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 Forum index » DIY Hardware and Software
Mixing 40106 oscillators
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egasimus



Joined: Feb 11, 2011
Posts: 112
Location: Bulgaria

PostPosted: Mon Sep 09, 2013 3:02 am    Post subject: Mixing 40106 oscillators Reply with quote  Mark this post and the followings unread

In a hurry so will be brief: I wanna build a small 3- or 6-key 40106 drone box and I can't find any know-how about mixing the oscillators. I understood that I need to use two gates per voice to buffer the output so they don't interact.

Also, since the synth will be key-driven (i.e. not constantly droning all the time), can I connect the keys in the feedback loop (where the frequency pot is), instead of after each oscillator's output? I would imagine this would be easier on the IC since all the oscillators won't be running all the time.

Here's what I have for now: http://imgur.com/6lWZrNI

I'm a bit confused about the resistor values, too. Any tips on what should I use in order to get the voices mixed without much issues?

I'd like to put Escobedo's Q&D VCF at the output, but I'll draw that later.

Thanks in advance for the knowledge shared : )
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PHOBoS



Joined: Jan 14, 2010
Posts: 1657
Location: Moon Base
Audio files: 333

PostPosted: Mon Sep 09, 2013 8:40 am    Post subject: Reply with quote  Mark this post and the followings unread

Well first of all in the schematic you got POTS 1 to 3 in the wrong place. But you probably know where they should be placed. There are several options
for mixing, and maybe a passive mixer will work too. I think this site might be useful. (easier to read if you select the text).
Those ciruits are for a dual supply btw, so if you're suing a single supply you'll have to create a fake ground.
Examples of single supply mixer can be found here and here.

I do remember that someone did it on a single supply without a fake ground and apparently that work (at least for squarewaves).


As for switching, I'm not sure how well it will work if you put switches in the feedback loop, might be a bit unstable, just try it Very Happy An easy way
would be to use NAND gates (4093) instead of inverters (40106) those are easy to turn on/off. The only downside I see with starting/stopping the
oscillator is that there might be a small delay, but if they're oscillating at audiofrequencies I don't think that would be noticable.

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egasimus



Joined: Feb 11, 2011
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Location: Bulgaria

PostPosted: Mon Sep 09, 2013 1:46 pm    Post subject: Reply with quote  Mark this post and the followings unread

I could imagine some odd phase issues turning up with stopping the oscillator, but I'm not sure if it would actually be an issue.

I drafted a 4093 version today, and I added the VCF between the two inverting stages of the mixer. Have a look.
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PHOBoS



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PostPosted: Tue Sep 10, 2013 6:55 am    Post subject: Reply with quote  Mark this post and the followings unread

egasimus wrote:
I could imagine some odd phase issues turning up with stopping the oscillator, but I'm not sure if it would actually be an issue.

The delay I meant has to do with the way this kind of oscillator operates.
Say it's oscillating and it's output is high. The capacitor get's charged untill it reaches the upper treshold level of the NAND gate, the output changes to
a low level and start to discharge the capacitor untill it drops below the lower treshold level of the NAND gate and so on,..

Now if you turn the oscillator off, it's output will be high and charge the capacitor. But it doesn't stop when it reaches the treshold level of the
NAND, it will charge it to almost the level of your supply voltage (+9v). So when you turn it on again it takes a longer time before the capacitor is
dicharged far enough that it's voltage drops below the lower treshold level. hence the delay. But as I mentioned you probably won't notice it at
audiofrequencies.

Quote:
I drafted a 4093 version today, and I added the VCF between the two inverting stages of the mixer. Have a look.

looks good to me,. Very Happy
just make sure R1 is small enough (with all switches closed R2..R5 are placed in parallel so R1 has to be smaller then 1/4 of the value you use,
or you could leave it out alltogether). And I think you'll have to place capacitors before P5 and the volume pot, or connect them to VB instead
of GND. But the more you connect to that point the more unstable it will get.

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egasimus



Joined: Feb 11, 2011
Posts: 112
Location: Bulgaria

PostPosted: Wed Sep 11, 2013 12:29 am    Post subject: Reply with quote  Mark this post and the followings unread

PHOBoS wrote:
The delay I meant has to do with the way this kind of oscillator operates. Say it's oscillating and it's output is high. The capacitor get's charged untill it reaches the upper treshold level of the NAND gate, the output changes to a low level and start to discharge the capacitor untill it drops below the lower treshold level of the NAND gate and so on,..

Now if you turn the oscillator off, it's output will be high and charge the capacitor. But it doesn't stop when it reaches the treshold level of the
NAND, it will charge it to almost the level of your supply voltage (+9v). So when you turn it on again it takes a longer time before the capacitor is
dicharged far enough that it's voltage drops below the lower treshold level. hence the delay.


Haha, that's something I need to get my head around. I think you might just have given me the key to starting to understand all those damn circuits.
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