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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: Thu Oct 27, 2016 7:08 am    Post subject: Reply with quote  Mark this post and the followings unread

Hi Chris,

Could you be more specific to what you don't understand, maybe I can help?
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Cfish



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

PostPosted: Thu Oct 27, 2016 4:09 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thanks grumble

I know the LM3399 is a comparator IC. I'm trying to understand if there is a good reason to use a designated comparator, instead of just using an opamp.

Not just in this schematic, that was more of an in general question.

I'm sure the much more educated person who designed this circuit had some reason for choosing the LM339. I'm just trying to figure out what that reason is.

Seeing the circuit is a dual power supply circuit, having a single supply comparator is not an advantage that I am seeing.

Im sure once I have built it as drawn and experimented with it, it may become clear.
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alanwilder81



Joined: Sep 03, 2016
Posts: 324
Location: italy

PostPosted: Fri Oct 28, 2016 11:23 am    Post subject: Reply with quote  Mark this post and the followings unread

to Grumble

thanks for the response Very Happy
As a matter of fact the whole analog switches thing is quite obscure to me, having never used one. Could you please explain what you mean by varying the 4k7 resistor if the slope of the CV is not steep enough ?

thanks
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Cfish



Joined: Feb 24, 2016
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Location: Indiana

PostPosted: Fri Oct 28, 2016 12:28 pm    Post subject: Reply with quote  Mark this post and the followings unread

I think grumble was pointing out that if the resistance acros the switch
Is greater. Lowering the value of the 4k7 would make the total resistance the same as design.

Starting at the switch. ( the switch makes contact when there is a gate signal from a key press) the tone signal passes through the switch, through the 4k7 and charges the capicitor to ground to the corisponding voltage. The opamp is then acting as a buffer.

When a key is released the switch opens, but the charge of the capicitor holds the reference voltage to the output opamp. So the opamp continues to output the voltage of the last key pressed. (Sample and hold)

The output opamp buffer setup I believe is called a unity gain follower. No gain is achieved from the opamp. However adding resistance before it will reduce the span of the output.

I have played with the beginning of this circuit already and it can produce close to 2 volt per octave of shift. So it shouldn't be a problem to leave the 4k7 as is.

There seems to be plenty of adjustment there already to take care of it.
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Grumble



Joined: Nov 23, 2015
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PostPosted: Fri Oct 28, 2016 2:41 pm    Post subject: Reply with quote  Mark this post and the followings unread

That is part of what I meant: The capacitor is charged and discharged via the 4k7 resistor, the R-on of the 4066 (or 4016) and the R-out of the opamp, and because all the resistances are in series (added), the charge/discharge current of the capacitor is limited by this resistance, resulting in a slope of the voltage at the output of the follower. So not the span is limited by this resistor, but the slope!
Like what is used when you build a glide for a control voltage, there the slope is intentional
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Cfish



Joined: Feb 24, 2016
Posts: 477
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PostPosted: Sat Oct 29, 2016 5:26 am    Post subject: Reply with quote  Mark this post and the followings unread

Thanks grumble

Your knowledge is greatly appriciated.

I am however curious as to why you would want to induce any slope at that point.

The 10k and what looks like 4.7n between the 2 buffer op amps before the switch, would slow the voltage change, to give time for the comparator to close the switch. (Same problem I ran in to trying to add sample and hold to other circuits)

I'm missing why you would want additional slope after that.

There is almost always a reason I am just overlooking.

Would appriciate any input on this
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Grumble



Joined: Nov 23, 2015
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PostPosted: Sat Oct 29, 2016 11:45 am    Post subject: Reply with quote  Mark this post and the followings unread

I don't want to induce a slope, the 4k7 resistor together with the capacitor following that create a slope and I thought that you want the same slope as the original schematicks where they use a 4066
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Cfish



Joined: Feb 24, 2016
Posts: 477
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PostPosted: Sat Oct 29, 2016 12:22 pm    Post subject: Reply with quote  Mark this post and the followings unread

Sorry grumble, didn't mean you personally. Was more refering to the design in general, and wondering if there was a technical reason that I was missing for adding slope at that point.
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Grumble



Joined: Nov 23, 2015
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PostPosted: Sat Oct 29, 2016 2:25 pm    Post subject: Reply with quote  Mark this post and the followings unread

