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FET Preamplifier FAQs, Part 4
Questions About Modifying & Troubleshooting
QUESTION. Can I use a different FET?
ANSWER. The 2N3819 works well in this circuit. Note the different pinout, though! (To make matters more confusing, some manufacturers reverse the source and drain leads. This page shows the Fairchild layout. Check your JFET manufacturer's datasheet!) Digi-Key and Mouser carry the 2N3819. Below are some other JFET types that might work in my circuit. (I make no guarantees, and haven't tried these parts.) Again, note that pinouts vary from FET to FET.
QUESTION. What's your technique for FET substitution?
ANSWER. Here's a start. Look for a match or near-match in these three datasheet parameters...
Got your three-way match? You've probably found a sub! Buy a few of the matching parts and try them. Remember, no two FETs are exactly alike. For your tests, I suggest breadboarding the circuit on a component plug board. Radio Shack and other vendors sell the ideal model. Click... Plug board.
QUESTION. Can you get me a schematic for a different preamp?
ANSWER. Sure. Use the Google "search" feature at the bottom of this page.
QUESTION. Can I run the preamp off some other voltage?
ANSWER. Yes. I've redesigned the bias circuit for the voltages below. These are the voltages that I recommend. Voltages below 9 volts are too low for most FETs. At 6 volts, you'd probably lose all your gain. Operation at more than 18 volts is probably too inconvenient for battery power. Anyway, you're getting close to the maximum for the FET, 25 volts. (Do you want to play through it, or detonate it?) For an MPF102, the ideal voltage is 15 volts.
Note that the source and gate resistors (RS and RG) remain the same as before. Just substitute the drain resistor (RD) below for the one in the nine-volt circuit...
QUESTION. The preamp doesn't work. Can you help me to debug it?
ANSWER. For $50 an hour, cash in advance, minimum $100, maybe. Anyway, save your money. Here are the usual problems...
Also see our troubleshooting page.
QUESTION. I reversed the drain and source leads. Will this reversal cause a problem?
ANSWER. Don't worry. The MPF102 and 2N3819 are symmetrical JFETs. With each device, the drain and source leads are interchangeable.
Here's what's going on inside these parts: Each device has an channel that's a bar of N-material. The drain and source are terminals at the ends of this bar. In each device, the channel has a consistent width. The gate terminal connects at roughly the middle of the channel. You can think of the channel as a "hose." The gate functions as a variable "hose clamp," increasing or decreasing current flow through the channel.
I've built several 2N3819 amps with reversed drain and source terminals. (I had to do that to qualify for the "all thumbs engineer" award. Joking!) By the way, my "reversed" amps all work normally. Caution: If your circuit malfunctions, a drain-source reversal isn't the problem.
Beware! Not all JFETs and MOSFETs are symmetrical. For example, the substrate of some MOSFETs must connect to a dedicated source terminal. Also, some JFETs have gates that connect to the bottom of the channel. We call these JFETs "assymmetrical." Such devices operate best when you follow the manufacturer's drain and source connections.
Simulation. In Bill's simulation, the LND150 matched the performance of a 12AX7. Bill's 6BQ5 output now clips nicely. At more conservative input voltages, the LND150 amplifier produces good sine waves. Here are the maximum specs for the LND150...
Breadboard. I breadboarded the circuit on June 7, 2010. During tests with an old Conar® tube power supply, I ran several resistors through the solderless breadboard. I started with a 1 mA maximum current, and parts as in the above table. The 1 mA circuit didn't respond to changes in the source bias resistor. I increased the current to just over 2 mA. Then the LND150 started to behave in a proper, tube-like manner. Yippee! The parts values below produced the best results. P.S. Does anybody out there remember Conar? If so, flash your NRI class ring!
Wow. Does this sound good! For more about the final LND150 amp design, click: Solid tube preamp.
QUESTION. What's the easiest way to design my own preamplifier? A trimpot in the drain circuit?
ANSWER. We've all read about the trimpot-in-the-drain technique. This is an astonishingly poor design method. You can sometimes make a FET work that way. Yet you might not arrive at a useful circuit. The drain trimpot method ignores the JFET's minimum saturation current. If you fail to operate the JFET above minimum current, the device becomes unstable or won't operate. Any practical load will cause drain voltage to fall to zero. Conversely, should you exceed the maximum IDSS, you'll demolish the JFET. Below is a simple, rule-of-thumb design method. (In the worked examples, I use 9-volt power. The average current is 2 mA.)
The method above is a rule-of-thumb technique. Don't expect this method to work for all circuits. Tweaking part values might be necessary. If you're uncomfortable with experimenting, this method isn't for you. Try a cookbook Web site instead.
To vary the gain, you can bypass only part of the source resistor. Don't know how much gain you need? Replace the source resistor with a 500Ω potentiometer. Ground the bottom of the capacitor. Run the pot wiper to the top of the capacitor. Turning the pot will vary the stage gain. Word to the wise: The equalization will also change. That is, you'll get a variable treble boost and bass cut. To minimize the equalization effect, increase the capacitance.
If you want still more gain, increase the power voltage to 15 volts. Then use my 15-volt circuit on this FAQ page. The drain resistor will be larger, increasing your voltage gain. An 18-volt power supply would be fine, too. Please don't run the power up too much higher than that. The device turns into a pumpkin at 25 volts. (There's even a puff of smoke, but no bibbity bobbity boo.)
Don't know how much gain you need? Replace the drain resistor with a 1,500Ω potentiometer. Connect the top of the capacitor to VDD, the drain power supply. Run the pot wiper to the capacitor's negative terminal. Turning the pot changes the amount of bass boost from zero to maximum.
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JFET Preamp Pages
♦ WARNING. This is your project. Your achievement is entirely yours. I assume no responsibility for your success in using methods on these pages. If you fail, the same is true. I neither make nor imply any warranty. I don't guarantee the accuracy or effectiveness of these methods. Parts, skill and assembly methods vary. So will your results. Proceed at your own risk.
♦ WARNING. Electronic projects can pose hazards. Soldering irons can burn you. Chassis paint and solder are poisons. Even with battery projects, wiring mistakes can start fires. If the schematic and description on this page baffle you, this project is too advanced. Try something else. Again, damages, injuries and errors are your responsibility. — The Webmaster
Copyright © 2007 by James T. Hawes. All rights reserved.