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> Pcb Shock Mounts
kingneb
Posted: February 25, 2018 06:11 pm
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I am building a tube amplifier that contains a relay attenuator (see picture below).

https://ibb.co/kGfBOH

Electrically, I am having good luck but mechanically it is another story.

Whenever the relays switch, the mechanical noise they make is unacceptable loud. The aluminum chassis is the main problem. It acts as an echo chamber.

I mounted the board on rubber bumpers. This helped with the noise but the mechanical configuration is not ideal. Given that the rubber bumpers compress when the bolts are torqued, the board bows. This forces me to tighten each bolt while eyeballing the board so it does not bow.

I need a shock mount that best suppresses my relay noise and allows for easier, less potentially destructive, installation.

Does anyone know of a product that would best suit my application? My experience with mechanical engineering is limited. The mount needs to establish electrical continuity between the chassis and the board for EMI reasons.

My current mounting configuration uses the following stackup (see pictures below):

https://ibb.co/keWtLx

https://ibb.co/c3iUtH


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MacFromOK
Posted: February 25, 2018 11:58 pm
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Wouldn't it be better to mount just the relays on rubber/foam (or whatever) rather than the whole board? Or perhaps wrap the relays in foam?

Not a tube guy, just a couple thoughts.

Luck. beer.gif


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kingneb
Posted: February 26, 2018 12:04 am
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I do like the foam idea. It even occurred to me a time or too.

Mounting them on foam is not that simple as they are PCB mount. Foam around them is possible.

Is there any particular type of material anyone suggests?


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johninmi
Posted: February 26, 2018 03:03 am
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I think that as long as you have a rigid metallic screw attaching the PCB to the chassis, you're defeating your attempt to (mechanically) isolate the PCB from the chassis.. I would think the screw "shorts out" the effect of your rubber bumper..

Suppose you were to attach the PCB via plastic stand-offs such as one of these, then provide your electrical continuity by braid straps?
http://i1080.photobucket.com/albums/j332/j...20Stand-off.jpg
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Sch3mat1c
Posted: February 26, 2018 03:05 am
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Get the kind that's a metal collar inside (that the nut clamps down on) and a rubber grommet around. Finagle that into the board, screw it down and you're done. smile.gif

Or the axial kind, threaded stud on either end, rubber cylinder in the middle. You may want additional damping, they tend to be kind of wobbly and have a modest Q factor.

Add acoustic damping. Relay clack isn't very low frequency. Think conducted vs. radiated emissions. Which is, well, literally the case here, just acoustic instead of electromagnetic. Same thing. biggrin.gif

Isolation mounts will help, but not eliminate it, if the board itself is moving and transmitting that vibration, through the air, to the aluminum panel.

The converse is probably not true: acoustic damping will help, but not all that much if vibration is still being coupled through rigid screws/standoffs to the panel.

So both is ideal.

You could maybe combine the two by using foam tape (VHB or the like). Not ideal around hot parts or a need for servicing, but there are ways around that (depending on how fiddly you want to get). Example: PCB is taped into a metal bracket, bonded along the corners or edges. Most of the bracket is hollow, allowing access to the PCB solder side. PCB is not rigidly mounted, isolating some vibration (and also dampening vibration within the board, so it tends to go 'thud' more than 'ping').

There's also the traditional audiophool construction methods, like using ridiculously heavy plate, especially steel or copper, or machined billet. Akin to bypassing a supply with more and more capacitance, rather than simply adding another pole or two to filter it properly -- yes, it works, but not very well, and a two pole (or higher order) method gets there with much less material.

On a related note, how's tube microphonics treating you? If it's not ideal, you might cut some slots in the PCB -- make some PCB spring sections if you need traces to span the gap -- and support either side of the gap with mounting hardware, dampers or the like. Or a multi-board assembly with cables between

Tim


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CoulombMagician
Posted: March 09, 2018 05:54 pm
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Nice looking board. However you choose to mount the board it should not be bowed or under stress, it may take some time but that will affect the board reliability especially with thermal cycling.
You can get nylon standoffs and mix those in with the metal ones using the metal ones to make your chassis(safety) ground connection, that should help some. You might be able to use all nylon standoffs and make your chassis connection with a small lead wire?
Covering the Relays with foam should help a lot as long as the relays aren't dissipating much heat.
You can also add small patches of foam, they don't need to be too thick, to open spaces on the inside and outside walls of your enclosure, they will also help dampen the sound, kind of like the inside of an anechoic chamber.


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