Chasing hum: Two great iPhone apps to isolate hum in your analog chain

Grooves

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Feb 29, 2012
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Hi Ack,

For fun I re-read your orgin. post and I have to nod my head re: the R>L channel hum. I get a persistant ground (?) hum that is greater in the right channel. I have had this forever. What's interesting is that like you mention, if I unhook the tonearm cable from the phono preamp, this hum goes away. It is not evident in my digital set up at all. Boy, if only the analog output was a quiet as my digital (no source playing) I'd be an ecstatic vinylphile. Lately, I have been really playing around with trying to get my LP-S to sound how I want it and have been tube rolling and switching btwn two phono cables. It's interesting that my one cable, of which I have three, ( 2 of the same version one of a another version) all give R>L channel hum. Yet, my other cable from diff. manuf., is quieter and does not accentuate a right channel hum, but allows EM/RF "pops" from nearby kitchen appliances to enter the chain.
Recently I've found out (again) that touching the tonearm anywhere causes a mild hum. This I remember from before but forgot about and as far as I can determine is the direct result of the outside street powerline RF that plagues my system. I also assume that my low level ground hum in the analog chain is also related to the street powerlines. I have been unsuccessful in my many attempts to get rid of the effect w/o success. Short of moving my system to another room, maybe someday, I have little confidence I will ever rid my system of it. But, the little tool mentioned for meas. current may be a new start!!
 

ack

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Hi there, so it has also been my conclusion that phono cables have A LOT to do with hum, though most would tell you (and correctly so) that you have a grounding problem somewhere. Well, yes, grounding issues can be a big problem with phono, but I think I addressed this years ago. That left the phono cables, and indeed, like you, different cables will give me different hum results, and it all has to do with the shield they employ - even going XLR-to-XLR where there is no separate ground wire; for example, in your case where touching increases hum, try twisting the two channels around each other, and hopefully you should notice a big improvement (this worked well with the Valhalla and Shunyata XLR-to-XLR). Interestingly enough, the MIT MA-X seems to offer the lowest hum levels of any cable I tried, but... I had to wrap its box and ground it as shown earlier. At the end of the day, once you fix any grounding issues, one still has to shield things properly. It would appear shielding a phono cable is extremely challenging, especially when it comes to keeping overall inductance and capacitance as low as possible.
 

Speedskater

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Sep 30, 2010
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I see so many people doing all sorts of random things in an effort to reduce hum. At some point they should get down to business and do some serious troubleshooting.

First build a John Windt 'hummer' and then follow the troubleshooting instruction (page 99) in this Bill Whitlock paper.

"An Overview of Audio System Grounding and Interfacing"
by
Bill Whitlock, President
Jensen Transformers, Inc.
Life Fellow, Audio Engineering Society
Life Senior Member, Institute of Electrical & Electronic Engineers

An audio system is rarely a plug-and-play affair. Noises often plague the
system even if everything is done according to conventional wisdom. Did the
electrician screw up? Is the AC power dirty? Is the equipment to blame? Are
the cables poorly shielded? This presentation will discuss the "secret" forces
that drive ground loops, how they contaminate our signals, and new tools to
make troubleshooting faster and easier. Among the topics will be equipment
design errors that can make life miserable for installers and users, how
manufacturers manipulate "specs" to deceive, how new test standards and
instruments can make manufacturers honest, why expensive cables probably
won't help, and things your electrician can do to reduce or eliminate problems.


http://centralindianaaes.files.wordpress.com/2012/09/indy-aes-2012-seminar-w-notes-v1-0.pdf
 

XV-1

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May 24, 2010
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Hi Ack

Just read this. :D

I have a XP-25 and at first there was so much hum, I was going to return it. the umbilical cord is maxxed away from the control unit and virtually hum free. I think I have a little more noise from one channel, but only noticable if I go up to the speakers.

I will have to try these metal shields thou to see what they do.

Did you find any loss of upper frequency air and life or closed in effect with the additional shielding?

cheers
 

RogerD

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May 23, 2010
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I would not use any anti emi material internally be careful as it can effect high frequency reproduction. If hum is a problem check the IC's and on a turntable there should be a ground wire to the preamp. If a piece of a equipment produces hum and it is not cable related and star grounding doesn't fix it,I would return it. External EMI shouldn't really be a problem unless it is really bad. Any good double shield cable should guard against that. The real problem is the internal EMI produced by the transformers in equipment and even though grounding schemes can mitigate some but it will not get rid of the necessary amount for great fidelity. A adequate pathway back to ground is the only way to truly make EMI a non issue.
 

ack

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Hi Ack

Just read this. :D

I have a XP-25 and at first there was so much hum, I was going to return it. the umbilical cord is maxxed away from the control unit and virtually hum free. I think I have a little more noise from one channel, but only noticable if I go up to the speakers.

