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18th November 2008 06:42

Chris wrote...

Your thoughts about gate modulation of the FET are quite practical - however, the linearity is appalling, so you have to use large amounts of envelope feedback to get anything approaching reasonable quality.

The way I've done it is to gate modulate the final FET with an op amp - I used a TL071 for low noise, wide bandwidth and an output capable of driving into quite low impedance loads without distortion. I also used the non-inverting input to set the nominal (unmodulated) bias point, configured the op amp for a voltage gain of about 3, and fed the incoming (filtered, compressed) audio into the non-inv input. The output of the op-amp feeds the gate of the output FET through a resistor.

The output LPF has a capacitive tap feeding a diode detector, recovering the modulation envelope. This is amplified and filtered and fed to the modulating op-amp inverting input. A second TL071 provides this feedback amplifier and filter.

Setting the gain of the envelope recovery op-amp is quite critical, but once set, the results are remarkable. Without the envelope feedback, the audio is grossly distorted and switching on the envelope feedback makes the mod virtually "hi-fi" quality!