From an audio forum... FR4 question

But do your circuits vibrate the room and all it's contents?

Yes, a lot of audiophile stuff is bunk, but microphonics and vibration in fairly rigid materials are not exactly mysterious concepts.

Just use one of these:

But there has to be a specific responder to the vibration. Audiophooles (LOVE that term! Who came up with it?) who can't do without their tubes have to think about vibration because that's one of the main deficiencies of tubes, along with their built-in parameter corrupter, the filament.

Some cables have a piezoelectric response to vibration, and of course, anything mechanical has to consider it, but FR-4???

John Perry

I found this

"The FR-4 works very well in most audio applications. However, it is a very hard material and has a tendency to vibrate. This might cause tiny current changes in the components on the board, which, especially in very low level circuits like MC phono preamps and microphone preamps can cause unwanted ?sounds?. Teflon base-material, which has been developed for HF circuits, is much less prone to vibration. Consequently it is very much preferred for low-level circuits."

any comments?

martin

In phono circuits, even the cable can vibrate and cause effects. If you grab them and move them about, you will hear them. Probably much more serious than circuit board effects. I disagree with the statement about vibration stiffness. There is a difference in stiffness, but TFE generally is soft and damps vibration more.

greg

Well it seems like it would be easy to test... design the same pre-amp and fabricate it on FR-4 and teflon microwave material. Set each up with no input (or perhaps a clean source) near a speaker driven with a slow sweep and measure coupling to the circuit. If there is an effect, one would expect to see responses corresponding to mechanical resonances of the PCB and/or certain components, with the fiberglass board having a larger response.

Have to be carefull to mount components identically.

Hi John, I agree with you, but I've mainly used teflon devices for cooking, so I wanted to see what other people thought.

All these "audiophooles" keep going on about OFC etc. Then I try to imagine (badly) what the hell is going on at the molecular level inside a decoupling electrolytic. It must surely outweigh FR4/Teflon problems

Gullliby yours

martin

"John Larkin" a écrit dans le message de news: snipped-for-privacy@4ax.com...

I've already searched info about this "hook" but found nothing. Do you have some info/pointers?

SRBP is not as hard as FR4 either. Gotta wonder why audiophiles always choose the most expensive solution to their perceived problems.

Think it was me, or possibly John Woodgate

martin

nope, was not me, found references going back to 2001

martin

Some old Tektronix appnotes referred to the dielectric absorption of FR-4 as "hook" because it made ugly overshoots and undershoots on the rising edges of scope waveforms. FR-4 makes nasty capacitors, with lots of DA and TC in the +900 ppm/k range.

John

I had a pll running at 155.52 MHz with a narrowband (2 khz) loop, on a VME module. Whenever one pulled an SMB test cable out of its connector, the loop briefly lost lock. After some tapping and bending, we found it was the expensive Vectron crystal oscillator, not the caps. We designed some tiny springs to isolate the oscillator can, made it wobble like one of those gooney-head dolls you see in the backs of cars.

"Safe to say?" I can't imagine any significant pcb microphonics in an audio system in any real-world sense. More audio nonsense.

John

The oscillator was sensitive to high-frequency vibration, and the resonant frequency of the spring-mass system was way below the sensitive range, so the sharp shocks from connector mating couldn't penetrate to the crystal. The crystal *was* firmly mounted to the board, which was exactly the problem, and relocating it would have trashed the entire design. The springs work great.

I submit that if you open up your amp and start tapping parts with a pencil, the loudest noise you'll hear is the sound of parts being tapped with a pencil. To solve this particular problem, quit hitting parts with pencils.

My NMR gradient drivers have a couple of PPM noise, dc to 50 KHz, and use regular surface-mount parts on FR-4, with lots of noisy fans. Aside from pure semiconductor and resistor noise, the next biggest hazard is magnetic loop pickup from fields leaking out of transformers and fans.

How will a microphonic cap make hissy, buzzy audio?

John

You don't test audio equipment like this. You need meters for....

Musicality Etch Resolution Glare Detail Tightness Speed Depth Size Location Pace Brightness Balance Flavour Bloat Cleanliness Extension Energy Warmth Fullness Weight Smoothness Liveliness Sparkle Power Sweetness Correctness Naturalness

and that is just for cables.

I am not familiar with Teflon's vibration properties, but it is safe to say that ceramic caps mounted on FR-4 that is subjected to vibration will generate piezolectric effects that may distort high gain audio signals.

There are other capacitors more tolerant of vibration.

mw

Hey, Fred, see pics in abse.

Quoting Johnny Depp in the Willy Wonka movie,

"You are REALLY weird."

John

Consider that some external source is producing vibrations in the audible range. Assume the level in the listening room is X dB. How much additional sound level will be added to the X dB by the gain associated with a microphonic capacitor and the following amps/speakers? In other words, the truck itself will make a hell of a lot more noise than what the truck induces into the cap.

Ever hear a system howl from microphonic caps?

The superconductive magnets themselves are supported by pneumatic vibration isolators; my gear is not. Again, if your listening room is next to a railroad track, it's going to be noisy now and then, and microphonic caps won't add measurably to the din.

John

How can they do that? A coupling cap almost by definition is a low impedance at signal frequencies, so the voltage drop across it is small. And if it's inside an overall feedback loop, any nonlinearity is further reduced.

The only interesting case is a cap that itself defines a low-frequency rolloff point. If used at high signal levels, that one could in theory generate harmonics. In practise, I suspect the effects are generally small.

John