should, instead, be 1uF polypropylene, like a Wima 100v MKP, which is rectangular.
IF, however, the input filter cap is supposed to be electrolytic, it would be 4.7uF (or 3.3) and the NFB cap would be 47uF.


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