Being misled while running tests on preamps

[Potato Cakes]

Hello everyone,

I have made a fun and frustrating discovery recently when testing preamps for noise with a microphone, which I only partially understand. For starters, it seems that I have EMI radiating from somewhere in my basement/garage where I have my studio and  build/test recording gear. Even before I did the remodel where I added electrical lines for adding lights this was a problem, but I only really noted this with electric guitar. One would have to rotate on the stool they were sitting on until the noise decreased to a level that wouldn't affect the recording. Where one was located in the room did not mitigate the issue.

The past several times when I have completed a preamp build and finished doing any calibration/measurements with a voltmeter, I would test using an Earthworks SR40VL handheld mic, which draws an above average current of phantom power allowing me to check multiple aspects at one time. If the preamp was working properly then I would have a signal free of noise caused by the circuit. The discovery came a couple of days ago when I was working on a preamp that was known to be functioning properly and had already been used many times by a client for a tour. All it needed was some mechanical adjustments. I grabbed a the closest mic (Heil PR20 - dynamic) to do the initial function test and I heard a buzz, causing me to believe that there was also an electronic issue, but I found nothing wrong with the internals. I even went as so far as to make some new cables with Mogami 2791 (braided shield, quality copper) in hopes this would provide a remedy. I tried another dynamic mic which produced the same result. I spent a fair amount of time going over the circuit looking before I decided to try the SR40VL and no noise. Returning to the dynamic mics I found that when I was holding the body of it there was noise but when my hand was not making contact there was less noise, but that was also determined by how the mic was positioned in space. I also found that I got the same results with a commercially available preamp.

I just thought this bit of info might be helpful for anyone struggling with noise in preamps they build/test.

Thanks!

Paul

 

[Speedskater]

You can use that mic to do some detective work. Hold it near the ceiling and walls and look for a larger signal.

 

[Potato Cakes]

You can use that mic to do some detective work. Hold it near the ceiling and walls and look for a larger signal.

I have tried something like that. The only thing I find is that the noise increases when it is right next to the PSU, which is the case for any mic I use, naturally. I biggest concern I have is determining whether or not the piece of gear I'm testing is causing the noise or the test parameters.

Thanks!

Paul

 

[abbey road d enfer]

That sounds like your mains circuit is not properly earthed.

 

[Potato Cakes]

That sounds like your mains circuit is not properly earthed.

For the house?

 

[Speedskater]

That sounds like your mains circuit is not properly earthed.

in this situation, what does 'earthed' mean and how does it matter/

 

[abbey road d enfer]

For the house?

Yes.

 

[abbey road d enfer]

in this situation, what does 'earthed' mean and how does it matter/

For good performance and safety, mains distro must use earthed connections (3-prong sockets and plugs). The earth prong in the sockets must be connected to earth of reasonably low impedance (a few ohms).

 

[Potato Cakes]

Yes.

This is an older house (1956) and the first thing I did when I bought it 6 years ago is run all new wiring as nothing was properly grounded, but I did not put in a new copper grounding rod. I may need add that to my list of things to do around the place.

Thanks!

Paul

 

[Speedskater]

For good performance and safety, mains distro must use earthed connections (3-prong sockets and plugs). The earth prong in the sockets must be connected to earth of reasonably low impedance (a few ohms).

It's been a common misunderstanding in many fields of electricity about the connection to Planet Earth.
It's there only for safety during unusual events like thunderstorms & and power company high voltage problems also to keep the swimming pool and the Neutral at about the same potential.
Planet Earth does not act as a sink or sump for bad electricity.
The connection to Planet Earth has no impact on day-to-day AC power quality.

 

[abbey road d enfer]

It's been a common misunderstanding in many fields of electricity about the connection to Planet Earth.
It's there only for safety during unusual events like thunderstorms & and power company high voltage problems also to keep the swimming pool and the Neutral at about the same potential.
Planet Earth does not act as a sink or sump for bad electricity.
The connection to Planet Earth has no impact on day-to-day AC power quality.

I'm not talking about AC power quality. The equipment must be grounded in order to maintain their chassis to a near-zero potential, so there is no significant electrostatic field.
Many people make the mistake of analyzing safety and equipment grounding as if it were a single issue.

 

[JohnRoberts]

It's been a common misunderstanding in many fields of electricity about the connection to Planet Earth.

many books are filled with things people don't understand

It's there only for safety during unusual events like thunderstorms & and power company high voltage problems also to keep the swimming pool and the Neutral at about the same potential.

Lightning tends to make and follow it's own path. Earthed protective lightning rods can collect and discharge energy from local electrical fields (into the earth) reducing the possibility of lightning up-strikes, that ionize a path for subsequent larger current down strikes, (Ionized air is more conductive.)

Swimming pools are a human safety issues and water can be pretty conductive and that energized water can interfere with human nervous system function especially if completely immersed in the water.  By now most modern swimming pool installations should have GFCI/RCL current leakage protection, old pools not so much.

Planet Earth does not act as a sink or sump for bad electricity.

Kind of does, like a big capacitor with high ESR. It is where lightning goes.... For audio paths the earth is too high impedance to be significant compared to metallic wiring.

The connection to Planet Earth has no impact on day-to-day AC power quality.

Any direct earthing of safety ground (also bonded to neutral) will encounter a relatively high impedance to the dirt. Current will  generally flow in the lower impedance paths proportionately. I did encounter current flow once through my yard when I had a leaking water main causing a puddle of water in my yard. I tried to power a sump pump with a borrowed (mis-wired) extension cord. Either my neighbor was trying to kill me, or someone else was trying to kill him? The equipment safety ground lead was tied to line (hot) and while it did not draw enough current through my relatively high resistance yard back to the grounding rod and fuse panel to trip the fuse, I felt a tingle in my feet from several feet away. I wasn't dumb enough to stand directly in or too close to the wet puddle with electricity involved.  Wet dirt is more conductive than dry dirt. 

As has been shared multiple times; earth ground is different from safety ground which is different from shield ground, which is different from mains neutral. They differ mainly in function, while they are often all bonded together at some point. Then there is signal ground (I hate that nomenclature) which IMO is better thought of as audio low or audio 0V.

JR

 

[porkyc]

Once you have sorted out grounding etc. go to:-

<worldradiohistory.com>
go to R, go to REP, download Dec 77 and Feb 78.
You now have the best article on measuring noise etc in mic preamps, by Paul Buff.
Dec 78 is the reply part of the discussion, because Paul Buff did tend to ruffle feathers for some reason.

 

[emrr]

SM57's in my room have buzz if you are holding them, none if on a stand.  Only mic I have that does that.