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Why Musicians Love High-Impedance Preamps

Loading drags down the output of your guitar or mic. The effect is muffled, indistinct sound with flattened dyamics. But when you play into a high impedance, your tone has heart. You're in the groove. You connect with your audience. What was fog is clear. What was dirt is clean. What was drab and featureless is now texture and distinction.

MPF102, RIP. Until recently, a JFET was the perfect answer: Here was a solid-state device with a sky-high input impedance. Plus, it offered a sweet sound that many compare to the ideal: The angelic tone of a triode vacuum tube. The power requirements were so modest that you could build a small FET preamp into a stompbox. The battery would last for many concerts and jams. Plus, the idea that a local store could supply a replacement JFET was another huge advantage of the JFET. Unfortunately, ease of replacement is no longer true of the JFET. The local Radio Shack store doesn't seem to carry JFETs anymore. You can order JFETs online, but the venerable MPF102 is becoming scarce.

But wait! There is an answer! Gather 'round...

Can't Find the FET You Need?

Go Beyond JFETs! Use High-Impedance Bipolar Transistors!

Time to don the thinking cap! Let's see: How about building a high-impedance circuit with normal transistors? You ask: “You can do that?” Answer: “Yes, you can!” There are two ways to go. But the circuits are unusual and mysterious. A tiny cabal of wizened engineers knows the secrets. Welcome!

I've built both experimental circuits below. Both seem to work, although I haven't tried them with a guitar on a noisy bar stage. (That test's on you.) Shhh! Here are the big, dark secrets...

1. A normal, common-emitter stage with very large resistors: One transistor in an extremely simple circuit. The high-impedance performance will dazzle you. Very economical on battery power. Requires a high-impedance output or a follower circuit. The transistor should work fine into a tube guitar amp.

2. A bootstrap follower, feeding into a normal common-emitter stage. The second stage provides all the voltage gain. To my ear, this circuit sounds better than the one-transistor solution. Building and debugging takes maybe a half hour more than for the one-transistor circuit. Also, the bootstrap circuit isn't as small as the one-transistor solution. The input impedance is likely higher than that in the one-transistor circuit.

High Resistor Values & Low Current Drain Hi-Z plus gain, one stage. In one stage, you can combine gain plus a high-impedance (hi-Z) input! This stage isn't as stable as a typical, H-bias stage. Yet with just one hi-Z stage, you should be okay. (Caution: Please don't court disaster and use two!) You can even direct-couple to the input, but only if you're using a piezo pickup. I've drawn a schematic for a circuit that should do the job (right). I built the circuit on a plugboard and tested with a piezo pickup. For the test, I ran the output into my Radio Shack amplified speaker. Wow! The high-impedance circuit works beautifully! The output into a tube guitar amp should be even better. Circuit gain is about 7.2, which is better than for the MPF102 circuit. (If 7.2 is too much gain, you can reduce it by increasing the size of the emitter resistor.)

BJT stage with FET-like high impedance: 2N3904, 2N2222A, etc.

Follower converts high to low impedance: 2N3904, 2N2222A, etc.

Optional filter capacitor details: Parallel a small capacitance like 0.01 and a large, electrolytic capacitor such as 47 µF or greater. Some decoupling circuits also add a 100-ohm resistor between the power source and power rail. Wire the 100-ohm resistor in series with the collector resistor. The top of the 100-ohm resistor goes to Vcc. At the junction of the two resistors, connect the positive side of the filter capacitors. The other side of the filter capacitors goes to ground. This stage works best into a high-impedance. If the next stage is your tube amp, fine. On the other hand, if you're trying to work into a transistor amplifier, it might load down this preamplifier. In that case, you need a buffer amp (such as an emitter-follower stage) or a different preamp circuit.

• A general-purpose BJT isn't a low-noise device as a JFET is. You can certainly use a low-noise BJT in this circuit, but you'll have to hunt for one. Otherwise, a little grit isn't a bad thing. Maybe the noise will help to differentiate your tone. If you need the preamp to work and you can't find a FET, time to experiment with something new!

Tweaks. Tweaking the base resistor (R1) value is advisable. In this circuit, the value will vary according to the transistor that you use. I know: Resistors in the megohm range aren't exactly over-the-counter parts at Radio Shack. If you can't find the right base resistor, tweak one of the other resistors while meauring the no-signal quiescent collector voltage. Again, the ideal collector voltage falls in the range between 4.5 and 4.9 volts. Below is an easy how-to guide.

Formulas for Large-Resistor Preamps

This data might help (If not, skip ahead): I designed this circuit for transistors with a beta of about 200. Here's the formula for the base resistor value...

R_B= [β x (R_C + R_E)]
Where
• β is the transistor's beta, a voltage gain of about 200
Resistances are in ohms

Example
• R_B= [200 x (39,000 + 8,200)]= 9,440,000
• Round to nearest standard value: 9.1M (Note: I didn't have a 9.1M, so I used 10M. 10M is also close.)

