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

i'm getting a little buzz...so i'm going to open up my kit and spin the trafo a bit, maybe move some wires around and try to get rid of it.

two questions: how dangerous is this with the power on? should i not even try it? and two, what's the best way to terminate the input? i have a 150 Ohm resister, should i just stick it in pins 2 and 3 of the input jack?

oh, and is it odd that my preamp is quieter when i've got nothing plugged in?

nothing:
http://www.thebrotherkite.com/stuff/nothing.aif

150 ohm resistor on pins 2 and 3:
http://www.thebrotherkite.com/stuff/150.aif

You're dealing with 110VAC, so yeah be careful. You can always flip the power on/off switch before rotating it.

What's your gain setting when you're hearing the little buzz? 6dB or 72dB?

Is the pre in a metal/aluminum case? Input output wires twisted? Trafos far away from the PC boards?

> i have a 150 Ohm resister, should i just stick it in pins 2 and 3 of the input jack?

This is only for testing. Don't leave it there during normal operation.

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yeah, i just started fiddling and switching the power on and off in between moves.

here are some pics as a reminder of what i'm looking at:

http://www.fivefishstudios.com/forum/vi ... .php?t=392

i've been using the preamp with delight, but i've started to notice a little buzz at extremely high gain. those samples i pasted above are at full gain.

so i moved the trafo a bit, and the noise went down to -71dB (according to the continual reading on the RMS Buddy plugin i'm using). which is very good. but when i put the lid back on the case, a little buzz comes back and it goes up to -66dB...which is still pretty good. i just wish i could find out what's happening when i put the lid on...i suppose some wires are bending is some way. could the wires touching the underside of the lid be cause a problem? could i insulate the top of the lid with electrical tape or something like that?

either way, thanks! i've been enjoying my SC-1...just trying to make it even better since i've got nothing else to do tonight!

You know at max full gain, that's 72dB. In simple words, any little microscopic amount of noise you have, it will amplify it to 4,000X and that's the noise you hear.

You'd always have some little noise at the very max gain. (That's why most pres out in the market are limited to 54dB, or 60dB. or 66dB at most)

From 66db to 72dB, that's a doubling of amplification.

Try laying the wire flat/taped on the bottom case. Maybe the top cover isn't electrically grounded to the bottom case (i.e. paint acting as insulator?)

And lastly... you have a very small case, with a big transformer in it. It's going to be impossible to make it 100% noise free. But the numbers you're getting, if you say there were at 72dB gain, it's pretty good.

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Okay, I listened to the samples... they're definitely caused by the power transformer. Move the trafo away farther... but in this case, you're limited by the size of your chosen case.

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yeah, just by turning the trafo, i got it down to -71dB with the lid off and -66dB with the lid on. i can live with that, i think...but i got this thing dead silent with the lid off. pretty cool, maybe it's just a convertible now.

here it is after the trafo nudge with the lid on:
http://www.thebrotherkite.com/stuff/afternudge.aif

thanks!

My theory is this:

Without the lid: Imagine magnetic flux from the transformer radiating outwards... like a toroidal donut shape. No lid, the magnetic flux just goes out of the case.

Now put the lid on, and the whole top causes the magnetic flux to get deformed. and gets bounced back.

Of course those are just my theories.... don't know if it really happens that way.

You may want to try to make sure that the top panel/cover and your case is grounded properly. My guess is the top isn't being grounded properly into the same potential as your case.

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i'm back with another question...

i've been using my preamp a lot and loving it. but i've noticed that it makes a strange (possibly ground related) sound depending on where i am. at my apartment plugged into the wall, it's quiet and great...at my studio, it's also quiet and great. i brought it to a friends house, and it was making a strange "hissing" sound that went away when i touched the case. i brought it to another friends house, and it made the same noise. i just recorded everything while touching the case, and it was fine.

so it seems to be grounding related, but it's not a "buzz," it's a definite "hiss." is that strange? i made sure that it wasn't interference from something nearby by trying multiple locations in the room, and plugging it into different outlets. nothing helped. i followed the suggesting grounding scheme on this board...maybe i screwed up somewhere. i just wanted to ask if it's strange that it only seems to happen in certain places, and also ask about it's "hissing" nature (i thought grounding problems were typically a "buzzing" thing).

thanks again. good luck with the new products, they look great.

Insufficient grounding could cause not just hummm (which happens in the most extreme cases) but also buzz noise and RFI noise.

Have not heard of a hissing noise just because it's plugged into a different outlet at your friend's house.

But you said the noise went away when you touched the case... So that points to grounding issue... not good enough connection/contact?

My suspicion is the noise could be caused/coming from your friend's equipment/setup ?

Are you using my case or your own case?

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i'm using my own case. pics can be seen here:

http://www.fivefishstudios.com/forum/vi ... .php?t=392

i just used the continuity mode on my multimeter, everything seems to check out OK. touching the grounding loop or the ground pin on the power cord receptacle and the ground pin of the XLR connecter yielded a nice strong "beep." also got a good "beep" from the XLR ground pin and the case, and the power cord ground and the case.

strange.

Is there good contact (like really very low resistance, and not just continuity) between the rear panels, the main case and the front panel?

I'm concerned maybe there is not good contact between the rear panel and the main bottom case, (and top cover). That will break your case grounding.

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yeah, that's what i thought as well. so i took it apart, and filed away the gold paint at all the contact points (all along the edges and where the screws go). we'll see if that helps...but it's a tough issue to replicate since it only happens occasionally. but we'll see...thanks again.