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Old 14th April 2011, 03:04 AM   #1
ddietz is offline ddietz  United States
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Default CMOY pcb -my first pcb

Hi folks,

Thanks to all the terrific help I received in another thread, I have built a couple successful CMOY amps base don the tutorial at Tangents site. Now, I'd like to improve my original builds (or build improved versions) and make a few gifts. I'm using this as an excuse to try my hand at PCB design and etching.

I've attached below a couple screenshots of my board as drawn in Eagle (My first use of Eagle). I did not include part names or values as they are typical of all schematics and because I felt it would make the images to hard to read.

I'll try for a quick explanation of key parts, and if that is not enough I'll try something different. Basically, this will use one 9vdc rechargeable battery and is designed/sized to fit a Hammond 1593PBK (link). It uses a TLE2426CLP rail splitter. Recharge circuit is a LM317LZ and a 47R resistor (NOTE- I may have the pins drawn wrong, I need to check that) and a 1N5819 diode before it to ensure correct powersupply polarity. Zener after the battery in is a 12V/1W 1N4247ATR Vishay to limit input voltage to 12vdc.

Input caps are WIMA MKS2 1uF/63V. The PSU cap is a Panasonic 470uF/25V low ESR e.l. IN and output jacks are vertical Switchcrafts. Pot is a ALPS RK097. The 5 pads next to the pot are for LED and resistor. The red line to ground from each power pin on the IC is for ceramic or similar blocking caps ( could not find a library part that seemed to represent small ceramic caps). Red lines next to IC are jumper wires for potentially needed resistor or caps if IC oscillates. Resistors off of IC from bottom to top (in drawing) are 100K, 2K, 10K respectively. This sets input impedance of amp to 100K and gain of 6.

The other two red lines on the right hand drawing are jumper wires. One for Vcc- and the other for LED negative. I played with many iterations of this and could not do single sided w/o a jumper. I don't think I have the ability to DIY PCB at home using a two sided board. I plan to try the iron-on inkjet toner method.


I'd really like opinion about the traces!! Most are 0.031" (31mil) but the two input from potentometer are 0.024 to fit between the pins. I am anticipating having to trim back the ground pot bads after soldering the pot. Trace clearance everywhere else is at least 0.02". The distance between resistor bads next to IC are ~0.015". Is that enough or should I use smaller pads?

I will say, the screenshots make the pot pads look closer than they do when printed at 1:1 scale. Still, as I said, I am sure I will need to trim the two ground and two output pads back a bit where the clearance is close.

Thanks in advance and thanks to everyone who helped me get this far already.
Attached Images
File Type: jpg hammond1.1.JPG (51.6 KB, 572 views)
File Type: jpg hammond1.1_2.JPG (64.0 KB, 545 views)

Last edited by ddietz; 14th April 2011 at 03:07 AM.
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Old 14th April 2011, 05:16 AM   #2
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I found an old cmoy layout on my old drive, maybe it can give you some ideas.
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Old 14th April 2011, 04:35 PM   #3
ddietz is offline ddietz  United States
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Thanks Digits. It's always nice to see how other people do things.

I guess m two main questions are what is suitable minimum trace width and does my layout look like I would have problems due to interference, etc. Ive tried to lay it out allowing for many potential additions as necessry, such a extra resistors and blocking capacitors in the event of cranky opamps (I'd like to try a bunch of different ones), etc.

Thanks!
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Old 14th April 2011, 05:35 PM   #4
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If you are etching yoursef with say print and peel sheets, I won't go under .3mm traces. if you have boards made, its no issue and you can probably go down to .1 depending on the fab sepcs, generaly there is not much gained from too thin traces. As you can see, there is provision made for bypassing right at the chip pins for cranky amps (I like playing with the opa2227/8 series).You could make those input caps smaller and put the psu components left of the output jack, and just do two short wire jumpers to V+ and V-. Also those two resistors on the outputs can be low ohm resistors or just wire jumpers if your amps is silent enough.
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Old 14th April 2011, 11:21 PM   #5
ddietz is offline ddietz  United States
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Thanks again Digits. My smallest trace is about 0.6 mm, most are 0.8mm or larger.

I'd like to try the OPA2228, any particular tips for it?
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Old 15th April 2011, 04:04 AM   #6
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get the 2227 also, 8 is very tricky and prone to oscilate
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Old 15th April 2011, 04:42 AM   #7
ddietz is offline ddietz  United States
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I've heard really nice things about the 2228 but I ordered a 2227 also. I've tried to find AD825 based on a few reviews but can't find that one. I'll see how the 2227/8 suit me before trying others.

I've tried to leave space on my board design for both extra feedback resistors, output resistors and also parallel caps (for in- resistor) to cover all oscillation fixes I've read about. These can be easily jumpered or used as needed.

Is there a good place to get 2-4 boards cheap in the US? I'd like to make my own but if there is a cost effective solution elsewhere, I'd go that way instead as I don't have any etching abilities at this time and would likely try the toner method with RS etchant or muratic acid/peroxide/salt. I've got enough single sided, 1oz, 1/16" board to make 6 boards (or 6 attempts).
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Old 15th April 2011, 10:21 AM   #8
AndrewT is online now AndrewT  Scotland
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opax228 are not unity gain stable.
You must compensate them for low gain duty.
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regards Andrew T.
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Old 15th April 2011, 12:14 PM   #9
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If you can operate a lazer printer and a clothes iron you can make boards....
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Old 15th April 2011, 03:59 PM   #10
ddietz is offline ddietz  United States
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I will almost certainly try the toner method, unless I find some local hobbyist whole do it for me free

AndrewT, Thanks for chiming in here. The TI datasheet mentions stability issues at gains of 5 or lower. I have found gain 6 to be good for me and my applications using the 2132/2134 opamps, so maybe this will be similar. If not, TI recommends a parallel capacitor to the feedback resistor, which I have allowed space form on my board.
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