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!
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!
It's not for chosing the 'best', it's for calculating the resistor values.
Why don't you just copy that design?.
As in your other post, absolutely NOT!.
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.
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.
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 😉
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).
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!
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).
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.
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.
Audio uses voltage transfer (if that's the correct term).
Audio rarely uses power transfer.
Well, I have always seen the typical impedance of audio equipment as 47kOhms - seems to make sense with BJT inputs, which most audio equipment has (even IC based preamps). Are you saying that the input impedance of typical preamps is actually much higher than 47kOhms, say by a tenfold? I doubt... But I can be wrong. Obviously you know more about this I do, having corrected me on the "characteristic" capacitance of the phono cartridge, which I realise doesn't have to be matched (a select few audio components actually allow this, some Sony and Marantz devices come to mind).
Anyway, this has gone way off topic from the original topic, which was guitar preamps with JFETs.
all I said was that transconductance and Gate-source capacitance were determining factors in the choice of what transistor to choose.
It's not because I know less than you that I talk jibberrish... please understand this.
No, typical input impedance is roughly 47K, 100K, 220K, perhaps occasionally 470K. This is FAR higher than the output impedance of preceding equipment, which will normally be in the low single kilohms (or less).
Presumably you are confused about the output impedance of audio equipment?.
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