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Envelope Follower
Envelope Follower
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Old 21st July 2015, 10:21 AM   #1
fredcazaux is offline fredcazaux  France
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Join Date: Jul 2015
Location: Bordeaux, France
Default Envelope Follower

Hi everyone!

I'm working on a DIY synth controller, sort of drum pad on which I could strike, but also rub, play with a violin bow...

The first idea is to use an arduino to capture data from piezos. But arduino isn't fast enough to capture "simultaneously" 8 audio signals and track the envelope. And I don't really anything else than the envelope for my purpose. So I prefer save arduino ressources to compite antrhing else than high sampling rate data.

So, what I'd like is to build an envelope follower between the piezo and the arduino ADC.

So I would make one from this basic one :
Click the image to open in full size.

But I would like to improve it a bit if it keeps beeing quite simple.
What I'd like to add is a gain pot in the input of this follower, and a decay pot to be able to adjust the decay time of the envelope follower.

So I imagined this circuit. But I miss some skills to well understand everything. So I guess this could be better.
Click the image to open in full size.
I've added an inverting amp at the input with a pot to control the gain. I guess this is OK. I've let the unity gain amp the same, with this 10Kohms resistor (R3) which I just don't know what it is for? And I replaced the 2Mohms restistor and 0.1F cap by a standart 100Kohms pot (R4) and a 4.7F cap (C1) to control the decay time.
But my question is : is that modifications still OK to enter the arduino ADC? Is the impedance OK? WOuld there be something better to do? Simpler?

Thank you very much for your help!
Cheers,
Fred

Last edited by fredcazaux; 21st July 2015 at 11:04 AM. Reason: Invalid Image URL
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Old 21st July 2015, 10:43 AM   #2
DF96 is offline DF96  England
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I get an 'invalid attachment specified' when I click the image link.
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Old 21st July 2015, 11:09 AM   #3
fredcazaux is offline fredcazaux  France
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Quote:
Originally Posted by DF96 View Post
I get an 'invalid attachment specified' when I click the image link.
Thank you! Seems the attachment didn't work correctly. I uploaded the file again and updated so you should be able to see and open it.
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Old 22nd July 2015, 10:36 AM   #4
DF96 is offline DF96  England
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Why have you drawn a zener diode instead of a normal signal diode?

You may need to review the bias arrangements for a single-supply-rail opamp. I don't know what the 10k resistor is for, but I suspect it is an attempt at input bias.

Bear in mind that no envelope detector can follow an arbitrary envelope perfectly, so you need to think about attack time and decay time. The capacitor cannot charge up instantaneously, as the previous stage can only supply a finite current. I would use a smaller cap than 4.7uF, and if necessary add a buffer after it.
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Old 30th June 2017, 02:00 PM   #5
mjlyle is offline mjlyle
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How did this circuit end up working out for you? I am working on a similar project with a piezo transducer followed by an op-amp and envelope detector and wanted to get your thoughts on the configuration you suggested here. Thanks!
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Old 1st July 2017, 04:33 AM   #6
chaerin is offline chaerin
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Could it be preferable to use the capacitor voltage for nfb rather than the opamp output, to avoid diode-drop-related issues?
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Please note I don't really understand analog circuits at all, and I'm just hoping someone will correct me
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Old 4th July 2017, 03:40 AM   #7
JMFahey is offline JMFahey  Argentina
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Location: Buenos Aires - Argentina
Short answer: that schematic does not work at all so forget it.

Search for single supply envelope followers.

If nothing better, take a hard look at "Auto Wah" guitar pedals, they use an envelope follower to modulate bandpass filters and, by definition, are designed for single supply 9V because they are supposed to run on battery power; adapting them to just 5V is a reasonable task.

Heres one: notice the FET input, needed by the high impedance Piezo capsule, single supply operation, LM358 will work properly with just +5V supply, take your Arduino signal across the "range" pot, which can be a 22k fixed resistor instead.
Also use a conventional diode, say 1N914/1N4148 instead of the Red Led rectifier and replace 1N914 at the right with a wire link.
You may use the now unneeded second half of LM358 as a unity gain buffer if you wish, with +IN connected straight your output resistor hot pin, LM358 can read and follow 0V/ground voltages with no problem at all.

Click the image to open in full size.
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