Wiring circuit

Wednesday, November 5, 2014

Portable Microphone Preamplifier

This circuit is mainly intended to provide common home stereo amplifiers with a microphone input. The battery supply is a good compromise: in this manner the input circuit is free from mains low frequency hum pick-up and connection to the amplifier is more simple, due to the absence of mains cable and power supply. Using a stereo microphone the circuit must be doubled. In this case, two separate level controls are better than a dual-ganged stereo potentiometer. Low current drawing (about 2mA) ensures a long battery life.

Circuit Operation:

The 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 Microphone Preamplifier Circuit diagram:

Parts:
P1 = 2.2K
R1 = 100K
R2 = 100K
R3 = 100K
R4 = 8.2K
R5 = 68R
R6 = 6.8K
R7 = 1K
R8 = 1K
R9 = 150R
C1 = 1uF-63V
C2 = 100uF-25V
C3 = 100uF-25V
C4 = 100uF-25V
C5 = 22uF-25V
Q1 = BC560
Q2 = BC550

Notes:

Harmonic distortion is about 0.1% @ 1V RMS output (all frequencies).
Maximum input voltage (level control cursor set at maximum) = 25mV RMS
Maximum input voltage (level control cursor set at center position) = 200mV RMS
Enclosing the circuit in a metal case is highly recommended.
Simply connect the output of this device to the Aux input of your amplifier through screened cable and suitable connectors.

Monday, October 20, 2014

Modular Phono Preamplifier

High Quality Moving Magnet Pick-up module, Two-stage Series/Shunt feedback RIAA equalization

Any electronics amateur still in possess of a collection of vinyl recordings and aiming at a high quality reproduction should build this preamp and add it to the Modular Preamplifier chain. This circuit features a very high input overload capability, very low distortion and accurate reproduction of the RIAA equalization curve, thanks to a two-stage op-amp circuitry in which the RIAA equalization network was split in two halves: an input stage (IC1A) wired in a series feedback configuration, implementing the bass-boost part of the RIAA equalization curve and a second stage, implementing the treble-cut part of the curve by means of a second op-amp (IC2A) wired in the shunt feedback configuration.
.

This module comprises also an independent dual rail power supply identical to that described in the Modular Preamplifier Control Center. As with the other modules of this series, each electronic board can be fitted into a standard enclosure: Hammond extruded aluminum cases are well suited to host the boards of this preamp. In particular, the cases sized 16 x 10.3 x 5.3 cm or 22 x 10.3 x 5.3 cm have a very good look when stacked. See below an example of the possible arrangement of the rear panel of this module.

Circuit diagram :

Modular Phono Preamplifier Circuit Diagram

Parts:
R1_____________270R 1/4W Resistor
R2_____________100K 1/4W Resistor
R3_____________2K2 1/4W Resistor
R4_____________39K 1/4W Resistor
R5_____________3K9 1/4W Resistor
R6_____________390K 1/4W Resistor
R7_____________33K 1/4W Resistor
R8_____________75K 1/4W Resistor (or two 150K resistors wired in parallel)
R9_____________560R 1/4W Resistor
C1_____________220pF 63V Polystyrene or Ceramic Capacitor
C2_____________1µF 63V Polyester Capacitor
C3_____________47µF 25V Electrolytic Capacitor
C4_____________10nF 63V Polyester Capacitor 5% tolerance or better
C5_____________1nF 63V Polyester Capacitor 5% tolerance or better
C6,C9__________100nF 63V Polyester Capacitors
C7,C10_________22µF 25V Electrolytic Capacitors
C8,C11_________2200µF 25V Electrolytic Capacitors
IC1____________LM833 or NE5532 Low noise Dual Op-amp
IC2____________TL072 Dual BIFET Op-Amp
IC3____________78L15 15V 100mA Positive Regulator IC
IC4____________79L15 15V 100mA Negative Regulator IC
D1,D2_________1N4002 200V 1A Diodes
J1,J2__________RCA audio input sockets
J3_____________Mini DC Power Socket
Notes:

The circuit diagram shows the Left channel only and the power supply
Some parts are in common to both channels and must not be doubled. These parts are: IC3, IC4, C6, C7, C8, C9, C10, C11, D1, D2 and J3.
IC1 and IC2 are dual Op-Amps, therefore the second half of these devices will be used for the Right channel
This module requires an external 15 - 18V ac (50mA minimum) Power Supply Adaptor.

Technical data:
Sensitivity @ 1KHz: 4.3mV RMS input for 200mV RMS output
Max. input voltage @ 100Hz: 53mV RMS
Max. input voltage @ 1KHz: 212mV RMS
Max. input voltage @ 10KHz: 477mV RMS
Frequency response @ 200mV RMS output: flat from 30Hz to 23KHz; -0.5dB @ 20Hz
Total harmonic distortion @ 1KHz and up to 8.8V RMS output: 0.0028%

Total harmonic distortion @10KHz and up to 4.4V RMS output: 0.008%

Sunday, September 21, 2014

Electric Guitar Violin Preamplifier

Magnetic pick-ups in musical instruments have a relatively high output impedance. This can result in a reduction in treble response when connected via a long cable run or to equipment with a low input impedance. This preamplifier provides a high input impedance and a low impedance output, solving both issues. It has adjustable voltage gain and can run off a battery or DC plugpack. The input signal is AC-coupled to the non-inverting input of IC1a, part of a TL074 quad op amp. This has JFET input transistors and the input impedance is set by a 330kΩ bias resistor which also sets the DC level at this input to half supply (Vcc/2).

This is generated by a voltage divider comprising two 10kΩ resistors and bypassed by a 47µF capacitor to reject noise and hum. IC1a is configured as a non-inverting amplifier with a gain of between 2 and 20, depending on the setting of VR1. IC1a’s output is fed to VR2 via a 22µF capacitor, allowing the output volume to be set. The audio then passes to the non-inverting inputs of the remaining three op amps (IC1b-IC1d) which are connected in parallel to provide a low output impedance; it will drive a load impedance as low as 600Ω. The 100Ω resistors in series with the outputs provide short-circuit protection for the op amps and also prevent large currents from flowing between the outputs in case they have slightly different offset voltages.

Electric

The buffered signal is then AC-coupled to two output connectors using 47µF electrolytic capacitors. For Output 1, a 47kΩ resistor sets the output DC level to ground and a 220Ω series resistor provides further short-circuit protection. Output 2 is similar but includes another potentiometer (VR3) to allow its level to be set individually. Note that this means the impedance of Output 2 can be high (up to 2.5kΩ depending on the position of VR3’s wiper). The total harmonic distortion of this circuit is typically less than 0.01% with the gain set to six. If a TL064 is used instead of a TL074, the current drain will decrease but there will be more noise at the output. Finally, the input impedance can be increased by increasing the value of the 330kΩ resistor to suit high-impedance pick-ups.

Thursday, August 28, 2014

12AU7 ECC82 tube Preamplifier circuit

The following chart is a simple two-line preamp with 12AU7 / ECC82 tube. The overall gain of the schema is about 8 , The second phase of the schema is a cathode followers and a low impedance amplifier. This schema is from the RCA tube manual.

The schema is simple and requires no special components. For the power supply, you can create a Hammond 370BX transformer with simple CLC or CRC filtering to B +. You want to use a good quality capacitor C4 for the best results.