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Old 19th January 2010, 08:29 PM   #1
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Default JFET Mic Preamp Advice

Hey,
I am new to the forum here but have been slowly teaching myself audio electronics over the past few years (part time). My knowledge is somewhat lacking to say the least. I am trying to design a simple twin stage JFET mic preamp with components I have laying around, I have plenty of these (100 x 2sk170bl, 150 x 2sk117gr) so matching for Idss shouldn’t be an issue. Here is the circuit so far, am on the right track?


Not sure of:
Optimal supply voltage?
Biasing, should I go about this differently?
Am I right to test with a 5mvRms ac signal. Will a condenser mic output be more than this and lead to clipping in preamp?
I wish to eventually add a transformer on input and phantom power, what ratio should transformer be to avoid clipping, 1:1?
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Old 20th January 2010, 05:46 PM   #2
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anyone????
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Old 20th January 2010, 09:25 PM   #3
mjf is offline mjf  Austria
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hello.
why are there no source resistors?
greets
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Old 21st January 2010, 08:11 AM   #4
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hey,

well i've been reading up on jfet circuits and I believe that leaving out the source resistors lowers the noise. see Ultrasimple mm/mc RIAA preamp 2

not sure if this is a good idea so all suggestions/corrections are welcome.

The main confusing aspect for me is whether the Jfet is suitable for mic pre application, i'm not sure of the signal level to test the input of the circuit with, i'm currently using 5mV but I think a condenser mic will output more that this and overload the jfet input.
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Old 21st January 2010, 09:40 AM   #5
lohk is offline lohk  Europe
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Dynamic mics - the only one which you can attach here - have quite low impedances. I do not know if that works well with your circuit. And most microphones do have much more output if "driven" quite hard (loud).
But your idea is at least very interesting: Why should a really simple design like yours not work with microphones if it works with MC pickups? Why don't you simply try it?
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Old 21st January 2010, 09:54 AM   #6
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would you have any idea of the output voltage ranges of a condenser mic so i can do a few simulations?

I plan on adding transformer to the input somewhat like this, thought i would just upload the gain stage first for clarity though.

Thanks
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Old 22nd January 2010, 04:09 PM   #7
mjf is offline mjf  Austria
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hello.
mic amps were built with jfet's,found some schematics in old catalogs............
here is an example.
greets
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Old 22nd January 2010, 10:07 PM   #8
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thanks,

that looks usefull
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Old 27th January 2010, 06:38 PM   #9
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Quote:
would you have any idea of the output voltage ranges of a condenser mic so i can do a few simulations?
Yes, I design them, at least these days.

15 mV rms/Pascal is typical. 1 Pascal is 94 dBspl. So the 15 mV might be typical output for a fairly close miced loud singing voice.

However when used with loud sounds like drums as much as a volt can be obtained.

Dynamic mics usually put out about a tenth of that...say 1.5 mV/Pa.

Les
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Old 29th January 2010, 02:23 AM   #10
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Borbley has 2 versions of an all fet mic preamp - one with transformer front end and one with jfet front end. Neither is inexpensive, but would compete with the best commercial products. I have many of his kits and recommend them highly.

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