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 Forum index » DIY Hardware and Software » Lunettas - circuits inspired by Stanley Lunetta
First post - saying Hi and asking for help
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CHRISKELLY



Joined: Apr 08, 2018
Posts: 23
Location: England

PostPosted: Sun Apr 08, 2018 6:43 am    Post subject: First post - saying Hi and asking for help Reply with quote  Mark this post and the followings unread

Hi guys

Really pleased I found this sub forum! I’ve just started a cmos based synth project and this looks like a friendly place and hopefully you can help a noob out?

So far i’ve made an Atari Punk Console with a 16 step sequencer by chaining two 4017’s.

I wanted to add a second APC tuned to the same frequency as the first, which plays a sustained note, acting like a drone and is independent of the sequencer. I planned on using a stereo potentiometer so that both Astable oscillators were identical, but I found that even with a separate pot for each, changing one pot affected both Shocked

I don’t understand why this happens. Is this because both are effectively in parallel and by changing the voltage across one therefore changes the voltage available to the other?

Cheers guys
Chris
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CHRISKELLY



Joined: Apr 08, 2018
Posts: 23
Location: England

PostPosted: Sun Apr 08, 2018 6:59 am    Post subject: Reply with quote  Mark this post and the followings unread

Also is there a way to combine two oscillators at the output without them interacting or affecting each other?

When I tried to apply an LFO to my drone it affected the sequencer notes too.

I tried sending each oscillator through its own LM386 and then combining at the output but they still interacted somehow.

Sorry if these are noob questions or aren’t clear

Chris
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JovianPyx



Joined: Nov 20, 2007
Posts: 1612
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PostPosted: Sun Apr 08, 2018 10:15 am    Post subject: Reply with quote  Mark this post and the followings unread

CHRISKELLY wrote:
Also is there a way to combine two oscillators at the output without them interacting or affecting each other?

When I tried to apply an LFO to my drone it affected the sequencer notes too.

I tried sending each oscillator through its own LM386 and then combining at the output but they still interacted somehow.

Sorry if these are noob questions or aren’t clear

Chris


I think you need a circuit called an audio mixer. It will not allow one input to affect other inputs.

Try here: http://www.all-electric.com/schematic/simp_mix.htm

I googled "audio mixer schematic" to find that.

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CHRISKELLY



Joined: Apr 08, 2018
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PostPosted: Sun Apr 08, 2018 10:22 am    Post subject: Reply with quote  Mark this post and the followings unread

Awesome!

Thank you for the quick reply. I'll give that a try Very Happy
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Harry



Joined: May 09, 2017
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Location: Texas

PostPosted: Sun Apr 08, 2018 12:11 pm    Post subject: Reply with quote  Mark this post and the followings unread

Also, try adding a .1uf capacitor from pin 14 to ground on each 556 (pin 8 if using 555).
The two outputs can also be mixed with diodes.
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JovianPyx



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PostPosted: Sun Apr 08, 2018 12:23 pm    Post subject: Reply with quote  Mark this post and the followings unread

I wasn't aware that the VCO was based on a 555 type chip.

If so and if it's a bipolar 555 (not CMOS) then you can have trouble even with an audio mixer. That part has a nasty habit of crowbarring the PSU each time it resets.

Best to use CMOS versions of these chips as they won't crowbar.

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CHRISKELLY



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PostPosted: Sun Apr 08, 2018 1:38 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thanks guys

I just checked and i’m Using NE555P’s which aren’t CMOS.

This could be the problem then Confused

What did you mean by ‘crowbar’ btw?
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Harry



Joined: May 09, 2017
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PostPosted: Sun Apr 08, 2018 3:37 pm    Post subject: Reply with quote  Mark this post and the followings unread

When the 555 pulses it will pull current from the power supply, leaving any other chips with less power to work with. It can sound like a bleedthrough, even with its output shunted to ground you may still hear it through the rest of the circuit as it bleeds through the power rail. This is why i suggest the caps, it will hold a bit of power in reserve for each 555's own use and hopefully keep the power line clean.
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PHOBoS



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PostPosted: Sun Apr 08, 2018 4:01 pm    Post subject: Reply with quote  Mark this post and the followings unread

welcome party!

It depends a bit on how you are using it but you can probably get away with a passive resistor mixer for your purpose.
Diodes do indeed work too but give a different sound. You can also use different logic gates to mix the signals together.
(using diodes is the equivalent of an OR gate, although can also be used as an AND gate) You could use the XOA mixer for this.

Posted Image, might have been reduced in size. Click Image to view fullscreen.

you can find more info about it here.

note that you don't need the dual supply for the opamps and you could use the same supply as for the APC's. Ideally you'd
use rail-to-rail opamps for this but since these are digital signals it probably doesn't matter much. If you want to connect it
directly to an input of a mixer/amplifier it would be good to add some
attenuators between the multiplexer (U1) and the opamps.

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Steveg



Joined: Apr 23, 2015
Posts: 88
Location: Perth, Australia

PostPosted: Sun Apr 08, 2018 5:31 pm    Post subject: Reply with quote  Mark this post and the followings unread

Hi Chris, Welcome. You might find this a useful reference: http://castlerocktronics.com/modular/articles/CR-000_-_Power_Supply_and_System_Overview.pdf
Personally I'd add some capacitors across the power rails. You don't say how you are powering your boards but if you are using 9V batteries you may well be exceeding the rated output current causing noticeable voltage drops.
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CHRISKELLY



Joined: Apr 08, 2018
Posts: 23
Location: England

PostPosted: Mon Apr 09, 2018 5:32 am    Post subject: Reply with quote  Mark this post and the followings unread

Thanks for all the suggestions.

I'll double check the caps across the power supplies to each chip and I've ordered some cmos 555's.

I'll post a schematic too when I've figured out how to draw one!

That power supply pdf was helpful Steve Very Happy
I'm running everything off a 9V battery but I've seen some people using just 5V for everything, so if all my chips work off 5V then maybe i should consider reducing it down using a voltage divider.
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Steveg



Joined: Apr 23, 2015
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PostPosted: Mon Apr 09, 2018 6:11 am    Post subject: Reply with quote  Mark this post and the followings unread

I suggest moving to a plug pack. A 9V battery just doesn't have the current capability and my well be responsible for the crosstalk symptoms you are seeing. 5V is perfectly fine but a voltage divider won't fix your problems. A regulator will be better but a plug pack will be far better.
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