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Author Topic: KM84 FET Replacement: Type and Procedure  (Read 12154 times)

squareneck

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KM84 FET Replacement: Type and Procedure
« on: January 17, 2013, 09:02:51 am »

Hello!

I have a Neumann KM 84i that had no output, I found out that it was the FET that was broken. I bought a Fairchild 2N3819 and replaced the broken one wich was a SK107. Now the mic is working again but the output is low and quite noisy.

Do I have to change some other component to get the 2N3819 to work properly?

best regards/ Bo Savik
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klaus

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Re: KM84 2N3819
« Reply #1 on: January 17, 2013, 01:41:47 pm »

Assuming that the Fairchild FET you bought is low noise, you still need to individually bias any FET (as Neumann originally did in your mic when they built it.) That's why the existing bias resistor (RS) needs to come out, and proper biasing needs to be set up for the new FET.

Here is a simple way to get very close to the ideal biasing point:

Attach a wire to the gate of the FET (or take the head off, and squeeze the wire into the slit of the two gold-plated wires the head's contact needle attaches to), attach this wire to a signal generator putting out 1kHz @ ca. -25dB or lower (you will adjust the exact output level of the generator later), remove the current bias resistor, and solder a wire to that connection point on the circuit board. Attach the other end of that wire to a multi-turn trimpot (10-20kΩ), and attach a second wire to a convenient ground on the mic's circuit board whose other end will be soldered to the second contact of the trim pot.

Plug in the mic into proper 48V phantom (no need to squeeze the housing tube over the test wires), and observe the meter of your mixing board/pre-amp rise as you increase resistance on the trimpot.  Turn the trimmer from zero Ω until you hear a faint (and very distorted) signal of the test tone, continue increasing the resistance, until you get the highest output with the least amount of distortion at 1kHz.

As you keep increasing resistance, you will notice two things happening:
1. the signal gets lower again, and 2. the signal starts to distort again.

Back off the trimmer to the least distorting point, unsolder the trimmer wires, measure the resistance you have dialed in, and find a fixed 1/8W or 1/4W resistor that comes closest to that value.

Done.

Some final notes of advice: use headphones for this procedure, at very low, barely audible listening levels, and don't have any other noises in the room that will interfere with your hearing. It makes the final set point that much easier to find.

Speaking of final set point: this process explained above is even more precise and yields lower distortion than how most manufacturers do it.
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Klaus Heyne
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squareneck

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Re: KM84 2N3819
« Reply #2 on: January 18, 2013, 03:01:16 am »

Thank you so much Klaus for your fast and professional response! I will try this, I'm really looking forward too it.

Is there some way I can tell if the 2N3819 is noisy? I have four of them so if there is a simple way to test them maybe I should do that to get the best one into the KM 84i.

best/Bo
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klaus

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Re: KM84 2N3819
« Reply #3 on: January 18, 2013, 05:10:10 am »

You will know once the FET is properly biased.
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Klaus Heyne
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squareneck

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Re: KM84 2N3819
« Reply #4 on: January 18, 2013, 05:25:02 am »

Ok. Thank you!
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Jim Williams

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Re: KM84 2N3819
« Reply #5 on: January 21, 2013, 12:59:27 pm »

Listening tests have shown that any THD below about 3% is very difficult to identify, especially if you are attempting to find the lowest THD setting.

This is where the test equipment is used. Any decent THD analyzer can find the bias spots easily, without any guesswork. Do it by ear, then re-check with the test gear. You will find your settings are probably off to some degree. This should be done by any mic moder or repair person as certainty is what is being paid for.
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klaus

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Re: KM84 2N3819
« Reply #6 on: January 21, 2013, 04:43:20 pm »

I guess, I am one mic modder who does not use a distortion analyzer anymore. I found the exact same value of the correct source resistor through both methods enough times, that I leave the analyzer in the basement these days, and just do it by ear.

Besides: if I cannot hear distortion, who cares what I might see? Granted, measuring instruments can quantify what I hear, if I hear it., but that to me seems a secondary pursuit, after detecting anomalies by ear.
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Klaus Heyne
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Kai

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Re: KM84 2N3819
« Reply #7 on: January 24, 2013, 05:46:23 am »

if I cannot hear distortion, who cares what I might see?
If you like to actually hear distortions way below 1%, use the differential distortion method.

