js201 vs bf245 FET?

[jasonconanan]

which is the better FET amplifier for a bass guitar? or for any guitar as a volume control amplifier.
the js201 or the bf245
im thinking of using either FET as a volume amplifier using a potentionmeter at the base.

ive seen both JFETS with the same configuration, with a pot at the base and then a 2.2kohm resistor at the emitter, and a 6.8kohm resistor at the collector.

albert kreuzer has a small article on how to pick the best bf245, im also curious if there is a way to pick out the best js201?

also, a quick question,
if i have a pickup with around 20kohm resistance, i am supposed to use a 20kohm linear potentiometer as the input to the volume control stage right?

merry xmas!

[jasonconanan]

edit: make that the j201 FET. not JS201

[Nigel Goodwin]

Originally Posted by jasonconanan

albert kreuzer has a small article on how to pick the best bf245, im also curious if there is a way to pick out the best js201?

It's not for chosing the 'best', it's for calculating the resistor values.

Why don't you just copy that design?.

also, a quick question,
if i have a pickup with around 20kohm resistance, i am supposed to use a 20kohm linear potentiometer as the input to the volume control stage right?

As in your other post, absolutely NOT!.

[Jarno]

They are both JFETs so the procedure for selecting the correct one for the biasing resistors is the same.
The other way around is to choose suitable biasing resistors for your parts (I usually stick a trimmer in and then solder in the correct value).
You need to select or adjust because of the quite large tolerances on JFETs in general.

[gain-wire]

Originally Posted by jasonconanan

also, a quick question,
if i have a pickup with around 20kohm resistance, i am supposed to use a 20kohm linear potentiometer as the input to the volume control stage right?

merry xmas!

thank you!
Happy new year.

Although it is true that impedances should be matched, in the guitar world, the typical arrangement is that you NOT have the pickups seeing a volume control right after the jackcord. There are already volume controls and tone controls (let's just stick with volume for simplicity's sake) on the guitar itself, and those are susally ten times the value of the DC resistance of the pickup (usually 5-8kOhms for single coils and 8k to 12kOhms for humbuckers.) Remember that the voltage is split between the resistance across which it is developped: the bigger the input resistance (potentiometer) the greater voltage is developped across it. That's V=RI.

anyhow, the volume control or gain control in your JFEWT amplifier will be after the first stage because the first stage usually is a source follower, well, it isn't the case always, sometimes it's just a class A amplifier. you should use, with most FETs, a value of potentiometer between 100k and 1 Meg, as JFETs somewhat work like tubes.
Have you ever looked at a Marshall schematic? First stage, gain, then volume, the gain stage again, then Cathode follower.

I'll leave you with that.

Also, the FET choice should be determined by input Capacitance and Transconductivity. Transconductivity determines the gain of the transisitor. Capacitance will take away high frequency, you want to have the lowest input capacitance.
alot of other members here have better knowledge of FETs, I suggest you post your question in the electronics forum..cheers,

gain-wire

[Nigel Goodwin]

Originally Posted by gain-wire

Although it is true that impedances should be matched

No it's NOT - it's only in a small number of cases where you need impedance matching - almost always it's the wrong thing to do. Impedance matching provides maximum POWER transfer, with the theoretical highest possible efficiency only 50%. This is obviously pretty dismal, and almost everything uses VOLTAGE transfer, where maximum theoretical efficiency is 100% - and in ALL cases will greatly exceed the 50% maximum of power transfer.

Examples where you DO use impedance matching are RF systems (to prevent reflections back down the cable) and speaker connections to valve amplifiers (which is one reason why they are so inefficient).

[Mr.Chamber]

DEFINETLY the J201 FET hands down !

[gain-wire]

Originally Posted by Nigel Goodwin

No it's NOT - it's only in a small number of cases where you need impedance matching ..
Examples where you DO use impedance matching are RF systems (to prevent reflections back down the cable) and speaker connections to valve amplifiers (which is one reason why they are so inefficient).

Well, I've been working in RF/optical so long that it just came natural to me to say so.
Obviously guitar systems use voltage transfer. However, it is common practice to have the preamp inputs on audio equipment match the typical output impedance of the source it is connected to. And in the case on Phono cartridges, the capacitance should also be matched.

I don't mean to sound all-knowing, and those here who know me know that I don't mind being corrected, but I tend to want to know the truth also!(when it appears I'm saying weird things)

cheers!

[Nigel Goodwin]

Originally Posted by gain-wire

However, it is common practice to have the preamp inputs on audio equipment match the typical output impedance of the source it is connected to.

No it's not - it's VOLTAGE transfer again, not power transfer, which would upset the sound quality. Can you give any examples? - certainly CD players don't, tape decks don't, tuners don't, mixers don't, record decks don't - I can't think of a single example? (or any reason to do so).

And in the case on Phono cartridges, the capacitance should also be matched.

PU cartridges aren't 'matched', they have specific load requirements, both resistive and capacitive - which the preamp is designed to meet, but it's not the output impedance of the PU.

[AndrewT]

RF and similar UHF frequencies are a completely different animal from audio.
Audio uses voltage transfer (if that's the correct term).
Audio rarely uses power transfer.