I will have to try these metal shields thou to see what they do.

Did you find any loss of upper frequency air and life or closed in effect with the additional shielding?

cheers

There is no loss of anything, just gains (there are no power transformers in the main unit and no internal magnetic fields where I placed them, therefore, you are not re-shaping existing magnetic fields, just plain shielding from external influences). This phono is very susceptible to external hum, unfortunately, but it can be fixed (others on the net have simply surrounded it with steel plates, or placed it far far away from anything else) - you just don't put sensitive electronics in a plain aluminum case (the XP-25 or the MIT cable). By contrast, the Moon I had in for evaluation features what appears to be a proper stainless steel case (pretty much the quality case you'll find in the Spectrals and the Shunyata boxes, to name a few) and it was dead quiet from that perspective. Again, the hum I have described was only evident at max preamp gain, a total gain of about 100dB. Still, hum should only be as high as hiss, if not lower (where I ended up). The other thing to keep in mind is that the XLR cables I tried were in pairs, thus not true "phono" - good phono cables are actually one cable, not two (this is one of the reasons I had to twist those XLR cables around each other).

The PDF that Speedskater posted is an excellent read and I would urge everyone to spend the time to go through it (took me about 8hrs, so be prepared). Here are some relevant excerpts:

WHY STAR-GROUNDING IS IMPORTANT (something discussed on WBF a number of times):
If unbalanced interfaces exist between two grounded points in a system, hum and buzz can easily become larger than the signal! - Ground voltage differences between outlets is effectively applied across the ends of the audio cables and directly added to the signal
- Other “alien” ground connections, such as CATV or DBS, can create comparable voltage differences

ON SHIELDING:

CABLE ISSUES
. A cable is a source of potential trouble connecting two other sources of potential trouble.
. Electric Field Shielding
. Provided by copper braid or aluminum foil
. Resistance critical for unbalanced
. Construction critical for balanced
. Capacitance
. Can reduces audio bandwidth in long runs
. Magnetic Field Shielding
. Provided by coaxial construction for unbalanced
. Provided by twisting of inner signal pair for balanced
. Copper and aluminum have no effect on LF magnetic fields

Strange that cable manufacturers still don’t “get it” ... they seem to think “floobydust” is more important.

Electric Field Shielding
. Electric fields exist when voltage between two points is high
. Capacitance Cc is formed by air space between conductors
. AC voltage causes AC current flow through Cc
. Current flows in signal line without shield, creating a noise voltage
. Grounded shields divert noise current harmlessly to ground
. Without shielding, noise current may couple unequally to the two signal conductors of a balanced interface (creating noise signal)
. Twisting improves match by averaging physical distances to the field
. Braided shields with >= 85% coverage are usually entirely adequate
. An issue with high I/O impedances in vintage vacuum-tube gear but trivial in modern systems
. Some "audiophile" cables have no shielding at all - wires are woven
Capacitance exists between any two conductive objects, whether intentional or not.
Closer physical spacing and increased surface area both increase capacitance.

ON DIRECTIONAL CABLES (shield connected at one end only, and why connecting at the source is better):
. A shield grounded only at the *receiver* forms pair of low-pass filters for common-mode noise - bad
. The two mis-tracking filters will convert a portion of common-mode (noise)to differential-mode (signal), degrading CMRR at high audio frequencies
. A shield grounded only at the *driver* eliminates both filters - good!
. Because all filter elements move together at the driver ground voltage


Shield Connections and Crosstalk
. Driver signal asymmetry and cable capacitance mismatch cause signal current flow in the shield at high audio frequencies
. Shields grounded only at the receiver force this audio current to return to the driver via an undefined path – often resulting in crosstalk, distortion, or oscillation problems as it flows in sensitive circuitry!
. Shields grounded only at the driver allows this audio current to return directly to where it came from – the line driver and avoid problems
. Always ground the driver end of a balanced cable, whether or not the receiver end is grounded
. It’s OK to ground both ends – experience has proven it but, for the ultimate attainable interface CMRR, ground only at the driver end

I remember a prototype console checkout at Quad-Eight (when I worked there around 1973) where the internal wiring grounded shields only at the receive end. As the fader was pushed up, with no signal input, suddenly the output VU meters slammed into the pegs. The entire signal chain was breaking into oscillation at 300 kHz!

