Home made PCB
THD and IMD plots - IRFP240
THD and IMD plots - IRFP250N
THD with IRFP240 OP transistors
THD with IRFP250N OP transistors
VP0109 - VAS cascode MOD
I was always wondering what is so special in quasi complementary designs. I have read some comment posted by one of the diyaudio.com (thanks Hugh) forum member and I started to work on a new project. The plan was to create some simple, kind of retro looking design. I decided to go for ECC88 double triode as an input differential amplifier and hybrid mosfet cascode for VAS (Voltage Amplification Stage). Input capacitance's of the cascoded mosfets are not bootstrapped with output voltage so it is working quite well. Bad example here is my educational project GrandMOS where mosfet VAS is operating very nonlinear with a bare drains connected to the next stage of the amplifier. Some of people like highly distorting amplifiers - I think it is matter of taste. For anode load I have selected current mirror, so the anode voltage have higher value with lower PSU voltage. Input stage requires stabilized power supply, the easiest way was to make Voltage doubler PSU . For output stage I have selected IRFP240 (IRFP250N for second version). The only modern part of the design is OTMC compensation which keeps OLG at reasonable high value (60dB at 10kHz according to the simulation) and helps to reduce distortions. The amplifier is sensitive for parts selection and small changes, with a bit of effort it can be tweaked to own preferences. Good example is substituting IRF9610 with VP0109 vertical mosfet (resulted with high HD2 and HD4 products), with a bit small headroom (approx 5V = two blue LEDs in series) in VAS cascode and small bias current (10mA) it can work more or less nonlinear, depend on selected part. The part selection is quite crucial, with some of them parasitic oscillation may occur at the output. I must say that popular IRF9610 is doing really decent work here.