DIY Mic Pre Kits, Lunchbox Mic Preamp Kits, Power Supply Kits

so I finally finished building a rack case for my SC-1s...it's all steel and aluminum, and unfinished except for the face, which is really a Raxxess 2-space cover panel that's powdercoated steel

Of course I want to paint it - both for looks and to keep the steel from rusting - but I had been wondering about grounding/noise stuff, and particularly whether painting the steel (therefore presumably affecting it's conductivity) would have any negative consequences....so...

i slapped the case together and installed the guts...WOW...AM radio.
somehow i've turned the thing into a big antenna.

on the good side, both channels sound otherwise as they did in the plastic testbox, and both appear to have consistent and reliable control functionality - more or less what i expected.

i found that removing the steel top cover of the rack case substantially improves reception...yikes. alternately, placing the cover back on and holding onto it with both hands reduces the noise and radio by quite a bit.


also, i bent the little grounding tab over and soldered it to the shield pin on all the XLR's. was I supposed to do that?

From what i understand, its usually a good idea to grind the paint off of your chassis grounding point so you have a good, reliable connection to ground.



Did you use the grounding scheme suggested by owel in one of his stickies around here? Did you twist signal wires together?

yeah...uhhh...i needs me a slap upside the face

(yes, my grounding is up to spec, and my signal wires are twisted)

after a few entertaining hours trying to figure this out, i realised the mic cable i had been using was not correctly wired..(the shield was pin 2!)

so that changed everything......still getting some AC hum, but that's at ridiculous gain settings. may have to spin the Xformer a little or whatever, but at least i'm getting closer. i'd love to make it as quiet as it was in the plastic box.

today i took the guts back out and dismantled the entire thing, and now i'm almost done cleaning the edges up and brushing it. i think if i can't find some sort of conductive paint (i'm sure it exists), then I'll just "wire" the steel panels to one another after painting.

onward

yes yes, indeed...right now the whole thing is unpainted bare steel and alum., so the point of the assembly and test was to determine the results of a situation where all of the case components are freely conductive.

assuming that any normal non-conductive paint will affect the free-conductivity of the bare metal, i can imagine the RF and AC noise rejection will suffer, so some sort of solution is in order.

i'm pretty familiar with shielding and grounding in an electric guitar or bass, and i'm assuming here there's a similar goal of enclosing the sensitive circuitry in a continuous metal barrier that is common to both the mains earth and the XLR shield.

i need a beer

Grounding... make sure the rack is TRULY grounded. Noise should be lower or about the same with a properly cased preamp.

Did you shunt pin 1 on the XLR terminal straight to the XLR case?... and the XLR jack has good contact with the case?

Distance is also your friend. Position the transformers and AC part (fuse, IEC) as far away from the preamps as possible.

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yes, i did. my DMM shows good continuity between the face panel and each XLR case.






for sure...this metal enclosure is 8" deep, just like the plastic test box, and the first two channels of preamp are farther away from the trafo than in the test box. the last two channels (when i build them) will be about the same proximity as in the test box.

one difference between the metal case set and the plastic test box is that in the metal case i have the PSU board mounted vertically on the rear panel, as opposed to horizontally on the floor in the test box. i shoud try repositioning the PSU board to see if there's any noticeable difference in AC/RF noise.


finally, i have also decided to replace my manually-twisted signal wires with some Canare StarQuad...it's got two insulated twisted pairs inside a tightly braided shield - at this point i simply don't want to wonder if my wiring is as quiet as it can be.

I have better results using twisted wires than shielded cables inside the case. Your mileage may vary.

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really? any theories?

It's not a theory... it's been discussed in electroncis books. See "The Circuit Designer's Companion" book.

Twisted pair is effective and simple way of reducing both magnetic and capacitive interference pickup.

Twisting the wires ensures a homogenous distribution of capacitances, both to ground and to other sources are balanced. This means CM capacitive coupling is also balanced, allowing high CMMR.

Twisted pair most useful in reducing low freq magnetic pickup, because it reduces magnetic loop area to almost zero. Each twist reverses the direction of induction, so two successive twists cancel the interaction with the magnetic field.

Effective loop pickup is now reduced to the small area at each end of the pair.

When combined with an overall shield, the resulting connection reduces both radiated noise and induced interference to a minimum.

.... excerpts from the book.

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ok, that book is now on my wish list


thanks for the always pertinent info, Owel. You rock!