Here’s the circuit I’ll use for my dalek voice modulator (still have to buy a microphone , the ring mod circuit is in my notebook but yes I’m pretty slow….).
The principle is pretty basic . Converting a AC signal into a DC signal to turn on and off a light emeter element (LED , bulb , ect…) .
here’s the very simple schematic :
At the input , a buffer (not necessary if the incoming signal is low impedance). Not a lot to say , it’s a buffer separating the input of the output. Then a big gain Op-Amp . Its gain is variable with P1 . P1 can be used a sensibility control or to make the circuit work with small signal . the AC signal gets out via C3 and then get rectified by D1/D2 . They cut out one half of AC signal. The RC filter is here to give a constant time which is the wanted effect.The capacitor will take time to get fully charged and discharged because of VR2/VR3 which modify theses parameters. The voltage across it will raise slowly and the LEDs will follow that (relative) slow change .
Finally , The transistor is here to drive the LED . An Op-Amp isn’t really capable of giving large amount of current that the LEDs will ask . A transistor is a current device , it manipulates current . It will be perfect for that task. Therefore you can control the current flowing trought its collector and emetter. To simplify ,we can say that the transistor acts like a dynamic switch.
The issue the circuit may have is the threshold voltage of the diode (0.7V , meaning that voltage coming into the diode should be above 0.7V (700mV) to get through) but the signal is strong enough to avoid this issue . Nevertheless , some more elaborated circuits can be used but I like keeping things simple .
Note the schematic is more the gerenal idea than a real working circuit hence the several pots (VR1, VR2 , ect…) Values might need some tweaking depending of the component. The circuit will work but decisions have to be made on component values (so much potentiometers for such simple circuit si not good)
To finish , a Rail-to-Rail op-amp as the LM358 will work better I think. What Rail-to-Rail means ? simply that the ouptut can go really close to the negative and positive supply , what a regular op-amp can’t do (It’ll chop the signal quicker ) . The LM358 (or any other Rail-to-Rail op-amp ) would give a better response for the application . Here’s a video that I found which explains the difference (thanks to the author!)
https://en.wikipedia.org/wiki/Precision_rectifier (for the rectifier circuits)
oh and by the way, ac rectifier is also used in guitar ! First of all ….in your amp ! the small diode brigde which transforms the AC signal from the secondary power transformer into a DC voltage. You can also find AC to DC converter in the enveloppe filter effect. You know the wacky funky wah wah. The enveloppe detector which controls the filter is the same. Actually there’s two DC detectors used in almost every enveloppe filter for guitar , this one and the one used in the MUTRON 3 . There are some others but they’re pretty rare .
The AC rectifier works also….in the vintage fuzz Octave up effect , it has the same topology than the diode bridge used with power transformer . If you look closely to the Tycobrahe octavia and the Dunlop octovia (which are the same by the way) there’s a small transformer with two diodes which transforms the AC signal into a DC signal , hence the octave up effect !