LM317 As Electret Mic Amplifier

mrfeedback · 23rd May 2003, 10:23 AM

A useful little circuit maybe - kills two birds with one stone.

Eric.Electret Mic Preamp

davesaudio · 23rd May 2003, 12:28 PM

rather clever design
those panasonic capsules are pretty amazing for the price

Circlotron · 23rd May 2003, 12:34 PM

But you'd have to be a bit of a sausage to use a voltage reg as an amp...
Stop posting silly circuits, mrfeedback.

dhaen · 23rd May 2003, 12:37 PM

It's amazing it works with C1 in circuit!

Cheers,

capslock · 23rd May 2003, 12:42 PM

Ever seen an amp with 15 - 47 µF on its output?

dhaen · 23rd May 2003, 12:44 PM

Perhaps if it's a Black-Gate, it'll be ok

capslock · 23rd May 2003, 01:05 PM

Look at http://www.linkwitzlab.com/images/graphics/microph1.gif for the two standard ways to hook up an electret.

1) suggested by Panasonic uses the FET in common source mode, with a gain resistor in the drain connection:
+ gain
- distortion of the FET in common source mode

2) suggested by Sennheiser and Siegfried Linkwitz, uses the FET as a source follower with a 10 k load impedance:
+ lower distortion
- no gain

3) I have been thinking about replacing the 10 k with a constant current source:
+ even lower distortion
- higher parts count
- still no gain

The LM317-circuit gave me another idea:
4) Use the FET as it is, i.e. without disconnecting the membrane from the source. Connect the drain to a stable 2 V supply. Connect source to a virtual ground (negative input of op amp). The gain can be set through the feedback resistor. This hookup keeps the voltage across the FET constant and measures the current, just like the LM317. Haven't had time to look at typical FET curves. Would this by chance be more linear than 2) or 3)?

cm961 · 23rd May 2003, 01:29 PM

Me and a buddy are working on an electret mic project for recording drums. We plan on hooking up one little capsule per drum/cymbol, and mixing it all down to two channels. We had to modify the electrets a la linkwitz because the loud sounds were saturating the FET stage inside the capsule.

Although we're still putting it all together, the one mic that I did make was decent. They are supposed to have pretty flat response, so I'm wondering why studios etc don't use them regularily. At like $0.50 a pop, you can't go wrong.

Pete

capslock · 23rd May 2003, 01:51 PM

schematics 3 & 4:

mrfeedback · 23rd May 2003, 02:40 PM

Quote:

Originally posted by Circlotron
But you'd have to be a bit of a sausage to use a voltage reg as an amp...
Stop posting silly circuits, mrfeedback.

LOL.

23rd May 2003, 12:28 PM	#2
davesaudio diyAudio Member Join Date: Feb 2003 Location: GTA	rather clever design those panasonic capsules are pretty amazing for the price __________________ intentionally blank

23rd May 2003, 12:34 PM	#3
Circlotron diyAudio Member Join Date: Jun 2002 Location: Melbourne, Australia	But you'd have to be a bit of a sausage to use a voltage reg as an amp... Stop posting silly circuits, mrfeedback. __________________ Best-ever T/S parameter spreadsheet. http://www.diyaudio.com/forums/multi...tml#post353269

23rd May 2003, 12:37 PM	#4
dhaen diyAudio Moderator Emeritus Join Date: Jun 2002 Location: U.K.	Sausages and 'roo's united... It's amazing it works with C1 in circuit! Cheers,

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23rd May 2003, 12:42 PM	#5
capslock diyAudio Member Join Date: Feb 2002 Location: Germany	Ever seen an amp with 15 - 47 µF on its output?

23rd May 2003, 12:44 PM	#6
dhaen diyAudio Moderator Emeritus Join Date: Jun 2002 Location: U.K.	Perhaps if it's a Black-Gate, it'll be ok

23rd May 2003, 01:05 PM	#7
capslock diyAudio Member Join Date: Feb 2002 Location: Germany	Look at http://www.linkwitzlab.com/images/graphics/microph1.gif for the two standard ways to hook up an electret. 1) suggested by Panasonic uses the FET in common source mode, with a gain resistor in the drain connection: + gain - distortion of the FET in common source mode 2) suggested by Sennheiser and Siegfried Linkwitz, uses the FET as a source follower with a 10 k load impedance: + lower distortion - no gain 3) I have been thinking about replacing the 10 k with a constant current source: + even lower distortion - higher parts count - still no gain The LM317-circuit gave me another idea: 4) Use the FET as it is, i.e. without disconnecting the membrane from the source. Connect the drain to a stable 2 V supply. Connect source to a virtual ground (negative input of op amp). The gain can be set through the feedback resistor. This hookup keeps the voltage across the FET constant and measures the current, just like the LM317. Haven't had time to look at typical FET curves. Would this by chance be more linear than 2) or 3)?

23rd May 2003, 01:29 PM	#8
cm961 diyAudio Member Join Date: Jun 2001 Location: Peterborough, Ontario	Me and a buddy are working on an electret mic project for recording drums. We plan on hooking up one little capsule per drum/cymbol, and mixing it all down to two channels. We had to modify the electrets a la linkwitz because the loud sounds were saturating the FET stage inside the capsule. Although we're still putting it all together, the one mic that I did make was decent. They are supposed to have pretty flat response, so I'm wondering why studios etc don't use them regularily. At like $0.50 a pop, you can't go wrong. Pete

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