Replacing jFET

[überfuzz]

I have an application in which I need to replace a jFET. It's a phantom (standard recording equipment 48V) powered cg-stage. I've found an interesting jFET. However, it comes in four versions, different Idss intervals. How do I determine which I should go for?

versions
1. Idss : 2.6 to 6.5 mA
2. Idss : 6 to 12 mA
3. Idss : 10 to 20 mA
4. Idss : 18 to 30 mA

[Jazz P Bass]

What part number are you trying to replace?

How much current will be across the jfet?

The current, IDSS, is important because it's the maximum current that a FET can reach without entering the restricted breakdown region.

[überfuzz]

I guess the jFET will see a theoretical max current of I = U/R = 48/6800 ≈ 7 mA I.e. B+ = 48V and the anode resistor is 6800 Ohms.

Edit: This would suggest that the 2.d-version is okay, right?

[J M Fahey]

Please post the schematic.

Basically if it's a FET gain stage we want lowest IDSS simply because it's the simplest to bias and provides highest gain (in general has highest transconductance) while if it's a source follower (unity gain, just matches impedance) it's way less critical (all will work) but highest IDSS usually means highest Vp (pinchoff/cutoff voltage) and that in general allows for highest headroom (Vp ~maximum peak voltage it can handle) .

So, POST THE SCHEMATIC, capisce?






EDIT: also not sure the 6800 ohms resistor is a load resistor , that value is suspiciously similar to the typical 6800 ohms resistor used to limit current pulled from Phantom Power, go figure.

Used so even if that particular mike shorts, it won't steal more than 7mA from Phantom supply.

[überfuzz]

Please post the schematic.

Basically if it's a FET gain stage we want lowest IDSS simply because it's the simplest to bias and provides highest gain (in general has highest transconductance) while if it's a source follower (unity gain, just matches impedance) it's way less critical (all will work) but highest IDSS usually means highest Vp (pinchoff/cutoff voltage) and that in general allows for highest headroom (Vp ~maximum peak voltage it can handle) .

So, POST THE SCHEMATIC, capisce?






EDIT: also not sure the 6800 ohms resistor is a load resistor , that value is suspiciously similar to the typical 6800 ohms resistor used to limit current pulled from Phantom Power, go figure.

Used so even if that particular mike shorts, it won't steal more than 7mA from Phantom supply.

I have a hard time to understand exactly what you're getting at... But thanks for the reply, I guess... I thought I mentioned that it is phantom powered, standard recording equipment phantom power. The layout of the CG-stage is as such:
1. Anode resistor 6k8 Ohm - fixed in phantom power supply
2. Cathode resistor 300 Ohm
3. Gate resistor 300 kOhm

Hope this increases your level of capiscieness. Cheers!

[g1]

I think what he might be getting at is...
Post The Schematic!
If you don't have it, draw one up. If it's top secret, then probably no one can help you here.

[überfuzz]

I think what he might be getting at is...
Post The Schematic!
If you don't have it, draw one up. If it's top secret, then probably no one can help you here.

Eh, did you or anyone else read my posts. Well third times a charm I guess, here it goes again. :-)
The CG-stage is as follows:
Anode resistor: 6k8 Ohm - sitting in phantom powered leads
Cathode resistor: 270 Ohm (updated from 300 Ohm)
'Grid' resistor 300 kOhm

[J M Fahey]

Eh, did you or anyone else read my posts. Well third times a charm I guess, here it goes again. :-)
The CG-stage is as follows:
Anode resistor: 6k8 Ohm - sitting in phantom powered leads
Cathode resistor: 270 Ohm (updated from 300 Ohm)
'Grid' resistor 300 kOhm

Yes, we read your posts.

No, we see no schematic, sorry.

[überfuzz]

Yes, we read your posts.

No, we see no schematic, sorry.

...

[J M Fahey]

Sorry, that schematic is incomplete/wrong , that is not what you have inside your device.

By the way,what do you mean by "cg stage?".

Hints:
* you are not showing the audio source feeding it
* you are not showing where the audio passing through that Fet is going to
* phantom powered circuits are balanced and are fed from *two* 48V lines ... where is the other?
* phantom power standard calls for two 6k8 resistors, which are usually *not* "plate resistors" but current limiting resistors, and two are needed to keep circuit balance.
* if you have two 6k8 resistors(which you should), where is the other?

If you don't like drawing, at least post a couple sharp, well illuminated pictures from both sides, *maybe* we can guess the circuit for you.

Why do you think the Fet is bad?

Post a few voltage readings.

EDIT: I'm not being particularly obnoxious, but you seem not to have fully "lifted" the schematic , just saw a Fet and a couple resistors nearby and *assumed* it's a gain stage similar to your drawing.

As said, that drawing is very incomplete and does not match what's minimally needed or functional in a Phantom powered device.