Vcc – Pin 1 IC1
+12v – Pin 8 IC2, IC3, IC4
-12v – Pin 4 IC2, IC3, IC4
Ground – Pin 8 IC1

IC1 – 4049 CMOS Hex Inverting Buffer
IC2, 3, 4 – 5532 Dual Low-Noise OpAmp
D1, 2 – 1N4001
D3, 4, 5, 6, 7 – 1N4148
All 1uf caps 50v all others 25v unless
marked otherwise.
All resistors 1/4W, 5%

Channel 2 is identicle to Channel 1, and uses IC4b for the clipping meter and IC3 for the input/output driver. The input impedance of the TubeHead is about 20k ohms, which is consistant with most gear like Synths, Effects Processors, Mixers, EQs, and so on. 20k is too low for a proper match with high impedance sources like guitar pickups, but a few minor changes take care of this. To use the Tube Head as a instrument pickup preamp, remove the 47k* resistor and the 20pf capacitor from the feedback loop of the driving Op-Amp. Then change the 47k” resistor to 680k and the 22k resistor in the feedback loop of the driving OpAmp to 100k. Now the Tube Head can be used to warm up a cold sounding guitar amp or just provide a great preamp tone.

Designed/Manufactured by PAiA Electronics

Download Tube Head circuit + components list and explanation in pdf file:

Tube Head Pre-amp Circuit

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All capacitors with value of 33uF are 16V, while the all others are 50V unless marked otherwise. Resistors marked with “#” are 1% metalfilm resistor type.

The “Drive” LED indicates how hard the tube is being driven. The “Blend” control allows for a mixing of SS and tube coloration. Symmetry controls the relative amounts of even and odd harmonics, CCW the Tube Mic Pre-Amp may sound punchier, while CW it may sound warmer. The 12VAC needed for pin 5 of the 12AX7 can be obtained from point G while pin 4 should be connected to point A.

There are two options for line level signals. First if you know that you”ll be using line level signals all the time with the TMP then you can change the two 33k 1% resistors to 1k 1% types and your done. Alternatively if you want the option of line level or low level signals then you can sacrifice the polarity switch and and rewire it here as shown. Notice that the 47k resistors are again of the 1% variety.

Download the schematics and circuit explanation of Tube Mic Pre-Amp circuit:

Tube Mic Pre Amplifier

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Power supply circuit for bass guitar preamp also included in the PCB design. Simply connect the power supply section to the output of transformer with the secondary 15 + 15 V / 500 mA.

Bass Guitar Preamp Pedal PCB Design

Bass Guitar Preamp Top PCB

Bass Guitar Preamp Top Component Placement

Bass Guitar Preamp Pedal Connection

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This small portable mic preamp circuit is based on a low noise, high gain two stage PNP and NPN transistor amplifier, using DC negative feedback through R6 to stabilize the working conditions quite precisely. Output level is attenuated by P1 but, at the same time, the stage gain is lowered due to the increased value of R5. This unusual connection of P1, helps in obtaining a high headroom input, allowing to cope with a wide range of input sources (0.2 to 200mV RMS for 1V RMS output).

Portable Mic Preamp Parts List:

Resistors:
R1,R2,R3 = 100K
R4 = 8K2
R5 = 68R
R6 = 6K8
R7,R8 = 1K
R9 = 150R
P1 = 2K2 Linear Potentiometer

Capacitors:
C1 = 1uF/6.3V polyester
C2,C3,C4 = 100uF/25V elco
C5 = 22uF/25V elco

Transistors:
Q1 = BC560C
Q2 = BC550C

Misc:
J1 = Jack socket (Mono 3 or 6 mm.)

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schematic diagram:

The leftmost 10k resistor supplies plug-in-power to the electret, forming part of the FET amplifier in the electret capsule. This could be anything from 2k to 10k, the higher the better the stereo separation (another mic derives bias from the same rail). Apparently higher values also lower distortion, and the best bias power circuits involve actually breaking a trace on the electret capsule to allow the use of both a drain & source resistor, but I”m not going that far.

The leftmost 2.2uF cap blocks the bias voltage from the input. In conjunction with the following 27k resistor it forms a high pass filter, but cutoff is essentially near DC.

The input impedance is set by the two 27k resistors and the 10k resistor. The +ve rail is also connected to ground as far as the AC signal is concerned because of the power supply cap. So there are two 27k resistors in parallel, making 13.5k, in parallel with the 10k, making about 6k or so for the input impedance. But if you’re making it proper dual supply, you don’t need the upper 27k resistor, as the input doesn’t have to be biased mid rail anymore.

The feedback loop has two resistors 27k & 1k5 from the inverting input to ground. When they are both in circuit, the gain is a bit under 2 ((28.5/33)+1). The 27k resistor can be bypassed with a switch, then only the 1k5 sets the gain, to 23 ((33/1.5)+1).

The 10uF cap in the bottom half of the feedback loop reduces DC gain to ~1. The value isn’t very important. If any DC input offset were amplified it would create a larger output offset, pushing the output toward one of the rails and reducing headroom. (At a gain of 23 with the expected input levels it probably doesn’t matter.)

The optional 2pF cap in relation to the 33k resistor sets the high frequency rolloff. The cutoff frequency is in the 100″s of kHz. It has to go further than 20kHz to keep the phase shift at audio frequencies small, and also because output starts falling long before cutoff. The op-amps cannot maintain enough gain at these frequencies anyway and their output will already be falling, but the cap makes the circuit more stable, though it will probably work without it. There will probably be 2pF of capacitance just from the PCB traces, and op-amps tend to be fairly well compensated these days so it”s really not needed. I think in retrospect this cutoff frequency should be much lower, say 30kHz-50kHz.

