100watt MOSFET Power Amplifier

[MagicBox]

I agree. High frequency components do affect the sound and sound stage. For the fast transients in a music signal and all its combined components as you point out may require a large change in conductance from the Mosfet in a short period of time which requires charge.

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Also it is prudent to dampen the high frequency oscillator that is formed by the internal capacitance and lead/bonding wire inductance. Placing a zero between the gate and drain pin along with local decoupling will accomplish this. A very simple easy way to prevent local oscillation around the device. (I use COG SMD caps for this filter.)

This appears to be a very effective technique to get the parasitic oscillations in the output FETS to "shut up". In my current design under development I applied this and experimentally determined the local feedback caps for the FETs on the dev PCB I had made. The gate stoppers are really low. Without the caps the FETs would oscillate up to 80MHz (!!) at the slightest quiesent current. For these particular fets, 47pF appeared to be just right. Without low-pass input and a square applied I was unable to entice the output stage into oscillation.

The amp was working very well already as it was fast by design (~100V/uS slew), no phase shift at 20KHz with a low-pass of 800KHz. But it was neat to get it even faster.

It's a 80Watts into 4 Ohms. Current 10KHz THD is 0.00018, about -114dB. 1KHz THD is 0.0013 something, still a good > -90dB. The increasing THD the lower the frequency is attributed to the NFB cap, although it's cut-off frequency is sub 1Hz.

 

[U102324]

Well done magicbox

 

[MagicBox]

Thank you. It's an exciting result. Figures look good so now I can make the final PCBs one of these days. If there's interest from MOSFET lovers I might even compile this amp into DIY kits. The next revision might receive a JFET input stage. However, I want to finish and case this amp first 🙂