Re: Basic questions about microphones and mic preamp design...
From: Ban (bansuri_at_web.de)
Date: 12/13/04
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Date: Mon, 13 Dec 2004 10:36:37 GMT
Jaye Gallagher wrote:
> Hi there,
>
snip
> Essentially, I need a microphone pre-amp of some kind, it's just a
> matter of what kind I want, and how on earth to scrounge a design for
> it. My questions are:
>
> * There seem to be two kinds of microphones in use -- condenser and
> dynamic. Can a single pre-amp design support both of these? What is
> the difference in a electrical sense?
>
A dynamic mike works like a loudspeaker, a diaphragm moves a voicecoil in a
gap with a magnetic field created by a permanent magnet. There are several
different condensor mikes
1.) Electret, which requires only a lowish(1.5-5V) voltage to operate a
built-in FET stage which is supplied by batteries or +5V on the center pole
of the connector(Sony). The output voltage of 1/4" diameter size is around 3
to 10mV/Pa. Output is unbalanced jack or RCA. Price is very low. There are
also musicians grade electrets with a built in transformer for balancing and
phantom power or 2 AA cells.
2.) real Condensor, needs a higher operating voltage, normally +48V phantom
power. Has balanced output with XLR-connector. Price from 150$+.
3.) RF-Condensor, made by Sennheiser, modulates a high frequen.cy. Balanced
XLR, phantom powered. Price is high 300$+
> * There also seem to be two different ways of connecting a microphone
> to stuff -- single ended or differential. Again, can a pre-amp
> support oth of these, or only one?
>
Can do both if done properly.
> * What is phantom power, do all microphones require it, and it is
> only used in differential microphone connections?
>
It is only used with balanced lines. +48V, negative to Pin1(gnd), positive
through 2 resistors 6k8 into both of the balanced conductors (Pin2 and 3).
> * All the pre-amp designs I've found seem to require at least +9V and
> -9V supplies to operate. I'm hoping to produce a portable device, and
> so operation from, say, 2 AA batteries is desirable. Is this
> impossible? What's going in things like my little Minidisc player
> that has a microphone input, and thus a preamp, but runs from a
> single AA?
>
Depends on the dynamic range you need. You can make a preamp with a +2.5V
only supply, but then you are more limited with the max. Sound Pressure
Level vs. gain setting. If the mike puts out 20mV/Pa (1Pascal equals 94dB
SPL) and your pre can output 0.31Veff without limiting, you can have +26dB
gain, if your input stays below 94dB. A singer can easily output 120dB SPL,
which limits your gain to 0dB, generally you want some additional
"headroom", so the gain must be even lower. This raises OTOH the noise
floor. Depending on the sensitivity of the mike and the required max.SPL,
you have to find the best compromise.
> * Are there any IC based solutions to the above, since, as I'm hoping
> for a portable device, board space is at a premium.
>
You can use any low noise opamp depending on the supply voltage range. You
will need to decide for an electret with headphone jack only, because the
XLR connectors are big and heavy.
> Any help, opinions or rampant speculation greatly appreciated.
> Miraculous delivery of useful schematics may be rewarded with sexual
> favours ;).
If you are Jaye Foucher, you are very welcome. :-])
>
> Thanks,
> Jaye.
-- ciao Ban Bordighera, Italy
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