How to keep the Party noise down?

So for my first build, I housed a mixer in there with it, and had to wrestle with all sorts of noise/clockbleed issues (which I resolved most of in the end), but this time I want to go all out.

I also read on the narrat1ve blog that this one is quieter than the last one, which is champ.
http://blog.narrat1ve.com/2010/09/20/wtpa2-proto-starting-to-pwn/


So a couple of things. For offboard buttons, just regular, non-shielded wire is groovy as there is no audio stuff running through. Offboard pots however, are carrying signal.

I read that the master volume control is particularly important.

Precautions to take.

Grounding all pots/enclosure?
Running shielded wire to pots? (which pin(s)?)

Moving the clock/speed knob offboard. I remember this one causing a ton of bleed problems with my first build. I ended up wrapping the wire in copper foil, and grounding the pot. Would running this as shielded be good?

What's the rule on grounding on both ends? Like if you're running an audio cable from one part of the circuit to another, you leave one side ungrounded to avoid a ground loop, but when connecting to I/O jacks, you connect ground on both sides as it's leaving the circuit? How about pots, if going shielded for a couple of the wires worth?


Next, for I/O. Running shielded wire for both. Do you ground both sides?


Ok, what are the particularly noisy parts of the board to avoid. It looks like most of the analog audio stuff is bottom left, and the noisiest stuff(?) (the speed/pitch knobs) to be avoided with analog signal.

What about the main MCU and SRAM? Are those noisy?
MIDI I/O?

Basically, what can't you run audio near (shielded or unshielded)?


I also plan on building in a second MCU (arduino), and LCD and a mixer. I plan on sticking the mixer circuit in a small enclosure (inside the bigger enclosure) and using shielded cable to/from everything going to it (though tricky with 3wire pots, can you use 3core+shielded?). With the LCD/Arduino. Are those noisy? Should they be moved over near the onboard MCU to keep it away from audio stuff?


Lastly, power. I know the WTPA has some nice regulation going on, but is there such a thing as too much regulation? For example, my mixer has none (other than a couple bypass caps Todd suggested) and the Arduino has it's own power regulation, but just a 7905 I think.

Like would adding this before all of the circuits (right at the power supply input) be of any benefit?

http://i198.photobucket.com/albums/aa78/xgoddess1/ciatpowersupply.jpg

It's from one of the ciat-lonbarde circuits and I have one built already (from years ago).

Basically I want to try to make this thing as quiet and tidy as possible!

(btw, this thread isn't meant to be me asking Todd a jillion questions, more as a people sharing tips/tricks on keeping shit quiet)

Comments

  • Rules with noise --
    The coupled noise in WTPA is typically electrostatic. WTPA has plenty of its own internal noise sources (on-die, which I've taken pains to minimize, but you can look into too).
    Look up electrostatic (voltage) noise and understand what it means:

    http://www.analog.com/static/imported-files/application_notes/41727248AN_347.pdf
    http://www.analog.com/library/analogdialogue/cd/vol16n3.pdf#page=16
    http://www.analog.com/static/imported-files/application_notes/6001142869552014948960492698455131755584673020828AN_345.pdf

    Know that distance matters. The farther something is from a noise source the better.
    Know that preventing capacitive coupling matters. You do this with distance or by introducing a shield (usually at ground potential) between the things you want to isolate.
    Impedance matters a LOT. A 500k node (like a 500k pot) will pick up noise much more easily than a 10k or 1k pot. Don't use high value resistors if you can help it (sometimes you can't).
    Fast digital signals are noisy.
    Some places in the circuit are way worse to end up with noise in. Anything with gain after it is very bad. The master control on WTPA(2) is not such a bad spot, since it is in a feedback loop of an amplifier with a gain of 0.5.
    Getting noise BEFORE the pre-amp in WTPA(2) is bad.

    Wall warts can be noisy, yes, it helps to get a linear wart (as opposed to a switching one). A pre-regulator can't hurt, although Peter's is a bit excessive and also awesomely old-skool.
    In particular, the diode across the 780x looks to be the wrong way around. I suppose he could be trying to limit regulator current if there's a large capacitive load on the ouptut, but I'd put the diode the other way around to protect the regulator in case the input side got grounded and the output side had a large cap.

    TB
  • its interesting, I got my WTPA fired up again (after breaking it and then being without a workshop for half a year.)

    I have ALL offboard pots & switches and jacks and really none of noise or clock bleed issues you mention. I am using a linear regulated power adaptor. I got a tiny bit of oscillator bleed but only at certain settings and I could not make the issue repeat. I'll keep an eye on these solutions though as I start to put this in the enclosure.

    the only issue I haven't been able to resolve yet is the LED behavior. Still no LED activity, although on the most recent plug-in, I got random LED behavior. Everything else works great so this may just be an "unlit" WTPA....depends on how comfortable I get memorizing the controls without their help!

    Strategy
  • @strategy
    For me the problem was pronounced as I had a mixer circuit right smack against the clock knob. It was bleed city particularly if I turned up the multiple channels.

    @Todd
    "Impedance matters a LOT. A 500k node (like a 500k pot) will pick up noise much more easily than a 10k or 1k pot. Don't use high value resistors if you can help it (sometimes you can't)."

    My mixer circuit has 1M resistors on the input. Should I go with a smaller value there? (I sometimes use piezo/guitar input)


    "Fast digital signals are noisy."

    So the Arduino should be away from stuff?


    "Some places in the circuit are way worse to end up with noise in. Anything with gain after it is very bad. The master control on WTPA(2) is not such a bad spot, since it is in a feedback loop of an amplifier with a gain of 0.5.
    Getting noise BEFORE the pre-amp in WTPA(2) is bad."

    Is this down in the bottom left of the WTPA?


    "Wall warts can be noisy, yes, it helps to get a linear wart (as opposed to a switching one). A pre-regulator can't hurt, although Peter's is a bit excessive and also awesomely old-skool.
    In particular, the diode across the 780x looks to be the wrong way around. I suppose he could be trying to limit regulator current if there's a large capacitive load on the ouptut, but I'd put the diode the other way around to protect the regulator in case the input side got grounded and the output side had a large cap."

    It's from the Fyrall circuit which I got back in um, 2004ish? That 2200uF cap is a big fucker too! So does "can't hurt" mean, help keep noise down down the line, or won't make things worse?
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