Grey Box transistor preamp with buffers

Frequency response of the CE amplifier

C1 C2 have a high pass effect

C3 has a high pass shelving effect on the gain

Raising C1 (input cap) lowers cutoff frequency for the initial high pass filter.  Keeping Cin = Cout keeps the high pass roll off frequency (HPRF) low, while just changing one lowers the HPRF only just before the first gain stage, and at the output, the HPRF is much higher (proportional to 1/2piRoutCout)

For the best frequency response:

C1 = 10uF

R4 = 2.2Meg

C2 = 1uF

R2 = 10k

C3 = 10uF

R6 = 2.2k

3 dB rollof = 20 Hz

6.6 dB gain, all the way to 20kHz and beyond

Design 2 - Best One!

Design parameters:

2.76V/V gain, 8.87 dB.

20 Hz High pass 3 dB roll off frequency

DC bias at 2.5V

Max input voltage: 900 mV (1.8 Vpp)

Min input voltage: 20 mV

Max output voltage: 2.5 V (5 Vpp voltage swing)

Design 2 actual parameters:

3 dB rolloff frequency at 19 Hz

8.7 dB gain flat frequency response throughout, up to 20k

DC Bias: 2.5V

Time response outputs

Typical input: 300 mV

Frequency = 1kHz

With 20 mV input:

In line with text | Fixed position

Output at max input 900 mV:

Design 3 - Simulation Winner!

Added a low pass filter with 3 dB roll off frequency at 20 kHz


Frequency Response

Transient Response

Output when input is 1 KHz, 900 mV (max input)

Quinn:  Use FET input opamp low noise TL072 dual audio op amp, don’t use bipolar


Emitter follower is the second stage is can have current gain, depending on what it is driving.  not voltage gain

high in impedance, low out impedance, buffer

use to drive a speaker

hfe = common collector current gain. related to voltage gain

CEAmp has high output and low input, so surround with buffer stages.  

To check the noise, put a voltage follower on the output,  2.2 uF, (as big as i can find thats nonpolar).  OR put two resistors on a FET input opamp and make a preamp.  

Put low input pot at the input of Q3 as volume control (1k)

Preamp 5:

For op amp stage:

1. Use 2A333K cap (large green one) to filter out DC before sending to speaker

2. DC bais + terminal of op amp wth 5.1K resistor coming from 5V voltage regulator, gets rid of the clipping on the negative side of the signal on the output of the opamp.

3. You need both caps for the voltage regulator to work correctly, 10uF on the output to ground and a 104 from input to ground.

4. Use large 10 uF (100V) cap between the mic signal and the input to the preamp, increases the volume, without it, the mic is too quiet.

Only mic or guitar can be running at one time.  Implemented a switch

For Preamp stage:

1. Volume pot inserted with wiper on Q3.

2. Increased gain by lowering R7

3. increased allowable output current without clipping by lowering R10 (decreased? current gain on emitter follower, Q3).

4. slightly increased 3dB rolloff frequency on LP at output by lowering R14

5. added caps between power and ground near sensitive parts of circuit to decrease random spikes and noise from power supply.

Preamp 5

Q and A:

1. Why is a JFET the input stage?

        High input impedance as opposed to a mosfet and BJT.  Acts as an input buffer and the high impedance input stage.  Mosfet is cheaper, but has lower impedance and higher capacitance ratings at the gate.

2. What are the emitter followers for?  

The emitter followers are adding the current gain in order to drive the small output impedance without impacting the voltage gain.  They act as buffers after the common emitter stages.

3.  Why didn’t you use an emitter follower configuration as the input stage?  It also has a gain=1.

The common emitter has a larger input impedance than the emitter follower configuration, so a common emitter configuration is used as the input stage  

4. What is the feedback loop at the input stage for?


The 9.1k R2 decreases the noise of the common collector configuration because it is large and creates a feedback loop.  It also increases the linearity and stability of the circuit.  

Used the 2.5V regulator, now lots of things have changed.  Go over all resistor values again and make sure that all are accurate

Preamp 6:  Added diode protection and diode created DC bias of 2.5V

Using VISHRAY Resistors

Using for cap references

Capacitor dielectrics and their effect

Low leakage, low dielectric absorption and a shallow, flat temperature curve makes these capacitors suitable for timing circuits, filters, integrators, and sample-and-hold circuits.  Moisture absorption is very low.

Some advised materials,

polypropylene polystyrene ceramics

I wonder paper in oil...

sample preamp schematic

Slides on preamps

Wein Bridge oscillator

Background:  A winebridge oscillator produces a very low distortion sine wave.  This can be used to make a synth, except you cannot really control it as well as digital.  However, here is how they made a synth originally, analog.  THey made the circuit oscillate, which is a case of “instability”

Maxim:  Analog clickless switches and mux

Summing Amplifier:

Rail Splitter TI 2426

Design tradeoffs for single supply designs

Will this op amp work if I bias it at the positive input terminal with a rail splitter?  I believe I am use two of these op amps to give my differential mic input a gain?

op amp:

rail splitter:

TI free samples order number: 2038976





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Application Documents


chip document


test results

Order confirmation:  2046409

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Most recent Preamp plan:  Use BOM to get a +/- 12V supply on the board from the +12 wall wart.  Then, use +5V regulator and -5V regulator to step down voltages and use for opamps and low power modules.  This will keep the cost low (free samples, and small size)

+12V to +/-5V DC power supply



- Volume pots are tapered (logarithmic) in order to adjust volume logarithmically, rather than linearly, so the perception of loudness and the percent rotation are proportional