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 Forum index » DIY Hardware and Software
Envelope Generator For 555 Based Synth
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NarcolepsyKhan



Joined: Nov 02, 2013
Posts: 4
Location: Pakistan

PostPosted: Sat Nov 02, 2013 3:28 am    Post subject: Envelope Generator For 555 Based Synth Reply with quote  Mark this post and the followings unread

I am trying to make a 555 timer based synth for a school project. My design is very simple. It makes use of different resistor combinations to emulate each note frequency on the 5th octave.

Here is the circuit:

Posted Image, might have been reduced in size. Click Image to view fullscreen.

Main Question:

I want to add an envelope generator in there. A simple attack/release type deal to make the sound more realistic. Can you guys please suggest designs that will work with my circuit? It doesn't have to be a full on ADSR envelope. Attack and Release are fine.

Secondary Question:
I would also like to mention that I have been able to generate an Attack/Release envelope using the 555 as a one-shot. I am using a switch as the trigger. It works fine but I can't come up with a constant envelope. Keeping the switch on just makes the RS flip flop in the 555 toggle and it ends up losing the envelope. Should I use a voltage buffer?

Thanks in advance for your help.
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PHOBoS



Joined: Jan 14, 2010
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PostPosted: Sat Nov 02, 2013 6:07 am    Post subject: Re: Envelope Generator For 555 Based Synth Reply with quote  Mark this post and the followings unread

NarcolepsyKhan wrote:
I want to add an envelope generator in there. A simple attack/release type deal to make the sound more realistic. Can you guys please suggest designs that will work with my circuit? It doesn't have to be a full on ADSR envelope. Attack and Release are fine.

welcome party!

ok let's see. Using those different resistors will work but you're not going to get those values (unless you combine several resistors). You could
use values that are close to it, but it will probably be out of tune so you I would suggest using trimpots with a series resistor instead. (maybe
you've allready done that).

The speaker should not be connected directly to the 555 but with a capacitor (10uF) in series. Which is better for both your speaker
and the timer chip (does the chip get hot right now ?). And don't connect it to pin 2/6 but to GND or Vdd. (I don't know if that's just an error in the
schematic or not).
Posted Image, might have been reduced in size. Click Image to view fullscreen.

A simple envelope generator is possible with a 555 but you would also need a simple VCA if you actually want to control the sound level with it.
And then you probably also need an amplifier allthough it might be possible to make a VCA with a heavy transistor so it can still drive the
speaker. You also need to trigger the envelope generator with each keypress. Because you allready have the keyboard switches connected to
Vcc this shouldn't be to hard. You could do it by using diodes and a pulldown resistor to create an OR gate to trigger the envelope generator.

I made a simple synth from a toy keyboard myself which has an envelope generator and VCA. It cant drive a speaker though,
but here's the schematic for the ADAR/VCA part:
Posted Image, might have been reduced in size. Click Image to view fullscreen.
more info here

And here's a version with an improved trigger section so it doesn't need 2 seperate input signals for AD or AR mode.
And it uses a transistor instead of an inverter (CD40106).
Posted Image, might have been reduced in size. Click Image to view fullscreen.
you might need to adjust the values of zenerdiode and the resistor attached to pin5 depending on your supply voltage.
But you can also leave them out completely to keep it simple. And you can leave out U2b and all the components attached
to it.
more info here

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NarcolepsyKhan



Joined: Nov 02, 2013
Posts: 4
Location: Pakistan

PostPosted: Sat Nov 02, 2013 6:27 am    Post subject: Reply with quote  Mark this post and the followings unread

Thanks for the reply. I am sorry about speaker connections. I was in a hurry. Razz

I really like your ADR circuit. It looks easy enough to implement. So all I basically need to do is feed the output of my 555 into the trigger input on your ADR circuit? Or do I need additional circuitry?
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PHOBoS



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PostPosted: Sat Nov 02, 2013 9:11 am    Post subject: Reply with quote  Mark this post and the followings unread

NarcolepsyKhan wrote:
I really like your ADR circuit. It looks easy enough to implement. So all I basically need to do is feed the output of my 555 into the trigger input on your ADR circuit? Or do I need additional circuitry?

no you trigger it with the keyboard keys (you'll need to add a diode to each key), and you connect the output of the 555 synth to transistor that's used
as a VCA. I can't guarantee you that section will work, those simple 1 transistor VCA's often need some experimenting to get the right values, but
it should at least help you get started.


