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 Forum index » DIY Hardware and Software » Lunettas - circuits inspired by Stanley Lunetta
Noob having a little issue with a simple 40106 circuit
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Collman



Joined: Jun 30, 2013
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PostPosted: Thu Apr 24, 2014 6:06 pm    Post subject: Noob having a little issue with a simple 40106 circuit Reply with quote  Mark this post and the followings unread

Hey guys, I'm just staring out and I've been doing some of the examples in Beavis Audio for CMOS circuits.

I'm doing number 7, which consists in 2 oscillators each with a pot or frequency going through a diode into another pot for volume and then to the output.
I've checked and double checked, and I'm pretty sure I have everything wired up the right way, but when I turn one of the volume pots all the way down both oscillators go silent which I thing shouldn't happen. Is this normal or is there something wrong?

I'm not 100% sure that when I change the volume they don't modulate each other in some way to be honest, since I have very little experience listening to these. I thought that might have something to do with crosstalk since I was using two oscillators next to each other (read about something like that somewhere), but moving one didn't change much.


I attached a recording since it may clarify what I'm saying. It starts with both volume knobs all the way up and I turn one all the way down, then it turn it up again and repeat the same with the other knob.

Any help and comments are more than welcomed. Excuse me if my writings have some errors, english is not my first language.


Schematic-40106-Dual-Oscillators-With-Mixer.png
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Schematic-40106-Dual-Oscillators-With-Mixer.png



Exp 7 volumen knobs test.mp3
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gdavis



Joined: Feb 27, 2013
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PostPosted: Thu Apr 24, 2014 7:57 pm    Post subject: Reply with quote  Mark this post and the followings unread

Ya, the way this circuit has been designed it will do that - turning either volume pot all the way down will completely silence all output.

It's basically shorting the output to ground when one volume pot is turned all the way down.

Putting 10k resistors between each wiper and the output would probably help.

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-minus-



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PostPosted: Thu Apr 24, 2014 8:06 pm    Post subject: Reply with quote  Mark this post and the followings unread

Try taking D1 and D2 to the wiper or middle pin of the pots. Earth the pots as you have and take the output from the remaining outer pin of the pots. Or you could try using a couple of resistors to where the wipers are linked to the output. Maybe 51K or something? I think what is happening is when one pot is turned up and the other is turned down, you are sending the signal from the pot turned up to ground, if that makes any sense?
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Collman



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PostPosted: Thu Apr 24, 2014 9:14 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thanks a lot guys, I didn't consider that it may be grouding the signal.

I tried with a couple 10k resistors that I had handy and it "solved" it. Tomorrow I'll try some other values since the output is a little low now.

Again, thanks a ton.
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PHOBoS



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PostPosted: Fri Apr 25, 2014 2:35 am    Post subject: Reply with quote  Mark this post and the followings unread

-minus- wrote:
Try taking D1 and D2 to the wiper or middle pin of the pots. Earth the pots as you have and take the output from the remaining outer pin of the pots

If you do that you can short the outputs of the oscillators to GND (through the diodes), which is not a good idea.

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-minus-



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PostPosted: Fri Apr 25, 2014 8:01 am    Post subject: Reply with quote  Mark this post and the followings unread

Ah. Yes I see that now. Embarassed I was going off a drawing I did years ago for a similar drone box/machine. I did not use the diodes, I used resistors.
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gdavis



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PostPosted: Fri Apr 25, 2014 10:20 am    Post subject: Reply with quote  Mark this post and the followings unread

PHOBoS wrote:
-minus- wrote:
Try taking D1 and D2 to the wiper or middle pin of the pots. Earth the pots as you have and take the output from the remaining outer pin of the pots

If you do that you can short the outputs of the oscillators to GND (through the diodes), which is not a good idea.

You can still short one of the oscillators output to ground in the original design by turning one pot all the way up and the other all the way down.

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gdavis



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PostPosted: Fri Apr 25, 2014 10:45 am    Post subject: Reply with quote  Mark this post and the followings unread

Collman wrote:
Thanks a lot guys, I didn't consider that it may be grouding the signal.

I tried with a couple 10k resistors that I had handy and it "solved" it. Tomorrow I'll try some other values since the output is a little low now.

