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JFET guitar Pre-amp
JFET guitar Pre-amp
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Old 4th February 2011, 11:07 AM   #21
AndrewT is offline AndrewT  Scotland
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If you go to using a 2sk170 then I think you would be better with a gr grade and preferably towards the low end of 2sk170gr.
A BF862 might also work, but again a low Idss device.

There is a thread on using the BF862 in a low gain low noise amplifier.
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Old 9th May 2011, 07:51 PM   #22
dabelectro2501 is offline dabelectro2501
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Hi there does anyone have a layout showing the tilman connected together in a Pedal form with LED and 3pdt switch to use like a stompbox? I have the schematic layout, just need to see how to hookup to everything else Thanksd!
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Old 9th May 2011, 09:48 PM   #23
Fenris is offline Fenris  United States
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If you're not afraid to try something, here's a little circuit with three 2sk170s. It only needs 5 resistors, one capacitor, three JFETs and one LED. Gain is about 2, with plenty of driving current. The LED should be a low brightness type with about a 2v drop. It functions as both a power indicator and as bias for the first JFET. The other two JFETs are a source follower and constant current source. Total current draw is about 13ma. You could probably just knock together this circuit "bugsplat" style in a few minutes to see how well it works.
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Old 22nd September 2011, 05:17 PM   #24
JT Hawes is offline JT Hawes
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Default Correction on CS vs. CD gain; FET circuits

Quote:
Originally Posted by JT Hawes View Post
One potential disadvantage of common-source circuits is that they offer no voltage gain. In fact, CS circuits are slightly lossy.
Oops. The above quote requires a small edit. I meant to refer to common-DRAIN circuits (CD). Common-source circuits (CS) can indeed have gain.
I apologize for the confusion.

FET gain depends to some extent on the ratio of the drain and source resistors. (Rd / Rs: The internal resistance and load also alter the gain.) For more gain, decrease the source resistor (Rs) or increase the drain resistor (Rd). There are limits, though: The average current must exceed the minimum Idss spec for the FET. Otherwise, your FET will pinch off, and probably quit amplifying.
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Old 14th April 2018, 03:47 AM   #25
miked33 is offline miked33  United Kingdom
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Apologies for restarting a very old thread, however...
I am just learning JFET biasing theory and building up my understanding.

...I have come to understand that every individual JFET has a specific measurable value of IDSS which is a fixed and inherent property of that individual JFET. The datasheet shows a range of values in which an individual sample may fall (not a range of ID which can be used for any JFET of that type).

I can not match my understanding as described above with the comment in post #24 which says

'The average current must exceed the minimum Idss spec for the FET. Otherwise, your FET will pinch off, and probably quit amplifying'

Can anyone clarify?

Many Thanks...
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Old 14th April 2018, 11:19 PM   #26
PRR is offline PRR  United States
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Instead of "average" it should probably say "peak".

Say the sheet gives "Idss = 2mA to 6mA".

Say you design for 4mA average, but grab a specific part that only does 2mA.

You will only get 2mA, NOT 4mA.

Say you design for 2mA average and grab the same specific part that only does 2mA. It will idle at 2mA, and swing "down" to zero mA, but will not swing "up" above 2mA (much).

Assuming symmetric swings (close enough for jazz), if the spec says "2mA to 6mA" then you should design to idle at 1mA max.

Or else sort-out a baggie of parts to find the higher Idss parts. (Which may be tolerable in DIY but not for mass production.)
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Old 15th April 2018, 01:51 AM   #27
miked33 is offline miked33  United Kingdom
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Many Thanks PRR for a very clear answer,

......When you are learning a concept and think you have nailed it and then read something which contradicts completely you tend to doubt yourself.

The specific question I was really trying to ask but didn't express very well is whether it was correct to attempt to run a JFET at a current greater than its IDSS.

You have confirmed that you can't, which is what I thought, whereas the post originally mentioned implied that you must do.

Cheers again,
Mike

(I tried to edit my previous post to clarify it, but the edit does not seem to have appeared)
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Old 15th April 2018, 02:30 AM   #28
PRR is offline PRR  United States
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Quote:
Originally Posted by miked33 View Post
... the post originally mentioned implied that you must do....
Ah, I see that now. I believe the post is missing a "not".
> The average current must not exceed the minimum Idss spec for the FET.

Maybe line-noise, maybe forum corruption, maybe the mind got ahead of the fingers.
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Old 15th April 2018, 04:53 AM   #29
EUVL is offline EUVL  Europe
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A typical guitar preamp should be able to take 0.25Vrms input and has a gain of 3.
So the JFET in the circuit shown should have a Vto of at least -2V, preferrably more.

Using a J201 as shown above, the output will have a distortion of ~0.5% with 0.25Vrms at the input.
By raising the voltage to 18V and using a J111 instead, you can reduce this to ~0.02%.
Adding one more J111 and a resistor will also allow you to lower Zout dramatically.

Total current consumption is about 3mA.
And you can build the entire thing in SMD as well, so it will be tiny.
The J111 should have a Vto of about -4V.

Patrick
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Old 16th April 2018, 07:17 AM   #30
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by EUVL View Post
Using a J201 as shown above, the output will have a distortion of ~0.5% with 0.25Vrms at the input.
By raising the voltage to 18V and using a J111 instead, you can reduce this to ~0.02%.
Speaking as both a guitarist and a person who knows a fair bit about audio electronics, lowering JFET distortion is the last thing you want for a JFET electric guitar preamp.

The 12AX7 triode usually used for this job in good guitar amplifiers produces several percent of low-order harmonic distortion, and this softens the harsh sound of a naked solid-body electric guitar appreciably. It is a good thing, and the reason guitarists still put up with fragile, expensive, bulky, heat-generating valves.

When you use a JFET in place of the 12AX7, it would be wonderful if the JFET could be coaxed into producing as much distortion as the half-12AX7 did. We would rather have 5% THD than 0.5% if possible, and a distortion of 0.02% would be a shame, as it's too little to make an electric guitar sound good!

Electric guitar amplification is nothing like Hi-Fi. The goals are very different - not to accurately reproduce an audio signal with minimum distortion and noise, but rather to take an inherently rather cold, harsh, unattractive-sounding instrument, and intentionally mess with its signal until it actually sounds quite good. We aren't reproducing a good musical signal, we're trying to create one!

-Gnobuddy
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