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  #21  
Old 10-29-2010, 03:02 AM
domct203 domct203 is offline
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I can't wait to hear how it sounds. The only real concern I have is the wah inductor picking up the "digital noise" RFI of the MPA. As a precaution I used shielded cables for input, output and pot wiring. I guess I could try a metal divider to shield the audio PCB's if there is any problem. If the digital pot is a source of un-tolerable noise my backup plan is to use a LDR as the wah pot and control the LDR's LED voltage with the MPA. I have a couple of Silonex NSL-32 & NSL-32SR2 Optocouplers ready to go! The regulated 5V would be a great voltage source for the LED!

We'll see what happens when I hook it up to my Mesa Boogies Wednesday night. The high gain channels will bring out any noise for sure.

If all goes well I'll make a few quick audio clips Wednesday as well.

Dom

Last edited by domct203; 10-29-2010 at 03:23 AM.
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  #22  
Old 10-29-2010, 01:16 PM
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Not sure about noise, but you may hear some sort of artifact from the "steps" in the digital pots...I've found this sort of thing to be an interesting effect, rather than a drawback. Interested to see what you think.
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  #23  
Old 11-04-2010, 01:05 PM
domct203 domct203 is offline
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John,
The MPA is extremely quiet. No perceivable "zipper noise" from the digital pot. Great job with the board design.

The only real issue I have is the range and taper of the pot. It seems the sweetspot for the wah is right at the end of the pots range, and it comes on quickly.

I noticed the pot did not go from exactly 0-ohm to exactly 100K-ohm. Is there any way to tweak that?

Dom
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  #24  
Old 11-04-2010, 09:32 PM
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Dom,

Glad to hear that the sound is solid.

Unfortunately, the digital pots have a very loose tolerance for actual value vs nominal value. You will see this in the MPA hardware manual and also in the digital pot datasheet (Microchip MCP42xxx).

My best suggestion is to use some fixed resistors in series with the A/B terminals to "bias" the output in one direction or another.

You could also give two pots an identical configuration and wire them together to expand the resistance range...
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  #25  
Old 11-27-2010, 12:15 AM
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Dom,

A blog post for this project will appear on Nov. 27 at this location:

http://blog.highlyliquid.com/2010/11/27/diy-midi-wah/

If you have a website or another link that I can include in the post, please let me know.

Thanks again for posting all of the detailed photos.
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  #26  
Old 11-27-2010, 12:30 PM
domct203 domct203 is offline
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John,
Thanks for posting the project. I ended up changing a capacitor in the wah to tweak the wah's frequency range and she sounds great. I may still try some small resistors in series with or parallel across the digi-pots to see if that helps to tweak the wah's pedal position.

Once I get some sound clips I am going to make a web page for the project and will post the link here.

Thanks again for your help,
Dom
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  #27  
Old 12-01-2010, 03:15 AM
bryanjbenting bryanjbenting is offline
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Cool project! Nice documentation as well.
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