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Old 28th October 2012, 05:22 PM   #28581
PMA is offline PMA  Europe
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From new AD products, try ADA4898. Even better is ADA4899, but low supply voltage.
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Old 28th October 2012, 06:58 PM   #28582
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From new AD products, try ADA4898. Even better is ADA4899, but low supply voltage.
Yes, so if we measure 10psec PIM with these, what problem next? RFI/EMI comes to mind.
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Old 28th October 2012, 07:03 PM   #28583
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The PIM is only scarecrow today. RFI/EMI is the key, IMO.
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Old 28th October 2012, 07:55 PM   #28584
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The PIM is only scarecrow today. RFI/EMI is the key, IMO.
Time to invest in a mine of mu-metal. (it is a joke)
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Old 28th October 2012, 08:36 PM   #28585
gpapag is offline gpapag  Greece
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(it is a joke)


May be we should ban emoticons. They make things look too obvious

George
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Old 28th October 2012, 10:50 PM   #28586
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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.
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.

Last edited by kgrlee; 28th October 2012 at 11:05 PM.
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Old 28th October 2012, 11:00 PM   #28587
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Originally Posted by scott wurcer View Post
Yes, so if we measure 10psec PIM with these, what problem next? RFI/EMI comes to mind.
Only scratching the surface, got to get thermoelectric, electrochemical, piezoelectric, triboelectric and pyroelectric effects on board as well, ... -->> <<--

Frank
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Old 28th October 2012, 11:27 PM   #28588
RNMarsh is offline RNMarsh  United States
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Default Just for the heck of it -- what if ....

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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?
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Old 28th October 2012, 11:41 PM   #28589
bcarso is offline bcarso  United States
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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.
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Old 29th October 2012, 12:28 AM   #28590
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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.
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