How to properly ground an audio chain that is not in a rack

[K1JJ]

I’d like to start a dialog about audio chains and grounding practices to set up the rig here and maybe help some newcomers at the same time.

After getting most of the bugs worked out of the new 24 pill class E rig, (Rico Suave) I found I had some audio hum generated by a ground loop. It was suggested to run a balanced pair from the chain's audio output to the rig. This solved the problem. The hum is now below the quiet room noise.


However, I've noticed after many years of changing around gear, my ground strapping was getting sloppy. I tore down the audio connections and grounding and decided to rewire everything.

Questions:  I notice that there are no ground studs on any of the audio gear. Are all of the cabinets of the individual components supposed to be in a rack, thus all cabinets bonded together?  If they are all sitting on wooden shelves, should I run an individual braided shield from each audio component cabinet (screw on the cabinet) to a central ground point?

I am using a mic preamp > 32 band EQ > effects box > 6-band compressor.  All of the gear in connected together using unbalanced cable except for the final run to the rig, which is balanced.  I have no problems with hum or RF in the audio, but want to set the grounding up properly.

I find it curious there are no gnd studs on any of the gear. Not everyone uses a rack to mount this stuff.


Another point:  All of my transmitters use an unbalanced input. I used the balanced output from the processor (two wires and a ground) to the rig and then ground one of the wires to the shield/chassis at the rig. It killed the hum and there is no RF in the audio. However, the Tron said this may be hard on the processor since one balanced lead is essentially shorted to ground at the rig. I could easily make Rico Suave a balanced input, but the other three rigs wud be more difficult and I’d rather stick with what I have. Any opinions on the “shorted wire” load on the processor?  The processor is a Behringer 9024. I can plug in either a mono or stereo ¼” plug to the output and it “senses” whether it is a bal or unbal. Not sure how this is done or how it wud like having a stereo plug with one lead shorted to ground at the rig..

I do not want to use an isolation transformer. There's no transformers in the audio chain or drivers now and the difference is remarkable.

T

[Jim KF2SY]

Tom,

Here's a rather nice legthy technical note by the folks at Rane:

http://www.rane.com/note151.html

[W1RKW]

Tom,
Maybe grounding methods have something to do with pro gear having 3 prong power cords and consumer gear having 2 wire power cords?  Just guessing.

This is what I use on GORT since his audio input is unbalanced too and his audio chain is balanced.

http://www.mcmelectronics.com/product/MCM-CUSTOM-AUDIO-555-8485-/555-8485

It's an active device rather than a passive device like a transformer.

Works well with no RF or hum in the audio and it's mounted inside the transmitter too.

Something to note. Some audio racks and equipment enclosures are anodized so grounding between components and the rack may not be what one may think it is.  There may be no bond at all.

[ka3zlr]

For the Economically minded fixed income Ham:

 http://w2ihy.com/ibox.asp

It works great an you can build it yourself 

73

Jack.

[flintstone mop]

Sometimes more ON-purpose grounding creates the ground loops.
And sometimes you might need a three-prong adaptor to cheat.............hi

Fred

[K1JJ]

That was good reading there, Jim.  It appears that bonding all the cabinets togther is important.

Bob, that board seems like a good solution too.

I tied all the cabinets together at a central point. The hum seems even less than before. It is barely audible, even at higher volumes.  Most likely it's just power supply residual hum now, not ground loop hum, which is a good thing.

All set.

T

[K5UJ]

I'd run balanced cable between your processing boxes.  Most broadcast and studio boxes have the audio at the rear panels at XLR jacks and males or they have terminal barrier strips.  Your inter-box grounding gets taken care of this way.  Your safety ground should be in the power cord.  Broadcast processing gear almost always has small ceramic bypass caps on the audio + and - lines and little ferrite beads on them at the back panel just inside near the jack leads.   If there is RF in the audio you can try adding these to your processors.  I have my processing chain out of a rack and simply stacked on a table with rubber feet cushioning each cabinet.  Only inter-cabinet ground is in the balanced cables.  All the power cords go to a single terminal strip; everything is hot, neutral and ground.  I have an isolation transformer doing the ground loop interruption between the final peak limiter and the rig and it also gets the balanced line to unbalanced into the FT1000MP exciter.

