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 Forum index » DIY Hardware and Software » Circuit Bending
Bending a Casio SA-65 and 3/4 of a synth
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Rolbista



Joined: Nov 17, 2012
Posts: 21
Location: Poland

PostPosted: Thu Jan 17, 2013 2:28 pm    Post subject: Bending a Casio SA-65 and 3/4 of a synth Reply with quote  Mark this post and the followings unread

Hello, I got this little SA-65 that I wanted to bend into a synth and plug it into a few modules that I have like a VCO and ADSR. The problem is Gate, I don't have any CV keyboard or anything, so i hoped I could gate the ADSR with amplified signal from Casio. Here's my idea: audio output-> opamp stage amplifying it to 5V-> bridge rectifier with a cap and voila! signal was converted into Gate voltage. Erhm, nope. The problem is the voltage is constant, whether the key is pressed or not it's at 5V at all times. And I managed to read 5V on the output only once, at other times my DMM just shows 0,14V or something like that, but it could be my faulty breadboard. I used datasheet's application schematic for non inverting amplifier with R1=10k and R2=150k, tl072 powered with +5/-5V. Any ideas on how to use signal as gate?
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PHOBoS



Joined: Jan 14, 2010
Posts: 1647
Location: Moon Base
Audio files: 332

PostPosted: Thu Jan 17, 2013 3:42 pm    Post subject: Reply with quote  Mark this post and the followings unread

I have been thinking about the same thing after I bend a keyboard which allready had a LED that lit up everytime you push a key
which was very usefull for controlling a filter. So how do you do this if a keyboard doesn't have a LED ?

my idea is using a comparator which can detect when there is sound, but that just made me think that it probably should
be set at a certain level else you have to wait untill the sound is completely gone before pressing another key.
But that's easy with a comparator and the output will be either low or high no varying voltage. You can then convert this
into a pulse. a reliable way is using a 555 timer but there are several ways to do this, using a capacitor and a schmitt
trigger gate or a transistor.

btw did you put a capacitor in series with the audio output to block any DC voltage ?

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Rolbista



Joined: Nov 17, 2012
Posts: 21
Location: Poland

PostPosted: Thu Jan 17, 2013 4:00 pm    Post subject: Reply with quote  Mark this post and the followings unread

you mean a capacitor after the op amp? I want DC on the output, I 'm not so sure i understand correctly how gate works, so far, I thought that as long as the gate is on (5V), the envelope is going on from attack to sustain stage, and after the Gate is released (switched to 0V) the Release stage begins. Is that right?

EDIT
http://sound.westhost.com/appnotes/an001.htm what about this
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PHOBoS



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PostPosted: Thu Jan 17, 2013 5:08 pm    Post subject: Reply with quote  Mark this post and the followings unread

Rolbista wrote:
you mean a capacitor after the op amp? I want DC on the output

I mean a capacitor between the audio output and the opamp. I don't know how it works with your keyboard
but it could be outputting a DC voltage on the output at all times, allthough that depends where you take the audio from.
If you take it from the speaker there is probably allready a capacitor in place or at least no DC voltage, since
that would waste power and could fry the speaker. But it might help.

I was also wondering if you might need a fake/virtual GND but I see you are using a dual supply for your opamp(s), so that's perfect.
The gain on your opamp might be a bit high with those resistor values, but that shouldn't be a problem, it might actually be helpfull.
Not so sure about the bridge rectifier. The page you linked to has some nice circuits for full wave rectifying that will probably
work but a simple diode should do the trick too. And a capacitor of course.

It might need some tinkering,. but you should be able to get a voltage when there is a sound (key pressed)
and no voltage when there is no sound with that setup. There might be another way too,. taking the signal from
the keyboard but that's a whole different method, and I have no idea how to do it at the moment. (assuming it's wired up in a matrix)


Quote:
I 'm not so sure i understand correctly how gate works, so far, I thought that as long as the gate is on (5V), the envelope is going on from attack to sustain stage, and after the Gate is released (switched to 0V) the Release stage begins. Is that right?

you are correct about that, with a standard ADSR it will go through attack and decay stages (both times can be controlled)
when the gate input goes high, then it stays at the sustain level (level can be controlled not the time) untill the gate goes low,
then it will go into the release stage (time can be controlled).

