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

Alright, I'm finally done with the spring semester and I've had a chance to test out my 2-channel jobber.

For some reason or another, my kit is generating hiss through both outputs regardless of the setting of gain or trim controls. The gain and trim knobs could be set completely counterclockwise and the hiss remains. (both channels pass around -50 db of the hiss according to Audacity)
It passes signal just fine, but the hiss is present the moment the device is turned on. It is constant and unaffected by the type of microphone or the use of phantom power.

I've managed to rule out a few things so far: My ins and outs are all grounded as per the diagram on the forum and they're all connected to the proper terminals. The cables to the ins and outs are twisted more or less properly and the PSU is putting out the correct voltages.

Another weird thing that I noticed (which may help point this in the right direction) is that the LED2 spot on the PSU had 18 volts running across it for some reason (which makes a short a suspect). That didn't make sense to me since the LEDS I'm currently running across it are 3.5 volts and they're doing just fine. Everything seems to be running fine despite this—and the hiss is there, of course.

So, any ideas about what could be causing this? A short? Bad soldering? Karma?
I'm thinking that some bad soldering somewhere and/or a short is the culprit, but I'm no expert. I'm worried, though.

Thank you very much,
-Ryan Murdock

If the gain and volume controls are all set counterclockwise, and you still hear hiss... then the hiss is not coming from the preamp.

This is a typical example of the amount of hiss you should hear.
http://www.fivefishstudios.com/audio/kol/Silence.mp3
Gain=48dB, max volume (max. Clockwise), attached to 15ft of cable, laying on the floor, surrounded by AC wires, etc...



As I said, the hiss doesn't seem to be caused by the preamp electronics.

I would look more into grounding, etc... case grounded, wiring, cabling, etc. ... or maybe even to the device where this is connected to.

What kind of housing/case are you using? Is your case properly grounded? (But even then, an improperly grounded case will be more of a humming noise and just bunch of RF junk... not hiss.)

I'm assuming you're connecting the balanced outputs of the preamps to some balanced inputs?



If there's a short, you'd see 0 volts. But I don't see this related to your hiss problem.

The spot for LED2 is connected to one of the 18V outputs with a series limiting resistor. So yes, you'd measure 18V across it.

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Yep.

Right, but it's getting to the outs somehow, right?

I connected it to three different devices, all yielding the same result. I even tried different power outlets in different locations.

I had tested it previous to grounding it and once I had grounded everything all of the RFI stopped.
My case is admittedly a bit unorthodox but it's conductive, grounded and everything inside is on stilts.
I'll take a look at it again, try some new things (double-check a few things) and I'll get back if I find anything new out. If I don't find anything else out, I'll be back just the same...

Thanks again,
-Ryan Murdock

If noise isn't being shunted to ground, yes, it will find it's way to the input of the next gear.



I think this is a clue. Like I said, it must be a grounding related issue.

Whatever you did that made the RFI noise stop, is in the right direction.




You have to really make sure your metal case is grounded, and making good contact on all six sides of the box. Sometimes a case is painted and the paint acts as an insulation and what you thought is a good grounding isn't.

We're talking here about a preamp that can amplify signals 1000x, 2000x up to 4000x. Any bit of noise caused by not proper grounding/construction/wiring is going to get amplified alot.

Also, make sure your transformers are positioned far away from your preamps. And try the "rotating the toroid" trick to really squeeze out the last remaining noise in your system.

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Ahh, this is probably the problem. The case is conductive on bare parts (screwholes and edges), but it has a goldish finish on the rest of it so it isn't actually conductive on most of the sides (and the top is indeed coated with a layer of paint). Live and Learn, I suppose. That's what I get for trying to be thrifty. I found an old Realistic equalizer for 10 bucks that had the perfect size factor I was looking for. Is there any way that lining it with a conductive sheet of some fashion—then connecting it all to ground—could help salvage my case (and by extension the time spent fitting everything in it)?

The toroid and PSU are both about 2.5" from the nearest preamp. I was going to try messing with the toroid a bit as a last-ditch effort today.

Thanks,
-Ryan Murdock

There's nothing wrong with a painted case. What I'm saying is when you put your ground connection, make sure it's making good contact with the bare metal itself, and not on the paint. So try scraping the paint on where you bolt/attach your wire.

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