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 Forum index » DIY Hardware and Software » Lunettas - circuits inspired by Stanley Lunetta
[SOLVED] Sustain issues with 555 envelope generator.
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beepetc.



Joined: Mar 07, 2018
Posts: 6
Location: Denmark

PostPosted: Fri Aug 17, 2018 11:46 am    Post subject:  [SOLVED] Sustain issues with 555 envelope generator.
Subject description: CMOS 555 ASR and 1-transistor VCA.
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Hi guys!
I'm working on a mini 9v semi-modular synth voice, which based on a simple saw wave oscillator (courtesy of synthmonger), an ASR envelope generator and a 1-transistor VCA, which is based off PHOBoS' lun-a-key circuit.
Now, I've been going through heaven and hell with this thing, but I refuse to give up. A lot has been fixed and worked through, so I believe it's possible to solve. Cool

Here's the main issue [SOLVED]: The sustain stage of the envelope is more quiet, than the release stage.
Putting a 47uF cap instead of the 4,7uF cap, after the LEDs and pots, results in a proper sustain stage, but no release. The LEDs are an abstraction from the original design, which originally were 1n4148 diodes. I will try swapping the LEDs out with those, to see if it makes a difference.


Another issue is, that every time the envelope is triggered, the pitch of the VCO drops a bit. I'm unsure whether this is because the circuit is driven off a 9v battery, or lack of decoupling.

Any tips, help or ideas will be received with applause of gratitude and a 1000 virtual high fives.
Thanks, Tanis.

EDIT: After testing the circuit under the scope, it doesn't come as a surprise that the fault was lying in the 1-transistor VCA. The envelope is a beauty. I think it's the last time I'll use this kind of VCA, but for now, I'll just have to deal with it and fiddle with the resistor values.

EDIT 2: Alright! Problem 1, solved. Switching out the 10k at R5, with a 36k resistor solves the sustain issue. Now for the pitch drop issue.

EDIT 3: There it is guys. The grand finale. I've solved the mysteries and learnt why I need proper decoupling on my power supply. A 220uF decoupling cap, instead of the 2,2uF one, solved the pitch issue and this synth is now ready for our workshop.

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