PA2 Solid State Power Amplifier
This amplifier was inspired by conversation with John Curl in a ‘Blowtorch thread’, as well as by his thoughts ( http://www.parasound.com/pdfs/JCinterview.pdf ).
The aim was to build a reliable, solid, good sounding amplifier.
Topology is complementary differential, with JFET input, MOSFET cascode and VAS, and BJT multiple pair output stage. The circuit shown might be an inspiration for DIYers in their designs.
A functional sample was build, the circuit is very stable and rugged.
The project is not a commercial project, no PCBs or kits will be sold.
Schematics
R1, R5 and R7 values are not shown. J1 - J4, 2SK170BL/2SJ74BL are matched to within 0.5mA Idss, and Idss is about 9.5 - 10mA. You make a choice of R1 by measurement then, to get Id = 8mA approx. The R5 and R7 are chosen again by measurement, to get appropriate current through VAS Mosfets (check voltage drop across R6, voltage corresponds to Id of input stage + VAS current). You may get something like R1=7R and R5, R7 = 750R for 2SK413/2SJ118 mosfets as M1 and M2. Anyway, measurement is necessary. The DIYer should be at least average experienced when making a decision to build this amp. Supply rails - start with +/-30V and may increase to +/-50V if the amp is operating safely.
Last, but not least - voltage at gates of M3,M4 and M5,M6 is set to 13V approx by dividers R75/R76 and R77/R78. Do not forget this, not to thermally burn input JFETs. Before increasing supply rails, consider this.
First measurements
A functional prototype of the amplifier was build. Oscilloscope measurements performed at 10kHz, 4 ohm load. No output coil.
Slew rate was measured with the digital USBscope.
Square 20kHz, load 4 ohm, 37.8Vp-p :
Loop gain and open-loop gain
The loopgain is shown here It is a loop-gain, not open-loop gain, so have a look at amplitude plot crossing with 0dB and then find phase margin at the same frequency.
A loop gain is not the same plot as openloop gain. Loop gain tells about gain of the feedback loop only and is used to explore stability of the amplifier. Openloop gain of the PA2 is here. It is the simulation only, so it does not fit 100% to the real amplifier.
The amplifier prototype built
Based on PA2 amplifier, more powerful version has been built. It has 4 pairs of output devices, and output power is 150W/8 ohm and 265W/4ohm pc.
Pavel Macura, 10/2009. To be continued ...
