Mic capsules distorting at low level in binaural dummy head. Why :( - Page 3 - diyAudio
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Old 30th September 2012, 11:52 AM   #21
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So I am taking the reading correctly then?

You're right Pano, that's why I flagged this up, the datasheet says standard operating voltage is 3v, max 10. I've read that 5-6v is good.

Raising that 33k resistor is exactly what I thought I needed to do. Annoyingly I dont have any higher ohm resistors in my parts box and the shop is closed until tomorrow. Drat! I could just insert another resistor in series though right?

I'm trying to calculate the appropriate value resistor to use using the VIR equation but not succeeding. I can't figure out how to implement the equation for this problem. How do I calculate the required resistance to drop the voltage to say, 5v?

Last edited by Poxican; 30th September 2012 at 11:54 AM.
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Old 30th September 2012, 01:02 PM   #22
Mooly is offline Mooly  United Kingdom
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How to calculate...

the problem is the capsule may or may not behave as a pure resistance. What you could try.

A quick experimentation is probably the quickest way.

I'll think about calculating but it may not be ideal as the FET capsule is probably non linear in its current draw against voltage.
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Old 30th September 2012, 02:48 PM   #23
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Originally Posted by Mooly View Post
A quick experimentation is probably the quickest way.
I kind of figured that actually. Shall I just experiment with inserting a resistor and measure what the voltage reads between the capsule terminals?

In this picture, A and B are both valid places to insert an additional resistor in series right? Pos A would mean putting it off the board, just inline in the lead the runs from the board to the capsule; B would be on the board, though trickier as the board is glued into it's casing now

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Old 30th September 2012, 03:08 PM   #24
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Must be at point B really. Anywhere else upsets the balance of things.
If you are good at working with small things then snip the 33K close to its body and tag one in there.

And yes, experiment.
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Old 30th September 2012, 03:18 PM   #25
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There really isn't space on the board to get it right in there next to the 33, but I can snip the lead that runs from R4/R5 to point B and stick it in there. That'll make it easier to experiment with different values too.

I'm going to give it a shot now, will let you know how I get on.

[edit] I'm getting this advice from Reddit, not sure what to make of it:
Quote:
[–]obsa

Don't use a resistor, use a voltage regulator. Especially because this is audio-related, you want your power as clean and stable as possible. Linear regulators are cheap and effective, but wasteful. Switching regulators are typically much more efficient under load but more expensive and often require an extra inductor.




[–]fatangaboo
Perhaps you can install a 5 volt zener diode in parallel with C1? This would prevent the microphone's voltage from exceeding 5 volts. Digi-Key will sell you the "1N5231" , a 5.1 volt, 100 mA zener diode, for twelve cents. Or 100 of them for $7.23. Jameco will sell you the "1N4733A" which is a 5.1 volt, 200 mA zener diode, for 6 cents (if you buy ten). Tayda Electronics is even cheaper [1] (IMAGE)
Surely if a voltage regulator was necessary it would have been in there in the first place? As for the Zener, the original circuit featured a 12v zener, but Bill Ward told me to ditch it and raise that resistor to 33k.

Last edited by Poxican; 30th September 2012 at 03:28 PM.
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Old 30th September 2012, 03:27 PM   #26
Mooly is offline Mooly  United Kingdom
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I looked at that but isn't C4 connected there ?
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Old 30th September 2012, 03:33 PM   #27
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Don't think a voltage reg can be used here in this configuration.

Your new resistor has to go across a new cut in "row 11 B"

If super confident cut the 33K out at its body and tag a new value in soldering to the small wire ends that are left (or add wires to allow easy experiment of different values)
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Old 30th September 2012, 03:37 PM   #28
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I looked at that but isn't C4 connected there ?
I was thinking to do it like this, ie. the red line being the resistor. Is it no good there? But I see now that that wouldn't be quite where I thought it would be on the circuit diagram.

http://i560.photobucket.com/albums/s...tandBoard3.jpg

I'll get out my magnifying glass and try and slip it in with R3.
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Old 30th September 2012, 04:51 PM   #29
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OK, so I inserted a 22k resistor in there. Managed to make them both fit nicely standing on end. The voltage across the capsule reads at 6.4v, and I inserted the ammeter into the circuit and it now reads around .46mA.

Now, that seems to be a reasonable voltage, though I'm worried of starving the capsule of current, the datasheet says the max current consumption is .5mA. Am I better off slightly decreasing the resistor value to supply more current, although with a higher voltage across the capsule, or do you think I've arrived at a good compromise?
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Old 30th September 2012, 05:39 PM   #30
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Could you just make a resistor voltage divider to hit your desired voltage? Keep the values a bit low so as not to limit current too much?
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