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

Being OCD with all things audio, I have taken on a task to minimize or eliminate any residual hum in my Pass XP-25 (as rendered at max preamp gain) and have found a number of interesting things. For one, a lot of owners claim the XP-25 is very susceptible to magnetic fields, and place them as far away as possible from everything else; for another, Pass warns you to place the power supply unit far away from the amplification unit (some owners report no issues if it sits on the side); then, there is the whole different issue of hum picked up upstream by the arm, phono cable, etc; and finally, hum emitted by surrounding power cords, components and all other electronic devices.

Assuming one has properly star-grounded all components and properly grounded the analog chain, there are magnetic fields generated all over the place: from power supplies, to unshielded cords (or shielded cords with unshielded power connectors), even "mysterious" sources (see below). In my quest, I found two free relevant iPhone apps that are an eye opener in tracking down magnetic fields, and they both measure their strength in the X, Y and Z axes (I believe they just expose built-in functions in the phone):

1) https://itunes.apple.com/us/app/magnetometer/id342782714 - this will quickly tell you where the strong magnetic fields are, and it will show you a grand total in microTesla (see app's pics)

2) https://itunes.apple.com/us/app/sensor-kinetics/id579040333 - this one measures a lot of things, and also includes a magnetometer function, but it also graphs it - this is really important. If you happen to place your iphone over a power supply, you may just see the frequency fluctuations in all axes as a function of the change in the magnetic field strength. There will be no magnetic field leak if all axes converge on zero.

So here's what I found out in my system:

1) The Spectral amps are all "green" - this is really nice
2) The Spectral preamp's power supply area emits a "red" magnetic field, fairly high in intensity
3) The Spectral transport seems to have static magnetic fields in the back (ie. they exist even if not powered on)
4) The XP-25's power input section from its external power supply emits a "yellow" strength field - not sure what happens internally, though
5) The Alpha DAC is all "green" - consistent with my observations of its board layout and attention to noise attenuation throughout the unit
6) Now here is the most interesting thing of all, referring back to "mysterious" sources: the VPI's junction box (with or without a phono cable attached) - yes a passive component - emits a "red" field on the right channel area and a "yellow" one on the left.
7) I have posted before that the MIT MA-X phono cable's box appears to be acting as an antenna, and may not be extremely tight in its shielding

Before proceeding, let me repeat, I have no phono hum issues at high listening levels, I am just obsessing with what goes on at max preamp gain...

So #6 is really important to me and probably to other XP-25 owners as well, because most claim that the right channel has more noise than the left! In fact, with nothing connected to the XP-25, noise is exactly the same on both channels; it's only when you connect everything that the right channel all of a sudden has more hum than the left. The source, however, would appear to be nothing more than the VPI itself in my case.

The attack plan is to install mumetal (on order) where appropriate, to deviate unwanted magnetic fields away from the analog chain... and I have no idea what to do about the "magnetic" VPI junction box, other than attempt to demagnetize it.

BTW, if you want to test the effectiveness of the shield in your power cords, or discover other voltage leakages, get something like this http://www.homedepot.com/p/Klein-Tools-Non-Contact-Voltage-Tester-NCVT-1SEN/100661787#

Project KillNoise continues...

Grooves, great and informative response!

So the solution to my hum issues is so far three-fold:

1. RF-shielding of power cords' connectors and the MA-X phono cable's network box; the Shunyata power cords are shielded, but not the connectors
2. mu-metal shielding of the XP-25
3. mu-metal shielding of the MA-X's box

The RF-shield is simple copper tape and a metal braid for the power connectors (all RF shields are routed to a common ground) - using the Home Depot thingmajiggie, it's easy to verify the effectiveness on the power connectors (i.e. no buzzing from the instrument the moment you connect a braid to the ground). The audible benefits with phono are fairly evident, and more so with my FM tuner (probably because noise emission by connectors is greatly reduced).

The version of mu-metal I chose is 4-inch wide thin strips, cut to custom lengths: https://www.edmo.com/index.php?module=products&func=display&prod_id=18373&disp=desc&cat_id= . The material is easy to cut yourself, and has an adhesive backing. Again, with maximum preamp gain as a reference, hum reduction in the MA-X and XP-25 is simply staggering, about 10dB each. These products are not adequately shielded for my tastes. At this point, the used input on the XP-25 is quieter than the unused one, something that was never true before with any phono cable. Moreover, low-frequency noise is lower than high-frequency hiss - I call this success.

On the XP-25, I used two 13-inch strips on the bottom, and one 13-inch strip on the top, with the latter extending behind the unit's top plate as shown below:





Here's the MA-X with its RF-shield under one of the two required mu-metal pieces (didn't use the adhesive so I can slide those pieces, to access the articulation knob and impedance switch) - you can't see its ground wire, but it's there:



Here's a power cord connector with its braid shield under the heat-shrink tubing plus its ground wire:

Here's another view of the XP-25, along with copper tape around various power connectors during the testing and evaluation phase:



I have also put mu-metal strips over every other MIT interconnect and speaker cables' boxes, as insurance policy.

The ad-mu80 mu-metal costs about $4 per inch of length (standard 4 inch width). mu-metal must NOT be bent too much out of shape, otherwise re-annealing is necessary, and that requires a high-temperature furnace in a hydrogen atmosphere - impractical here

It's readily available from electronics outlets - just google it. I bought it from a local electronics store.

Shielded line-level cables

I just love coincidences... was just reading the Oct 2013 TAS and the review of the MIT SL8 interconnects says: The SL8 did have the poorest rejection of 60Hz hum and noise

So the order of the day is, shield your MIT cables, just in case

jap said:

There's a very simple solution, use balanced interconnection cables.

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Keep in mind my MA-X is balanced

BTW, keep in mind there is a fundamental problem when claiming that balanced cables can reject noise picked up by the cable itself: it assumes that the noise is in phase with the signal itself - i.e. the positive phase of the noise precisely coincides with the positive phase of the signal, and likewise for the negative part - which is a complete shot in the dark; this is, then, one of the reasons you still have to shield a balanced cable.

Speedskater said:

People often write about using elevator stand-offs to keep their cables off the carpet. A better plan is to run a grounded copper strap from one equipment location to the other and place the cables on the strap.

Click to expand...

Precisely!

EDIT: This is really why I don't care for any type of "cable elevator", even those that route static to the carpet - that static isn't effectively grounded to anything unless your carpet is on dirt or something, and moreover, static can appear ON the carpet and may be routed back up to the cables. Only a "cable lifter" that eventually properly routes to the ground is effective, and I am not aware of any.