For up-to-date information—see DX Blame.
Step |
|
1 |
Remove the rail fuses. |
2 |
Install DC voltimeter in the fuse socket with fuse previously removed. |
3 |
Switch on the amplifier power supply. |
4 |
Adjust bias trimpot to read from 3.3V to 6V over the 100 ohms series resistor. |
5 |
Check both rails. |
6 |
Move DC voltimeter to the output line. |
7 |
Off set reading should be from 3mV to 25mV, usually it is 12mV. |
8 |
Reduce it if you want, place 820 ohm, 1K, 1K2 or 1K5 resistance in parallel with R8. |
9 |
Switch your multimeter selector to ready AC volts and read the output line, no AC voltage should be there. |
10 |
Check once again your stand by current, if fine, them install fuses and enjoy your amplifier. |
If you experience overflow, or variations in the offset readings, them short the input to ground, after that, do not forget to remove the short you have made.
My home amplifiers, the prototypes, one used 1K resistance to reduce off set to 500mV, the other has used 1K2, so, as you see, this is variable, depends on the differential transistor matching and some amplifier components.
The stand by bias trimpot is also something that changes depending the circuit, a good starting adjustment is 250ohms, as one of my amplifiers asked for 160ohms and the other was 330 ohms.
Adjusting your bias at 38mA, your output transistor will be draining 27mA each one of them, the negative rail current will be 42mA, the 100 ohms protective resistance reading will be 3.8V up rail and 4.2V the lower rail.
The amplifier itself, has very low power consumption, the output is adjusted to dissipate, during stand by. 1 watt to each power transistor.
Driver transistors needs heatsinks, and it is a very good idea to install as small piece of aluminum, squared, 2 centimeters side, at the second voltage amplifier, the BD139.
Copyright Notice: The amplifier circuit designs presented on this website are the Copyright © 2006-2010 of Carlos Mergulhão. All rights are reserved. License is granted for anyone to use the circuits, drawings and information here only to build their own personal (non-commercial) amplifiers, but under no circumstances are these circuits, drawings and information to be used to produce a commercial product to be sold anywhere in the world, without first arranging a license agreement with the copyright owner. Use of the name "DX Amplifier" or "DX Amp" to represent a commercial product is fraudulent. Licensed vendors will all be listed here.
Warning: Be aware of unlicensed amplifier kits being sold under the misrepresentation that they are DX Amplifiers. These kits are currently not official, are not licensed and are not supported by the designer of the DX Amplifier series of DIY amplifier projects.