JLH-FAB CLASS A POWER AMP

Integrated in DH-200 Hafler Power AMp chassis 

Objective:

The goal of this project was to use an Hafler DH-200 Power Amp parts and convert it to another amp of very different topology using N channel V-MOSFET power transistors (IRF340) that I have since about 20 years in my inventory. These V-MOS transistors were retrieved from pcbs used to drive DC motor in a manufacture. These IRF340 are very robust. I have successfully used then in a quasi-complementary mosfet output class AB design driven by 2 differiential stages in series (input and VAS stages).

Topology:

The topology that I have been always curious about is the 10W class A "Single Ended-Push Pull" JLH Power amp of 1969 and upgrades. Refer to this site Class A amplifier Site for more details. The main original idea of the JLH 1969 design is the "phase plitter" driving the output stage. The JLH design used bipolar NPN power transistors in output stage. So, the first change was to use N channel V-MOSFET instead. The second change was the use a dual polarity power supply to get rid of the output cap. To get rid of the input cap, I change the input sate to a j-fet differential stage with cascode bjt. The input stage has similarities with Roender rmi-fc100 power amp (rmi-fc100).To reduce the load on the input stage, I used a CFP configuration for the "phase splitter". To increase the input stage open loop gain, I also used a current mirror in the input stage. Active current source was also used in the input stage. However, I kept the passive current source for the "phase spillter" stage and also the bootstrap cap to keep the high efficiency (for a class A amp having single ended mode behavior). I have also included the sensing current protection (like Nelson Pass Aleph and First Watt amp series does as example). The thermistor allows the bias current of 1.4 amp for 20 W class A amp max (thanks to push-pull operation mode portion and bootstrap cap) to be stable after about 5 minutes. Otherwise, without it, the amp would take about 1 hour to have a stable bias and it would start at about 60% of the bias and slowly reach the max bias of 1.4 A. The 1.4A was selected based on the DH-200 power amp heatsinks where the temperature is about 50 to 55 deg C. The power supply used a CRC arrangement followed by LM317/LM337 regulator circuit with power transistor boosting current to provide +/- 20 V DC regulated at about 1.4A.

Schematics:

 Photos of completed amp:

 

Photos of amp during construction:

 Scope traces:

Sinus 10KHz into 8 ohms at 16.5 V peak

Triangle 10 KHz into 8 ohms

Square 10 KHz (time scale x 10) into 8 ohms. Rise time of about 1.8 us.

Square 10 KHz into 8 ohms parallel with 2.2 uF at 2 Vac