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[Stephen Kirby]

Some vintage style amps can also pick up inductive noise.  Also, high gain rock amps.  Shielding is often pretty minimal and internet madness has led some folks to remove even that to "improve tone".  Shielding on some cables isn't that great either.  While a single coil guitar can be maddening around neon, fluorescent, and various sorts of dimmers, sometimes it's getting in downstream and the orientation of the guitar only makes a slight improvement.

Also, there might have been a pedalboard plugged into house power leaking something back into the ground.

[Mike Karseboom]

Active or passive DI boxes? Lots of cheap active DI boxes don't have an isolated pin-1 so they can send 48-volt power across the pin-1 lift. Also, there are imported DI boxes with rather crappy transformers that don't do well at lifting grounds. If pulling out the XLR cable from the DI box stops the hum/buzz, then stick a mic in front of the offending amp and get on with the show. Sound check is no time for mental exercises. Just get it done and run...

Also, check to make sure your XLR cables don't have a pin-1 to shell bond. If they do and you're plugged into a DI with a metal XLR jack, that bond will be in parallel with the DI's ground lift and stop it from lifting properly.

I had a confounding situation like this with a bass guitar.  Originally a DI out from an Eden amp was used with no hum issue.  The amp died during the show and we switched to a whirlwind hot box active DI directly from the guitar into the PA with the bassist getting his stage volume all from the monitors.


The bass was producing a noticeable 60 cycle hum.  Muting that channel mutes bass in both monitors and mains and stopped the hum, so definitely coming from the bass channel.  This was with the entire system running off a spider box right next to the stage and iPad mixing so no long AC runs.  We tried the ground lift on the DI with no change.  I also tried moving the physical location of the DI box around a few feet without change.  Signal path was bass > 1/4" cable > active DI > 20' XLR > drum snake > X32 Rack.  From the DI to the mixer is all my gear which in general good quality and in good condition and has not had this type of problem before.


The bassist thought his batteries might be low so we changed those  (2x 9V) and that seemed to clear it up slightly but it was still there.  We double checked connections and disconnecting the bass from the 1/4" cable stopped the hum.  At that point we needed to get on with the show so I just EQ'd out what we could and played on.


This was outdoors at a very basic stage that it does not seem would have magnetic field problems and that I have worked at before with no issues.  The 6/4 feed cable was behind the stage about 10' from the bass player and the spider box was on the opposite side of him about 20' away.  A 120V power run was along the back edge of the stage about 2' away and terminated in a drop box behind the unused bass amp. Monitors were all active and had not presented problems with hum before or since.  The only other amp on stage was a guitar amp about 5' away.  Lights were 4x LED washes a good 15' away. 


Like Stephen K I was too tired at the end of the evening to do any testing and did not bother to mark the DI or cables.  But I still wonder if there is any deductive logic that would help identify the problem now.


If the bass did not hum when going through the 1/4" cable > amp > amp DI > 20' XLR > drum snake > X32 rack, does that mean none of those components were the problem and that the whirlwind DI must have introduced the hum?  If so, what testing could I perform on the DI to check it? 


Or could one of the other components, like the XLR cable or the drum snake have a pin 1 problem that was not an issue until an active DI with +48V required  was attached.  Would a pin 1 to shell connection even cause a hum in this situation where potential for a ground loop seems minimal?

[Mike Sokol]

Or could one of the other components, like the XLR cable or the drum snake have a pin 1 problem that was not an issue until an active DI with +48V required  was attached.  Would a pin 1 to shell connection even cause a hum in this situation where potential for a ground loop seems minimal?

A pin-1/shell XLR short can make some crazy hum stuff. Not quite related to your signal path, I know that early pro Sony video cameras had several separate ground planes, so plugging in a shotgun mic to the camera using a short XLR cable with a pin-1/shell bond would make them hum like crazy. I know because I accidentally did this with caused of troubleshooting time to find the source of the hum.

Another thing to be on the lookout for is a non-shielded speaker cable being used instead of a shielded instrument cable. Modern 1/4" speaker cable often looks pretty similar to instrument cable, and musicians don't always understand the difference. I had a keyboard player run one of these speaker cables between the keyboard output and a passive DI. Of course, lots of hum that they swore wasn't there last week. But I found the pile of random cables they would use for each gig, and the musicians would grab whatever cable was on top and plug in a keyboard or a speaker or whatever. There were both instrument and speaker cables in this same pile. We ended up using different color e-tape on the cables with a Sharpie INST or SPKR marking.

