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 Forum index » DIY Hardware and Software » The layout factory
basic volt per octave keyboard schematics
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Grumble



Joined: Nov 23, 2015
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PostPosted: Sat Nov 19, 2016 1:51 am    Post subject: Reply with quote  Mark this post and the followings unread

One of my favo sites when it comes to calculation of schematics behavoural calculations is THIS site.

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PHOBoS



Joined: Jan 14, 2010
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Location: Moon Base
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PostPosted: Sat Nov 19, 2016 5:12 am    Post subject: Reply with quote  Mark this post and the followings unread

Quote:
I have also a bunch of zener diodes of many values.Which do i pick and how do i connect it?
You need a 10.0V zenerdiode and a resistor, 1K will probably do but you could experiment a little. Connect the cathode of the zenerdiode
to GND, the anode to the resistor and the other side of the resistor to -12V. The point where the diode and resistor are connected will be
your -10V reference voltage.

Quote:
Now,looking for a definitive solution.I only have LM 317,(positive regulator), but not the LM 337 which will do the job from what i've heard.
That actually made me think of another method last night but it is a bit over the top for this purpose. Use the LM317 to create a +10v
voltage and then use an opamp configured as a inverting amplifier with a gain of x1 to invert it to -10V.

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Cfish



Joined: Feb 24, 2016
Posts: 477
Location: Indiana

PostPosted: Sat Nov 19, 2016 5:18 am    Post subject: Reply with quote  Mark this post and the followings unread

extremly useful link grumble.


Edit. That would be a great link in the (fun useful tips thread)
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alanwilder81



Joined: Sep 03, 2016
Posts: 310
Location: italy

PostPosted: Sat Nov 19, 2016 6:06 am    Post subject: Reply with quote  Mark this post and the followings unread

thanks Grumble Smile
designing and calculating things for now is a bit over my head, but this site will come handy in the future


to Phobos,

i've come across a website where they describe the inverting opamp method you mention to change the voltage polarity,Now you confirm it works.
I will also try the zener diodes thing.Good to know different approaches to get this matter around Smile
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alanwilder81



Joined: Sep 03, 2016
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PostPosted: Sat Nov 19, 2016 6:15 am    Post subject: Reply with quote  Mark this post and the followings unread

to Cfish
i can confirm that the keyboard circuits works decently.The VCO i used for testing it tracks ok.It s only three octaves but hey. Enough for a little monophonic analog synth.
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Grumble



Joined: Nov 23, 2015
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PostPosted: Sat Nov 19, 2016 9:49 am    Post subject: Reply with quote  Mark this post and the followings unread

But why do you want -10volt? you could just as easily put a resistor of 10k in series with the potmeter of 1k and connect it with the -12volt.
This resistor will prevent the -input of the opamp being directly connected to -12volt if you happen to turn the potmeter to 0 ohm.
Usualy opamps dislike to have the inputs connected to the power supply (pos. nor neg.) and the resistor prevents that from happening.
It may not be very stable, but you can play anyways.
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alanwilder81



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PostPosted: Sun Nov 20, 2016 6:51 am    Post subject: Reply with quote  Mark this post and the followings unread

i want -10 V because in the schematics its specified so
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Grumble



Joined: Nov 23, 2015
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PostPosted: Sun Nov 20, 2016 9:23 am    Post subject: Reply with quote  Mark this post and the followings unread

There is no law against changing diagrams to ones likings, in fact it realy helps you in understanding whats going on...
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alanwilder81



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PostPosted: Sun Nov 20, 2016 9:47 am    Post subject: Reply with quote  Mark this post and the followings unread

tweaking diagrams is well over my head.So,for the moment i'm settling for working with what's there and trying to wrap my head around that. Cool Cool
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alanwilder81



Joined: Sep 03, 2016
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PostPosted: Sun Nov 20, 2016 3:43 pm    Post subject: Reply with quote  Mark this post and the followings unread

to Phobos,

excellent trick, i got my steady -10 V reference voltage Smile
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Cfish



Joined: Feb 24, 2016
Posts: 477
Location: Indiana

PostPosted: Sun Dec 04, 2016 6:23 pm    Post subject: Reply with quote  Mark this post and the followings unread

So, experimenting with the yousynth schematic on here has led me to try to add basic trigger to the original schematic that has grown along the way.

Looks like it should work


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Cfish



Joined: Feb 24, 2016
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PostPosted: Sun Feb 16, 2020 4:16 pm    Post subject: Reply with quote  Mark this post and the followings unread

I’m still using that final schematic of this in my key pedals, never could get it to tune right for more that 3 octaves, but it’s great for what I use it for now.


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NuttyMonk



Joined: Jun 30, 2020
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PostPosted: Wed Jul 08, 2020 8:36 am    Post subject: Reply with quote  Mark this post and the followings unread

Hi there,

wouldn't it be easier to get a schematic for MIDI to CV? I'm sure there must be schematics out there where you can take a MIDI input and convert it to a 1 volt per octave CV output.

This issue of controlling analogue synths is something i'm just getting round to trying to figure out.

NM
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Grumble



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PostPosted: Fri Jul 10, 2020 12:41 am    Post subject: Reply with quote  Mark this post and the followings unread

NuttyMonk wrote:
wouldn't it be easier to get a schematic for MIDI to CV? I'm sure there must be schematics out there where you can take a MIDI input and convert it to a 1 volt per octave CV output.


No it wouldn't because after that you will need a midi to cv device and using this device you can directly control an oscillator (using the output voltage) and a envelope generator (using the gate signal).

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MissionTroll



Joined: Jun 07, 2022
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PostPosted: Tue Jun 07, 2022 12:03 pm    Post subject: I wanted to use your circuit for making a stlylophone Reply with quote  Mark this post and the followings unread

Cfish wrote:
Just in case anyone else was looking for something similar. This is what I finally got working. I'm sure I didn't invent in, but I sure couldn't find an easy solution that worked when I built it.

This works on my bass pedals and a 2 octave keyboard project I did.


(Update) when I tried this with a TL072 it caused an issue. I believe the heat caused by the comparator/gate side of the dual opamp caused a temperature drift issue in the voltage control side of the IC. I couldn't keep the span in adjustment.

(Another update) I have used this circuit all winter long in a set of bass pedals and a 2 octave controller with no problems. Turns out the radiant heat in my house is very stable at 72 degrees. Spring is here and the house got up to 78. And the span drifted substantially.

I have ordered some opamps with better drift ratings to try. I will post back with results.

I have high hopes for the OP177 op-amp. At first glance it appears it may go right in the IC socket and has much better temperature drift ratings.

I spent a couple of hours fighting that new TL072 circuit before I finally realized I wasn't trying the other circuits when it was that warm in the house.

(Update) OP 177 opamp cleared up the temperature issues.

Great thing is you can build this with IC sockets, try it out with TL071's, or something you have, and buy the pricier OP177s if you need the accuracy.

I believe the OP177s were about 2$ each American at Digikey.


I used a single TL072 because I don't need to much accuracy and I could change it with two TL071s if needed. The keyboard has 22 keys, I solder plated them for protecting copper. Only problem is I can't get it tuned for outputting positive voltages. It can be tuned for negative voltages and I can get 1V per octave but it gets closer to zero from -8 for example from right to left. I wanted to ask if that was normal or I messed up with my PCB design. Positive voltages require me to turn the pot to almost one side and I don't get much range. Also the Op Amp requires +/-12V right, if I only use 12V positive and ground the negative input I can get positive voltages set but can't get accurate results between notes. I can also share .gerber files for anyone interested.


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