John Curl's Blowtorch preamplifier part II

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You can make a DC microphone with a pressure guage, with a differential one there would even be a common mode pressure. I have not seen a condensor or dynamic mic yet with DC response.:confused:
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. :eek:


[*] 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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Just for the heck of it -- what if ....

And how about the relatively poorer performance of some of the audiophile certified class A circuits that Jan alluded to? If PIM is as bad as jitter (that's IF) then it applies here too, yes, no?

Personally I don't think this will lead anywhere but at least there is some technical content.

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
 
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Then the LF response is limited only by the electronics. A high Z FET amp will need a zillion G resistor to approach DC.

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.
 
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.
 
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...
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 ?
 
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R
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 ?

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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