No need for a sorry, I wasn't annoyed, if it looked like I was blame the language barrier Very Happy
That said, I think that maybe the reason for a resistor at that point is to limit the current thru the switch if the cap is fully charged (or discharged) and the next time the switch is opened the opamp before the switch is at gnd level (or at vcc).
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Cfish



Joined: Feb 24, 2016
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PostPosted: Sat Oct 29, 2016 3:00 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thanks grumble. That makes since.
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alanwilder81



Joined: Sep 03, 2016
Posts: 324
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PostPosted: Tue Nov 08, 2016 12:25 pm    Post subject: Reply with quote  Mark this post and the followings unread

thanks Grumble and Cfish !

do any of you have an idea as to how rise the voltage from -12 to -10 volts in the highlighted section ?
i ve been thinking to employ a voltage regulator with a LM 317. is it gonna make the job? cheers ! Cool


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Grumble



Joined: Nov 23, 2015
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PostPosted: Tue Nov 08, 2016 1:12 pm    Post subject: Reply with quote  Mark this post and the followings unread

Hi Alan,
You need an LM137 or LM337 or something like it, those are neg. voltage regulators.
But maybe those are not stable enough, so you should probably look into negative voltage reference circuits.
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alanwilder81



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PostPosted: Tue Nov 08, 2016 1:40 pm    Post subject: Reply with quote  Mark this post and the followings unread

hello Grumble,
you are right,the LM 317 is only a positve voltage regulator. Having a bunch of them at hand ,i assumed i could get away with it, but no luck.
i will have to get a negative voltage regulator.

thanks anyway !
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Cfish



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PostPosted: Tue Nov 08, 2016 4:42 pm    Post subject: Reply with quote  Mark this post and the followings unread

Hi Alanwilder81
I'm not sure how steady the -10v reference needs to be. But a 100k trimmer with the outer legs tied to ground and -12v then sweep the trimmer till you get -10 out the center leg. Quick adjustable voltage divider.

Will get you a -10 reference untill you can get the right regulator

Not sure what the current draw would be there seeing it is not just reference voltage for an opamp
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Grumble



Joined: Nov 23, 2015
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PostPosted: Tue Nov 08, 2016 4:59 pm    Post subject: Reply with quote  Mark this post and the followings unread

Current draw depends solely on the position of the wiper of the Echelle/1v per octave potmeter since the - input of the opamp following this potmeter is at virtual gnd level.
A better solution would be a resistor of say 10k with a trimmer of 5k of which the runner goes into a buffer which is set to output -10volt.
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Cfish



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PostPosted: Tue Nov 08, 2016 5:03 pm    Post subject: Reply with quote  Mark this post and the followings unread

I have the top section of this on breadboard right now.

It is behaving well with the 100k trimmer divider.

may not be stable in some circumstances I have not thought of
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Cfish



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PostPosted: Tue Nov 08, 2016 5:15 pm    Post subject: Reply with quote  Mark this post and the followings unread

good point grumgle

I was just thinking quick and dirty

I have had to do some adjusting to the top of this circuit anyway. My keyboards are already soldered up with 100 ohm 1% resistors.

Is really Very similar circuit in the top portion as the one I have been posting about for a while. Resistor chain in the feedback loop.

Different method to adjust volt per octave, but a lot of similarities
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alanwilder81



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PostPosted: Tue Nov 08, 2016 11:11 pm    Post subject: Reply with quote  Mark this post and the followings unread

thanks Cfish. i dont want to come across as dumb, but what does reference voltage mean ?
so, you seem to have started building the keyboard circuit already. and you said some modifications took place. what exactly is that about ?

i am now ready to build it. recently got the comparators, the CD4016 and the 47 ohm resistors Smile
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alanwilder81



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PostPosted: Tue Nov 08, 2016 11:27 pm    Post subject: Reply with quote  Mark this post and the followings unread

sorry folks for another dumb question, but bear with me, i've never used the CD 4016 nor CD 4066. i attached the 4016 pinout diagram below.
In regard to the keyboard circuit,should i connect the pins 1,2, and 13 ?
as well as the pin 7 (ground) and pin 14 (positive)

thanks !


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Grumble



Joined: Nov 23, 2015
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PostPosted: Wed Nov 09, 2016 1:19 am    Post subject: Reply with quote  Mark this post and the followings unread

alanwilder81 wrote:
what does reference voltage mean ?

