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In Reply to: RE: Unfortunately... posted by geoffkait on January 14, 2012 at 11:23:38
Hi, Geof,
Long time, no chat.
So, now that we've got a vibrating fuse clip....which is firmly attached to the board, we've got a vibrating board? The fundamental of all this is 60hz...the resistance of the wire.....nearly negligable....so, how much force could possibly be transmitted to the board? By the time you hit the 3rd or 4th harmonic of the powerline freuqency you must be in the sub-picodyne region..... certainly near the molecular noise floor....
Sorry, I don't buy it.
I would suggest an experiment! Put a fuse in a box of a couple cubic feet. The box also contains a speaker connected to an amp/source. Run 60hz thru the fuse. Don't pop the fuse. You'll also need a dual trace scope of as high a frequency as you can manage. certainly 100s of MHz. have a second fuse, wired as the first in a similar box with NO speaker. Put 60hz thru that one, too. Look at BOTH traces in some kind of either out of phase or 'subtract' mode in the scope, such that only differences appear. Nothing should be on the screen, the traces being identical. Turn on the speaker....loud...no...louder! See anything on the scope? Didn't think so.
From this I'd conclude the microphic fuse myth busted
Too much is never enough
Follow Ups:
Take any fuse, listen to it, paint it with AVM (Anti-vibration Magic) on the glass, listen to it again. Finally, explain why it sounds much better with the paint on it.
Of course a PCB can vibrate. Question is, if a fuse can do it? And if so, will the other sources of vibration overwhelm the fuses contribution? Will the multiple resonant modes of all the stuff attached and hung from a board not damp this whole system?
Now, yes everything vibrates and so what? TTs are vulnerable, of course, as are tubes. In both cases a clear feedback loop exists. A TT cart is designed to take very low level vibrations, connect to a pretty hi-gain circuit and put the result to the speaker.
Racks are well developed for seismic events. However, if you believe that fuse vibrations to the board (!) have an effect, than surely airborn vibes have an effect as well, and when the sytem is played loudly, will have more effect than a fuse. After all, the fuses contribution will be based on the mass of the wire, the amount of force it can exert and the frequency. I don't know what the fusing wire is made from....magnetic or not? But certainly all will agree that the amount of force a vibrating fuse wire can contribute is MINUTE a best.
I have yet to see a 'soundproof' rack which would be the next logical development.
And while we're talkin' vibes, how do you isolate your system from the Schumann resonance? I think that is somewhat <=to 8hz.
If someone wants to loan me a scope, I'll run whatever test I can devise and not only put the data up for review, but certainly remain open to experiments to run..
I asked for a used Tektronix for my birthday, but am not holding my breath.
Too much is never enough
"From this I'd conclude the microphic fuse myth busted"
From WHAT? Prey thee show us the data from your wonderous test...
You may be surprised if you actually did the work which is why most of us don't draw conclusions from thought experiments!
"So, now that we've got a vibrating fuse clip....which is firmly attached to the board, we've got a vibrating board? The fundamental of all this is 60hz."
Yes to the first part, no to the second. The fundamental is really 120 Hz unless you are using a half-wave rectifier.
You don't think PCB's vibrate? Stick it in your ear! Here's how:
- tie a foot long thread to one of the corner mounting holes.
- Wrap the other end around your little finger a few times.
- Stick that finger in your ear.
- Tap the board in various places with eraser end of a pencil or just a finger of your other hand.
Isn't that cool? If the boss doesn't catch you you can learn to play the melody and bass line at the same time by tapping the right spots for the melody at each beat. If sticking it in your ear seems crude and the board has any low level analog circuits you can probably extract the same information at their outputs but you may need some additional AF gain. Dies are intrinsically little strain guages and the newer packages provide rather tight coupling mechanically between the chip and the board.
Yes, the resonances are different when the board is screwed down. Just how they change depends largely upon the location of the mounting holes. I've actually had to have the PCB mounting holes moved on some gear to keep case flexure from modulating the offsets of op-amps and showing up as orientation sensitivity to users.
Rick
I guess I assumed everyone has been briefed on structural (seismic) vibration, motor and transformer induced vibration, not to mention vibration produced by acoustic waves from speakers. It's been painfully clear for many years that everything in the room, including the boards, chassis, wires, etc. are vibrating over a wide range of frequencies, not just 60 Hz. You're not suggesting we all throw out our vibration isolation stands, one hopes.Tootles
Edits: 01/14/12
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