Basic ADAR VCA.gif
 Description:
Basic envelope generator (Attack Decay / Attack Release) and a simple VCA.
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Basic ADAR VCA.gif



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NarcolepsyKhan



Joined: Nov 02, 2013
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Location: Pakistan

PostPosted: Sat Nov 02, 2013 12:05 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thanks, mate! You are a godsend!

I take it the 555 output goes at the transistor's emitter. So basically the VCA's amplitude varies with respect to the ADSR envelope and since my 555 is connected at its emitter, this subjects the 555 output to an amplitude envelope. Am I correct?
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PHOBoS



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PostPosted: Sat Nov 02, 2013 1:00 pm    Post subject: Reply with quote  Mark this post and the followings unread

NarcolepsyKhan wrote:
Am I correct?

almost, except that the collector is the input not the emitter Wink

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NarcolepsyKhan



Joined: Nov 02, 2013
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PostPosted: Sat Nov 02, 2013 1:09 pm    Post subject: Reply with quote  Mark this post and the followings unread

Awesome. Thanks. I should have looked at the schematic before posting. Smile
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littlem4tt



Joined: Jun 30, 2013
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Location: uk

PostPosted: Sat Jan 04, 2014 7:22 am    Post subject: Reply with quote  Mark this post and the followings unread

Hi Phobos, thanks for the ad/ar v1 circuit. The first time I built it I got it working great. However I have not managed to get it working again. Everything is fine upto the 10k feeding the base of the audio in transistor.
But the audio passes through the transistor unaffected by the signal coming into the base. I am running 12v on the supply and suspect I need to fiddle with the last set of resistors. What voltage were you working with in that design?
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PHOBoS



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PostPosted: Sat Jan 04, 2014 9:20 am    Post subject: Reply with quote  Mark this post and the followings unread

hmm, I probably used 12V aswell. What happens if you don't connect the 10K to the base of the transistor, so it's just connected with the 1M to ground,
does the audio still pass through ? If not then you might have to reduce the value of the 1M or increase the 10K. If it does pass through unaffected it
could be that the transistor is broken or you have it reversed* (allthough you no doubt checked that several times). If it still doesn't work you could try
changing the 47K resistor to GND connected at the emitter.

*I don't expect it to pass through unaltered when you have it reversed but without testing I can't say for sure.

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richardc64



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PostPosted: Sat Jan 04, 2014 3:22 pm    Post subject: Reply with quote  Mark this post and the followings unread

Every example I've seen of that configuration for a 1-transistor VCA had heavy attenuation of the input signal. I think it might have something to do with not exceeding the transistors' collector-to-emitter saturation voltage. This value varies from device to device, but generally it's less than 1volt.

The base resistor might need tweaking, too.


3series.gif
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littlem4tt



Joined: Jun 30, 2013
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PostPosted: Sat Jan 04, 2014 4:11 pm    Post subject: Reply with quote  Mark this post and the followings unread

You are totally correct and you beat me to the post! Actually there were 2 things going on.
1. I needed a 100n before trig in on the 555. Without this and even with a bare wire into the bread board was enough to open the gate.
2. Lowering the audio input make the system work correctly. I guess i got lucky when i first tried it hehe

I am using this circuit to control the decay of an 808 clap. ill make a vid when i have worked out the opamp resistors

thanks
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littlem4tt



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PostPosted: Sat Jan 04, 2014 6:34 pm    Post subject: Reply with quote  Mark this post and the followings unread