Using opamps and power them from say an LM7805 that's totally exceptable, even if the voltage coming from the LM7805 is (kind of) noisy, temperature dependent and not really accurate in terms of absolute voltage because an opamp has what is called a Power Supply Rejection Ratio what in short means that variations in the supply voltage wil cause a minimal effect in the (amplified) output voltage, this is usually expressed in dB.
But if you want to use this voltage as a reference voltage in e.g. a Digital to Analog Converter where the output voltage (or in the above posts where the accuracy of the 1V/oct circuit) behavior depends on the accuracy of this voltage you need a more constant, noise free and accurate voltage.
If an OUTPUT voltage of a circuit is REFERRED to a (constant) input voltage, this is called a Reference Voltage.

Compare the datasheet of a REF02 (5v reference circuit) and the datasheet of an LM7805 (5 volt power supply IC)
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Grumble



Joined: Nov 23, 2015
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PostPosted: Wed Nov 09, 2016 1:22 am    Post subject: Reply with quote  Mark this post and the followings unread

alanwilder81 wrote:
In regard to the keyboard circuit,should i connect the pins 1,2, and 13 ?
as well as the pin 7 (ground) and pin 14 (positive)

thanks !

This is correct! Be sure to keep the voltage to pins 1 and 2 within the power supply voltage (between pin 14 and pin 7) or else the switch will behave very nonlinear.
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alanwilder81



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PostPosted: Wed Nov 09, 2016 2:02 am    Post subject: Reply with quote  Mark this post and the followings unread

thanks grumble !

to be fair,i ve still got to build a power supply unit for my synthesizer.
it consist of the above keyboard controller (not yet build), plus a VCO,a VCF,a VCA,and an ADSR ( all successfully built, connected and tested).
it's recommended an amperage of 1.5 A to drive all those modules.
i guess voltage stability plays a key role in the keyboard circuit,right ?

so far, ive been using, as a power supply unit, the IDL-800 DIGITAL LAB which provides a mere 0.3 A 13 V, probably not enough to power all my modules together.
What would you suggest as a reliable, cheap and easy to build PSU ?
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Cfish



Joined: Feb 24, 2016
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PostPosted: Wed Nov 09, 2016 2:28 am    Post subject: Reply with quote  Mark this post and the followings unread

Hi alanwilder81.

Yes, you hook up the power and ground pins 7 and 14 on the switch IC.

As to your question about the reference voltage, Grumble can probably give a more accurate answer, but it is usually a constant voltage that any changes in a second voltage can be compared to.

As for the question about changes I was working on. Every other circuit like this that I have seen (accept for one used for a tube VCO) uses A 100 ohm resistor chain between each key. This calls for, I think it was 47 ohm resistor chain, on the left of the schematic. So I would half to set up anouther keyboard to use with this one. I already have keyboards set up with 100 ohm resistors, So it's way easier for me to try to adjust this circuit to my keyboards than to set up a whole keybed for it.

My big reason for experimenting with this circuit at the moment is to see if the main section for setting up initial CV from the keyboard, is less temperature sensitive than others I have tried. ( that is what the top left opamp is doing)

Because I have built so many of this type of circuit to see how they work. I started dividing them in to sectoons on small boards. Voltag controle section, sample and hold section. Gate and trigger section. So I can use one schematics sample and hold with another schematics voltage controle, or gate circuit. Sorta like a kid with toy blocks. I like this yousynth schematic really well because it's blocks already fit pretty well with the blocks I already have.

I'm actually glad I didn't find it sooner. I probably would have built it and been done. Instead I tried every simple circuit I could find and slowly started to see how the sections work to some small extant.

Last edited by Cfish on Wed Nov 09, 2016 2:37 am; edited 1 time in total
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Grumble



Joined: Nov 23, 2015
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PostPosted: Wed Nov 09, 2016 2:36 am    Post subject: Reply with quote  Mark this post and the followings unread

alanwilder81 wrote:
What would you suggest as a reliable, cheap and easy to build PSU ?

Hi Alan,
I'm sorry but I have no PSU suggestions for you, except to keep away from switching power supplies.

My synth is very different, I use 5 volt to power the whole thing because thyere are a lot of Arduino's in my build. In fact, I could use a usb power supply for it since it is using less than 500mA.

Grtzz,
Ed
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Cfish



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PostPosted: Wed Nov 09, 2016 2:38 am    Post subject: Reply with quote  Mark this post and the followings unread

LOL I type so slow that a hole discussion went on while I was typing
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