MosFET or JFET for source follower

[toshiba_nz]

Hi all.

I need a follower for my bass guitar (with passive pickups) that won't load down the input tube (CCS loaded 7b4) of my amp when I turn down the volume pots on the bass.

Should I go with jfet or mosfet?

PS I need extreme fidelity and neutral tone (the bass amp is all custom, with DHT finals)

 

[abraxalito]

What's the signal level? MOSFET is going to be noisy, especially in the lower freqs so low noise JFET will be the extreme fidelity solution.

 

[toshiba_nz]

Not sure exactly...the bass is a Fender jazz (mexico) , with a Fleor alnico pickup in the neck, and the factory ceramic in the bridge...maybe 1V RMS?

I can fit 2 x 9 volt batteries in the tone cavity if I need +/-, or 18V supply...

 

[adason]

https://www.muzique.com/lab/buffers.htm

 

[MarcelvdG]

JFET stage and a diode to protect it against reverse breakdown. Should be lower noise and less vulnerable than MOS.

 

[Osvaldo de Banfield]

I suggest a follower with 2SK241 MOSFET. Gate is protected inside the device, designed for low noise FM front end and 17mS of transconductance, this gives low voltage loss as follower.

 

[dkfan9]

I see 12mS max gm, 10 typical on the 2sk241 datasheet.

 

[Osvaldo de Banfield]

May be. I haven't the DS at hand. In any case, the higher the gm, the lower the output impedance, and the closer to unity the gain.

 

[toshiba_nz]

Thanks guys 👍

 

[MarcelvdG]

MOSFETs have more 1/f noise than JFETs. Whether that matters is a different question: electric guitars are also noisy (having 250 kohm to 500 kohm passive volume control potmeters) and I've been told that most players don't mind, because the noise is masked by the hum.

 

[toshiba_nz]

I suggest a follower with 2SK241 MOSFET. Gate is protected inside the device, designed for low noise FM front end and 17mS of transconductance, this gives low voltage loss as follower.

Can you kindly point me in the right direction (to find a genuine part)

 

[Osvaldo de Banfield]

I was searching between my printed data sheets, and you are true. My apologies. The JFET with 17mS is the 2SK427 between many others. My confusion was because it has same package as 2SK241.

 

[toshiba_nz]

So if I wanted to roll with a mosfet, that I could get from digikey, what would you use?

 

[WhiteDragon]

PS I need extreme fidelity and neutral tone (the bass amp is all custom, with DHT finals)

For fidelity and neutral tone and likely battery life
= opamp
Or just pretty standard low noise BC556

Or for low noise jfet, pretty normal source follower with
LSK489
Or common go too Mosfet be
2N7000

 

[Osvaldo de Banfield]

@WhiteDragon also took an important point: Current consuption. A single MOSFET or JFET needs 10 to 20mA to reach those mutual conductance. If you want to maintain FET category (Personally I love them toguether with tubes and strongly dislike BJT's), you can try one or more of the low voltage low current drain opamps. I may cite as example TS271 series from STM but there are several of them. If you have experience in SMD, you can go also for smaller micropower opamps. Search with Google or ChatGPT for a list of the thousands available from many manufacturers.