WHY I USED MU-METAL FOR THE MIT BOX AND THE XP-25 (given that hum varied by distance to magnetic field source, with either component):
Effective magnetic shielding for 60 Hz fields requires enclosing conductors in steel conduit or special magnetic alloys

ON THE IMPORTANCE OF SHIELD CONSTRUCTION:
Shield Current Induced Noise (SCIN)
. Current flow in cable shield creates a magnetic field extremely close to the twisted pair inside
. Physical imperfections in real cables result in unequal induced voltages, which adds the difference to the signal
. Dubbed SCIN in 1994 AES paper by Neil Muncy
. Best cables use braided or counter-wrapped spiral shields to create a very uniform magnetic field around the twisted-pair
. WORST cables use a DRAIN WIRE, which “hogs” the shield current and distorts the magnetic field (whether it has a braid or foil shield)
. For low-level signals, cables with drain wire should either be avoided or connected so that no current flows in the shield (i.e., grounded at one end only)

ON HYBRID SHIELDS (using also a capacitor) - OR WHY CONNECTORS THEMSELVES ARE REALLY IMPORTANT AS WELL:
“Hybrid” Shield Grounding
• Resolves the audio frequency vs RF conflicts about shield grounding for balanced audio cables
• At audio frequencies, grounding shields only at the driver produces the best possible interface CMRR [ack: Common Mode Rejection Ratio]
• But, at radio frequencies, grounding shields at the receiver often reduces or eliminates RF interference
• “Hybrid” grounding uses a capacitor to ground the receiver end of the cable at RF frequencies only
• AES Standard on Interconnections, AES48, issued in 2005 (after years of agonizing debate) includes this method
• The capacitor must be constructed and connected in a way that makes it effective up to at least 1 GHz … and that’s not trivial!

For an example of such a good connector, see PAGE 96 - The Neutrik EMC Connector - A THING OF ENGINEERING BEAUTY. Compare with the plastic, crap Amphenol XLRs that came with the Transparent Reference phono cable I also had in for evaluation.
 

Speedskater

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Sep 30, 2010
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Way off topic but the word "floobydust" does not mean what it appears to mean.

In the 1976 National Semiconductor - Audio Handbook the word “Floobydust” is defined as:

“Floobydust” is a contemporary term derived from the archaic Latin miscellaneus, whose
disputed history probably springs from Greek origins (influenced, of course, by Egyptian
linguists) - meaning here “a mixed bag.”

Later in 1991 Bob Pease (Staff Scientist at National Semiconductor) defined the word in his
Book “Troubleshooting Analog Circuits” as:

“Floobydust” is an old expression around our lab that means potpourri, catch-all, or miscellaneous. In this chapter, I’ll throw into the “Floobydust” category a collection of philosophical items, such as advice about planning your troubleshooting, and practical hints about computers and instruments.

Bob Pease (RIP) sometimes used the word in his columns in Electronic Design Magazine.
 

Speedskater

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For a mostly audio version of the Bill Whitlock Presentation see:

http://soundmarketingreps.com/bill-whitlock-presentation-available/

This is a huge Audio/Video file.
It's about 1 hour & 45 minutes long.
It will only be available through June 11, 2014
The video is just PowerPoint slides from the Seminar paper (only about 2/3 of the slides are used in the presentation).
http://centralindianaaes.files.wordpress.com/2012/09/indy-aes-2012-seminar-w-notes-v1-0.pdf

*********************************
There is an older 2005 version of the 2012 seminar paper. It has more words and less pictures, it is available at Jensen Transformers.
http://www.jensen-transformers.com/apps_wp.html

**********************************
In doing a search for Bill Whitlock you will find many of his papers from the last 15 years that expand on the different topics in the seminars.
 

ack

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And since we are talking about XP-25 modifications, another very significant one I did is to remove the 100pF load capacitors labeled C21 and C22 on the main unit's board (after consulting with Wayne Colburn at Pass), in order to raise the load resistance to 1K. For the explanation behind this, see this thread http://www.whatsbestforum.com/showthread.php?15077-Cartridge-Loading-A-Misnomer ; but briefly:

Increased capacitance between cartridge and phono amplification circuitry will lower the frequency of the ultrasonic spike, which requires the application of lower resistive values to dampen the spike. Reduced capacitance between cartridge and phono amplification circuitry will increase the frequency of the ultrasonic spike, which can be tamed with higher-value resistive values.

