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Author Topic: Do you need to do more than ground a mint tin enclosure?  (Read 821 times)
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Hi,

After making a couple of mintiboost kits I've been keeping my mint tins of various sizes in the hope I'll be able to use them as small project enclosures.  The first I'm trying is this Collin’s Lab: DIY Contact Mic which is a simple enough circuit that works fine outside of the tin.  But when I mount it in the tin, the gain on the little amp drops back to mostly terrible, with bursts of unbearably loud (granted the guitar amp is turned up to 11 to compensate).  There are enough metal enclosures out there (my guitar pedals even) in audio that mean I have hope it should work.

I expect this is something to do with grounding the tin, since the 1/4" jack for the amp is mounted through it, as is a 3.5mm jack for the contact mic.  I've experimented with additional earth wires mounted to the tin itself but so far am not improving things.  But take everything out of the tin, and have the jacks out in the air and all is fine, all the time.

The threaded part of these jacks is also the earth, thus my thoughts it is an earth thing.  So my question is - what is likely the cause of this, and what should I be doing to dampen this effect...and is there something you can point me at to read up on why my experiment of directly wiring the tin to the circuit's GND isn't helping? 

Thanks for your advice, Geoff
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Sounds to me like it is very high gain and perhaps is picking up alot of noise from something else
what if you were to isolate the mint tin? It doesn't have to be electrically isolated, this sounds like a physical problem in design
 Like use rubber washers on the jack or something else to prevent contact, since the tin is metal it is absorbing lot of sound and vibration which competes with the actual sound you want to amplify
try mounting it and leaving the tin on something really sound absorbant like foam and see if that helps too
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It sounds to me that mounting it in the tin has caused a short of some sort.
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Thanks for the quick replies - looks like I was barking up the wrong tree looking for a grounding issue.  I'll wrap the board and bits in more tape to see if I can isolate it further, and that should also eliminate any possibility of a short.

As a rule should you wire GND to a metal enclosure, or should that be avoided, or doesn't that matter?
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It would depend on the situation, some circuits would benefit from the shielding and larger groundplane while some circuits wouldn't care, although I imgine so long as your not doing anything wireless from in the tin grounding the box couldn't hurt
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Hi again,

Further testing on this identifies it's the grounding of the 3.5mm jack, the connection to the piezo pickup, that causes this symptom.  The intermittent loud bursts are not reproduced if the jack is mounted firmly, and I can kill the output by wiring the unmounted jack to the tin via alligator clip.  Adding a direct ground connection to the tin and/or the jack has no improvement.  I also tried an alternative 3.5mm jack part with the same result.

In this case it would appear one of the electrical properties of the mint tin is not tolerated in that part of the circuit so I'll try an alternative connector next that stays electrically isolated (pretty sure RCA or even a DC type might suit this) and failing that will directly solder the piezo wire to my board.

Thanks again for your collective advice, Geoff
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As a rule should you wire GND to a metal enclosure, or should that be avoided, or doesn't that matter?

Actually its quite complicated - you want to avoid (signal) ground loops, yet you may need to (mains) earth metal enclosures.  In general signal ground and mains earth should be separate (except at maybe one point)  Using chokes you can connect grounds at DC but effectively separate them at signal frequencies.   The main issue is ground loop currents flowing along the shields of screened cables between enclosures - the IR voltages then add to signal voltages typically leading to strong mains hum.

If the signals aren't low-level audio it will matter less, typically RF signals have case ground the same as signal ground for instance.  Logic levels have a volt or so of noise immunity so its less of an issue unless a very electrically noisy environment (lots of big motors in a factory).
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The noise can be reduce through proper plugin of wire and integrate the RF to its particular wavelength for the required signal but ut may also be due to other environmental diffraction so try to make the transmission signal more reliable for the signal   
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You Might also try some more by-passing capacitors. Guitar pickup voltages are fairly low in level and encasing the device in a metal case provides several issues. One is unwanted coupling through the can, you might try to make the ground connection at the case through the input and output cables only and it might be one or the other but not both... necessarily. The cables to the board should be shielded? or at least twisted 8T/30Cm any additional capacity here is negligible compared to the actual load and stray capacity. the ground and the board ground might well be the conflict that is causing you the feedback problems But don't forget the By pass caps at least 100uF.

Doc
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You Might also try some more by-passing capacitors. Guitar pickup voltages are fairly low in level and encasing the device in a metal case provides several issues...
Thanks again everyone.  I think I'm going to ditch the idea of using mint tins for this.  The last ones I made went into a smartphone belt clip and they worked perfectly, but weren't stiff enough for plugging in 1/4" jacks repeatedly.  These tins have the strength, but isolating the two jacks from the tin is a nightmare.  Overnight I've tried shrinkwrapping the first few mm of thread that goes through the tin, and using a plastic washer front and rear to avoid connections with the flat tin face inside and out, but still there's an intermittent connection to the tin there when you put force on the jack. 

I might try your big decouple cap idea Doc just for one last shot at it, but I'm now thinking for the sake of 5 small plastic enclosures (and the associated loss of cred without the re-used tin look smiley-wink) I'll get less headaches, and they'll still fit potentially inside a belt clip.

Cheers ! Geoff
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Try ungrounding the PCB as well. you might find the error there Ground loops and high gain amplifiers have been issues since I worked with vacuum tubes.
The comment about high level RF was not too far from the point either. Very frequently when there were "too many" grounds Local "Clear Channel' (From the conelrad (?) days remember  seeing an american AM< radio... All of them produced in the 60's had a mark at 640 and 1240 Khz and those marks were to designate the Emergency AM radio frequencies regular AM radio stations that could legally broadcast @ 50 - 100KW 24/7 all the rest had to lower power out @ 6 or 10 PM I don't now remember which and is immaterial to the topic They were the ones that you heard at night if you had built a crystal radio... I made a lot of them using Galena crystals embedded in lead (for mounting and electrical contact) A piece of pencil lead and a "Blue Blade" razor blade were also very good and very sensitive too. Gawd the old memories that get awakened here, I hadn't thought of that stuff for many years.... All of it including audio feedback and am radio's getting into PA systems... But that's where I first encountered the issue. We ised to add 1000 - 5000 pF caps on inputs to reduce the level of the offending AM signals... Sometimes the amplifiers had to "Go to the Shop" for adding capacitors to the control grids of the input stages of the amplifiers. Rf Chokes were commonly added as well to form "Low Pass" networks. Remember too that these were 50 to 200 watt tube type amplifiers with perhaps 100 ' of 600 ohm "Low Impedance" microphone cable attached to the inputs and the microphone had a switch that usually shorted the microphone to ground... So there was a "longish" Antenna connected to the Pre-amp input. This was why I didn't make too much ot the OP's comments about possible RFI. If that device works well in a plastic box then there will be a grounding issue and /  or a By-pass issue possibly both. Forget trying to insulate the jackks like you mentioned. The proper technique is to mount the connectors to a small bit of plastic and mount the plastic to the metal case. At one time (Back in the day...) there were available shoulder washers made of fiber based non conductive material commonly sold in any electronic's store just for this reason. It was Switchcraft in the US...
Well another perfectly good post shot to hell because of my reminiscing...

Doc
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