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[Worklog] Faraday Cage for Vacuum Tube Headphone Amplifier


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#1 TheFrunj

TheFrunj

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Posted 13 August 2013 - 10:00 PM

ElectroMagnetic Interference EMI, Radio Frequencies, 'noise' - I got some.

My equipment:
PC -> Coaxial Digital cable $9.95
DAC -> Matrix Mini-i 2012 ed (the internal headphone amp worked for a few months then died, but it was pretty meh). $340
RCA Interconnect -> Space HiFi 'Saturn' 0.5m cable $19.95
AMP -> Bravo Audio Ocean Vacuum Tube Headphone Amplifier $120
Genalex ECC82 12AU7 Replacement Tube - $45
$534.9

Headphones -> Audio Technica ATH-AD900 $270
Noise Cancelling when gaming (pc is loud with fans turned up) -> Audio Technica ATH-ANC9 $250

My setup is pretty budget, but I chose components based on a warmer sound. Some purchases were because of other things breaking, but overall it gives me great sound reproduction and many hours of enjoyment.

However, I have recently upgraded the RCA interconnect from the DAC to the Amp, and as it's a much better cable, I can hear much more interference from local sources than before - it's really annoying. I've isolated the amp from everything else, having it on with no RCA cables attached, and it's definitely the amp picking up noises. Whether it's my mobile phone calling a tower (which I never really noticed before), the wireless remote for my ceiling fan causing a whir, the air conditioner on the other side of the house causing a hum, or even my Varta AA battery charger - it all makes juuuust enough noise to be annoying.

The first thing I tried to fix it - swap power points - didn't work, nor did buying ferrite chokes to clamp onto my cables. Desperate times call for overly elaborate and potentially silly measures, so my latest plan is to create a Faraday Cage around my amp. To do this I have purchased some supplies, as pictured below (click for fullsize):

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Faraday Cage Supplies

Non-stick bread loaf baking tin $14.15
21.5cm L x 10.5cm D x 10cm H
I messaged the eBay seller about the material, they said it was anodised aluminium, but magnets stick to it so I imagine it's powdercoated steel. Peeerfect.

Xilence Red Wing 80mm Hydro Bearing Fan $8
Perfect size fan, vibrates a bit when run on 12v but has little movement when at 7v or lower. Will run off 5v USB using an adapter cable. Possibly overkill but I want the thing cool (as evidenced by the large number of heatsinks I stuck to the amp).

80mm fan grille $1.69
To be used on the inside of the enclosure to keep cables out of the fan blades.

80mm Stainless Steel Fan Filter $4.88
On the outside of the enclosure, the steel mesh will hopefully close the cage while stopping dust and cat hair from entering.

Metal 'L' Brackets for Shelves $2.23
Sturdy protrusions to attach the amp to and suspend it in the center of the enclosure.

16mm M4 bolts with matching M4 nuts $5.70 + $3.86
Mount the fan and brackets to the enclosure.

2.5mm wide 100mm long cable ties ~$1 or so worth

Monoprice 6ft 3.5mm Male to Female 22AWG Extension Cable $12
Super heavy duty cable, will be threaded through the enclosure wall so I can easily swap headphones.

EcoSwitch 240V AC remote switch $10 on sale, normally $20 or so
Sometimes I use my computer without wanting audio, so being able to control the amp's power status without accessing it directly is a must. Thankfully this does just that.

Total cage enclosure setup cost: $63.51 (I'll have some nuts and bolts leftover for other projects so really it's a few bucks cheaper).

Planning Stage And Test Fit

To test that my idea was even feasible, I downloaded a neat app called Real Signal that graphs phone reception over time, set it running, and then chucked my phone into the baking tin and closed the lid. Waited a few seconds, opened it up, and hey presto - zero signal.

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Next step was to test-fit the Bravo Audio Ocean Amp to make sure I had measured correctly. As luck would have it (and measuring about three times before ordering), it did.
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I then plugged in the Monoprice headphone extension cable and routed it underneath the amp along the path I'm going to have it go, and it all fit with plenty of clearance.
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Grabbed a pencil and the 80mm fan grille, and used it as a stencil to mark the holes I'll need to drill to mount the fan.
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I also used the grille as a stencil for the fan cutout itself, so I know how much metal to remove later on. Ideally I would have applied masking tape to this area before marking with pencil, and before cutting, but the roll of masking tape from my PC build worklog was MIA and I was lazy.
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Then I measured 9.5cm from the opposite end of the tin (i.e. the end that the power and audio cables will go) and made a mark on both sides. I made another mark 3.5cm in from the end. This is where I'll drill to mount the L shelf brackets.
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Finally I measured the amplifier's connectors and spaced out the power socket (the small square to the right), the RCA input jacks (the tall rectangle) and a 2x1cm hole for the headphone extension cable to pass through underneath the amp. I fully anticipate to have to finagle the holes a bit wider with a file later on, but I'm not too worried.
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More to come!

I apologise if the formatting isn't as nice as my previous worklog, Photobucket have disabled direct linking of images so I'm using imgur and have a choice between 90x90 thumbnails and these big'uns. I've uploaded the pics to Facebook as well, but they reduce the quality so it's just a back-up in case this thread is found in a couple years and imgur has lost the pics. Hope it doesn't bother anyone too much!

This worklog has taken a lot of time to throw together, so please post below and let me know your thoughts on the project, any tips or suggestions are welcome :) More to come when I get the drill and dremel out.

Edited by TheFrunj, 13 August 2013 - 10:26 PM.

"TheFrunj doesn't guess, when he forms words with his lips, the world will Warp and change to do his bidding." - Shikimaru No longer writing for Atomic :( http://forums.atomic...showtopic=49899




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