The best base resistor value will allow a no-signal output voltage of about half the power supply voltage. A bit more than that is even better. Our one-stager has an input impedance of 1.4 megohm. Here's the input impedance formula for this circuit...

Z_IN= [(β x (R_E + R_E-INT)) || R_B]
Where
• β is the transistor's beta, a voltage gain of about 200
• Resistances are in ohms
• R_E-INT = (26 / I_E) in mA (Shockley's Constant)
• “||” indicates parallel, or the product over the sum

Example
• Z_IN= [200 x ((8,200 + 273) || 10M)]= 1,449,044.8 Ω

If you require even higher impedance, you can use these values...

Bootstrap Follower & CE Voltage-Gain Stage Possible: Impressive high impedance. The input impedance of a bootstrap amp can be very high: As high as that of tube or FET preamps. The bootstrap follower stage produces no voltage gain. The impedance multiplier effect depends on positive feedback between the emitter and base circuit. Since the stage is regenerative, it can't have any gain, or you'd hear a squeal. Instead, the positive feedback multiplies the input impedance. Afterward, a voltage preamp provides ample voltage gain.

Experimental bootstrap amp for high impedance with common-emitter voltage amp stage: 2N3904, 2N2222A, etc.

I built this preamp on a plugboard. I tested the circuit with a piezo pickup. First I directly coupled the piezo to the input. Then I tried the piezo into the 0.0033 uF base capacitor. I couldn't hear a significant difference in performance (and didn't expect any). I didn't test the volume pot, but that part of the circuit is conventional. It should work just fine.

Loading Remedy. I calculated the input impedance of the bootstrap amp at about 3.2 megohms. This is a no-load value, though. The output voltage preamp slightly loads the bootstrap stage. This loading will reduce the input impedance. You can considerably reduce the loading by increasing resistor values in the second stage by a factor of 10. Unfortunately, doing that increases the output impedance by ten times. If you're working into a tube amplifier though, the increase probably doesn't matter.

Bootstrap Formulas

Optional reading. For those who want to roll your own, below are some formulas for bootstrap parts.

Z_IN Formula. Here's the input impedance formula for this bootstrap amp...

Z_IN= [R₃ / (1- α)]
Where
• α is the transistor's alpha, a voltage gain of about 0.98
• α (formula)= h_FE / (h_FE + 1)
• Resistances are in ohms

Example
• α = 200 / (200 + 1)= 0.995
• Z_IN= [((68K || 56K || 33K) / 1- 0.995)]
• Z_IN= 3.2 MΩ

•NOTICE. This formula assumes that the parallel R1, R2 and R3 are one-tenth of (β x R4) or less. If not: Parallel (β x R4) with the other parallel resistors!

Bootstrap C_BOOT Formula. The bootstrap capacitor is C2 or C_BOOT. The circuit's input voltage divider is in parallel with the bootstrap resistor. At the lowest desired frequency, C_BOOT must short this parallel impedance. (So that the circuit will work for both guitar and bass, 20 Hz seems like an appropriate bottom frequency.) Here's the bootstrap capacitor formula...

C_BOOT= [160,000 / (F x (R3 || R1 || R2) / 10)]
Where
• C= Capacitance in microfarads
• F= Frequency in Hz (lowest frequency)
• R= Resistance in ohms
• “||” indicates parallel, or the product over the sum

Example
• C_BOOT= [160,000 / 20 x ((68,000 || 56K || 33K) / 10)]
• C_BOOT= 4.7 µF (rounding to standard value)

Base Input Current I_BB. The necessary input base current limits the size of the bootstrap resistor and input bias resistors. A bootstrap resistor must permit sufficient current for stable transistor operation. The practical I_BB limit is about 10 times the minimum quiescent (q) current for the device. This minimum q-current is about I_E / β. For the circuit on this page...

•I_E is close to one milliamp.
•The β (beta or forward current gain) value is about 200.
•The minimum quiescent current is about 4.7 microamps.

Example
• R_E-internal= Shockley's Constant: 26 / (1,000 (4.5 / 4700)) = 27 Ω
• I_BB= 5.2 volts / (68,000 + 4,700 + 27)
• I_BB= 71 µA: PASSES!

Loading. Again, consider reflected impedances. Loading the output of the bootstrapped first stage will affect the input impedance. Ideally, the output load should be 10 times or more as large as the first stage emitter resistor. The output load is the input impedance of the second stage. That is, the parallel combination of the base resistors. This parallel combination is also in parallel with the second stage's input impedance. To find this impedance, multiply the Stage 2 emitter resistor times β (beta). For the bootstrap circuit on this page...

Example
• Load= R5 || R6 || (R8 x 200), assuming a β of 200
• Load= 11,765 Ω
• Reflecting load in parallel with R4: 3,358 Ω

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