Stop Press: Since the paX protection system was published in the May 2008, Elektor issue, one or two persons have reported that sometimes the protection system fires prematurely. René van Heijningen has done some tests for me and found out that the cause was leakage on the detector side of the opto-coupler U1, an MCT6. There appears to be a large spread in this parameter between devices.
The following change has been tested (thank you René!) and takes care of the issue:
Change resistor R1 from 10k to 18k;
Add a resistor R1a of 3.3k from ground to pin 5 or 8 (this is point ProtN in fig 2) of the optocoupler. You can put this resistor on the bottom of the circuit board.
My Error Correction Power Amplifier:
has been published in April and May 2008 by Elektor Magazine.
Part 1, the output stage, is published in the April 08 issue.
Part
2, the complete amplifier, and
Part 3, the comprehensive protection
system, is published in the May 08 issue. As far as I know it has appeared in the
English, Dutch, German, Spanish, Portuguese and French language issues.
There are separate
Construction Guides in Dutch and
English.
The amplifier and its supply are sized for about 100W in 8 ohms / 200 watt in 4 ohms. You can also run the amp at lower power and a single output pair.
The protection system design is based on an XL spreadsheet available for download. I have made some changes since the article publication to slightly increase the available power in complex loads.
Pilgham Audio has prepared complete kits for the paX amplifier and the protection system for those who would like to build the design.
Errata:
There are two errors in Bill Of materials as published in the Elektor article (the schematic drawings are correct, though):
R33 = 100 ohms;
R49 = 22k ohms.
This does not effect performance, but increases the input impedance of the amplifier and brings the gain closer to the design value of 20.
Slight improvement: The transistors Q9 and Q10 in the final stage of the voltage amplifier, become quite warm. I have not heard of any failures, but the higher temperature is a bit unsettling. By changing resistors R6 and R17 from 47 ohms to 39 ohms, the temperature of the two transistors drops approximately 12 degrees Celsius without any performance change.
Running the amp at lower power
