Saturday, 21 March 2020

Synthesizer Build part-24: ADSR with 7555 (YuSynth design)

A great ADSR. Works perfectly and it's a  very simple design, no trimmers to set. With verified stripboard layout.

What does an ADSR or Envelope Generator do?
The Envelope Generator is generally better known as the ADSR which stands for Attack, Decay, Sustain and Release. These are the four phases a note goes through when you press a key on the keyboard. If we didn't have this in combination with the VCA, we would constantly hear the oscillator sound but we only want to hear it when we press a key on the keyboard right? 
The attack is the speed of the initial rise of the note, once you press the key. Set it to zero and the sound is instantly there. Turn it open and the sound is going to take a while until it gets to full volume.
Decay is the time it takes for the note to go from the peak attack level to the sustain level. 
Sustain is the level of the note as you keep the key pressed down. It is usually set a bit less loud than the peak Attack level. (If we set Sustain fully open it will be on the same level as the peak Attack level and then it won't matter how you set the Decay because there's nothing to decay to.)
Then we have Release and that is the amount of time it takes for the note to fade out once you let go of the key. So the envelope generator produces a signal that determines the volume of the note over time and this signal is being used by the Voltage Controlled Amplifier (VCA) which interprets it as an output level. In some Minimoog synths it is also called the Loudness Contour.
Now of course the envelope output is a control voltage so it doesn't mean that you need to use it for the above mentioned purpose. You can connect it to anything that can be controlled with a control voltage like the filter cut-off or the pulse width of a squarewave or even the pitch of an oscillator. This opens up a miriad of options but let's not get ahead of ourselves here. If you're just starting out with synth building, you need the ADSR to open the VCA and the fancy stuff will come automatically with more experience. And this ADSR is perfect for beginners but also for seasoned builders in need of a good working ADSR. This is my ADSR of choise really.

My building experience:
This is the fourth Envelope Generator I present on my website and I think this one is the first that worked as it should straight away. No trimmers to set in the circuit either. I just used the schematic from the Yusynth website made a layout and built it. On the website he has two versions, an old and a new one. I built the new one. I can say without any doubt that this design is perfect if you want a good and reliable ADSR to pair with your VCA or to drive a filter. And because the circuit is so simple, even a stripboard version like this one would be robust enough to put in a rig you take on tour with you because, providing the panel is sturdy enough, there's practically nothing that can go wrong on the print.

This is the schematic.

And this is the stripboard layout I made for it. It is verified, I used it for my build and it worked first time. I didn't even test the print after building it. I made a frontpanel and wired everything up and then I plugged it into my synth and it just worked.

Print only:

Bill of Materials:

The wiring of the potmeters may look a bit strange with pin 3 left unused on three of the four potmeters, but I assure you that this is the way it should be wired up. Just follow the layout. It'll work fine I promise you. You can see in the schematic drawing that these pins are left dangling in the wind so that's what we do.
The ADSR triggers with a gate signal with a threshold of 2 Volt. The output envelope is 10Vpp. There's a manual trigger button on the panel (which is useful for testing). The envelope generator has two outputs. There's a normal output and an inverted output with a switch that lets you choose between +10V to 0V or 0V to -10V. There's also a switch to change the duration times with 'Fast' and 'Slow' settings. Use a DPDT ON-ON switch for the Fast/Slow function and a SPST (ON-OFF) or SPDT (ON-ON) switch for the Inverter voltage function.
In Fast mode the duration for Attack, Decay and Release can be set between 1mS and 1Sec. In Slow mode they can be set from 5mS to 10Sec. These times are generated by the 1µF and 10µF electrolithic capacitors C4a and C4b. In the text on the Yusynth website it says to use Tantalum caps for this but I used normal Electrolithic Caps and this works just fine. I hate Tantalum caps anyway, they always blow up on me, LOL. If you want longer times you can install bigger caps. You could even take a 3 position switch and add a third cap of, for instance, 47µF to generate really long times. I haven't tried this myself so I can not guarantee it works but I don't see why it shouldn't.
There's a LED to indicate the level of the envelope. I'm not sure why, but on my print the LED does not go fully out. It indicates fine but it keeps glowing faintly, if the ADSR is not used. There's no DC offset voltage on the output tho, so that's not it. It probably keeps glowing a bit to indicate the ADSR has power. Anyway, it doesn't matter.
Make sure you use three logarithmic 1 Mega Ohm potmeters for Attack, Decay and Release. Otherwise it will be difficult to set the parameters accurately. For Sustain we use a normal linear 10K potmeter.
You can run this envelope generator on a dual 12 Volt powersupply without any changes only the envelope output levels will go from 0 to 8 Volt instead of 0 to 10 Volt.

Here are some screen shots from the oscilloscope:

The normal envelope:

Inverted 0V to -10V:

Inverted +10V to 0V:

Well, that's all there is to say about this project really. A very satisfying build because everything worked as it should right from the start. I would say that this is the perfect ADSR. The panel potmeters work over their complete throw, unlike some other E.G.'s I built, and you can set all the parameters very easily. If someone would ask me what ADSR to build I would certainly recommend this one. You can easily add on extra outputs if you so desire. You can add a TL074 for instance and wire up some extra outputs and/or inverted outputs. That's easy enough to do.
Okay, to close off, here are some pictures of the finished product. I made a copper bracket to keep the print in place behind the panel. That way I could use just one M3 bolt. I soldered all wires straight to the copper side.

Okay, see you on the next one. If you have any questions or comments please leave them in the comments below.


  1. Hey Eddy had a question about switches, are you using on-on-on for dpdt and spst or on-off-on, thanks again. Does it also matter really what switches you
    use between the two?

    1. Good question, I should have mentioned it. Use DPDT for the fast slow and SPDT for the inverter voltage.

    2. They are the ON-ON type btw. I should have mentioned that in the first reply. ^^

  2. appreciate the photos of the finished boards, nice to have a real life reference when you're just starting out.

    1. Thank you! I'm glad you're finding it helpful. If you run into any problems building this, just let me know and I'll try and help. I love this ADSR. It works really well.