electrical engineering: April 2007

Monday, April 30, 2007

100W OCL Amplifier circuit by A1215 / C2921

Saturday, April 28, 2007

Dynamic Microphone Preamp

Circuit : Andy Collinson
Email: anc@mitedu.freeserve.co.uk

Description:
A low noise pre-amplifier suitable for amplifying dynamic microphones with 200 to 600 ohm output impedance.

Notes:
This is a 3 stage discrete amplifier with gain control. Alternative transistors such as BC109C, BC548, BC549, BC549C may be used with little change in performance. The first stage built around Q1 operates in common base configuration. This is unusuable in audio stages, but in this case, it allows Q1 to operate at low noise levels and improves overall signal to noise ratio. Q2 and Q3 form a direct coupled amplifier, similar to my earlier mic preamp.

Source: http://www.mitedu.freeserve.co.uk/Circuits/Audio/dyn_mic/dyn_mic.htm

15 Watt Amplifier

Circuit : Sergio Garcํa de Alba Garcin from Guadalajara, Mexico
Email: turboman80@hotmail.com

Description:
A 15 watt amplifier made using discrete components. Sergio designed this circuit for his Electronics Level II course.

Source:http://www.mitedu.freeserve.co.uk/Circuits/Audio/amp15.htm

Hi-Fi Preamplifier (transister)

Design: Graham Maynard
Email graham.maynard1@virgin.net

Notes:
This circuit was submitted by Graham Maynard from Newtownabbey, Northern Ireland. It has an exceptionally fast high frequency response, as demonstrated by applying an 100kHz squarewave to the input. All graphs were produced using Tina Pro.

Source :http://www.mitedu.freeserve.co.uk/Circuits/Audio/gmpre.htm

Pre Tone Control Circuit (Transistor)

Notes:
Based on the classic Baxendall tone control circuit, this provides a maximum cut and boost of around 10dB at 10K and 50Hz. As the controls are passive, the last transistor provides a slight boost. The output is designed to feed an amplifier with input impedance of 10k to 250k. Both tone controls should be linear potentiometers.

Source : http://www.mitedu.freeserve.co.uk/Circuits/Audio/t-ctrl.htm

Thursday, April 19, 2007

Motorola Hi-Fi power amplifier

This is a very simple, low cost, Hi-Fi quality power amplifier. You can build it 5 ways, like it�s shown in the table (from 20 W to 80 W RMS).

Some comments:
- The first thing that you must do, is to measure the end transistors (T3 and T4) amplifying coefficient, the hfe or β. If their disagreement is bigger than 30 %, the amplifier would not give a clear sound. I used MJ3001 and MJ2501 transistors, and this disagreement was around 5%.
- Before the first �turning on� you must short circuit the inputs of the amp, and put a mA-meter on the output, than turn the amplifier on, and tune the R13 pot, to decrease the DC current on the output, to some uA-s, or in a lucky situation to zero. I was able to decrease it to 10 uA.

60W Linear amplifier

The 60 Watt linear amplifier is simple all solid state circuit using power mosfet IRF840. The IRF series of power transistors are available in various voltage and power ratings. A single IRF840 can handle maximum power output of 125 watts. Since these transistors are used in inverters and smps they are easily available for around Rs: 20/-.

The IRF linear amplifier can be connected to the out put of popular VWN-QRP to get an output of 60 Watts. The circuit draws 700 ma at 60 Volt Vcc. Good heat sink is a must for the power transistor.

Alignment of the circuit is very easy. Connect a dummy load to the out put of the circuit. You can use some small bulb like 24V 6Watts as the dummy load. I have even used 230V 60Watts bulb as dummy load with my IRF840 power amplifier working at 120Volts. Adjust the 10K preset to get around 100 ma Drain current. I used gate voltage of 0.8V with my linear amplifier. A heigh gate voltage can make the power transistor get distroyed by self oscillation. So gate voltage must be below 2V and fixing at 1V will be safe.

Bifalar transformaer T1 is wound with 8 turns 26SWG on 1.4 x 1 balun core.
The coil on the drain of IRF is 3 turns 20 SWG wound on 4 number of T13.9 torroids (two torroids are stacked to form a balun core). The RFC at the Vcc line is 20 Turns 20 SWG wound on T20 torroid.

2N3055 Power Amplifier

Simple and low cost. The optimal supply voltage is around 50V, but this amp work from 30 to 60V. The maximal input voltage is around 0.8 - 1V. As you can see, in this design the components have a big tolerance, so you can build it almost of the components, which you find at home. The and transistors can be any NPN type power transistor, but do not use Darlington types... The output power is around 60W.

Some comments:
- capacitor C1 regulates the low frequencies (bass), as the capacitance grows, the low frequncies are getting louder.
- capacitor C2 regulates the higher frequencies (treble), as the capacitance grows, the higher frequencies are getting quiter.
- this is a class B amplifier, this means, that a current must flow through the end transistors, even if there is no signal on the input. This current can be regulated with the 500Ω trimmer resistor. As this current incrases, the sound of the amplifier gets better, but the end transistors are more heating. But if this current decrases, the transistors are not heating so much, but the sound gets worse...

copy from : http://www.electronics-lab.com/projects/audio/042/index.html

thank you.