I have it all working now, i just have a thump on the output now which is louder than the clap. I am not using the attack so i have shorted that connection. With the attack pot in, slowing it does remove the thump but obviously this handy for a clap. I have tried putting bypass caps here and there but to no avail. Does anyone have any ideas? I love the simplicity of this circuit i cant find a simpler A/R any where.
Is the signal after the pots suitable for driving an lm13700 vca? I have a spare side of one?
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PHOBoS



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PostPosted: Mon Jan 06, 2014 2:24 am    Post subject: Reply with quote  Mark this post and the followings unread

Good to know it works when attenuating the input signal. It does make me wonder why it worked for me with a squarewave, which has an
amplitude that´s almost the supply voltage. If it was distorting I can understand how I woudn´t notice it. Maybe a different transistor ?

The LM13700 needs current control, which can be done with just a resistor, but If you drive it directly from the signal after the pots it's probably
too much of a load and will affect the behaviour of the AD/AR. You should however be able to use the voltage available at the collector of the
first transistor. (the one that's connected to the + rail). Or you can use an opamp to buffer the voltage instead as I did here:
Posted Image, might have been reduced in size. Click Image to view fullscreen.

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analog_backlash



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PostPosted: Mon Jan 06, 2014 3:59 am    Post subject: Reply with quote  Mark this post and the followings unread

Hi PHOBoS.

Could you see any problems that would arise if you substituted a 556 for the two 555s in your LM13700 circuit above? I only ask as I have quite a few of these and I tend to substitute them for 2 x 555 whenever I get the chance. I've even got some 558 quad timers, but they share CV and Reset pins, so they may be less useful.

Thanks for the design by the way.

Gary
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PHOBoS



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PostPosted: Mon Jan 06, 2014 4:28 am    Post subject: Reply with quote  Mark this post and the followings unread

I haven't tried the NE556 myself (allthough I have one laying around) but I took a quick look at the datasheet and it looks like they don't share any
pins,. so I don't see a reason why it wouldn't work (unless some internal noise causes false triggering but I doubt that will happen). It would make a
nice combo with an LM13700, especially with the mirrored pin layout they both have Very Happy.

One thing I want to note, if you use a different supply voltage you'll have to adjust the 8.2V zenerdiode. The threshold voltage of a 555 is internally
set at 2/3 of the supply voltage by 3x 5K resistors (CMOS version has higher values).
With a 12V supply this sets the treshold voltage at 8V, so in AD mode the voltage will rise to that and then the decay starts. in AR mode the voltage
will rise to a voltage of 8.2V max. limited by the zenerdiode. So with a lower supply voltage it will still work but the voltage will be higher in AR mode
then AD mode. If you'd use a higher supply voltage the voltage never reaches the treshold level so it would rise to 8.2V and then just stay there.

Of course with a different supply voltage the VCA section would need some adjustment too.

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richardc64



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PostPosted: Mon Jan 06, 2014 8:07 am    Post subject: Reply with quote  Mark this post and the followings unread

littlem4tt wrote:
I have it all working now, i just have a thump on the output now which is louder than the clap.


The thump is something that unfortunately is inherent for a 1-transistor VCA. Since you're going for Clap sound which is mostly high frequencies, it may help to use a smaller cap, say .1uF or less, on the output. That will block most of the low frequency content of the thump.

Or use the better VCA Phobos posted Wink

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analog_backlash



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PostPosted: Mon Jan 06, 2014 3:51 pm    Post subject: Reply with quote  Mark this post and the followings unread

PHOBoS wrote:
It would make a nice combo with an LM13700, especially with the mirrored pin layout they both have Very Happy.

Yes, that's exactly what I was thinking. Come to think of it, I did something like this when I built my "Super Stylophone", which has a 556 dual A(S)R envelope generator (but that was on a single rail 12V supply). It's definitely on my list of "stuff to breadboard".

Thanks again,

Gary
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