My Mic Experiment: In Progress. KM84-ish

Anyone know of a decent host without a tiny tiny DL limit I could host my stuff on?

Here is a pic of my mic:



And here is a sound clip:

http://www.geocities.com/microphoneland/Audio/CLSMicClip1.wav

Made from a Carvin branded China mic. I used what I had laying around, MPF102 FETs and an oooold Sescom transformer. Rudimentary and preliminary frequency response testing shows -3dB @ 34Hz-17kHz

Recorded 6' in front of my drumset, no EQ or FX on this clip.

I want to see if anyone notices anything peculiar about this setup, other than putting the whole can & octal base into the body.

Here it is closed up:


And a sound clip WITH a bit of bass boosting EQ & a touch o' 'verb.

http://www.geocities.com/microphoneland/Audio/CLSMic2.wav

I think you can upload audio files to twin-x. It's free.

http://www.twin-x.com/groupdiy/index.php

That mic looks exactly like the standard china body, but that grill is fooking weird. Looks like a speaker grill on a bass cab.

Your samples are a little to short to get a good idea of the sound. Kinda mid-range-y. My guess would be that the sescom might hold you back. What ratio, and what recommended impedances?

Does the capsule look like this?

[quote author="skipwave"] That mic looks exactly like the standard china body, but that grill is fooking weird. Looks like a speaker grill on a bass cab.
[/quote]
Came with the mic. A Carvin CM-80 or something or other.
[quote author="skipwave"]
Your samples are a little to short to get a good idea of the sound. Kinda mid-range-y. My guess would be that the sescom might hold you back. What ratio, and what recommended impedances?
[/quote]

The impedances were worn off. It's a TR164-A, which I looked up on the Sescom site. The Sescom site is probably wrong, I think the ratio is not 15k:150 but more like (at least!) 30k:600 as I have it wired.

I didn't test it before I put it in, but now I have to because output is a lot lower than expected. This and measuring the voltages on the FET are the next order of business.

[quote author="skipwave"]
Does the capsule look like this?
[/quote]
Yes.

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The mic is basically a KM84 copy with a large diaphragm. I made some slight design concessions to provide for my own learning and to match the FET to the +48V power supply. Oh yeah, the capsule is floating from ground so I could skip the input cap!

http://www.gyraf.dk/schematics/Neumann_KM83-84-85.GIF

Sounded pretty OK with an electret capsule on the breadboard - better than the Behri mic that the capsule came off of.

Cool. I just compiled all of my notes about the KM84, mostly comments from Gus and Dale.

Cheers to you for taking initiative and making something. I'm curious to know how the FET biased. The lower output could be the result of bias point and not the transformer ratio. FET characteristics are all over the place.

A more in-depth frequency response test confirms my dilemma. Why does everything I build lately roll off @ 70Hz? :shock: And now I'm outta 1 Gig Rs.

Voltages - Now on to do the math.

[EDIT] R6 and C3 are simulating the capsule.

c3? might try it without..

oh and I use netfirms. cheap and reliable.

I see a number of issues

The MPF102 is a P 50 Die (from fairchild site)

Look close at the voltages at the FET and then look a the neumann schematic

You will find a hint in the phantom C thread.

Tim could you link the corrected schematic of the simple KM84 type. This is as simple as I make a KM type circuit and I often like to use the min amount of parts.

Here it is...

What I gather from this is:

1) I could strap the output for 150 or 300R using the matched 2.2k R for the power tap.

2) I should be regulating the voltage supply, filtering it, and perhaps even loading the FET a little higher w/less current for small signal stuff.

3) I should have an AC shunt on the polarized side of the capsule

4) Where's that thread for places to order good 1Gig resistors? A few 680Megs would be cool too.

Neumann specs 10V at the drain for a good reason.

[quote author="Gus"]Neumann specs 10V at the drain for a good reason.[/quote]

I'm still a newb at this, but looking at the 2n3819 datasheet it appears that:

1) 1V Source bias seems to be the point of least deviation due to temperature changes

2) Working backwards I would say that 47K load R seems to be a convenient point to place the load resistor to create an output impedance of @470R.

3) Lower current = less noise.

4) I could use some reading links and such on JFET audio amps- I have nothing in any of my books on FET amps x'cept for my AARL which is focused on radio oscillators & AGC etc.

5) I'd love to investigate the possiblities of a Vbe multiplier or superdiode regulator above the drain, but once again am stuck without reading for truly understanding what this FET is needing for optimal performance.

Neumann specs 10V at the drain for a good reason.

Click to expand...

Because 10V is roughly half way between 0 and 21.5V (the maximum available drain voltage)? This will yield maximum output before clipping.

When in doubt, you can think of the FET as a little, cold, vacuum tube.

In the case of the 2n3819, with 250ua flowing in the SD circuit, the FET will yield a voltage gain of about X40. The transformer attenuates by a factor of X10 so the net gain of the mike is X30 or approximately 30db. The gate voltage can't be much higher than -1V because this transistor cuts off at about -3V. The maximum drain voltage can't be much higher because the maximum GS/GD voltage is 25V.

http://socrates.berkeley.edu/~phylabs/bsc/PDFFiles/2N3819.pdf

Datasheet for reference

There is another reason besides being about 1/2 power supply voltage.