crunchy Jfet boost

[JasonG]

Ok
Simple Jfet boost. I cant stop it from distorting. It seemed ok before but , then again I don't remember if I had the volume on my guitar all the way up when I tested it.
These are my voltages
G =0
S = .21
D= 4.15
battery =8.9v
I am playing a guitar with humbuckers.

http://www.aronnelson.com/gallery/v/DRAGONFLY-LAYOUTS_0/album87/JFET_BOOST.gif.html

[brett]

Hi
Something's wrong.  The source should be at much more than 0.2 V. 
With JFETs, but the input voltage must be considerably less than the source voltage.  Yours is the other way around (assuming your pickups put out about 0.3 V).  A good voltage on the source would be 1 to 2 V. 
Check that the source resistor isn't one tenth of the value you thought it was (e.g. 220 ohms instead of 2.2k).  The voltage on the source moves (non-linearly) with the size of this resistor.
cheers

[Dragonfly]

check pinout ?

[JasonG]

fairchild J201's when looking at the flat side are DSG right ?

[brett]

Sounds right.  How big are the source and drain resistors?
Is the electro bypass cap oriented correctly?  Does it have plenty of resistance?

[JasonG]

I tested the cap on the source and its good. the drain value is 33K source is the stock 2.2k. The layout said adjust till you got about 4.5v  thats what it took to get close.

[Gus]

J201s have a IDSS spec of .2 to 1ma.  What does this mean? It means you can't have more than .2 to 1ma Id (drain current)

VERY SIMPLE way to work with this circuit (It might be a god idea to install a cap in series with the 22K between R1 and R3)

Do you know ohms law?
  Lets take worse case IDSS at .2ma (.0002A)
Lets make it real simple  9volts /.2ma =45K(not taking into account other losses,  for people who know more, Yes this is very simple)
9V /1ma =9K)

So I would take a 47K and make that the drain resistor.  You want the Id (drain current to be less than IDSS)

47K and a 5volt drop =about .1ma

Then I would take a 10K pot and wire it in as the source resistor 0 to 10K.  Then adjust it for about 3VDC to 5VDC (taste) drain to ground.
Remove the pot and use a fixed resistor(s) of or close to the value of the pot measured on a meter after you carefully remove it without moving the shaft.

When I work with FETS I often measure them before placing them in circuit.

[JasonG]

Well after doing some reading , I realized that this booster is basicly all about getting the most gain it can out of one transistor. With the ( what seems to me) very hi input impedince and the 10uf bypass cap on the source giveing the gain a good kick. I built this for a friend who is a bass player.
A bass guitar would probably demand more from the booster so it might not be as cruchy with a bass guitar. Right?
So if I wanted a really booster with the gain boost I seem to be going for,I have to have 2 stages.

[petemoore]

Well after doing some reading , I realized that this booster is basicly all about getting the most gain it can out of one transistor.
  Well a 1 mosfet can get more.
  With the ( what seems to me) very hi input impedince and the 10uf bypass cap on the source giveing the gain a good kick.
  I built this for a friend who is a bass player. The plot thickens...
A bass guitar would probably demand more from the booster so it might not be as cruchy with a bass guitar. Right?
  Bass tends to be higher output than guitar, and if the peaks go over the rails of the boosters PS voltage, you may have a distortion instead of a booster, probably not what you're goal is.
  Making more swing room before railing [by increasing supply voltage] prevents that.
So if I wanted a really booster with the gain boost I seem to be going for,I have to have 2 stages.
probably the signal will exceed the PS potential, again it may clip.
  A mosfet or an opamp might get the amount of clean boost which meets your needs, BTW what is that...I mean to say that when cascading clean' gain stages very soon a high voltage output can be obtained, and the 'high output booster' is more of a 'pre-amp', and driving a pre-amp with a pre-amp can also cause distortion.
ie much more than a mosfet boosts and 'pre-amp' might be what you're looking for.
  A bit of info about the application chain [amp/etc.] might be useful for refining the design goal outline.