You can make your transfer from balanced to unbalanced at the transmitter audio input if it takes unbalanced audio only, by using an isolation transformer.   You need to get a quality audio transformer from Jensen if you are having distortion problems with the one you are using or have used.   You can mount it inside the cabinet of your transmitter if there is room, or put it in a project box with whatever cable jacks you want on the box.   Most modern audio is 600 ohms.  If you need to go from low Z to high for the transmitter you can get a Jensen transformer that will do that.
If it is a 1:1 transformer pick any side as primary and put your audio + and - on each end of the winding and ground to the box.  the two lines to the rig are ground shield and your single hot audio.  This connects to one end of the secondary winding; the ground shield isn't connected to anything at the transformer; only at the rig's ground.

I'd take a look at these; I think they will allow for a transfer from balanced to unbalanced and provide ground loop prevention.

http://www.jensen-transformers.com/ln_in.html

Rob

[K5WLF]

Tom,

The bal/unbal output of your Behringer only requires that you use the proper cable for your intended input. IOW, since Rico Suave requires an unbal input, simply run a 1/4" mono cable from the 9024 to the xmtr. If Rico had a balanced input, you would use either a 1/4" stereo cable or a 1/4" stereo to XLR cable if Rico had an XLR input. The output circuitry senses the type of cable you connect and adjusts to bal or unbal without the need for you to short a hot lead to ground at the xmtr input. You won't need any transformers or anything. Only the appropriate cable for Rico Suave's input preference.

Fred,

As a professional electrician for about a dozen years, and a live sound and recording engineer off and on for most of my life, I hope you were kidding about the 3-prong to 2-prong adapter. The safety ground is there for the safety of the operator and the equipment and will not compromise the audio in a properly installed system.

73,
ldb

[k3zrf]

Hi Tom,

This is a chart from Rane that I use all the time for reference. It's good schtuff.

As you see Rane's grounding scheme you may also use in reverse of their charts.

"We" always grounded from the send end or outpoot side...........

http://www.rane.com/note110.html

[W1RKW]

"We" always grounded from the send end....

And floated the return end of the shield.

That's how I learned too. I learned the hardway when working for a commercial station that had a digital cart system.  Somehow a ground loop was introduced into the audio system by another station engineer that installed it. We kept wasting ADC boards in the digital cart system. We found out we had a ground loop between it and a reel to reel machine. Just a couple of volts, enough to kill the output stage.  Isolated the R to R machine and all was good.  

My system is shielded the same way.  Got no hum whatsoever.

[k3zrf]

I forgot to mention floating that ground, thanks....implied but not mentioned.

With any audio gear whether in a rack or on a wooden shelf I always bond the chassis to ground....if there is a three prong connector on the AC I use a ground lift...the chassis is grounded.

I remember years ago installing a switch that would take pin #1 from a balanced input to ground, float or connect to a .1mfd mica cap to chassis ground in each unit. If floating the ground on the input didn't do it the AC ground path thru the cap would.

I am uncomfortable mixing balanced and unbalanced stuff together. Following a balanced line to an unbalanced input (usually the unbalanced input of the xmtr) I apply a xfrmr....usually a 1:1.....keep in mind the polarity shift.

Generally I have found grounding at the send end and dropping the shield at the input of the next device has always kept me out of trouble.

And of course there are always exceptions to any rule.

In my station the last audio device in the line B4 the xmtr goes into a 10K:10K xfrmr switching its output to various rigs....only have to adjust the gain at each rig.

[K1JJ]

Thanks for the extra info, Dave.

After rewiring everything in the audio chain, installing new grounding that made sense and getting rid of accumulated "junk" in the audio system, I now have no hum and the best reports I've had to date. Even the 4X1 rig is accepting shark fin audio phase. This is something my old system would not do for whatever reason.

The direct coupled E-rig (no transformers anywhere) passes audio I never  knew was there in my voice...

Just doing fine tuning and bells and whistles now.   BTW, I just added an audio delay that turns on the audio ~200 ms after the heavy relay acoutics die off in the room. I even had an unkey spike. The delay made it quiet for both key and unkey.

T

[WA1GFZ]

Good app Dave!