B&K 4133/4134 omni capsules (and other B&K omnis) have a little hole which is sized to limit the LF response. If you block the hole with a drop of cyanoacrylate, the capsule becomes a barometer; ie responds to DC. This is fairly permanent so you might want to use a blob of Unobtainium .. or even Blu-tak at a pinch.[*]
Then the LF response is limited only by the electronics. A high Z FET amp will need a zillion G resistor to approach DC.
An RF solution, modified from Baxandall or the Sennheiser, can give true DC performance.
The Helmholtz Wave Eqn precludes a velocity mike with DC response but in practice, a fig-8 flat to below 30Hz has serious problems with subsonic noise even with 30dB/8ve filtering below that. You get Proximity with 'distant' thunder. 😱
[*] You can do this with other omni condensors but B&Ks have a single well defined hole instead of the several uncontrolled leaks on lesser capsules.
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Only scratching the surface, got to get thermoelectric, electrochemical, piezoelectric, triboelectric and pyroelectric effects on board as well, ... -->> 🙂<<--
Frank
Just for the heck of it -- what if ....
Since few have seen The Paper -- moving right along, then. What if we used a signal which only had a zero to + (or -), repeated for awhile. Like a pulse. There is an average level that is + or an equivalent dc level. What does this do to the circuit... including Golden Class A or SE circuits? Does it cause any operational shifts which could increase thd?
-RNM
Since few have seen The Paper -- moving right along, then. What if we used a signal which only had a zero to + (or -), repeated for awhile. Like a pulse. There is an average level that is + or an equivalent dc level. What does this do to the circuit... including Golden Class A or SE circuits? Does it cause any operational shifts which could increase thd?
-RNM
A floating gate JFET will be fine. But since when we say "d.c." we really mean no highpassing after we take the first measurement, something to initialize the input voltage, say a reed relay, having determined in advance what the drain-gate leakage and gate-source diode characteristics are, would be of great utility.
Guru Wurcer and at least 1 other MicBuilder has done some experiments along this line. The results are not very satisfactory and there are issues with RFI & other stuff to add to temperature drift.
My vote still goes to the RF methods but as this is of interest only to omni fans, I'll probably never try building a DC mike.
Well...if it is a symmetrical amp, DC coupled, we can believe one side of the power stage will heat more and the other refresh ?
Offset can increase, or decrease...depending how it was ? or the level ? Or the one after the other ?
Distortion can increase slightly, or decrease, depending how the parts mismatch ?
Of course, the thermal inertia is slow enough for any servo can correct this DC (because your signal is a symmetrical signal with a DC equal to half its peak to peak value) before it become important, while the power units are thermally coupled with the radiator ?
But the main question i ask to myself is: What kind of instrument would produce such a signal ? Even Kick drums, with its first attack always in the same sens is globally near symmetrical, main dis-symmetry on half a period...
I noticed that kick drums can sound a little different, switching their phase, but i believe it is due to loudspeakers, as i never noticed impact on other instruments.playing in the same time.
Did i miss something ?
And destroy all those years, training ourself to never smile at our own jokes.
Animated emoticons are nice, Christmas night.
R
Too broad... maybe this, maybe that. (many SE and other audiophile circuits are not symmetrical). Looking for numbers... after-all I am looking at .001 and less thd numbers for SOTA circuits. Looking for CMRR of at least -100 db.
It may not take very much asymmetry in waveforms to cause increase in distortion.... for some circuit topologies. Assume 1k-10K source Z for worst case. Also, for worse case, assume C/L gain is on the low side. Thx - RNM
Too broad... maybe this, maybe that. (many SE and other audiophile circuits are not symmetrical). Looking for numbers... after-all I am looking at .001 and less thd numbers for SOTA circuits. Looking for CMRR of at least -100 db.
It may not take very much asymmetry in waveforms to cause increase in distortion.... for some circuit topologies. Assume 1k-10K source Z for worst case. Also, for worse case, assume C/L gain is on the low side. Thx - RNM
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Fiber-Optic microphones: If you shine a light on a vibrating diaphram and pick up the reflected light via a f.o. to a detector.... you pretty much dont have to worry about noise, amps and Z and cable pick up of hum emi/rfi et al. Why arent more using them and developing them further for more recording appl? Why, why, why? -RNM
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Not asking for you to take my word for it. But without details, falling back on photonics as if it were some sort of panacea is naive. And when one adds the realistic details, it becomes much more pessimistic, most of the time.
I will believe you only if you have tried to make such f.o.microphone. Otherwise, speculation means nil.
In other words you dont know what you are talking about. Ever used one? Go buy one and use it, then tell me about how naive it is. -RNM
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