I need a simple non tube large membrane mic schemo

[tommytones]

 

[Marik]

Another KM88:

http://home.comcast.net/~markfuksman/km88sch.jpg

and KM86:

http://home.comcast.net/~markfuksman/km86sch.jpg

 

[tony dB]

a little OT here...

André get back on your bicycle and work on your training program. :twisted:
Tour de France is not so far away anymore... :razz:



Tony dB

 

[adrianh]

Late to the party.
freetradezone.com lists 12 manfactors for the 2N3819
that MAY be active. No I do not have the full version
great design and data sheet resource if you had the money.
It used to be all free.

 

[frob]

i found the 2n3819 at mouser.com no problem but i was wandering if there was a difrance in bands it list four, also i was planing on useing a large diafram capsule are thier any values that should be changed? and what about types of caps and resistors i only want the best, could any one help point me in the right direction? thanks youve always been a great help.

theise are the brands it list

fairchild
central
NTE
and
Vishay/Siliconix

one more thing the tranx it says on the scematic 7:1 will that hold true with the large diafram?

 

[Gus]

I bought the fairchild ones $.06each if you buy 1000. You are going to need to buy a few for the right bias point. Tim's schematic of the circuit I built needs one small correction the 5.6k should be 56K. I used a ll1636 wired 10:1, With a LD you will get more signal from the capsule so 7:1 to 10: should be fine. The KM84 circuit can be reduced in parts with a LD one side of the capsule might not need to grounded, look at the circuit I built.

The cap from the drain to the capsule needs be be a very good one it is in the charge amp feedback path. I could not find a polystyrene at the vaule I wanted so I twisted up some wirewrap wire and measure and cut to the value I wanted. You might want 5pf to 10pf.

 

[andre tchmil]

Ah, the NT 1000 schematic with mod would be nice

 

[DerEber]

I have a housing and capsule left and will build a KM84 circuit inside.
I allready selected a right FET. My first 100-Fet´s order was very dissappointing. Nearly all 100 pices were the same specs. Nearly no spread and no singe usefull Fet. After ordering from another source I had much more luck. specs all over the place :thumb:
For the feedback cap (4pF) I could only find ceramic ones
So I think I will firstly stick with them.

Now as completing my first Tube mic recently, the whole circuit was easy to understand for me. Only one thing is constantly on my mind. It is regarding the feedback cap. I insist on thinking that the cap will give feedback wich is dependent to frequency. So for high frequencies there should be a higher feedback than for lower freq.
If I calculate AC-resistance of a 4pF cap at 20kHz I will get nearly 2M Ohm
for 20Hz I calculate around 2000M Ohm.
I just cant get how this leads to a flat response in the end. :roll:
Is the response of the capsule opposide to this?
Maybe someone could lead me to an explaination.

Another thing I will try is: to avoid the 470pF copling cap between the capsular and the Gate and connect on side of the capsular directly to Gate.
any comments?
:green:


Thanks to you all,
Stephan

 

[Gus]

go to neumanns site find the PDF "microphones" your answer is inside the PDF/book for the feedback cap.

 

[burdij]

The 4pf capacitor provides negative feedback to the fet input. One side forms a voltage divider with a 1G Ohm resistor so a range of 2M to 2000M would not be odd. Condenser capsules have a rising response with respect to frequency so the 4pf in that range of impedance values is going to tend to level the output vis a vis frequency.

BTW - you can make low value and variable caps by twisting two pieces of insulated solid wire - #26 or #28 - together. This produces approximately 10pf per inch of twisted wire, as I recall. Once you get the right value, stablilize it with a drop of clear nail polish.

 

[Gus]

or you can use the metas

http://www.groupdiy.com/index.php?topic=2539

 

[DerEber]

strange.... I can´t access the Neumann Site. I will try again tomorrow and read it up.
Thanks for the tip on the twisted wire. I wounder if this homemade cap will be better quallity then the ceramic. But I will try it and listen. What Cap did Neuman originaly use?..... Anyway it does not matter.

