jfets

[guitarfreak666]

hello everyone,

i'm new here but i have seen a lot of good advice so id like to ask you all something. i am now building a  jfet buffer but i cant get j201 jfets in my area. does anybody of you know a jfet that can directly replace them. will a j300 work, i have some of those at home now.

greetz guitarfreak

[J M Fahey]

Probably, but to check, post the schematic of what you intend to build.

[guitarfreak666]

the schematic is under here but i have noticed that a lot of pedals use j201 jfets, if they are interchangeable, are there some conditions that i need to know about that i dont blow them?

[J M Fahey]

It seems that you will have to get a J2101 after all, it was not chosen by chance.
Most others work much worse.
This guy experimented and studied it a lot:
http://www.hawestv.com/amp_projects/fet_preamp/fetpreamp1.htm
Recommended reading for anybody interested in Fets for guitar use.
Personally I much prefer working with Op Amps, much more flexible, predictable, and, to boot, cheaper.
Yesterday I bought a "stick" of 50  TL062, I´m interested in building some active guitar preamps.
I paid around U$ 16 for all 50.
Not the greatest Op Amp in the world, not bad either, and optimized for low voltage (9V batteries) plus 1/10 the current consumption of TL072.
And it has FET inputs

[Loudthud]

The J201 is a favorite for stompboxes because it can be used in circuits that require very little in the way of current at low voltages. Typically 0.1mA to 0.5mA at 9V. This translates to battery life of more than a month if you left it operating all the time.

JFETs can vary widely on the amount of current they draw. Usually a 5 to 1 range is all the manufacturer will specify unless you get specially selected parts. The J201 Idss is spec'd at 0.2mA to 1mA. The generic J300 specifies the Idss current at 4mA to 45mA. That's over an 11 to 1 range. There are tighter spec'd units, suffix A, B, C and D. J300A: 4mA to 9mA, J300B: 7mA to 15mA, J300C: 12mA to 25mA, and J300D: 21mA to 45mA. Do your parts have one of these suffixes?

To operate at 9V, the J300 will have to opreate at significantly higher current. At that current, battery life could be only several hours. Another option for the J300 would be to operate it with two 9V batteries connected in series. That will help reduce the current. If you will be using an external power supply, that won't be a problem. How do you intend to power this device?

To make the J300 work in that circuit, two resistors will have to be changed to customize the circuit for your J300s. The J300s will need to need to be tested to determine those resistor values. These tests will require you to connect the FET in a simple circuit with one or two resistors and measure a voltage or two. If you want, I will walk you through this process.

Note: The circuit you posted has no protection on the input. A simple static discharge could kill the FET. Also, if a J300 is operated at a high current, it can pickup any nearby radio station, even in the FM band. Some simple changes to the input circuit of the FET would be wise.

[guitarfreak666]

i have tried to build the circuit like it is on the schemtic and it works wery good, nice clean boost with lots of extra volume. i use it with a power suply.

[guitarfreak666]

btw thanks for the info about the j300. i didnt know that i could blow it. what changes do you suggest then?

[Loudthud]

The gate of a JFET is basically a diode to the source and drain. On most data sheets for JFETs, there will be a current rating for the gate, usually between 10mA and 50mA. For an N channel JFET, the gate diode is turned on by positive voltage with respect to the source/drain voltage. So in a normal stomp box with a +9V supply, any input to the gate higher than +9V will turn on the gate diode, probably less in most circuits with gain. All that is necessary to protect the gate for positive inputs is to limit the current to a safe value, usually with a series resistor on the gate. The attached schematics use 68K. 100K or more could be used.

On negative input voltages, the story is different. The gate diode will breakdown usually between 25V and 40V. The diode is not very rugged, it doesn't take very much current at all to destroy the junction. A diode from the gate to a negative voltage (-15V) would work, but in a stompbox, that option is not available. So a zener diode with a signal diode in series or just several signal diodes in series is required. In some circuits where the gate is not at ground potential, but is biased above ground, a diode to ground could be used as long as it doesn't interfere with normal input voltages.