[Gus]

As posted by Petemoore Bass signals can be larger than guitar.   I would try removing the source bypass cap to lower the gain a bit.

Question why a boost for a bass?  At 9V it is very easy to clip boosters with a bass level signal. 

[snoof]

Or put a pot wired as a var resistor before the bypass cap so it's adjustable.

[JasonG]

This booster has given me a good reason to learn about Jfets. Pete your right I was confusing distortion with other unwanted characteristics.( Generally speaking you will have a quieter pre  amp if you use more than one stage.) I built another booster on the bread board last night and it had the some problem basically the source was about at about .2 v. Its just one 2.2 k reistor and one 10uf cap with one end connected to the source and one to ground. I tested the parts they are good.
Thats whats getting to me. If the voltage at the source was good maby the booster would work sound closer to what I expected. Maby I have gotten a bad batch of transistors. I guess I need to build a tester and get to work.

[petemoore]

G =0
S = .21
D= 4.15
battery =8.9v
  These voltage readings don't look too bad to me but I need to refresh my Jfet understandings to be sure .21 is enough 'source above gate' room for the bias to work with, the fact that 4.15 comes up on the drain makes me think the circuits wired up correctly, and that maybe a little bias tweek toward 4.5v on drain might not help that much.
  Thats whats getting to me. If the voltage at the source was good maby the booster would work sound closer to what I expected. Maby I have gotten a bad batch of transistors. I guess I need to build a tester and get to work. 
  Tester? [as in..] Good? [as in source above gate?] Expected? [you should get a clean-ish output, similar to input but higher volume/amplitude]
  Bad Batch [New? 8 Ball and Ime says: Doubtful.
  First is it working...it should mostly just boost, and Jfet voltage readings should be within good bias parameters [I need to refresh-read at GEO 'foolin with Jfets' IIRC]. Everything shown on the schematic should reflect what the board 'exhibits' as far as visual scanning, DMM voltage, resistance and connection readings, anything marked Gnd. should test to connect with all other grounds.
  Signal path goes through caps which can be ear tested, though they are 99.999% reliable, especially when new and not overvoltaged...caveat being electrolytics, which can be damaged by reverse polarity.

[Gus]

JasonG I think the fets are in spec you most likely have a batch at the lower end.  Measure the IDSS with the simple test in this post

Lets do the math voltage divided by resistance = current.  The source current is very close to the drain current so
.2V / 2.2K = .09 ma about .1ma

Like I posted above the IDSS has a range of values the j201 specs are .2 to 1ma and yfs 500umhos

FWIW the J202 is .9ma to 4.5ma and the same process 52  and yfs 1000umhos

For all you jfet builders find the sweet spot by subbing in different fets note the  gfs yfs, Vgsoff, IDSS or do the math and select a range to install in different places in the circuit

VERY SIMPLE TESTER/selector that often works with the same small signal n channel brand and batch fet numbers used in effects

Set you DMM to current

connect the source and gate together
connect the Drain to a 9V battery
connect the red wire of the DMM to the source and gate
connect the black wire to the - terminal of the battery
The current displayed is the IDSS at 9V

FWIW When I am building/modding  fet condenser microphones I often only get maybe 2 fets out of 100 that work at the operating points I want.  I purchased 1000 of a number that has a large range to find what I like that gave me a range that allows me to order another number with a tighter spec range.
I built a circuit fragment as a tester as well as other simple testers And when I have time I will build a simple fet curve tracer for my scope.

[Dragonfly]

fairchild J201's when looking at the flat side are DSG right ?

http://www.fairchildsemi.com/ds/J2/J201.pdf

Heres a good reference for jfets and understanding them....I can't find the original links where I found these PDF's, so I'll just host them in my gallery.

http://www.aronnelson.com/gallery/v/DRAGONFLY-LAYOUTS_0/JFET+INFO/

[Gus]

google vishay fet ap notes

[petemoore]

  Taking into consideration points made above, consider GEO 'Updates for Mosfet boosters', AMZ Mosfet Boost...