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 Forum index » DIY Hardware and Software » Lunettas - circuits inspired by Stanley Lunetta
AC Coupling, a mixer and a reverb..!
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joeconnor90



Joined: Mar 07, 2022
Posts: 5
Location: London, UK

PostPosted: Mon Mar 07, 2022 10:31 am    Post subject: AC Coupling, a mixer and a reverb..!
Subject description: 40106 synth with envelopes, mixer, LFO, reverb
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Hi! Have been lurking for a while learning from this awesome forum!

I had a couple of questions and I've tried to find the answers on existing posts, but thought a specific post would be maybe easier...

Summary of what I'm working on -

- 8 x 40106 oscillators (two x 40106)
- simple AR envelope per voice, with a button to play each voice (https://www.lookmumnocomputer.com/simple-envelope-generator)
- each voice going to one TL072 mixer (https://www.lookmumnocomputer.com/projects#/mixer)
- mixer output into ms20 style LP filter
- filter output into reverb (clone of DBA reverberation machine)
- reverb to main output

- also a bit of a dodgy LFO going to 6 of the voices - different speed 40106 oscillators, with 1 master speed knob that sort of makes everything go crazy.

For the envelopes and LFO, I'm using LED/LDR combinations and that's all working pretty well.

Main questions are about buffering the oscillator voices, AC coupling, and potential impedance mismatches going from oscillators to filter to reverb...

Should I be buffering each oscillator voice before each envelope LDR? Or would the TL072 mixer function as a buffer for all voices (I'm guessing the latter isn't correct).

In terms of AC coupling, would you generally do this after each voice before a mixer, or is one capacitor/resistor combination after the mixer stage enough?

Then for any issues going into filter and reverb, I'm not sure where to start in terms of measuring input/output impedance etc..!

Appreciate that's a massive load of questions - any suggestions would be very helpful!

Joe
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piedwagtail



Joined: Apr 15, 2006
Posts: 297
Location: shoreditch
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PostPosted: Tue Mar 08, 2022 2:11 pm    Post subject: Reply with quote  Mark this post and the followings unread

Quote:
potential impedance mismatches


suggest you build bit by bit and test with a cheap oscilloscope
then join together bit by bit and test with a cheap oscilloscope
you'll see the above if they are there.

Then write down what you did/saw/learnt
because you'll likely forget why you did certain things
and you don't want to fight the same battle again later

R
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joeconnor90



Joined: Mar 07, 2022
Posts: 5
Location: London, UK

PostPosted: Wed Mar 09, 2022 9:28 am    Post subject: Reply with quote  Mark this post and the followings unread

Thanks for the reply!

Cheap oscillator is now in the post.. been putting off getting one for a while because I didn't know how helpful I'd find it. I guess I'll find out!

Any thoughts on the above points about ac coupling or buffering?
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gabbagabi



Joined: Nov 29, 2008
Posts: 651
Location: Berlin by n8
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PostPosted: Thu Mar 10, 2022 9:47 am    Post subject: Reply with quote  Mark this post and the followings unread

Hi,

Are you aware of the big question that is always there in the beginning :::: what are u planing as power supply?

the 40106 will take single supply like 9V or 12V, (may 15V, dunno)?

the mixer and AR are running on dual supply means +12 AND -12V!
(they will run at +/-9V as well, of course!)

are the 40106 are osc or VCO?

Cos i dont see a VCA (or eight of them), and if the 40106 are not VCO`s - i cant see where the envelopes would go?

GLHF
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joeconnor90



Joined: Mar 07, 2022
Posts: 5
Location: London, UK

PostPosted: Sat Mar 12, 2022 1:44 am    Post subject: Reply with quote  Mark this post and the followings unread

Oh yes I forgot to mention power. I built a frequency central 12v supply, which has +/-12v. So far so good running oscillators and envelopes together, but I haven’t built everything and powered all together yet… I’m assuming it will be ok based on this being a lot simpler than a massive euro rack setup for example.

I guess they’re oscillators rather than VCOs - they’re constantly running. The way I’m using the envelopes is to let the sound through, rather than trigger the 40106s. So I have a vactrol coming from each envelope, and each ldr is stopping each voice. I don’t know if this is the best way, but it seemed simpler to me and is working so far!
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gabbagabi



Joined: Nov 29, 2008
Posts: 651
Location: Berlin by n8
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PostPosted: Sun Mar 13, 2022 2:49 am    Post subject: Reply with quote  Mark this post and the followings unread

you have dual power! thats good.

Dont know how good the LDR´s are acting as VCA,
but,
if you are planing to use only the rectangle Waves from the 40106,
may that little circuit would be interesting for you,
it acts like a inverting VCA for rectangle waves.

Other good thing, you feed in an unipolar rectangle, output is bi-polar,
no offset or decoupling needed

GLHF


super simple RECtangle VCA gabbagabi.png
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super simple RECtangle VCA gabbagabi.png


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joeconnor90



Joined: Mar 07, 2022
Posts: 5
Location: London, UK

PostPosted: Mon Mar 14, 2022 12:55 am    Post subject: Reply with quote  Mark this post and the followings unread

Thanks gabbagabi!

Does the circuit you posted also act as a buffer for each voice?

I made this on breadboard and it made my envelope work in reverse, so I’ll come back to it and try again!
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gabbagabi



Joined: Nov 29, 2008
Posts: 651
Location: Berlin by n8
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PostPosted: Mon Mar 14, 2022 2:10 am    Post subject: Reply with quote  Mark this post and the followings unread

Indeed it serves also as a buffer.

Regarding the reverse of your envelope,
This circuit acts as an inverter if the signal
on the transistor is steady high ( above 0,7V).
If its low it acts non inverting.

If you would raise the resistor value
to something like 1M you cold probably
even save the Buffer on the envelope.

So you would only need 2 opamps for vca and env.
That's only four Quad opamps like TL084 or LM324,
for your eight "voices".

Will post a Simulation later.

GLHF
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gabbagabi



Joined: Nov 29, 2008
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PostPosted: Mon Mar 14, 2022 6:27 am    Post subject: Reply with quote  Mark this post and the followings unread

attached the simulation,

it is a little variation which ,in my opinion, is worth to try,
instead of the transistor we have now a diode on the
REC Input of the "so called" VCA,
which makes a potential stripboard design even more simple.

in the simulation there are a lot of "glitches",
not shure if they are appear also in reality,
but you have a breadboard Smile

the input resistance of the VCA with 1M does not take a lot of current out of the Envelope, so there is no buffer anymore.

(Since there is no Schemo for the LMNC-Envelope, i used my one meanwhile Cool )


REC VOICE DIODE.png
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joeconnor90



Joined: Mar 07, 2022
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Location: London, UK

PostPosted: Tue Mar 22, 2022 3:40 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thanks so much, that’s super helpful and I’ll give it a go on the breadboard!

Haven’t had much synth time since you sent this Sad
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