Feed in a two tone signal like 19kHz + 20kHz sine.
You probably won't hear these any more if you are male and older then 25.
But you can easily hear the distortion, because it's 1kHz.

The same applies to musical signals; sibilants e.g. are suffering from these kinds of distortions, because they aren't harmonic and their spectrum is far apart from that of the original signal.
The strongest test I know: use a triangle (perc. instrument).
You shouldn't hear any "wooden" knock together with the clear hit.

Explanation: differential distortions aren't separate from harmonic distortions, it's just another and sometime more obvious way to show the same nonlinearities of devices.

Regards
Kai
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Jim Williams

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Re: KM84 2N3819
« Reply #8 on: January 28, 2013, 12:10:10 pm »

The CCIF twin tone IMD tests are best for that. Shove a 15k sine with a 14k square wave and you will find anomolies. This test also has far higher resolution than any common THD+noise test which also incorporates the noise readings. The CCIF method eliminates the noise contributions so measurements can be seen at a far lower level. Typical Audio Precision resolution of the THD+noise tests are .0005%. CCIF goes down to 1 PPM.

If you can't hear these non-linearities that doesn't make them magically go away. Just because you can't hear the residual THD doing a bias set by ear also doesn't mean someone else with finer tuned hearing won't notice.

"Trust, but verify".
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klaus

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Re: KM84 2N3819
« Reply #9 on: January 28, 2013, 12:49:35 pm »

...If you can't hear these non-linearities that doesn't make them magically go away. Just because you can't hear the residual THD doing a bias set by ear also doesn't mean someone else with finer tuned hearing won't notice.
I have done enough of these over the years that I feel comfortable about passing on this simple FET biasing method without reservation. Let's talk again about 1PPM THD resolution once we have eliminated MP3 encoding and 20khz brickwall filtering.

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Klaus Heyne
German Masterworks®
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Jim Williams

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Re: KM84 2N3819
« Reply #10 on: January 29, 2013, 11:00:07 am »

Some don't do MP3, there are some people doing high resolution recordings. After investing thousands in an old mic, I believe they would appreciate the extra fine tuning.

If things down stream are so destructive to the sonics of a well tuned microphone, you might as well save those thousands of $$$ and buy Chinese stuff if it doesn't make any difference.
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klaus

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Re: KM84 2N3819
« Reply #11 on: January 29, 2013, 01:47:15 pm »

Final note from me on this: to my ears, biasing FETs beyond the level of resolution that my method yields has not shown an audible improvement.

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Klaus Heyne
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squareneck

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Re: KM84 2N3819
« Reply #12 on: January 29, 2013, 03:48:34 pm »

Now I have done what you told me and found that 4.8 kOhm gave me the best result. I have now ordered a new resistor á 4.7 kOhm. The one that belonged to the old FET was 3.9 kOhm so I guess I will have more signal and less noise now :-) I'm very happy for your support and will report again when the new resistor is in place!
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Kai

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Re: KM84 2N3819
« Reply #13 on: January 30, 2013, 09:10:18 am »

The CCIF twin tone IMD tests are best for that. Shove a 15k sine with a 14k square wave
I've mentioned the twin tone method mainly because an untrained ear can hear the distortion much clearer.
No analyser needs to be used.
For that purpose, the exact frequencies are of minor importance.

The CCIF method mentioned is even more demanding, as the square wave produces several different (sub-) harmonics. For listening it might be a bit too much.
You need a generator capable of producing a 14kHz square wave - this is not possible if, e.g. a CD is the source, because the 20kHz band limit converts the square into a sine.

The results of different distortion measurement methods, btw., aren't much different, if the harmonics lie within the usable bandwidth.
1% nonlinearity stays the same, however you measure it.

Regards
Kai
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Jim Williams

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Re: KM84 2N3819
« Reply #14 on: January 30, 2013, 12:09:26 pm »

With modern test gear like the Audio Precision 2600 series you can set the test up and see and measure all the pertanent perameters at the same time with one sweep.

You can see THD+noise, frequency response, IMD and phase response, you also get a noise vs frequency plot to sus out the quietest transistors.

Do that with a set of jfets and you will see they are all over the place in spec. This is the most precise and predictable way to find the best parts and set bias for the highest performance, if you care to do that.
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