Wednesday, July 11, 2018

building a JFET amp no math just logic

I'm going to piece together an audio amp to drive my Low Z earbud and my Hi Z earbud without any math other than in my head (guessing).
I selected the 2SK2539 simply because sells a 3000 reel for $15 dollars with free shipping. A reel should be a lifetime supply for an amateur. You can use whatever JFET you have on hand. The first step is to determine the Q point. I put a 1K resistor in the source circuit to set the bias and a 100k to tie the gate to the circuit. I wanted about 1 ma. This gave me just over 1 ma so far so good. The drain needs room to swing so I select a drain resistor that will drop about half the supply at 1 ma. 4.7K should work nicely. so ready for the next step.
Hows that for dumb luck? The drains Q point is about 4 volts. Now to try some signals to see how it functions.
1mv in 800 mv out.
3 mv in 2 volts out, starting to distort.
5 mv in flat topping. Now to look at some loads. I have standard earbuds which are 60 ohms and a high Z earbud which is 2K. I will make and output port for each.
Yea that will drive my earbud to hard. I am hard of hearing but I can still hear thunder;).
Yes the Hi Z phone will work too. I have some transformers and will probably put one here but it will work without it.
Time for a build!

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