Isolation/coupling: basics?

El: Not only can individual components and circuits be microphonic, you can literally hear the sounds eminating out of transistors that they are amplifying during certain conditions. In some RF amplifiers, this can be heard quite plainly and is referred to in the industry as "transistor talk". The only way that one could hear this is if the conductors inside the transistor casing were vibrating enough to resonate the outer case in time with the signal that was being amplified. If those vibrations can be transferred from inside the transistor case to the outside case, and be done in a fashion loud enough to be easily heard and understood, what is stopping external vibrations from taking a reverse path from the outside of the transistor case and modulating the conductors inside the case ??? My guess is that the same thing occurs in audio gear, but with audio gear, you typically have the system running in the same room. The sound from the speakers will smother the sound of the transistors "talking".

I've seen / heard / could not believe this, but one can talk into an inductor that is coupled to a high gain audio circuit and hear your voice come out of the speaker. It is not clear, but it is plainly audible. The inductor that i heard this demonstrated on was wound on a plastic former and dipped in wax to try and minimize microphony. Even with all of this, it was still quite sensitive to outside stimuli. Inductors are commonly found in the majority of amplifiers that are currently on the market.

As far as other "passive" devices being microphonic, much of this comes from the wire leads or legs attached to the component vibrating. As such, it is best to keep the leads on each component as short as possible in order to avoid this. Many higher quality capacitors actually come with a sleeve over the legs so as to damp any external vibrations from being transferred into the body of the cap. Sean
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Sean...Thanks for your input. I take your comments under consideration and will respond later.

Sean...When you lift your coffee cup to take a sip, you change the mass distribution of planet Earth, and change the distance from the earth to the Moon. This is absolutely certain, and the exact change of the distance could be calculated, but who cares? Effects such as those mentioned by sean do exist at some level, but is that level one that we should care about?

Inductors and capacitors take their values from the geometric dimensions of their construction. Unless an inductor is fully potted (filled and encapsulated in plastic) it is much more prone to dimensional changes due to vibration than is a rolled mylar capacitor. I have no doubt that one could actually create an inductive microphone (capacitative ones are common) by deliberately increasing the design features leading to microphonics, and placing the device in a circuit designed to be sensitive to inductance value.

I forgot the output transformers of tube amps. Power transformers all hum, so I suppose that audio transformers must sing. These effects are not necessarily reversible, so I don't know if an audio output transformer is microphonic. If so, I expect it is a lot less so than the tubes associated with it, and therefore of secondary importance.

It is necessary to consider HOW the electronic component is used in the circuit. I have not built or repaired any amplifiers for a few years so I am not familiar with recent circuits. As I recall, inductors were used in the signal path only at the amp output, to limit bandwidth to 200KHz or thereabouts so as to preclude possibly damaging oscillation. If such an inductor were to change value the effect would be to vary the HF rolloff slightly around the 200KHz value, which would have no practical impact on a 20KHz signal.

Do transistors "talk"? Well yes...in fact most electronic circuitry makes various buzzing noises that can be heard if speakers are not swamping it out. But do transistors "listen"? I don't accept this without evidence.

Such evidence would be surprisingly easy to gather. I suggest that sean (in his spare time) should take some of his numerous spare audio equipments, and throw together a Preamp/Interconnect/Power amp/Speaker wire system, and put a dummy load (resistor) at the end of the speaker wires. This rig should be located in the listening room and, with the preamp input shorted, turn it on, and then crank up the regular audio system to about 90 dB. The output of the test rig will now exhibit its total microphonic effect from all causes (except from a turntable/phono pickup that we all agree is vibration-sensitive). You could look at this output on a scope, listen to it with headphones, or record it on a cassette (and send it to skeptics like me). Measure the microphonicly induced output. Also measure the output when the main system is not playing, as this is the background noise level. Measure the output of the power amp that is driving the speakers: this is what the test rig would be doing if it were connected to the speakers. Now you can calculate how many dB down is the system noise level, with and without microphonic contribution. I suspect that a tube preamp will measure significantly worse than a transistorized one.

"Worse"?? Well that's another issue. Some have suggested that a little microphonics is good. Adds "body" or some such thing.

Are we having fun yet?

For basic information on resonance transfer vs isolation, see this link

http://www.audiopoints.com/proving.html

Eldartford, do you mean that for SS gear this issue is not really an issue? Do you use any kind of isolation/coupling yourself?

In regards to CD players, I just don't get it. Optical reading mechanism reads data, and it doesn't really know what kind of data is it: audio or data files. Processing of that data actually determines it. And optical systems of CD drives seem to be tolerant enough to vibrations to be able to read the data with no errors even in computers, where drives vibrate like crazy. The only explanation I could throw is that in spite of the fact that vibration defocuses the optics and causes read errors, a computer, reading data in X speed, is able to read, check CRC, re-read again and again until he gets the perfect read. Audio CD player has only one attempt, and if it isn't lucky there will be a error. Do I miss anything?