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Old 6th October 2003, 10:51 PM   #1
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Default PCB Layout check

Well I took a PCB I found posted on this board and attempted to modify it to better suit my components. Can somebody please take a look at this layout and see if I have missed anything. I build the original circuit and the PCB came out fair, hey it was my first time, but it was just too cramped. The Riken resistors are pretty big.


Thanks in advance.

Eric

BTW the my first PCB gainclone worked and sounded good, but after a few minutes would start to distort at low volume, turn it up and it sounded decent again.

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Old 7th October 2003, 02:26 AM   #2
BrianGT is offline BrianGT  United States
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Default Re: PCB Layout check

Quote:
Originally posted by supereri
Well I took a PCB I found posted on this board and attempted to modify it to better suit my components. Can somebody please take a look at this layout and see if I have missed anything.
With a quick look, the routing looks correct for the inverting gainclone (thorsten) schematic.

As for the routing, I would make the traces thinner under the chipamp, as it looks like you have clearance issues. A better option might be to move the .22 ohm resistor, crossing over the V+ trace, and run the output down, instead of running under the chip. This would avoid the clearance issues and still allow you to use your current trace thickness.

What program did you use to make the layout? ExpressPCB?

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Brian
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Old 7th October 2003, 02:33 AM   #3
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Default gainclone

Yes PCBExpress. Thanks for the input on the routing.
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Old 7th October 2003, 09:08 AM   #4
elib is offline elib  China
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Hi,
How about this one? any comments welcomed.
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File Type: jpg 3875pcb1.jpg (56.3 KB, 982 views)
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Old 7th October 2003, 05:47 PM   #5
antomas is offline antomas  Europe
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Hi Supereri,

being the author of the original version of this pcb I felt somewhat responsible for your troubles... on the other side, I couldn't find the time to build the circuit, yet, so please consider my suggestions just as an anthology of other people's posts and not first-hand experience. Keep also in mind that I'm not an EE. Hope it helps.

A question: did you measure the quiescent current drawn by the amp immediately after power on and when the problem arises? According to the datasheet, normal values should be between 30 e 70 ma. A larger current draw could be a sign of oscillation.

If this is the case, you could do various things.

1. implement the clever suggestions from Brian;
2. streamline the traces from the chip to the decoupling 1uF capacitor, or at least remove the weird "elbow";
3. considering that you're not using a power supply connector (the big square box), you might make components closer, shorten quite a bit the traces and make a smaller pcb;
4. you could replace the 1uF electrolytic capacitor (see #2) with the Siemens stacked cap recommended by Saint Thorsten. Don't know where to find it in Arizona, but Newark stocks a similar cap "manufatured" from Farnell (yes, www.farnell.com).
You might check if this is in fact a Siemens cap with the Newark people before buying it.
This is the real thing: www.rswww.com (search for product code 191-3452)
This is the "mysteryous cap": http://www.newark.com/NewarkWebComme...jsp?id=25H9503
5. According to Joe Rasmussen, the resistor values you choosed could help the amp oscillating when the pot is in the intermediate position. If you look at my thread, there is an Excel file listing some combination of values that should hopefully not "help" the amp to oscillate.
6. Don't trust too much my suggestions. There is something against nature in giving advice about an amp not yet built...

Ciao,
Massimo
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Old 7th October 2003, 05:51 PM   #6
antomas is offline antomas  Europe
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Default One more thing...

...you used a big heatsink for your LM3875, isn't it?
Massimo
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Old 7th October 2003, 07:37 PM   #7
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Antomas,

First of all thanks a bunch for posting the pcb files, they provided a great place for me to start. I really liked your grounding layout.

Second I have modified the circuit as sugested by Brian as I think he suggested.

Also I had forgotten the 22k resistor to Signal ground, which I have added below.

Quote:
A question: did you measure the quiescent current drawn by the amp immediately after power on and when the problem arises?
I am not sure how to do this. I am a complete newcommer and I am trying to learn by doing.

All in all before distortion set in I was quite impressed by the sound. I had heard reports of lack of bass, but I was pleasantly surprissed by the amount.

Thanks for all of the great replies!!!

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Old 7th October 2003, 10:30 PM   #8
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supereri

Ideally the 0V connection between your main psu caps should not pass through the star-point since there are large charging current pulses on this track.

It is best to connect the capacitors directly together and then join to the star-point via a short stub.
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Old 7th October 2003, 10:37 PM   #9
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Also, your input ground and feedback ground should be at the same potential so rather than run each to the star-point seperately it is better to join these together and then run a single track to the start-point.
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Old 7th October 2003, 11:53 PM   #10
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Is this what you had in mind Richard C?

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