Essentially, higher resistive loading results in better transients and higher resolution; as you will read, effectively, resistive loading has nothing to do with the cartridge and everything to do with the phono stage.
 
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XV-1

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And since we are talking about XP-25 modifications, another very significant one I did is to remove the 100pF load capacitors labeled C21 and C22 on the main unit's board (after consulting with Wayne Colburn at Pass), in order to raise the load resistance to 1K. For the explanation behind this, see this [hijacked] thread http://www.whatsbestforum.com/showthread.php?14082-Sme-30-2&p=257062&viewfull=1#post257062 on down; but briefly:



Essentially, higher resistive loading results in better transients and higher resolution; as you will read, effectively, resistive loading has nothing to do with the cartridge and everything to do with the phono stage.

Interesting. I find the bass gets a little sloppy when it gets to 1k loading. Does the bypass of the 100pf load help that?

Do you have any further information on the modification, or best I contact Pass Labs directly?
 

ack

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Interesting. I find the bass gets a little sloppy when it gets to 1k loading. Does the bypass of the 100pf load help that?

I guess I never had sloppy bass, so don't know; and I can't see how loading would do this

Do you have any further information on the modification, or best I contact Pass Labs directly?

There isn't much to it, you just unsolder these two caps - take off the top plate and then all allen screws on the board plus the allen screws on the back, in order to release the entire board plus the back plate and get access underneath the board to unsolder; takes about half an hour
 

XV-1

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I guess I never had sloppy bass, so don't know; and I can't see how loading would do this



There isn't much to it, you just unsolder these two caps - take off the top plate and then all allen screws on the board plus the allen screws on the back, in order to release the entire board plus the back plate and get access underneath the board to unsolder; takes about half an hour

That is a job for a technician, not me :D

I will email Pass and have them pass the appropriate info across

Thanks
 
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ack

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Not a bad idea to leave it to a pro
 

ack

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Final mods

During this year-long investigation, I realized the following:

a) the XP-25 is not shielded well at all (unlike, say, the exceptionally-constructed Moon 810LP) - not surprising, Pass are very well made, just not to fanatical standards (perhaps the Xs line is better?!?). It also appears to be more susceptible to hum pick-up from underneath than above.

b) the Alpha DAC's transformers are the main source of very significant amount of environmental hum, that the XP-25 picks up, especially when the DAC is under it, even if set 2ft apart; the DAC sits now well above the XP-25. In sharp contrast, the Etude tuner sitting directly on top of the XP-25 contributes no hum at all (not surprising perhaps, it is extremely well shielded as to not affect its own FM section)

These final modifications have rendered the XP-25 as quiet as perhaps possible (and certainly A LOT quieter than out of the box), to the point it's not affected by whether the DAC is powered on or not anymore. In addition, at max preamp gain, switching between the two inputs on the XP-25 (used and unused) makes for a very clearly audible difference, with the used one having significantly lower noise than the other. At normal and even loud listening levels, up about ~75% preamp gain, there is virtual digital silence with the XP-25's used input.

XP-25 shield top and bottom - copper over mu-metal (mu-metal is usually used for shielding below 100Hz, copper over that and really for RFI/EMI shielding):

xp25-shield1.jpg
xp25-shield2.jpg

JMW 10.5i - additional 0.5g copper mass still keeps overall resonance in the green zone (at this point, the arm is a combination of steel, aluminum and now copper):

jmw-shield1.jpg

mu-metal separating the Alpha DAC's transformer and main board - you feel the magnetic field as you are about to place that vertical mu-metal strip between them, therefore you know it's leaking over to the main board:

alpha-shield1.jpg

the single ground wire plug where all power-connector shields shown upthread finally meet:

ground-plug.JPG
 

ptss

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Any experience with the 3M EMI isolation and metal foil damping products. Could the tin plated copper be effective as mu metal with a few layers? It might be easier to work with?
 

Speedskater

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Sep 30, 2010
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3M makes almost countless products, which are you thinking of?
Mu metal and copper foil work in very different frequency ranges.
What sort of problem are you trying to solve?
Moving components around is often the easiest solution.
 

DasguteOhr

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LL21

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great thread! Thanks for taking the time...interesting.
 

ack

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Holland Shielding makes great products; the mumetal and copper shields that they make should be really effective, based on my own experience with the shields I have made; for example:

Theirs:


Mine (shielding of the RF section in my tuner):

etude-shield.jpg
 

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