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U87 transistor inquiry

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Can any one give a spec and description of component T1 in the schematic below. I have a U87 microphone with component S2436/TIP645. Searching for this transistor using typical cross reference charts leads to nothing. Is the S2436 transistor a proprietary one ?

Klaus Heyne:
The 2436 is similar to the 2N3819 for which you will find specs by Googling it.

Both transistors were used by Neumann interchangeably for at least 10 years, from the late 1960s to the late 1970s, and occasionally thereafter as well.

The two transistors mentioned above are also compatible with a couple of other ones Neumann used after the Texas Instrument 2N3819 became obsolete: K105 and SK107.

The specs are pretty close on all four of them, and you may liberally substitute them in any Neumann FET mic without negative consequences for the circuit or the sound of these mics, provided that you custom bias each FET, just as Neumann used to do, regardless of the brand of FET it used.

Thanks Klaus for the quick reply...

I believe the TIP645 or TIP64 is extinct. I cannot find any info on it anywhere.

I was'nt sure if the S 2436 was a 2Sk2436 or so.

I will certainly study the different data sheets on these.

Interestingly, The 2N3819 is also used in my KM84.

Is there a good primer on biasing fets on the web, I have seen a few but not sure how well they are discussed/written. Can one be recommended ?


Klaus Heyne:
Don't know hat you mean by "TIP645"...

The 2436 is not an SK-type. Neither is the SK 107, despite its letters...

Biasing of single FETs in mic applications is done to minimize distortion and maximize output. As FETs vary, it needs to be done for each individual FET in each individual mic.

I set these FETs up the following way:
I feed a 1K sine wave signal to the gate and slowly increase the source resistance from 0-10k Ohm with a temporary trim pot (10k) until I reach the lowest distortion of the FET.

This will be a pretty precisely defined point right after the highest output of the mic, right after the first heavy distortion audible when the FET turns on, and right before the more gradual onset of the distortion as the FET's output decreases again.

The trick is to feed enough 1k into the gate that at this ideal bias point the distortion is indeed barely audible or gone altogether, and clearly audible to each side of the ideal bias point.

All of this is best done at very low listening levels over headphones in a quiet environment. If you don't want to do it aurally, you can use an oscilloscope.

I then copy the ideal value I found  with the trim pot and choose a fixed source resistor of the same value for final installation.

If others have found another or better way to bias source-biased FETs in condenser mics, please post them!
But please be conscious of the forum's rules: keep the explanations simple!

John Monforte:
I have used Klaus' method and it works well. But I would add that you might also want to test for noise.

Buy a bunch of these (they are cheap) and run through them to listen for 1/f noise. Sometimes this is called flicker noise or popcorn noise. It has a low level popping sound. It comes from contamination of the die when it is packaged and a device will either have it or not for its whole life. You will find some duds because noise is not a guaranteed spec on those parts.

You can also measure the "good" noise and find differences of a dB or two. To do this, you first need to bias it up and then replace the capsule with a 50pF capacitor (so you get rid of the acoustic stuff) and carefully shield your test setup to eliminate hum. You can measure a couple dB difference among a normal batch of parts.

If this last test sounds tedious to you, well it is! I built a little test fixture with a special test circuit which speeds up the process greatly. But now I am getting too techno...


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