So the capsular has a "rising response with respect to frequency"?
Interesting, but I just wounder. If I hook up an Frequency-Generator instead of the capsular on my tube mike I will get a nearly flat response up to 20kHz (-0.9dB) so ........
wait, now I got it..... the capsular itselfe is shunting(?) the higher feedback Voltage at higher frequencys to ground.
Damm...... I blame myselfe here

 

[DerEber]

Btw.: I found an interesting read on microphones. Sadly (for most of you) it is in German.
But it has a lot of schematics inside:
http://www.kgw.tu-berlin.de/forschung/magisterarbeiten/historische_kondensatormikrofone/Magisterarbeit%20Volker%20Meitz.pdf
It has the Km84 schematic drawn with a FET called "S 2496"

 

[Gus]

P 50 fets are used in a few microphones

Most of the circuit schematics can be found on the web in the paper/link posted.

Look for circuit schematic I built I made the 84,87,86 gain stage as simple as possable.

in the meta use the search

I have posted this before In the 87s and 84s and 86 neumanns I have been in fixing cleaning I have seen 2.2K to 11K source resistors.

Come on people drop a trim pot in adjust for 10V at the drain remove the trim pot and measure it replace with resistor(s) I have seen two used to get the right value in an old stock 87.

hint look at the 4pf and 20pf and think.

Why I use a certain value source R I don't care to share anymore. To much work thinking and testing.

Anywhat the km84.... circuit is nice but it has hidden limit that matters big time with microphones and fets like the 2N3819. Read the fet spec look at all the curves

Just checked the Neumann site both german and english PDFs

downloads select all

About 8/10ths of the way down

 

[burdij]

Thanks for the link. My German is a bit rusty but I was able to read most of it with my trusty dictionary at hand.

There is a bit about the capsule response in the original Von Braunmühl and Weber patent. I had also found a couple of other articles and studies done on the diffraction effects in condenser capsules. I will continue the search.

Here is an interesting article that some engineers at Shure published concerning ideal and actual capsule response in dual membrane capsules.

http://www.shure.com/stellent/group...b_resource/us_pro_dual_diaphragm_paper_ea.pdf

 

[nhaudio]

did you check the shoeps circuit?

http://www.stefanavalos.com/microphones/smallcardioids/diymic5.jpg

and this thread

http://www.groupdiy.com/index.php?topic=18920

 

[dale116dot7]

The 1G resistor rolls things off at the low end, but since both the feedback capacitor and the capsule 'capacitor' see the same (ac) signal, their impedances (vs. frequency) change by exactly the same amount (the ratio of capacitive reactance remains the same). So it works as a voltage divider just like if you used two resistors - for frequencies above maybe a couple hundred Hz. The amplifier will be linearized by the feedback, as Gus has mentioned.

-Dale

 

[DerEber]

Look for circuit schematic I built I made the 84,87,86 gain stage as simple as possable.

Was it the schematic with the 5,6kOhm wich supposed to be 56k?
Yes, I allready read the topics about it. Sadly the link to the schematic is broken :?

Read the fet spec look at all the curves

This emerges lot of questions :green:
Sadly (for this matters) I some years ago decided to focus on recording and did not start to study EE after school. So this very nice field of DIY is only some sort of hobby for me. A very nice and interesting on indeed :grin: So this is my first FET situation.
I printed out the 2N3819 datasheet some weeks ago and I still have some questionmarks. But I don´t think you like me to bother you all with this basic questions.
I found that the "Transfer Characteristics" temperature wise is having a nice spot at about -0.9 Vgs. But this may not have anything to to with what you trying to spot me at. And I need to understand why I have two graphs for differen Vgs(off). I´m not familiar with this Vgs(off)deffinition.

did you check the shoeps circuit?

I did not note this before. But it is very interesting to see other FET-mic schematics for me! I wounder how one calculates amplification on the Fet here. It would be nice if I had two identical bodies and capsulars to build this circuit and the KM84 to compare.

So it works as a voltage divider just like if you used two resistors

Yes, I was questioning about what they meant by "capacitive-voltage-divider". Now I wounder how I could not instantly understand. :green: Thanks to pointing me to it. It´s a nice Idea also!!

The Neumann book is very interesting and answerd many questions. The part about dynamic mics is also very interesting!
Gus, I am not sure what you meant by pointing me at the 20pF and the 4pF? It´s the Pad and it rises the amount of feedback..... and one is able to calculate amplification if I know my calpsular capacidence. Anything more

Now I even can imagine how I could avoid the 470pF coppling and go with my capsular directly to the gate. If I had a scanner here right now I would draw you a quick variation.


Thanks to all of you, who take your time for answering my questions or giving me some directions. I appreciate this a lot!!
Now I think I have to help myselfe some little bit, by reading more and more. :guinness:
Stephan