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Microphonics? Take them away!

Posted by Bambi B (A) on August 19, 2010 at 06:11:42

In Reply to: MICROPHONICS TAKE AWAY posted by Anacrusis on August 18, 2010 at 16:03:37:

Anacrusis,

You write:

"If microphonics are the audible result of the excitation of the internal structure of a tube caused by external stimuli ie mechanical vibration doesn’t calling a tube microphonic incorrectly imply that the tube itself is at fault. Is this really any different than saying a phono cartridge that exhibits audible feedback because it is insufficiently isolated from external vibration is at fault?"

Active microphonics in a tube is caused by external excitation of the structure, but it is the characteristics of the bottle/plate structure interaction- a resonance inherent in the structure that creates the conditions for microphonics. In that sense, it is appropriate to call the tube "microphonic" as it is the component "allowing" vibrations to excite the structure and become amplified. Logically it is the tube that possesses the microphonic mechanism.

One can see it is the tube that acts in this way as the most effective resonance mitigation- tube damping rings- are applied directly to the tube. Of course, chassis typically have attention to vibration damping to some degree.

Likewise, I would say that a phono cartridge that feeds back vibration is the component at fault also, but a better term than "fault" would be "engineering challenge". However, in that case, the damping is applied to the chassis so as not to increase the cartridge mass.

"Why would a designer use ceramic tube sockets when ceramics are also used as transducers (tweeters)"

The use of ceramic tube sockets is due to ceramics having a high resistance to the considerable heat. If your intuition suggests ceramic sockets may be a better conductor of vibrations into the tube than a plastic- it's very possible, but remember that the socket is tied to the mass of the chassis and that lowers the resonant frequency and damps vibrations as well. I've seen ceramic sockets that are mounted to the chassis with a metal retaining collar that has a high temperature elastic damping ring sandwiched between the socket and chassis. Those may be used for mobile use- car and military/police radios were tubed into the 60's so there must have been some attention to vibration. And, I never remember seeing tube dampers until the 1980's.

Keep in mind that the ideal speaker cone would be infinitely light, infinitely rigid, and have no resonance, plus the cone is mounted on a flexible ring that has a considerable damping factor.

Microphonics is one of those interesting collisions of the electronic and mechanical effects as are certain vibration effects in turntables- they have damped chassis suspensions and the tonearms are sometimes damped- to lower their resonant frequency below the audible. Transformer buzz is another- a mechanical oscillation caused by electrons arguing while at work! When CD's first arrived, there were some naive few that thought that finally the need for such care to vibrations as necessary for turntables was all over, but take a look at those $XX,XXX CD players and the efforts to optimise mechanical vibration damping of the chassis, isolating and damping the transport appear to have a substantial cost.

I once tried some unselected Sovtek non-EB 6DJ8's in an Audio Research SP10 phono stage- and without damping rings. Every movement of the stepped gain control was clearly audible in the speakers as was just brushing a finger on the gain knob. If I shouted very near the tubes, this would be picked up and amplified as well!

In analogue, there is inevitably a mechanical data storage device- whether, wax cylinder, LP, tape and with that comes vibration (and speed variation) considerations. With digital there is a choice of mechanical storage device- hard drive- or chip. I think of the mechanical HD as the signpost of the primitive computer era. When the HD is gone- which should be only a few years, much will be possible,..


Cheers,

Bambi B


My personal motto: No matter what is your wealth, power, or strength, the cheapest things in life are free."

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