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Step By Step Hum Test-Simplified Tags: hum

STEP BY STEP HUM TEST

Simplified for quick and easy location of the source of unwanted hum.

There are two other articles on this site discussing hum tracking in detail.  This is the "quick and dirty" page for those who wish to run the easiest tests, quickly.  They're usually effective in the majority of cases.

An article on Wikidpedia is unusually well written, helping even beginners to understand what causes hum and how to deal with it.  It's not too technical, not overly long, and very informative.

https://en.wikipedia.org/wiki/Mains_hum

Note while a bad pickup can cause all sorts of problems... in the majority of cases the source of hum is not the pickup.  In two places in that article it states that even humbucker pickups are designed to "reduce hum" (not eliminate it 100%).  If a pickup itself were enough to eliminate all hum, companies couldn't sell hum-killer devices for high dollar.  Obviously hum must be met head-on, at the source.

FIND THE SOURCE

Blaming a pickup for hum is often pointing a finger the wrong direction.   Even a single-core pickup can have zero hum if the line power is clean.  At the same time, even a humbucker will hum if the line is "dirty".   So it's necessary to identify the true source of the hum and work to overcome it.  Thus this article.

The two primary sources of hum is ground-loop issues, and RFI/EMI (wave interference).  The source of the hum could be either or both.  But RFI/EMI can be shielded and sent to ground, so in the end it still comes down to making sure your instrument, amplifier and power source are properly grounded and your power clean.

I recommend Uninterruptable Power Supplies for ALL important electrical equipment (music equipment, televisions, computers, etc).  They are no more expensive than a good surge protector... and work better.  They are available for under $50 (the higher price version are to provide more power and remain online longer if the wall power goes out).  Of course a UPS is no magical solution to all hum.  In music there are many variables.  But employing a UPS to filter your power source is just one of those no-brainer wise decisions. 

 

THE STEP-BY-STEP TEST

The above principles and concepts understood, following is the step-by-step test to trace down the source of any hum issues. 

1. Install the pickup on your instrument as normal.  Your pickup may already be installed. In such case it may be necessary to check all wire connections and make sure the solder joints are neat and solid.

2.  Turn the volume to minimum, turn on your amp, and bring up the volume to normal playing level.  The GAIN should be set to minimum (gain by nature often causes noise).  Bass, mid and treble should be set to mid-range or lower (on most amps a setting of 3-5 is considered good testing range). 

3.  Touch the end of the guitar cord to make sure you have continuity and that it doesn't "roar" (too much power from the amp).  You just want a mild sound when the end is touched.

4.  Plug in your guitar.  Ignore any noise or hum (that's what we're checking and we take it for granted it's there).   Strum a few chords and notes to see if the sound is normal, acceptable sound without excessive "noise".   Excessive noise indicates an instrument, amp or external problem.   Most instruments when played should pretty much override any noticeable hum.   That's the purpose here: to make sure there's no *excessive* source of noise that might indicate more serious problems than basic hum.

5.  Quiet the guitar strings (mute with your fingers).  When the guitar is quiet, cease touching the strings.   You'll be able to hear hum (if there is any).   At this point, touch the metal sleeve on your guitar cord jack to see if the hum remains or vanishes.   If the hum vanishes when you touch the metal sleeve, that indicates a ground loop issue that will need to be traced and corrected / counteracted (thus the other two articles).

6. If there is still hum when you touch the sleeve, that is indication the hum is internal to the instrument.  The instrument itself should be checked for proper internal grounding (pickup, bridge, strings, controls), and that all areas are properly shielded against RFI/EMI.  It is also possible in this case there is a problem with the amp.  To test this plug in another instrument and see if the problem persists.  If it does, look to the amp as the issue.

7. If your amp has 3 prongs on the cord, turn it off, unplug it and plug the cord into a 2-prong wall adapter (available cheap, anywhere).  Plug the amp back in and turn it on.  If there is no hum-- the problem is definitely a wall-based ground-loop issue.   Whether there is hum or not, after a few seconds turn the amp off and remove the adapter.  A 3-prong amp should not be operated for any length of time using a 2-prong adapter (even if the hum vanishes).   But hum vanishing when using a 2-prong adapter definitely isolates the cause of the hum... and indicates how it needs to be fixed.

IF ALL ELSE FAILS...

8. This test sounds strange, but if hum persists, wrap the body of your guitar with aluminum foil and make sure the foil connects with the cord jack sleeve (for grounding).  If the hum disappears, the problem is very likely RFI/EMI interference. 

9. Then un-ground the aluminum foil from the cord and see if the hum situation changes.   This is basically a "let's see what happens" test to see if there is significant external interference.  Of course you can't play an instrument wrapped in aluminum foil-- but it will indicate if the problem is a lack of shielding... or trying to play the instrument near an excessively "noisy" device such as a computer or other electrical device that is emitting undue RFI.  If your guitar is properly grounded there should be no difference between grounding the foil to your guitar cord or not doing so. 

 

THE ABOVE STEPS should in the majority of cases help you find at least a hint of where hum problems may be originating.   In most instances the source of hum is unfiltered wall power that is creating electrical hum.  There are ways to overcome such (UPS devices, Hum-X filters, etc).  If the problem is indicated to be the amp or instrument, then of course more professional steps will need to be taken to isolate such issues.  Your local music store or a good friend with electronics / guitar knowledge may be able to help you there.

 

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April 2020 (1)

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