2SK246 substitute for FA-1 build

[AllenM]

Hey guys.. need some transistor help / education.

I am working on the Aion Prism (Boss FA-1) and wondering if there is a substitute part I can use for the 2SK246 and what other changes may be required. I did a little google research and most of the info I found gave recommended substitutes and even mentioned biasing but did not have any details. I would like to learn from this experience as much as get the answer as it were.

Any help or advice would be greatly appreciated.

Link of the project... https://aionelectronics.com/project/prism-boss-fa-1-fet-amp-pcb/

AllenM

[AllenM]

Well after some mote google-fu I decided to search FA-1 schematics and found several that had substituted the 2SK246 with a J201 (not any easier to find) and one used a MPF102 which I am  sure I have stock of and also easy to get still. I will print off the spec sheets and see if I can make sense of the information.

AllenM

[somnif]

In this case the JFET is just a simple input buffer, and the particular choice of part does not have an enormous effect on the audio. Its just "take signal, make bigger".

The MPF102 will work fine. You can get SMD versions of the J201 if you want to use an adapter and that will work fine. No real need to worry about the drain resistor in either case as the transistor isn't really doing anything magical, its just there to keep the input impedance up, and output impedance low.

A bit of background: http://www.muzique.com/lab/buffers.htm

"Probably the easiest buffer is the basic jfet common drain amplifier. The input impedance is determined by the value of R1 and is 1M as shown in this example. The value of R2 is not too critical and may be any value from 3.3k to 10k without much change in the sound. I prefer to use lower values since this allows more drive on the negative portion of the audio cycle where the only pull-down is the source resistor. This configuration has the least number of parts but is limited in that if the input voltage exceeds the gate-source forward voltage plus the bias voltage at the source, the signal will be clipped. This configuration is not normally useful with bipolar or mosfet transistors, which require a positive bias voltage (for N-type devices). Input impedance is approximately the value of R1. The output impedance will depend on the jfet but is on the order of a few hundred ohms. "

[AllenM]

somnif,
thanks for the information, this is helping me understand a little more. I may look into the SMD - J201 solution if I do not find another option. The MPF102 pin out does not match (DSG) and I am not all the that keen on twisting two of the legs to make it work. I would either like to find a direct pin out match (SGD) or at least a reverse match (DGS).

I will check my inventory at home and see what I have in JFETs.

AllenM

[somnif]

https://oshpark.com/shared_projects/7TAIpS4u  There are the adapters I use, $4.65 gets you 27 of them.

The SMD part is MMBFJ201, and can be found many places. Don't know what part of the world you're in, but they're 36 cents on Mouser, 17 cents on Tayda, etc.

[AllenM]

Somnif thanks for link! I'm actually in Fort Texas and mouser is like 20 minutes from my house.

To try and continue my education I went thru my parts stash am done came up several JFets ( at least I think they are).

J201
2n5952
2sk30
Mpf102

I have a tester I bought on Amazon (and have no idea how to use yet) and noted these readings.

J201 - I = .34ma Vg = .24v
2n5952 - I = 1.6ma Vg = 1.1v
2sk30 - I = 1.0ma Vg = .70v
Mpf102 - I = 1.9ma Vg = 1.3v

Now I am wondering does this mean anything to me, or how do I compare this to the 2sk246 spec sheet. Again any help / pointers appreciated.

AllenM

[midwayfair]

Once again, it literally doesn't matter what you use. It's just a buffer and there aren't any meaningful characteristics between FETs to compare. Just read the datasheet to get the pins right.

There are plenty of things to fuss about, but transistor part numbers for a buffer aren't one of them.

If you have very hot pickups and you care about such things, a J201 is more likely to clip at the gate than the others.

[lrgaraujo]

You should be fine with the 2sk30, since the pinout should match

[AllenM]

Thanks everyone for the help!

AllenM