so you don't need a pulse for that, I was thinking about that because of the other keyboard Embarassed

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Rolbista



Joined: Nov 17, 2012
Posts: 21
Location: Poland

PostPosted: Thu Jan 17, 2013 5:15 pm    Post subject: Reply with quote  Mark this post and the followings unread

now i looked at some Schmitt trigger circuits, and maybe it would be a lot less fuss to just set it to be activated by the signal of certain level and output 5V when output is high and 0V when low. And yes, the signal is taken from the speaker, the service manual states that output should be around 1000mV.
Also, can i measure signal level with a DMM set to AC?
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PHOBoS



Joined: Jan 14, 2010
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PostPosted: Thu Jan 17, 2013 5:26 pm    Post subject: Reply with quote  Mark this post and the followings unread

yep,. that's what i mean with a comparator, if you add hysteresis you have a schmitt trigger with adjustable trigger level.
this page might be usefull.

I doubt measuring the signal level with a DMM set to AC will work, since it's a complex wavefrom and you won't be able to measure if
there is any DC voltage in there.
Best way would be to use an oscilloscope. But you'll need to rectify it anyway and then you can measure the DC voltage.
Still a scope would be best here, but you should be able to get some idea of the level.

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Rolbista



Joined: Nov 17, 2012
Posts: 21
Location: Poland

PostPosted: Fri Jan 18, 2013 12:52 pm    Post subject: Reply with quote  Mark this post and the followings unread

well, i tried a cap on the input and I also measured the rectified signal without amplification but still there is constant voltage, around 1,5V whether the key is pressed or not, so i guess I'll have to handle without ADSR for now. I'll be building an oscilloscope plug for a TV, so I'll try measuring the Casio with that later. I have a question concerning the op amp however, are my resistor calculations for the noninverting amplifier correct? will the tl072 give me a 5V output? I'm asking because the CEM3391 that I used for modules requires a 10V peak to peak signal, so i thought the opamp stage would be handy for that too.

EDIT
I looked further into google with "audio to DC converter" and found this: http://www.rajkumarsharma.com/circuits/Audio%20to%20DC%20voltage.pdf It works, this time when a key is pressed the voltage is up, when released, down, maybe my earlier problems were due to using 1n4007 instead of 4148, or wrong arrangement. I removed the caps however, because the rise time was too long. Will post an update on how it turns out with the actual module.
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YamahaPSR6



Joined: Feb 10, 2013
Posts: 23
Location: Greece

PostPosted: Fri May 17, 2013 9:14 am    Post subject: Reply with quote  Mark this post and the followings unread

very interesting!

I think I have been wanting to do this also, but could not even bring it into words.

An update would be awesome!
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Rolbista



Joined: Nov 17, 2012
Posts: 21
Location: Poland

PostPosted: Fri May 17, 2013 9:55 am    Post subject: failure and shame Reply with quote  Mark this post and the followings unread

well, I wasn't able to get the EG to work, I guess I must have messed up the wiring but got a little lost in it and couldn't troubleshoot it. The VCAs never worked, all I got from it was the filter (which sounded very good). Unfortunately, after an attempt to repair the EG, even the filter stopped working (complete silence) so I gave up having no idea how to troubleshoot this kind of an integrated circuit. I still have the chips though but can't say if they work at the moment. I ended up building discrete state variable filter and a octave mixer for this Casio.
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YamahaPSR6



Joined: Feb 10, 2013
Posts: 23
Location: Greece

PostPosted: Thu May 23, 2013 1:47 pm    Post subject: Reply with quote  Mark this post and the followings unread

I have a couple of devices I made myself

one is 40106 based

and one is a 556 based atari punk console variation

I have made a 'powersupply out' in the 40106 unit so as , so as so as to power up the atari punk console , but then one affects other, which is quite welcome if you think the circuit bend mentality I am coming in all these.

https://www.youtube.com/watch?v=2pKQYRTreQY

the demo is not good

question is

how easy it to connect a spare part keyboard / midi controller to controll this through voltage.

and first of all how can I test it?

and a really a newbie, but have an education in electronics, so I can really miss some colloquial terms


cheers
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