Author Topic: JFET buffer help!!!  (Read 1828 times)

thedavidbeam

JFET buffer help!!!
« on: June 12, 2012, 10:18:21 PM »
hi guys. I was working on a JFET buffer using a 2n5457 transistor. I can't seem to get it to work. it has VERY low output. my amp has to be set on 6 and it doesn't even sound like it's at 1! any idea what's wrong? I can upload a picture if needed. I built it point to point.

the circuit begins with a .1 uF input cap which has two 1m resistors soldered to it. one goes to ground, and the other goes to +9v. I have the input cap then connected to
pin 3 of the transistor, and pin 1 goes to +9v. pin  goes to a 1 uF output capacitor, which has a 3.3k resistor connected to it which then goes to ground. the negative lug of
the output cap goes to the output. I have built several of these, and never had any trouble. any idea why i'm having so many problems? I can't get any to work today, and have tried starting over many times.

any help is much appreciated.

thanks!

sault

Re: JFET buffer help!!!
« Reply #1 on: June 14, 2012, 07:22:05 PM »


Check your battery, try using a different Jfet (a different 2n5457 I mean), then upload a picture, because the only other thing I can think of is a Jfet with extremely high/low values that might be throwing you off... e.g. a very low Idss (1 mA rather than 3-5 mA) might be affecting your output, for instance, or a much lower Vp than you're used to. Maybe double-check the values of your input and output caps, too...

Tony Forestiere

Re: JFET buffer help!!!
« Reply #2 on: June 14, 2012, 08:01:49 PM »
Manufacturer may have a different pin-out?
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thedavidbeam

Re: JFET buffer help!!!
« Reply #3 on: June 16, 2012, 01:38:53 PM »

Earthscum

Re: JFET buffer help!!!
« Reply #4 on: June 16, 2012, 03:56:32 PM »
Check your input jack. I think your ground lug is touching the tip. Other than that, looks correct.

eta: you should clip all the ends off, those may cause problems for you down the road. For instance, if that one hanging off the Source/Resistor/Cap is close enough to the case, it could sink some signal to ground.

eta (another thought) Check the voltage of "pin 2" (Source). This could tell you what's going on... but I'd definitely make sure nothing is touching, or even within about 1/10 of an inch from eachother if you can. Also, check with and without the bottom on... sometimes you may not catch something hanging a bit low in the case. Remember gravity is working against you here. It may look good until you flip the sucker over and something moves just enough to make contact.
« Last Edit: June 16, 2012, 04:08:20 PM by Earthscum »
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Jdansti

Re: JFET buffer help!!!
« Reply #5 on: June 16, 2012, 04:41:57 PM »
+1

Also, there's a lot of stuff soldered to the transistor. Unless you're experienced and quick or you used a heat sink, you could have damaged the transistor.

Another thing to check is your grounds between your jacks.  I can't see both jacks in your photos or a switch or pot. Can you provide photos of the other components?
R.G. Keene: EXPECT there to be errors, and defeat them...