Hawes Amplifier Archive by James T. Hawes, AA9DT
FET Preamplifier FAQs, Part 2

Basic Preamp Questions

QUESTION. What type of battery should I use with the preamp?

ANSWER. We're talking about the nine-volt, Type PP3 battery that powers the preamp. The usual battery choices are alkaline and heavy duty (zinc chloride). I'll compare these two.

I prefer the "heavy-duty" battery. A heavy duty cell fails gradually and allows continued performance. When you hear distortion or the amp seems weak, replace the battery. Alkalines fail suddenly. You could be in mid-performance and lose your lead guitar!

An alkaline cell might last longer. Yet the gradual failure of the heavy duty cell is a tremendous advantage. Advertising claims that alkaline cells last "six times as long" seem to be exaggerations. An arbitrary test might prove such a claim, but real life tells a different story.

True, neither battery type fails very fast. The reason is that the preamp poses only a light load. The preamp's average current drain is only 2 milliamperes. Some heavy duty batteries have a 400 mAh rating. With this rating, your average battery life could be 200 hours! (Actually, the way that you use the battery affects how long it lasts.) A heavy duty battery will save you some money, too.

When you aren't using the preamp for an extended period, remove the battery. Battery leakage can destroy the preamp. I've seen battery acid completely eat through the leads of a semiconductor.

QUESTION. Can I buy a preamp from you? Will you build me one?

ANSWER. What are you offering me? Bids start at $100, cash in advance. But hey, this is really a do-it-yourself project. Before sending money, please read the next question.

If you'd still rather buy a preamplifier, why not buy a commercial one? The Para Acoustic includes a class-A, FET amplifier, plus an equalizer. You power the circuit with a 9-volt, internal battery or 48-volt, phantom power. The unit is stingy on the power requirement, too. This amp only uses 2.9 mA. The manufacturer claims that with an alkaline cell, you can get 200 hours of operation. The output impedance is 600 ohms, about a third of what my JFET amp develops. You can order from this site: Commercial JFET Preamp.

QUESTION. I don't know electronics. Can I build your preamp?

ANSWER. FETs can really challenge a first-time project builder. FETs are fragile. No two are alike. They're sensitive, temperamental and mischievous. Touch a FET the wrong way, and it goes away and never comes back. Can't cuss your way through a little old-fashioned trouble? Then don't bother with FET preamps. Those are the negatives. Here's the positive: A FET can produce the most melodious tones that you can imagine. Make a special effort. If you want to succeed badly enough, you will.

QUESTION. Where can I buy the MPF102 JFET?

ANSWER. Check out the sites below. Based on my personal experience, I highly recommend these vendors. I'm very proud to have Allied as an advertiser on this site. The parts are top quality and the service is second to none. Visit these sites and then please, come back...

QUESTION. What are some advantages of working with FETs?

ANSWER. There are several advantages...

  • Small size. With FETs, you could compress a tube preamp design down to a two-by-four inch circuit board. Not only would the resulting circuit be smaller. It would also be much lighter than the original. Yet the FET circuit would sound and perform just like the original.

  • Low power. You could power a FET amplifier with the 12-volt filament supply from a tube amplifier! (Be sure to rectify and filter the supply voltage.) If you power the circuit off batteries, expect the batteries to last a long time. For instance, a good MPF102 design only draws three milliamps per device.

  • Simple construction. You don't need a PC board or a device socket. If you're careful, you can just solder a JFET into your project. Buy parts with leads. They're quite compact, but large enough for assembly by human beings. You only need to connect three leads. The leads are nice and long. You can bend them and tack solder on resistors or capacitors. You can easily build FET circuits on unplated perfboard or even terminal strips.

  • Serviceable. With today's microscopic, surface-mount devices and large-scale integration, many circuit designers neglect serviceability. Leaded JFET designs are different. They're easy to service. Passive components, such as resistors and capacitors can be large enough to grasp with your fingers. You can easily replace leaded, discrete parts. You certainly don't need a pick-and-place machine! If an IC goes out of production, you might never get its circuit working again. JFET circuits are another story. Choose carefully, and you can replace one JFET type with another type. You might need to change a bias resistor. But you won't have to start over with a new circuit.

  • Single power supply. Unlike many op amps, FETs don't require split power supplies. One power supply is enough, and works just fine. Of course, if you want to build a complementary circuit with FETs, you can still use dual supplies.

  • Almost no heat output. FETs have no filaments to waste half of your power supply's output. The lack of filament heat means far less wear and tear on your amplifier. For example, heat dries out capacitors. Capacitors in a FET circuit should last much longer than capacitors in a tube circuit.

  • Sensitive. FETs are voltage-driven current sources. They're extremely sensitive. They draw no current from the input circuit. Here's a comparison of different devices. Typical transistor circuits have input impedances in the thousands of ohms. (Bootstrapping or Darlingtons can raise that impedance to a few million ohms.) Tube circuits have input impedances of up to a few million ohms. JFETs have input impedances of about a billion ohms. MOSFETs have input impedances of several billion ohms.

  • Low noise. FETs are far less noisy than either tubes or transistors are.

  • Graceful aging. Also, JFETs age far more gracefully than tubes do. Experts consider JFET aging as unnoticeable. Like tubes, MOSFETs age in a noticeable way.

  • NTC. With bipolar transistor circuits, you must protect against thermal runaway by adding external resistors. JFETs don't have this problem. What they have instead is a negative temperature coefficient. This coefficient protects the devices from thermal runaway.

  • Like designing with tubes. JFETs act like tiny tubes. If you know anything about tube design, you'll be right at home with JFETs and depletion MOSFETs. (Enhancement MOSFETs are different. Somewhat like transistors, enhancement devices require positive bias.)

  • Easier than tubes. JFETs are easier to connect than tubes are. You can solder JFETs right into your circuit. A JFET has no filament to burn out. With a triode tube, you connect five pins. With the equivalent JFET, you only connect three pins. A FET power supply can be more compact than most tube power supplies are.

QUESTION. Will the JFET preamplifier work with a piezo pickup or a piezo element?

ANSWER. Yes, of course!

If you're building a guitar pickup, you might be using a piezo element. You can find such elements at Radio Shack, All Electronics and Electronics Goldmine. Make sure that you're getting an element and not a buzzer. You don't want the buzzer circuit. You just want the transducer.

Piezo elements are easy to work with, and you don't need to wind any coils. The soldering is the tricky part. You might need to buy a small amount of silver solder. Make sure that your iron is hot and clean. If the element has attached wires, use those wires. Splice a shielded cable to the element wires. Connect the cable to the guitar jack.

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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

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