The funny thing is, if you're going to reshape the radios audio stages for a Hi-Fi passband, you shouldn't need a ton of outboard audio equipment to drive the rig. Conversely, if you use external processing and equalization, you don't need a radio that has very many modifications whatsoever, to transmit that Hi-Fi on AM.
To elaborate further, when the AF stages of the rig have been properly modified to match the impedance and frequency response of a decent microphone, you shouldn't need to do that all over again, with outside equipment. You can literally plug a D-104 (modified with a high impedance FET pre-amp) right into the front of the radio and get all the useful Hi-fi that is achievable.
The flip side to this, is not changing any parts inside the radio and just injecting quality audio directly into the series pass modulator stage. You just have to amplify the line level output, to easily reach 12 volts of peak to peak audio without flat topping. Line level is not enough to drive the series pass modulator at 3 volts peak to peak. This stage has no voltage gain and can only provide current gain.
To elaborate further, when the AF stages of the rig have been properly modified to match the impedance and frequency response of a decent microphone, you shouldn't need to do that all over again, with outside equipment. You can literally plug a D-104 (modified with a high impedance FET pre-amp) right into the front of the radio and get all the useful Hi-fi that is achievable.
The flip side to this, is not changing any parts inside the radio and just injecting quality audio directly into the series pass modulator stage. You just have to amplify the line level output, to easily reach 12 volts of peak to peak audio without flat topping. Line level is not enough to drive the series pass modulator at 3 volts peak to peak. This stage has no voltage gain and can only provide current gain.