Just yesterday I was on a big install gig troubleshooting an audio hum in the video feed. Turned out the video director had swapped in a Roland HDMI audio box that would merge analog or digital audio with the master video stream. They simply grabbed a set of RCA outputs from the audio DA, stuck on a pair of RCA to Phone plug adapters, then plugged it into the phone jacks on the Roland box. This hummed like crazy, maybe 10 or 15 dB below O dB VU. A little looking at the schematic of the Roland box showed me it had TRS balanced inputs and it sounded like true Ground Loop Hum, so I just ran a new pair of Phoenix to TRS cables from the DA to the Roland input, lifting the shield at DA side of the cables. Ta Da... We now had zero hum with tons of signal. The extra 6 dB level was because we now had both sides of the balanced input being fed a proper push-pull signal. Again, not obvious to the casual tech AV sticking in a piece of gear, but it drove them crazy for weeks before calling me.  This took me maybe 2 hours to diagnose and correct, which I charged them top-tier rate for since they screwed this up themselves.

However, this level of troubleshooting is almost impossible during or after or gig, so some of the simple run-n-gun techniques such as looking for speaker/instrument cable swaps and checking for pin-1/shell shorts is about all you can do in a time crunch.

[John Roberts {JR}]

Musical instruments (like bass guitar) are relatively high source impedance (compared to mics or active gear), mid level amplitude (more than mics but less than line leve)l, and unbalanced (single ended).

This allows for multiple vectors to corrupt the signal integrity. Hum/noise can couple directly into the source, and because of the modest levels and higher impedance be more audible that line level feeds. The unbalanced/single-ended signal interface means that any common current flowing in the grounds will induce a voltage drop signal that is superimposed on top of the audio signal. Even noise that the shield absorbs while protecting the signal flowing inside the cable, can develop a voltage drop, and subsequent noise. In balanced interfaces the shield currents are dumped separately. In single ended interfaces the shield current must be managed along with the signal.

Since there are multiple possible vectors, troubleshooting means eliminating as many variables as possible.

Good luck.

JR 

[Steve M Smith]

Active or passive DI boxes? Lots of cheap active DI boxes don't have an isolated pin-1 so they can send 48-volt power across the pin-1 lift

No active DI* can have a full lift on pin 1.  It is usually still connected via a 100R resistor and a 0.1uF capacitor. Otherwise there is no return path for the phantom power.

(* I am aware of one which has a high frequency inverter with full transformer isolation for the power supply).





Steve.

[Mike Sokol]

No active DI* can have a full lift on pin 1.  It is usually still connected via a 100R resistor and a 0.1uF capacitor. Otherwise there is no return path for the phantom power.

(* I am aware of one which has a high frequency inverter with full transformer isolation for the power supply).

Steve.

Yes, I think the BSS boxes do the inverter/transformer trick, and maybe a few others. That's why I typically use a bunch of passive DIs for most instruments that have active outputs with low-z outputs, and save the active DIs for passive instruments with high-z outputs.

[Frank DeWitt]

Yes, I think the BSS boxes do the inverter/transformer trick, and maybe a few others. That's why I typically use a bunch of passive DIs for most instruments that have active outputs with low-z outputs, and save the active DIs for passive instruments with high-z outputs.

the Radial J48 is another one that uses a DC to DC converter but as Mike points out, in most cases you don't need it.  It is hard to beat the performance and isolation of a good quality transformer.

Disclaimer, I build and sell a passive DI

[Steve M Smith]

Passive DIs seem to be much rarer on this side of the Atlantic than yours.  Most companies here use the BSS (not sure if it's the one with an isolated dc supply).  EDIT: It is.

This one:
 

The only passive DI I have is one I made for connecting laptop and i-thing headphone outputs for music playback.

If I'm working with my own equipment, everyone gets one of my own high impedance FET input DIs.


Steve.

[Stephen Kirby]

Passive DIs seem to be much rarer on this side of the Atlantic than yours.  Most companies here use the BSS (not sure if it's the one with an isolated dc supply).

This one:
 

The only passive DI I have is one I made for connecting laptop and i-thing headphone outputs for music playback.

If I'm working with my own equipment, everyone gets one of my own high impedance FET input DIs.


Steve.

I have one of those and love it, especially on bass.

There is a sliver painted clone from a company known for cloning things.  I wonder if it also has the same isolated power supply.

[Frank DeWitt]

A active DI is actually a preamp.  It dosn't have gain but it is a amp before your board.  That brings up questions

How does this amp sound?
What is it's noise floor.
How flat is it?
What does it do when over-driven?
What does it do when it's powersupply changes?
None of the above are show stoppers. Just things to find out.

With a good transformer, you look up the numbers, say "that will work"  Now as long as you know that transformer is in your DI and all the switches that change the sound are off you know what will happen when you use it.

[ProSoundWeb Community]