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Conference hips::uk_audioo

Title:	You get surface noise in real life too
Notice:	Let's be conformist
Moderator:	GOVT02::BARKER

Created:	Thu Jul 28 1988
Last Modified:	Mon Jun 02 1997
Last Successful Update:	Fri Jun 06 1997
Number of topics:	550
Total number of notes:	3847

253.0. "Crystal mikes kill my cassette recorder" by NEARLY::GOODENOUGH () Tue Jan 15 1991 15:51

    I have a small JVC cassette recorder (Walkman-size, but records as
    well).  I want to be able to have two people with lapel mikes
    recording, one on left channel and one on right channel.
    
    I picked up a couple of very cheap crystal lapel mikes from Maplins to
    experiment with.  I found that using one mike at a time (either mike,
    either channel) it works fine, but when I plug both in at the same
    time, the recording level is so low it's useless.
    
    So a couple of questions:
    
    1) Why is this happening at all? Why should one mike affect the other?
    
    2) Would a cheap preamp chip solve the problem? (I've seen several in
    the Maplin's catalogue, one is dual).
    
    You may gather that (a) I'm not a microphone expert (though impedance,
    or lack of it, would seem to be the problem  and (b) I'm not prepared
    to spend very much money, e.g. on a couple of Electrets.
    
    Jeff.

T.R	Title	User	Personal Name	Date	Lines
253.1	How are you wiring the mics?	EDSAC::MARSHALL	What she needs, I don't have....	`Wed Jan 16 1991 10:44`	7
	How are you connecting the mics? If you get the connections the wrong way round, this could cause the problems you describe. If you're in DECPark (sorry, can't be bothered to find out where NEARLY:: lives) bring the kit round to me and we'll have a look... Scott (REO D4/3A-2)
253.2	Microphone/audio amp/recorder expert needed!	NEARLY::GOODENOUGH		`Tue Feb 05 1991 12:43`	15
	The problem is simpler than I thought. Neither mike works. What I thought was one mike at a time working was in fact the other channel of the built in mike! I should have sussed that when I went to see Scott, and told him that the L and R sockets appeared to be labelled back to front! So, to ask a couple of different questions: What sort of external microphone does a pocket tape recorder expect? What type are the built-in ones? "Condenser" seems to ring a bell. Is there any way I can get the crystal mike to work with it? I tried the LM382 dual pre-amp chip, but that didn't work either - I suspect the impedances are all wrong. Jeff.
253.3		FORTY2::SHIPMAN		`Tue Feb 05 1991 19:00`	19
	re .2: Yep, that's much simpler. You probably won't get the crystal microphone to work at all. These things put out a high voltage - maybe 1V - but with a very high output impedance - maybe 1M. This is the sort of mic you'd plug (almost) directly into the grid of a triode. The built-in mic will probably be a condenser (capacitor) type. This also has a very high output impedance, but the capsule will come with its own FET preamplifier to fix this. I think condensers are used because simple ones are small and cheap and they're immune to the magnetic fields in tape decks. For an external mic, a dynamic (moving-coil) type should be OK. An electret type would also do but these are likely to be a little more expensive. Their internal pre-amps generally make them look like dynamic mics, with an impedance of 50R to 500R, sometimes switchable. Can't remember the output voltage, but it's low. Nick
253.4		NEARLY::GOODENOUGH		`Wed Feb 06 1991 11:08`	6
	Thanks. Would a FET preamp do the trick for the crystal mic? If so, could you let me have a simple circuit? I looked at electrets and found them far too expensive for the application I have in mind. Cheers, Jeff.