i frequently make audio effects that involve envelope detectors. These always full wave rectify then low pass filter. At low frequencies the DC level is distorted as the audio is below the cut-off frequency, but i don't want to make the cut-off any lower as it affects the minimum response time

does anyone have any clever techniques for improving on this - quite crude - circuit? I'd rather not use a RMS detector chip as they are expensive. maybe im just cheap but its fun to see what you can do on a budget

 

i should add, the frequencies i want to detect are 50Hz-5kHz

 

Low-pass filtering of the full-wave rectification converts modulated DC to a fairly constant DC voltage, is that correct?

For your effects, could you make use of a peak detector? That is, the peak detector gives you a DC voltage equal to the maximum positive or negative amplitude of the envelope. This would work equally well 50 Hz to 5 kHz.

 

Cheap way.......adjust the RC ratio of a half-wave rectifier for the best linear response for those frequencies.

Plot slope of frequency vs output voltage. Build buffer amp with a mirror image of that slope.

 

Actually a peak detector would probably not be correct for what you want to do.
Something very simple that you could do is-

audio source signal ----> power amplifier ---->incandescent light bulb, the bulb illuminating a photo-resistor

Then resistance of the cadmium sulphide photo-resistor is inversely proportional to RMS voltage of the audio signal.

But you had better act soon while there are incandescent light bulbs still available.

 

Cheap way.......adjust the RC ratio of a half-wave rectifier for the best linear response for those frequencies.

Plot slope of frequency vs output voltage. Build buffer amp with a mirror image of that slope.

What is the RC ratio? Would this apply to an op-amp configured as a half-wave rectifier?

 

An envelope has two parts. It has an underlying steady tone(frequency). This tone can be filtered and gives a steady DC voltage or current output.

When you amplitude modulate a tone.....the tone develops an envelope. The other part of the envelope......is the RATE of change of that steady tone.

An envelope has a steady part and a changing part. The changing part is much lower in frequency than the steady part.

So.....there is two frequencies coming into the rectifier. We want to filter and rectify the steady part........BUT.....we want to NOT filter....but pass along the changing part to the audio amp. The changing part of the envelope is what carries the information or data.

AND therefore....we need a rectifier that will filter and smooth the steady tone....but pass along the change ripples.

A RC network(resistor/capacitor)......can divide the circuit frequency scale into two parts......the low part and the high part. The RC ratio determines the center frequency that divides low frequency from high frequency.
The RC configuration determines if the low or high frequencies pass.

Are you with me? Have you got a scope, sig gen and meter?

 

i frequently make audio effects that involve envelope detectors. These always full wave rectify then low pass filter. At low frequencies the DC level is distorted as the audio is below the cut-off frequency, but i don't want to make the cut-off any lower as it affects the minimum response time

If you use a 2-, 3- or 4-pole filter instead of a simple 1-pole RC filter, that would allow you to raise the filter cutoff frequency (and thus shorten it's response time) while still giving good attenuation for ripple at audio frequencies.

 

An envelope has two parts. It has an underlying steady tone(frequency). This tone can be filtered and gives a steady DC voltage or current output.

When you amplitude modulate a tone.....the tone develops an envelope. The other part of the envelope......is the RATE of change of that steady tone.

An envelope has a steady part and a changing part. The changing part is much lower in frequency than the steady part.

So.....there is two frequencies coming into the rectifier. We want to filter and rectify the steady part........BUT.....we want to NOT filter....but pass along the changing part to the audio amp. The changing part of the envelope is what carries the information or data.

AND therefore....we need a rectifier that will filter and smooth the steady tone....but pass along the change ripples.

A RC network(resistor/capacitor)......can divide the circuit frequency scale into two parts......the low part and the high part. The RC ratio determines the center frequency that divides low frequency from high frequency.
The RC configuration determines if the low or high frequencies pass.

Are you with me? Have you got a scope, sig gen and meter?

As I understand it, you are saying use a first order low-pass filter to attenuate the steady, higher frequency component, and pass the lower frequency changing amplitude modulation of the steady tone.

Yes, I do have function generator, scope and DMM.

 

Yes. I'm not sure the context of your sound effects. I'm taking about detecting an rf envelope.

If you want to detect a baseline envelope.......and then manipulate it.......DSP would probably be the way to go. They make special chips for it and there are loads of information.

To see what I mean......feed your span of frequencies into a RC network and watch the response on the scope.
Experiment with R and C values.....and series and parallel configurations.