abbey road d enfer

Re: Mic Biasing
« Reply #20 on: August 28, 2019, 07:08:15 PM »
I got 10K at source by tweaking pot for maximum gain and headroom. I was tweaking drain pot at the same time. It's Sterling Audio ST51, not u87. Similar, but not the same as u87.
Then what are the source and drain voltages?

Quote
I am quoting you saying 2sk30 should have bottom resistor:
Yes, I stand by it. Most FET's need a source resistor, unless their gate is negatively biased.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.


Re: Mic Biasing
« Reply #21 on: August 29, 2019, 01:46:57 AM »
Well, we are on the same page. That is why i quoted you in the first place.

Do you have any suggestion on how to pick a FET that is suited for zero volts at gate?

Khron

Re: Mic Biasing
« Reply #22 on: August 29, 2019, 02:16:53 AM »
"Almost all JFETs are what's known as 'depletion mode' devices.  This means that they conduct with no gate voltage (typically gate shorted to source or at the same potential).  This is the maximum current region, and usually must be avoided for linear operation.  The FET is biased off by applying a negative voltage to the gate with respect to the source, and they can be biased in exactly the same way as a valve. "

Source: https://sound-au.com/articles/fet-applications.htm , emphasis mine.

Absolute gate voltage (ie. with respect to ground) is NOT important or relevant. What IS, is the gate-to-source voltage, ie the RELATIVE voltage between them (which will change depending on the source resistor, drain resistor, transconductance and possibly / likely some other parameters).

Well, we are on the same page. That is why i quoted you in the first place.

Do you have any suggestion on how to pick a FET that is suited for zero volts at gate?
Khron's Cave - Electronics - Audio - Teardowns - Mods - Repairs - Projects - Music - Rants - Shenanigans

abbey road d enfer

Re: Mic Biasing
« Reply #23 on: August 29, 2019, 04:36:47 AM »
Well, we are on the same page. That is why i quoted you in the first place.

Do you have any suggestion on how to pick a FET that is suited for zero volts at gate?
+1 to what Khron wrote.
I would just add that  you need to make sure the gate-source voltage needed for proper operation is not too high, just because the whole circuit operates on low voltage. Since source bias is used in most cases, increasing the  source voltage also reduces the available voltage for the FET, which results in loss of headroom. FET's are available in several grades; you want the lowest Idss.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

Matador

Re: Mic Biasing
« Reply #24 on: September 12, 2019, 05:45:02 PM »
If it helps, my experience has been that when contemplating different JFET's, it's important to draw the intended load line on a typical VDS/IDS plot, then consider how that same line will look on the replacement JEFT.  Often, tweaking of drain AND source resistances is in order.

To make things more difficult, JFET parameters across a handful of the SAME DEVICES can span a 300% range across common parameters like gate-source off voltage, IDSS.  For example, for the J305 device, VGS(off) can span from -0.5V to -3V, and IDSS can span from 1mA to 8mA!  For the 2N3819, VGSoff can be anywhere between -0.2V to -8V, and IDSS can be anywhere between 2mA and 20mA!  This often means that you need to buy a handful and measure the one that will work best in your intended circuit.

Re: Mic Biasing
« Reply #25 on: September 13, 2019, 05:06:37 AM »
May the JFET rolling begin!  ;D

It seems to make more sense than tube rolling :D

Khron

Re: Mic Biasing
« Reply #26 on: September 13, 2019, 05:16:03 AM »
... Or [ahem] cap rolling? ::)

May the JFET rolling begin!  ;D

It seems to make more sense than tube rolling :D
Khron's Cave - Electronics - Audio - Teardowns - Mods - Repairs - Projects - Music - Rants - Shenanigans


 

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