The 100ohm resistors are there partly to limit current to protect the op-amp if the output is shorted, but the op-amps have internal protection anyway. They mainly allow the op-amp to drive capacitive loads (long/cheap cables) without oscillation.

The 2.2uF cap on the output blocks DC and the value is not specially important. It forms a highpass filter with the 10k pot, the cutoff is virtually at DC.

If you think you might accidentally start connecting the battery the wrong way round, you’d better put a diode in series with the battery clip, or you’ll smoke your ic. Put your IC in a socket too just in case you do want/need to change it. You could try several dual op-amps against each other, they’re all direct plug in replacements.

PCB layout:

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Dynamic microphones are versatile and ideal for general-purpose use. They use a simple design with few moving parts. They are relatively sturdy and resilient to rough handling. They are also better suited to handling high volume levels, such as from certain musical instruments or amplifiers. They have no internal amplifier and do not require batteries or external power.

Schematic diagram:

Circuit Notes:

• This dynamic microphone amplifier circuit has a total gain of 200 times.
• If you use 200? microphones R4 must be 220? and C1 is 4.7uF.
• For the best performance, use metal film resistor, MKM type for unpolar capacitor and tantalum type for bipolar capacitor. Use stabled and regulated power supply.

With a 3mVpp input signal the output will be 800mVpp. The maximum output voltage was 10Vpp when the input is 50mVpp. The frequencies domain is between 50Hz and 100KHz.

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This is the circuit diagram of mic condenser amplifier. The low-cost and compact mic condenser audio amplifier described right here is deliver good-quality audio of 0.5 watts at 4.5 volts. It could possibly be applied as a part of low-power transmitters, packet radio receivers, intercoms and walkie-talkies.

Transistors T1 and T2 form the mic preamplifier. Resistor R1 gives the required bias for the mic condenser whilst preset VR1 works as gain control for adjusting its gain level. As a way to boost the audio power, the low-level audio output from the preamplifier stage is coupled via coupling capacitor C7 to the audio power amplifier constructed close to BEL1895 IC.

BEL1895 is really a monolithic audio power amplifier IC intended specifically for sensitive AM radio applications that delivers 1 watt into 4 ohms at 6V power supply voltage. It exhibits very low distortion and noise and works above 3V-9V supply voltage, which would make it perfect to be supplied with battery operation. A turn-on pop reduction circuit prevents thud once the power supply is switched on.

Coupling capacitor C7 determines low-frequency response of the amplifier. Capacitor C9 acts as the ripple-rejection filter. Capacitor C13 couples the output accessible at pin 1 towards the loudspeaker. R15-C13 combination acts as the damping circuit for output oscillations. Capacitor C12 presents the boot strapping function.

This circuit is appropriate for low power HAM radio transmitters to supply the important audio power for modulation. With easy modifications it could possibly also be applied to intercom circuits.

Mic condenser amplifier circuit source: “Electronics For You” magazine, December 2001

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Designed by Don Tillman, this guitar pre-amp circuit design is dedicated for people who don”t like op-amps module. This circuit is a discrete JFET pre-amplifier design, use 2N5457 as the main component. It has low noise, low distortion, low feedback, overloads gracefully, is small, etc.

Overall gain is 3db (2X) or so. It uses about 1/2 ma, so a 9V battery will last a long time, you may also use simple regulated power supply with LM7809. It is possible to add a high boost switch in case you like by having it shunt the 2.2k ohm resistor with a 0.05uF cap (or other value; smaller cap = boosts only higher frequencies, as well as the reverse). You may simply placed in a 10uF capacitor across the 2.2k ohm resistor in order to up the gain.? You don”t like pre-amp circuit with op-amps..? then, just try to build this circuit.

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Low Impedance Microphone Amplifier Component list:

PCB layout (bottom):

PCB layout (top-component placement):

Good luck with this low impedance microphone amplifier circuit project

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Stereo Tone Control Component List

INTEGRATED CIRCUIT

• TL072 : 2
• TL071 : 1

RESISTORS All 1/4W

• 22K ohm (red, red, orange) : 10
• 10K ohm (Brown, Black, Orange) : 10
• 1K ohm (brown, black, red) : 3
• 100K ohm (brown, black, orange) : 3
• 100 ohm (Coffee, Black, Coffee) : 4
• stereo/dual potentiometers 100K ohm : 3
• stereo/dual potentiometers 10K ohm : 2
• mono/single potentiometers 10K ohm : 2

CAPACITORS

• 1uF/25V electrolytic capacitor : 4
• 10uF/25V electrolytic capacitor : 1
• 4.7uF/25V electrolytic capacitor : 2
• 6.8nF (682) polyester capacitor : 2
• 1.5nF (152) polyester capacitor : 2
• 47nF (473) polyester capacitor : 2
• 1nF (102) polyester capacitor : 2
• 100nF (104) polyester capacitor : 4
• 2200uF x 25 V electrolytic capacitor : 2
• 1uF nonpolar capacitor (105) : 5

Stereo Tone Control PCB Layout and Component Placement

Power Supply Circuit

The power supply type for the stereo tone control is symmetrical and regulated and on its output we use +15 and -15 VDC. The LM7815 will deliver stabilized +15VDC and the LM7915 will deliver stabilized -15 VDC. For symmetrical source, the TL072 VCC range will be 7V up to 36V, so you may use a pair of regulator IC LM7805/LM7905 up to LM7815/7915.
Below is the schematic diagram, pcb design and component layout:

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