Setting Bias in FET847 with Multimeter Only

[Wordsushi]

Forgive the noob-ness of this post, but I'm ready to embark on building a  Poctop FET847 but what's holding me back is figuring out exactly how to set the bias with just a mutimeter.  Poctop says: ● 2n3819 FET should be selected and biased using the same methods as a U87

So in Andrew Shaap's 87 build (http://www.vintagemicrophonepcbkit.com/Neumann%20U87%20Build.htm),  it says:

Connect your mic preamp of choice, preferably one that works, and turn on the phantom power.  We’re going to calibrate the drain on the microphone to 11.5 volts.  Take your multimeter and put the red probe on r6. This is right next to the FET.  Take your black probe and put it on the ground hole.  Take a small flat-head screw driver and tweak the pot until you measure 11.5 volts.

My questions are:

a) If R6 is the red probe point of contact for the U87, what is the corresponding probe point for the FET847?
b) Would I still be measuring the drain to 11.5v on the FET847?

I've read using an oscilloscope and injecting a 1K tone into the circuit is the best way to bias the FET, but I don't have an oscilloscope so I was going to do what Schaap did.

Any help at all would be very much appreciated. I've been very pleased with my previous experiments swapping out capacitors and capsules from inexpensive mics and also building a couple of microphone-parts kits (I'm actually very interested to hear how the FET847 compares to the MP T84) and I'm ready to try my hand on builds like those from vintagemicrophonepcbkit.com

Thanks in advance!

 

[0dbfs]

That may be a good starting point but its going to depend on your selected FET.

Without a scope try recording a loud transient signal like a kick or snare drum. One that will clip the mic signal. Then adjust for symmetrical waveform clipping. Or a tone.

Good luck!
-jb

 

[Wordsushi]

Thanks!
I was just going to go with the FET from Poctop's BOM. The 2N3819
https://www.mouser.com/ProjectManager/ProjectDetail.aspx?AccessID=f72374e439

 

[0dbfs]

Thats the FET in this circuit. It's going to fall somewhere within a range of attributes which means that you need to select the one that is most desireable out of a handfull. Read up in the u87 thread on measuring IDSS value and try to find one with IDSS of 7.5? Based on the idss your calibrated drain and soirce voltages will be different yielding different amounts of headroom output and clip point.

You really should calibrate for symmetrical clipping regardless of what the idss is.

Dont worry too much about finding the perfect fet. I made 11 of these and was mostly concerened that they all be similar so i could use them interchangebly. Dont remember what idss i ended up with but it was not 7.5mA and they work exceptionally well. I ordered a couple hundred 3819's from different sources and measured each one and sorted them into ranges.

Good luck!
Cheers,
Jb

 

[Wordsushi]

Thank you, JB!

One other question. I'm now thinking I could attempt to use a software oscilloscope on my laptop an a tone generator app on my iPhone together. I know a hardware oscilloscope would be optimal but has anyone tried using software?

 

[Matador]

One other method that might work:

1) Using a DAW, send a 1kHz sine wave (using a free signal generator plugin) out of one of your recording outputs, and inject right into the capsule coupling cap
2) Record it back using another track
3) Export the track, and use RightMark Audio Analyzer (also available freeware) to measure THD

You should be able to tweak the bias for minimum THD and you should be very close.

 

[0dbfs]

That's a great idea and well worth checking the calibration using all available tools / methods.

I would like to note that there is a position on the PCB for a dedicated fixed resistor (precision) so that once you bias up with the trim-pot you can then measure the value and order a fixed resistor for permanent installation. There is a jumper in the area where you can jumper it for the fixed resistor or the trim-pot.

Cheers!
-jb

 

[Wordsushi]

Thanks JB!
I didn't know that. I saw a jumper in the BOM. Is that what it's for?

That's a great idea and well worth checking the calibration using all available tools / methods.

I would like to note that there is a position on the PCB for a dedicated fixed resistor (precision) so that once you bias up with the trim-pot you can then measure the value and order a fixed resistor for permanent installation. There is a jumper in the area where you can jumper it for the fixed resistor or the trim-pot.

Cheers!
-jb

 

[Wordsushi]

I have to figure out how to trim an audio cable to do that, but I think I can make that work.

Right Mark looks cool, but is it PC only? I'm on OSX and I saw this analyzer (and oscilloscope) program.
https://dogparksoftware.com/iSpectrum.html

Do you think this will do the trick?

One other method that might work:

1) Using a DAW, send a 1kHz sine wave (using a free signal generator plugin) out of one of your recording outputs, and inject right into the capsule coupling cap
2) Record it back using another track
3) Export the track, and use RightMark Audio Analyzer (also available freeware) to measure THD

You should be able to tweak the bias for minimum THD and you should be very close.

 

[Khron]

Might be a year too late, but here's a simpler method: skip the recording, and just use an oscilloscope plugin Most, if not all, DAWs come with one.

Or if not, there's definitely plenty of spectrum analyzer plugins - just tweak for minimal distortion.

Turn up the output (of the signal going into the mic, ie. the JFET gate), tweak trimpot until both the top and bottom of the waveform on the scope (plugin) are symmetrical and unclipped or minimal harmonics visible in the spectrum analyzer).

Rinse and repeat until you can no longer reduce distortion (and/or the top & bottom of the scope waveform are equally "flattened") ;D

One other method that might work:

1) Using a DAW, send a 1kHz sine wave (using a free signal generator plugin) out of one of your recording outputs, and inject right into the capsule coupling cap
2) Record it back using another track
3) Export the track, and use RightMark Audio Analyzer (also available freeware) to measure THD

You should be able to tweak the bias for minimum THD and you should be very close.

 

[doqmemory]

Might be a year too late, but here's a simpler method: skip the recording, and just use an oscilloscope plugin Most, if not all, DAWs come with one.

Or if not, there's definitely plenty of spectrum analyzer plugins - just tweak for minimal distortion.

Turn up the output (of the signal going into the mic, ie. the JFET gate), tweak trimpot until both the top and bottom of the waveform on the scope (plugin) are symmetrical and unclipped or minimal harmonics visible in the spectrum analyzer).

Rinse and repeat until you can no longer reduce distortion (and/or the top & bottom of the scope waveform are equally "flattened") ;D

Khron, should the capsule be replaced with a suitable (50pF or so) capacitor, or just disconnected when biasing the FET?

 

[Khron]

Disconnected. You'll have the test signal going into the JFET gate "anyway". You'll likely want to couple that through a 1-10nF cap, though.

 

[RuudNL]

In fact there are two 'best' bias settings...
You will find a setting if you adjust for maximum undistorted output or symmetrical clipping.
But if you lower the level (to a more realistic output), you will notice that there is a different bias point for lowest distortion.
So you will have to decide first what your priorities are.