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!
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
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)
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.
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 http://www.humblehomemadehifi.com/Cap.html 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... http://www.partsconnexion.com/capacitor_film_duelund_pio_electonic.html
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
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?
TI free samples order number: 2038976
TLE2426- The 'Rail Splitter' Precision Virtual Ground
LMV342- Dual Rail-To-Rail Output CMOS Operational Amplifier with Shutdown
LM4040B25- 2.5-V Precision Micropower Shunt Voltage Reference, 0.2% accuracy
OPA4140- 11MHz, Single Supply, Low Noise, Precision, Rail-to-Rail Output, JFET Amplifier
OPA4353- High-Speed, Single-Supply, Rail-to-Rail Operational Amplifiers MicroAmplifier(TM) Series
USB powered Bipolar Supply
12V output from 5V input
Max1896 step up DC DC converter Datasheet
Order confirmation: 2046409
TPS5401- 3.5V to 42V Input, 0.5 A Step Down Converter
UA7905- 3 Pin 1.5A Fixed (-5V) Negative Voltage Regulator
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