Again, thanks a ton.


Adjusting values might help a little but to really get it "right" you need something like a buffered mixer. That starts to deviate from the "simple cmos" trait of Lunetta inspired circuits though.

I'm not really that familiar with Lunetta circuits, but from what I've seen they're meant to be simple and quirky. Lot of designs you'll find aren't really very technically sound, and making them so might take away some of what makes these circuits appealing to many people.

As a formally educated electrical engineer, it pains me to see some of the "mistakes" that are made, but I'm trying to keep an open mind Wink My point is that you need to consider what your goals are - do you want the "simple and quirky" or do you want robust and stable. In my view, the two tend to be diametrically opposed.

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PHOBoS



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PostPosted: Fri Apr 25, 2014 11:06 am    Post subject: Reply with quote  Mark this post and the followings unread

gdavis wrote:
You can still short one of the oscillators output to ground in the original design by turning one pot all the way up and the other all the way down.

yep,. the original design is flawed too. It would make more sense to add the potentiometers directly to the outputs of the oscillators, and then mix them
together with the diodes (would require one more pulldown resistor). Or, as you suggested, resistors between the ouputs of the pots
instead of just shorting them together.

Collman wrote:
I tried with a couple 10k resistors that I had handy and it "solved" it. Tomorrow I'll try some other values since the output is a little low now.

hmm, although the output will be lower I would expect it's still more then enough. A simple buffer using an opamp as a voltage follower should help
you but I do wonder what you are driving with it (mixer, amp or a speaker directly ?).

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Collman



Joined: Jun 30, 2013
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PostPosted: Fri Apr 25, 2014 3:25 pm    Post subject: Reply with quote  Mark this post and the followings unread

gdavis wrote:
My point is that you need to consider what your goals are - do you want the "simple and quirky" or do you want robust and stable. In my view, the two tend to be diametrically opposed.


I mostly want to experiment. I like simple and quirky for now, but I'll do some reading about buffered mixers and consider going for it.


PHOBoS wrote:
I do wonder what you are driving with it (mixer, amp or a speaker directly ?).


I got it connected to a Emu 0202 soundcard. It doesn't sound that bad too low that way, but I also tried it through a Monotron Delay (and into the Emu) and there it gets pretty quiet for some reason. As far as I understand there shouldn't be a problem with this setup, but please correct me if I'm wrong.

Maybe I had a loose connection that got disturbed when I added the resistors, but it didn't seem like it.

Hopefully I'll find time to experiment with it a bit during the weekend and maybe even start with the next project.

Thanks a lot for the help and pointers guys.
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gdavis



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PostPosted: Fri Apr 25, 2014 5:47 pm    Post subject: Reply with quote  Mark this post and the followings unread

When making audio connections like this you ideally would want the output impedance to be 0 and the input impedance to be infinite. Of course this isn't realistically possible and there are compromises that need to be balanced against each other.

The effect of a relatively high output impedance driving a low input impedance is a drop in volume. For this reason, audio interfaces like your Emu try to keep their line input impedance high, usually on the order of 1M, in order to work well with a variety of sources. A quality line output may be on the order of 100 ohms. This maintains a good strong signal transfer.

DIY synth circuits tend to compromise a little more, with output impedances of 1k and input impedances of 100k fairly common, but still reasonable at 100:1 considering they're not really "hifi".

Looking at the first schematic that google came up with, the line input impedance of your monotron is 10k. And passive mixers like the one you're implementing on the output of your oscillators tend to be high impedance, on the order of 10k with the resistors you added to "fix" your original issue. So now you've got a 1:1 input to output impedance ratio.

So, it sounds to me like you're not doing anything "wrong", the circuits are behaving as would be expected by there designs. Replacing the 10k resistors with 1k should help it work better with the monotron as it improves the ratio to 10:1. A buffered mixer as I mentioned before will be even better as it will lower the output impedance even more and provide more available drive current.

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PHOBoS



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PostPosted: Sat Apr 26, 2014 5:07 am    Post subject: Reply with quote  Mark this post and the followings unread

Here's how I would do it. I made some changes in component values to get a wider frequency range.
(also tested it, works fine Cool).


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