Amp design - sim problems

[Dzsoni]

Hi,

I just simulating my new amplifier design, that's going to be a 50Wrms/8Ohm studio amplifier. See attachement for the schematic.
As you can see it's looks like a modified Phase-Linear-300.

I'm using Multisim and Simetrix as simulation softwares.
The problems are the following while simulating:
- Multisim says THD at 1kHz is about 0.001%, but at 20kHz that's more than 0.1%. At 10kHz about 0.05%. How can I decrease THD in the circuit?
- when trying to simulating square-wave response at Multisim the sim stops, "timestep too small". What's wrong?
- at Simetrix the simulation won't start. Nothing happens when I press "run". I simulated many other audio amplifier circuits and I never had such problem.

So I looking for advices.
Many thanks,

Dzsoni

PS.: In the real life OP devices would be 2SC5200/2SA1943 but Simetrix hasn't got model for them so I used MJ15022/23 for simulation.

[djk]

I have to start Multisim at a lower frequency , and then move to a higher frequency, otherwise it gives all sorts of error messages.

[unclejed613]

one thing you need to do with most sims with square waves is to give a rise and fall time for the waveform, otherwise the sim can't generate "instantaneous" changes in voltage. your changes in THD with frequency are due to normal bandwidth limitations of op amp style circuits. you have less open loop gain at 20khz than you do at 1khz, so your distortion increases above the open loop corner frequency. other things that change are output impedance (damping factor) and IMD.