I suspect the topology is wrong on the QMX for a Voltage Mode Class D (VMCD), L502 simply should not be there.
Since it is there, it is forcing a constant current into the final, creating the voltage spikes observed by KB1NLW in post 136435.
The LPF should present a high impedance to harmonics (especially odd harmonics), so the voltage from the transformer can be a square wave.
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An alternative is Current Mode Class D (CMCD), where we no longer use the T501 center tap and have two L502's.
The LPF should short to ground the harmonics and pass the fundamental, allowing the current from the transformer to be a square wave.
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My guess is that the LPF does not behave appropriately for either VMCD or CMCD.
Perhaps a hybrid of the two was found to work best given the need to cover so many bands with so few LPF's.
One possible path forward might be to keep the current design for those that want all bands from 160m to 6m,?
but have an alternate build supporting only 6 bands where each LPF can be optimized for the frequency of interest.
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I'll run a few more simulations, using a series LC instead of the LPF for VMCD, and a parallel LC for CMCD.
Will use Evan's BS170 model, though I suspect the 2n7002 was close enough for the simulation.
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Thanks to both of you for responding.
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Jerry, KE7ER
