Group Actions
Engagement
  • Author Author: fmilburn
  • Date Created: Date Created
  • Views 2623 views
  • Likes 8 likes
  • Comments 10 comments
Related
Recommended

PCB Art:  Screaming LM386

fmilburn

10 Feb 2020 Update:  Schematic and Bill of Materials updated

 

In a previous post a method for transferring art work to PCBs was described.  I've been holding off sending it in until a circuit could be designed to go on the board.  In this post a completed board is presented and the issues encountered described.

image

Circuit

The circuit is yet another LM386 audio amplifier with a few twists.  Some of the usual modifications have been made:

  • Additional decoupling for the power supply
  • Addition of bass boost (fixed resistor)
  • Addition of gain control
  • Bypass capacitor on pin 7
  • Large output capacitor

image

For a bit of a change it will be made with SMD parts on a custom PCB (with artwork :-).

 

PCB Design and Layout

 

There are some issues to address when using artwork with a working circuit on the PCB.  The artwork itself is a footrpint and the Design Rule Check (DRC) may not catch errors when when vias or or through hole parts penetrate into the footprint.  Among the ways to address this are:

 

  1. Make a one layer board with the art on the back and don't penetrate the PCB
  2. Penetrate outside the PCB footprint - the PCB must be larger in area than the art
  3. Carefully penetrate only grounded areas for grounds in the artwork (may not look good)
  4. Carefully penetrate only bare areas for wires in the artwork (may not look good)
  5. Mask out of the art design for circuitry (may not look good but could be a feature)

 

Approach 1 only works if there are no through holes or vias.  Approach 2 means the artwork will be reduced in size.  Approaches 3 through 5 modify the artwork appearance and may or may not look good.

 

My approach turned out to be a combination of 1 and 5.  Approach 2 would enlarge the board and increase cost or reduce an already small art design further.  Approach 3 and 4 seemed to have risks for the first attempt at this and were discarded.  The circuit itself is all on one layer but because screw terminals and mounting holes were desired for this design it was necessary to have penetrations.  Accordingly, the art itselt was modified by masking out around all the holes in Photoshop.  The new images were imported into KiCad and converted into a footprint as described in the previous blog.  The footprint was then placed on the back of the PCB and the circuit drawn on the front layer.

{gallery} Artwork with penetrations masked out

image

Penetration Mask

REPLACE THIS TEXT WITH YOUR IMAGEimage

Copper Layer

image

Mask Layer

image

Silk Layer

image

PCB Layout

 

Renders of the design, front and back, are shown below.  It looks like a Screaming LM386 doesn't it?

{gallery} Renders of the Screaming LM386

image

Front

image

Back

 

In my opinion it does not degrade the design too much and with the screw terminals and mounting holes it is more functional.

 

Bill of Materials

image

 

Summary

 

The design was transferred over to what is hopefully a working PCB.  I think it still looks good although the artwork is not visible from the top.  I will know it is there though :-).  The breadboarding step was skipped and the parts aren't  in hand so there is some risk there.  I plan to sit on it a day or two and then come back and think through things again.  I may even change from a LM386 to a more modern chip with a Class D amplifier but it wouldn't be a Screaming LM386 then would it?

 

Comments, suggestions, and especially corrections are always welcome.

 

Other Posts in this Series

Screaming LM386:  An Audio Amplifier with PCB Art

PCB Art:  The Scream

 

  • in reply to fmilburn

    Hi Frank, this is an awesome project!

     

    Thanks for sharing the PCB on OSH Park.   If you email drew@oshpark.com, then I would be happy to send you a coupon code for future orders.

  • in reply to kmikemoo

    Hi Mike,

     

    Thanks!   If you breadboard it I would be interested to hear how it come out. 

  • in reply to shabaz

      wrote


    I like the 'what were they thinking' crazily-high gain

    One of the things I plan to do with this little board is test the performance as I have not done that sort of thing with an audio amplifier.  My going in thought was to dial gain down until there was little distortion and measure but after reflection on your comment that won’t be much fun.   That will just show low distortion.  I should crank it up and see how much distortion I can get :-)

  • I went ahead and made the order to OSHPark this evening but made two small changes:

     

    • Decided I was being too timid and went ahead and connected the back copper to the ground plane with two small traces.  I should be able to cut the traces if it screws things up but I think it will be OK
    • Determined that I had the volume pot hooked up in reverse and changed that

     

    This is what the renders from OSHPark look like:

    image

  • in reply to luislabmo

    Thanks Luis,

     

    You are correct, it is straight forward to incorporate it into empty space this way although tedious in places.  It should be possible to make it part of the circuit as well.  Since the copper area on the backside of this board is so large it would make sense to use that as part of the ground plane.  However, I want to make sure I am handling all the layers properly first because as mentioned in the write-up the DRC does not work when the layers have been abused this way.  Hopefully everything works as expected...