LM1875 PCB Layout

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
Hi Guys,

I'm currently building a 5 channel chipamp and have finally come up with a PCB design (took me a while to learn how to use Eagle).

I've attached the schematic and board for your perusal. Could you guys please go through my PCB layout and give me any hints and tips on ways to improve it.

Keep in mind that I'm planning on eventually having 5 of these PCB's side by side so the design is intended to be as small as possible long the width.

I've based my schematic on carlosfm's snubber design and Nationals datasheet schematic and will be using his design also for the power supply.

Amy I missing anything as I have seen in a few schematics for chipamps that they have around 2.2k ohm resistor inline with the positive input to the chip. What is this for and do I need it?

Thanks.
 

Attachments

  • lm1875 schematic.gif
    lm1875 schematic.gif
    9.4 KB · Views: 1,597
mangrovejack said:


Amy I missing anything as I have seen in a few schematics for chipamps that they have around 2.2k ohm resistor inline with the positive input to the chip. What is this for and do I need it?

Thanks.

this is a "safety" resistor used in series with positive input. it is used to reduce dc offset, or protect against some dc incoming mostly on power up/down. with a cap at the input i think you could get away without that resistor.
 
Tekko said:
Output2 should be going to ground and Output1 remains on the opa output.

I'm not sure what you mean by that?

Maybe I should have explained why I have two "outputs". It was because one wire pcb terminal wouldn't be stable enough (only one leg to solder), so I thought I'd just use a double one ant leave one output empty.

<Edit>Ah, I understand what you mean now. I'm getting the output ground from the star ground at the caps, and not on the PCB to reduce the possibility of hum.</edit>
 

BrianGT

Retired diyAudio Moderator
2002-01-21 2:42 pm
near Atlanta, GA
www.chipamp.com
Hello,

I made a LM1875 pcb layout back in April, and had prototypes made. I attached a pdf of the design. I have 5 prototype boards, if anyone wants one to try out. I also have the parts to put together a few kits. I haven't had time to test it out yet.

I will sell the pcbs for my cost of the prototypes, which was about $12 per pcb. Each pcb contains 4 amplifier boards and 2 power supply boards. I can take pictures of the boards when I get home if anyone is interested. They have no soldermask or silkscreen, as they are just simple prototype boards that I ordered from Advanced Circuits, where I get my other boards made.

As for the schematic, I followed the reference schematic from the service manual for the amp board. For the power supply, I put 47uF caps || 0.1uF caps on the amp board, and 1500uF || 0.1uF + 1 ohm on the power supply board to allow for the snubberized supply.

Let me know if there is any interest. I attached a pdf of my layout, to see if it will help you out at all.

--
Brian
 

Attachments

  • lm1875-pcb.pdf
    66.4 KB · Views: 761
BrianGT,

You ought to make your own boards! I used to use the MG Chemicals exposure method with bad, expensive luck. Now I use the Gootee method with nothing but luck! The best part is if you mess it up its 15 minutes of your time + 1 sheet of paper, unless you mess up the etching but that never happens. Last night I forgot about it for like 35 minutes and the boards are still fine!

Pete
 
I've etched and built one channel on the weekend for testing purposes and assuming everything is fine, then I'll be creating the proper 5 channel version. I used the press'n'peel blue paper method and ammonium persulphate to etch it.

I'll post some real pics once I get some photos.

Inspired by tobias_svensk in his OPA627 pre amp thread, here's a first attempt at using eagle 3d.

Note there are missing parts as I haven't fully sorted out the issues with POVray yet.
 

Attachments

  • 3dview.jpg
    3dview.jpg
    69.4 KB · Views: 902
:)

Yup, there's always like 50% that's the wrong size or package when you render the file from the beginning.

You'll have to manualy change things in your .pov file, when you have loaded it in your PovRay you see some text (or a lot of text to be right) in the first "page" of your .pov you can change like: angles/cameras/resistor color (i use blue ="2")/enviroment (that sea-background or not)

If you scroll down to the very last "page" of text you have all of your packages, here you can rotate them (they are often rotated 180 degrees wrong) and you can change text on your packages (like if you want to write OPA627BP instead of OPA627B for an example). But most imporatant of all, you can see the name of the loaded package so if your resistor/transisor/cap etc is somewhat wrong in it's size or footprint distance you can edit it in their dedicated *.inc file.

So if you want to edit a resistor open up resistor.inc and look up the exact name and there you'll have some numbers to edit.

Edit:
when you generate your .pov file in Eagle and you don't have the two boxes called something like "unkowned packages" it probably just work fine but you'll have some things missing, like your terminals on your picture above. If you check those boxes you can choose from a list wich ones to use.
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.