Electronics > Projects, Designs, and Technical Stuff
Light Bulbs and LDRs in Audio Oscillators
German_EE:
I'm looking at low distortion audio oscillators and all of them include either a small light bulb or an LED/LDR combination somewhere in the circuit. I'm guessing that it's some sort of level control or AGC because the filament in a light bulb will react too slowly to affect the quality of the output waveform.
Can someone give a more precise explanation of why a light bulb or LED/LDR combination are used here?
glarsson:
The oscillator must have an amplification of exactly 1. Lower and it will reduce in amplitude until it's dead. Higher and it will increase in amplitude until it over drives and generates square waves. The bulb (or LDR or JFET or...) stabilises the amplitude.
unitedatoms:
That's how Hewlett Packard started historically, with a bulb in oscillator. Genius. http://www.hp.com/hpinfo/abouthp/histnfacts/museum/earlyinstruments/0002/
ajb:
Commonly called a vactrol, it's quite a neat way to convert a voltage signal to a resistance. As you say, a light bulb will have a definite low-pass filtering effect due to its thermal time constant (although very small bulbs might pass some bass frequencies, I imagine), but LEDs will pass well past the audio frequency range. I've mostly heard of them being used in synth applications to allow, for instance, a low frequency oscillator outputting a control voltage to modulate the frequency of another oscillator in the audio range, or to allow control voltages to manipulate filters.
Benta:
--- Quote from: German_EE on February 17, 2019, 06:07:55 pm ---I'm looking at low distortion audio oscillators and all of them include either a small light bulb or an LED/LDR combination somewhere in the circuit. I'm guessing that it's some sort of level control or AGC because the filament in a light bulb will react too slowly to affect the quality of the output waveform.
Can someone give a more precise explanation of why a light bulb or LED/LDR combination are used here?
--- End quote ---
This is typical for Wien bridge sine wave oscillators. It's an AGC, as you suspect, and is there to keep the gain loop in the oscillator stable, otherwise it either doesn't oscillate, or the waveform is clipped.
I'm aware of at least three types of gain stabilization:
1: light bulb, due to its non-linear resistance/voltage behaviour.
2: LED/LDR, where the LDR is the non-linear element.
3: self-heating NTC, also non linear in the context.
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