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Noise/ground issue with midibox64


sidetrack

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Hi all,

So I'm building my second and third midiboxes here, I thought I had a full understanding on how to wire up the pots to avoid midi noise/jittering, but apparently I was wrong.

The way that I have hooked up all the grounds is that I have chained each pot's ground connection together then clamped it to the ground on one of the boards (not in a loop, its just one pot the next).  I have also clamped the unused outputs to ground, them being the last 4 on my second ainx4. I did this on a previous midibox64 and had no issues, mind you it only had one ainx4.  So I have been doing some trouble shooting trying to figure out what is wrong and I'm getting some weird things.  When i disconnect the first ainx4 and clamp its outputs to ground I get a stable second ainx4 without any noise (although I did get noise once, but it went away..found that strange).  So this leads me to believe there is something wrong with the first ainx4, but after switching it out with one that I know works, I get the same issue (so maybe it's safe to say the issue is not with the board itself).  Now after checking, rechecking all the solder points and ground connections I cannot for the life of me figure out what is wrong.  Additionally I had tested that board before hand (only a couple of the outputs, but it worked just fine).  Another odd thing that is occurring is that when i wiggle any of the ground connections between the pots I can see values being sent out like crazy (more so than just the normal noise), not sure if that is relevant. 

Anyways, can you guys please give me suggestions on how to fix this.  I need ways to test the grounds, the actual board itself etc.  Or maybe someone can just tell me the one thing that I need to hear to fix this :)

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The way that I have hooked up all the grounds is that I have chained each pot's ground connection together then clamped it to the ground on one of the boards (not in a loop, its just one pot the next).

Try star-like wiring.

Re: AIN ... entries talking ...

Re: The Dub Master MIDIbox 64 -work in progress-

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Looking at the diagram for star like wiring, I'm not really understanding what is happening here.  Or how it is different from what I'm currently doing.  It looks to me like the diagram tells you to chain all the Vd to one rail and all the Vs to one rail.  Currently, with my wiring, each 4051 output uses its own Vd ito power the 8 pots hooked up to it.  Then All the ground pins are chained together and directed to one Vs on one of the ainx4's. Maybe you can help me understand the diagram a bit more here.

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my problem must have something to do with the way that I'm hooking up the second ainx4, because I decided to continue building my second box the other day and got it up and running with no issues... using my previous approach to wiring the Vd and Vs to the pots.  Should the grounds on the two boards be kept separate or should they be combined?

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  • 2 months later...

OK.....I had to take a break for a few months as this was killing my brain.  I worked on it for so many hours straight to get it ready for shows I had, but in the end I couldn't fix the issue.

Now I'm back at it, with renewed interest.  So I have basically decided to start with one pot and continually add pots until I find the problems and can then fix them.  I believe I have a little more insight as to the symptoms of my problem.  Basically i got to about 5 pots in when the problem arose, noise issues when there should be none.  I checked and double checked everything... open connections are all sent to ground, every pot is correctly hooked up, solder points are solid... I even replaced many of the wires.  Now I noticed something by accident that seems to rectify the issues I have, when i pull on certain wires that are part of my ground loop between pots, the noise goes away completely.  That is if i just use my finger to pull the wire upwards and put stress on the two connections.  Can anyone explain to me what could cause this??? or if anyone has heard of this?  If I go through this simple circuit of 8 pots I notice that wiggling certain ground wires causes noise to happen, all the connections are well soldered... I just dont understand what this means.

Thanks for the help in advance.

R.

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i figured perhaps that might be a problem.  Speaking of which i noticed something strange happen with a couple of pots that I think might be problematic.  With the power on I turned the pot all the open and a bright light and puff of smoke came out, something definitely ignited when the flange turned all the way around.  That isnt normal as far as I know. I guess I could order some more pots, or go find some new ones. 

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I guess you mixed up something with the pot wiring - if wired correctly, a normal 10k pot can't ever get hot or even explode. But if you accidentally connected Vcc or GND to the wiper (the middle terminal), you can short the PSU by turning the pot to one end. And a little bit before reaching that shorting position, you've got only a few Ohms of resistance between the supply levels resulting in a lot of power flowing through the thin resistive layer inside the pot. That would be a reasonable cause why the pots went POOF :)

S

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That's the conclusion I came to - you've mixed up the wiper and fixed connections, so that as you turn the pot, the full voltage is applied to a lower and lower resistance, eventually you exceed the current rating of the track and it burns out like a fuse. If you  have a test meter, check that you have the right variable voltage coming from the wiper before hitching up to the AIN. Are using slider pots? These are much easier to make mistakes with, as there are many different wiring conventions for them.

Don't feel too back - just about everyone building an analogue synth has done this at least once! My favourite trick being to fit the pots one those 1mm plus systems, and mix up the crimped on connectors in the body. My usual one is the backwards pot, though occasionally I manage the full swap.

By the way. No-one seems to fit any small damping caps from wiper to ground on these things, which would certainly help cut the noise effects down. I need to look at this, because I've acquired a large batch of beautiful cermet mini pots with built in knobs, nearly free, that are just crying out to be made into a MIDIbox64. Trouble is that they are all 100K, so I need to look at the noise/jitter problem first.

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