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Author Topic: Noise, hum and other descriptive terms  (Read 8856 times)

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Offline Doc B.

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Noise, hum and other descriptive terms
« on: September 26, 2012, 06:21:09 AM »
This being a DIY electronics forum we see a lot of posts about hum or noise. And quite often I see someone chasing a problem by looking into aspects of the setup that would not create the particular kind of noise heard. So to make this noise troubleshooting process more efficient I propose a set of definitions of various kinds of noise and some suggestions as to where to look to reduce them.

Hum - this has become a catch-all phrase. Basically there are two kinds of hum, 60Hz hum and 120Hz hum (or 50hz and 100Hz in countries with 50Hz AC). Obviously one sounds an octave deeper than the other, but there are other characteristics to the sound that make them fairly easy to distinguish.

60Hz hum is a softer, more rounded sound. It is usually caused by magnetic coupling, i.e., something is picking up the radiated magnetic field of a power transformer or choke. Tubes and cables are common points of coupling. Distance is your friend here, though moving the cable or piece of gear that is picking up the hum around without adding distance from the transformer can help too. Cables that are radially symmetric like coax and high quality shielded twisted pair are better at rejecting this noise. Mounting inductors so that they are electrically isolated from the chassis can reduce eddy currents in the chassis that can add hum as well. This is done in all Bottlehead kits. Speaking of transformers, occasionally the hum is actually a mechanical vibration of the power transformer transmitted directly through the air rather than through the speakers. Tightening the transformer mounting screws can help and in more extreme cases some type of vibration damping may be necessary.

120Hz hum usually has a distinct buzzy content to it. This the buzzy content of the hum is electrostatic in nature rather than magnetic and is usually due to the power supply rectifiers' spikes not being properly shunted to ground. Thus 120Hz buzz is often associated with ground loops or lifted grounds, and is probably the most common issue. The best way to troubleshoot this is to disconnect any input cables and short the inputs of the device in question, to see if the buzz remains or if it is actually coming from the cable or equipment ahead of the gear. If the buzz remains, you may have a ground loop with other equipment. A "cheater plug", that converts a three prong power plug into a two prong plug, can be used to see if the problem goes away when the device is separated from the safety ground of the other gear. While this is not considered the safest method of operation and should probably not be left in this condition, it can help to establish if you have a ground loop that needs to be dealt with. The third and perhaps most common problem is that there is a cold solder joint or loose connection in the gear. Going over all of the solder joints with an iron to reflow them is a simple and most often very effective method of resolving buzzing issues. For some reason many builders are reticent to do this, to the point where they will spend several days asking for other suggestions on the forum before trying it. I have no explanation as to psychology of this, it seems about the easiest thing one could do to eliminate a problem without spending a lot of time tracing and measuring things.

Another rare occurrence that can cause 120Hz hum is a filter capacitor installed backwards. This can also cause a lot worse problems than hum as the capacitor shorts out or vents its guts all over your amp, and should be fixed before the device is powered up again.

Hiss or rushing sound - this is an entirely different phenomenon, due to self noise generated in a tube or in a SS device like a regulator. It is most often associated with preamp circuits that are amplifying very small signals. If it is due to a tube (this would be the most likely case in our kits) the solution is usually to simply replace the tube with one that is more quiet. Typically the more exotic high priced variants of a given tube are expensive because they tend to be more quiet in circuit than less expensive variants. If the rushing sound is from a regulator it may be that the regulator needs to be bypassed or that a SS component in the regulator has failed.

Crackling - this is perhaps one of the more unnerving sounds. It is usually due to an intermittent connection. That connection might be at a tube/socket interface, in which case inserting and removing the tube a few times may cure it. It may be a worn jack, in which case it is usually prudent to replace the jack. It may be a bad solder joint somewhere in the circuit, in which case the solution once again is to reheat the solder joints in the device in order to reflow any bad connections. It may also be in the cable ahead of the gear in question. Try a different cable and see if the problem goes away.

Popping - if there is a single popping sound sometime during or after warm up it may be a tube arcing over slightly as it comes up to voltage and the tube my need to be replaced. If the pop occurs exactly as the power switch is cycled on or off the solution is simply to have any equipment downstream of the device turned off when the power switch is cycled. If the popping is in a steady pattern it may be a failing capacitor which is charging to a certain potential at which point it discharges abruptly and then repeats this cycle. A capacitor in this state needs to be replaced. Tubes can do this cyclic popping as well. If it is mild it may go away as the tube heats up and its internal structure expands.
« Last Edit: September 26, 2012, 06:48:44 AM by Doc B. »
Dan "Doc B." Schmalle
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Bottlehead Corp.

Offline Grainger49

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Re: Noise, hum and other descriptive terms
« Reply #1 on: November 02, 2012, 03:00:11 AM »
Dan,

Excellent write up! 

I'm going to try to explain why no one wants to reflow all the solder joints.  They either think they are good at soldering or they are so bad they fear heating up the soldering iron again.  Still, it could be all sorts of other reasons.  I wholeheartedly agree, this should be the first step.  Anyone could have gotten in too much of a hurry when soldering.

I can't imagine what interesting construction you have seen at Bottleheadquarters.  I got a FP 2 which had what only could be called "solder balls" inside.  I know the manual did not call for solder balls. 
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« Last Edit: July 02, 2017, 11:54:36 PM by Grainger49 »

Offline Doc B.

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Re: Noise, hum and other descriptive terms
« Reply #2 on: November 02, 2012, 05:14:04 AM »
Solder ball, good name for that hot pitch that ended the World Series. Miguel Cabrera made an assumption similar to a lot of kit builders, that he didn't need to be careful about connecting...
Dan "Doc B." Schmalle
President For Life
Bottlehead Corp.

Offline Grainger49

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Re: Noise, hum and other descriptive terms
« Reply #3 on: November 02, 2012, 08:08:26 AM »
I'm a "mechanical connection first, then solder" kind of guy.  But it is a Bit** if you have to rework a terminal.

Offline alejon

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Re: Noise, hum and other descriptive terms
« Reply #4 on: March 06, 2017, 10:52:10 AM »
Dear Doc B.,
Excellent writing, indeed. I googled for "tube amp hum" and find  this thread. Perfect!
I am fighting with an old small Philips tube amp AG9016, it's got EL95 / 6DL5 output tetrodes, single-ended.
The output tubes give very low level (but still noticeable) 50Hz hum. It is NOT AC heater, as I replaced the AC with DC regulated 6.3V and it changed nothing, still hum.
Maybe I will need to move the power transformer around, just to see if the hum is magnetically induced (which I think it is, after reading your post). The chassis is extremely tight and there's not much space inside but I'll take it out to hear if there's any improvement.
In any other case I would just ditch the damn thing but this is THE sweeeetest amp I've ever heard,  so I kinda love it.
Regards!
alejon

Offline Paul Birkeland

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Re: Noise, hum and other descriptive terms
« Reply #5 on: March 08, 2017, 12:54:09 PM »
The hum you're talking about can be induced from magnetic coupling as you describe, and it can also be a consequence of poor grounding (rampant in vintage audio gear!).

When I rebuild vintage amplifiers, I will actually go through and drill out every rivet that holds down every terminal strip grounding lug that is serving as a ground.  Each lug is then isolated from the chassis by shoulder washers or nylon screws/washers, then each of those grounding lugs gets its own lead to a star ground.  This is a lot of work, but if you want to mess around with chasing down noise, it's a place to start.

-PB
Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man