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 Forum index » DIY Hardware and Software » MusicFromOuterSpace.com designs by Ray Wilson
making a 1/V per octave keyboard from a polyphonic one
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dylar



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PostPosted: Sat Apr 14, 2012 4:06 pm    Post subject: making a 1/V per octave keyboard from a polyphonic one Reply with quote  Mark this post and the followings unread

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There is isn't really a clear explanation of how to take a cheap polyphonic keyboard and turn it into a 1/V per octave controller on Ray's site. Here's how I did it.

This is a very easy process. The circuit I used was Ray's mini-controller. I just cut off the resistors I had on there. His other keyboard controller is the same circuit, but without the little keys on it.

1. Choose an older keyboard. The newer ones tend to have more complicated connections, which might require more precision cuts / jumpers. If you find a keyboard like the one in the image that A) has a diode for each key, B) Has a single connection at the bottom of each pad that can be run to a common bus, then you're probably good.

2. Connect all of the bus points together. You will see at the top of the image a set of 6 pads are already connected by a common bus (the trace that is a straight line). There will be a bunch of these groupings on your keyboard, and some keys might be set off by themselves. Connect ALL of them together. The easiest way to do this is to desolder the places where these bus connections end up, and then connect them all together with jumpers. Another way would be to file off the protective coating and solder jumpers directly to the copper traces--but this is a bit more risky.

3. Remove all diodes. Insert 100 ohm resistors. This is a bit tricky, because the spaces are usually pretty tiny--you have to watch for solder bridges. Only the side of the resistor that leads to the pad (the top end of the resistor connections in the image) needs to be soldered to the board. On the other side, you just need to make sure that the resistor ends are connected, although you can solder to the board to make things easier (we'll cut the connections later). Remember that the resistors are connected to one another bottom-to-top. Each resistor is adjoined to its neighbors, so that with every key you press on the keyboard one more 100 ohm resistor acts on the voltage running across the circuit.

4. Figure out which traces to cut. These will be ALL the connections coming off of the bottom side of the resistors (the side not connected to the pad). I used a dremel--you have to make sure the traces are completely cut.

5. Hook up the KBTP, KBUS and Ground as shown in Ray's circuit diagram. Basically, KBUS goes to the common bus, KBTP goes to the highest note, and the ground connects to the resistor on the lowest note.

Tips:
Make sure your solder points are not much larger than the originals. If there's too much space between the key and the pad, then they might not connect.

The rubber plunger strips are pretty easy to rip. Be careful with them. If they are attached to the circuit board when you open the keyboard, take them off before you start working.

Pick a keyboard that has a nice action in the keys.

While you can house the controller circuit inside the keyboard, I already had mine on my synth. Instead of removing it I just used banana jacks to connect the KBUS, KBTP and ground between the keyboard and the controller circuit. There is really no advantage to this--it's just how things worked out for me. If you do put the circuit inside the keyboard, obviously plan for space issues ahead of time and install your connectors (CV outs, Gate, etc.) before modifying the keyboard
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Cynosure
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PostPosted: Sun Apr 15, 2012 5:01 am    Post subject: Reply with quote  Mark this post and the followings unread

This is perfect. Thanks for posting it.
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Liquids



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PostPosted: Wed May 02, 2012 8:52 pm    Post subject: Reply with quote  Mark this post and the followings unread

Looks like the keyboard you are showing uses rubber/carbon/bubble switches like most cheap and or modern keyboards...

did this work for you? I have such a keyboard in hand, but with time invested in matching resistors, it seems a bit sad for each switch to have it's own resistance, which could affect CV & pitch. Have you or anyone else compared this to a 'true' organ style single buss keyboard?
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PostPosted: Thu May 03, 2012 3:52 am    Post subject: Reply with quote  Mark this post and the followings unread

Liquids wrote:
...it seems a bit sad for each switch to have it's own resistance, which could affect CV & pitch.

This is why these keyboards are more suited for digital scanning, where switch resistance is largely unimportant.

It's also why I inwardly groan whenever I see a post asking about the single buss resistor string circuit.

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Liquids



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PostPosted: Thu May 03, 2012 7:12 am    Post subject: Reply with quote  Mark this post and the followings unread

Trust me, I'm groaning here too. I tend to agree that I should give up on trying to make one of these keyboards into a CV keyboard, but Ray suggestest that 'some switch resistance shouldn't be a problem" in other threads. I can't reconcile my trying to get a string of resistors within .1% tolerance, but then using rubber contact switches for each key which have resistance of their own that also seems to vary over time.

If I had any concept or patience on how to make these sort of keyboards work with reed switches, and the money to do so, I'd even consider that...but I've not found a satisfying tutorial. Ray's site has some info, but the reed switch mechanisms and such just look ugly and or fragile.

Ironically, I started by looking for a such a keyboard for the picsynth project for a fun toy.

My buddy is a repairman who hoards musical junk. He has given me two different old organ keyboard (schober, crumar). I gave the first one back since it's PCB rig would be too much trouble to cut up the busses etc to turn into a diode matrix, the crumar is a single buss/octave...so I decided to take it and go true CV! Bought Ray's PCB and a bunch of parts to go 'true' CV keyboard.

The Crumar keyboard is a finicky unreliably piece of bad engineering. everything is loose and flimsy, at this point the PCB - which would be great for a resistor string - holds all the springs, but keeping it 'just so' is a nightmare since they used 'fitted' plastic with very poor tolerance from key to key. The single bus is 'held' by 6-8 separate spans of grooved plastic (grooved for the buss bar) that screw into the controller, don't fit together tightly at all have a lot of play, and are supposed to hold/secure the pcb!

I can't currently get some of the springs for each key - which are soldered to the poorly secured PCB - NOT to contact the buss when they're not being pressed! The poorly secured pcb has array of springs soldered into it, and the springs run through the less than precicely fitted plastic pieces, under the buss bar held across the same plastic pieces. The springs end by running through/into a hole in another plastic part that is semi-attached to each key. These are also poor tolerance so at resting point some springs are closer to the bus than others (already all over the place due to the plastic that 'secures' it). When a key is pressed, it causes the key's spring to hit the buss. Only, sometimes, if just properly propped, in certain weather, but not on all keys, on a good day. You can't believe the time and frustration (or maybe you can).

Simultaneously, someone just followed through with a freecylce request for keyboards from back when I wanted one for just a simple picsynth. I picked the keyboards up last night, and both are diode matrix out of the box, probably perfect for the picsynth (but 5 octaves not4).

So, I have one keyboard that is frustratingly inconsistent and poorly designed (at least after 30+ years of plastic degradation), but could be perfect for this, if I 'just' completely re-designed it's layout with tight tolerance machining and/or woodcutting skills. And now TWO matrix keyboards that would work quite well for the picsynth project, but I now want at least one CV keyboard as I've invested in parts and PCBs for it, even if I do also build a picysynth-based synth.

ARG.

The old schober keyboard I first had would probably work easily as a single but CV keyboard. I asked my buddy if he'd let me buy it back for cheap....that might be my quickest, best solution. Then later I can use the rubber bubble keyboards for a picsynth, for a different synth or to give as a 'gift' to someone.

ARG.

In the meantime I guess I'll just pretend one of the diode matrix keyboards is 'fine' and non-permanently connect it to the single bus keyboard PCB...solder up the resistors I've matched, knowing it will potentially be problematic, and go from there, at least as a starting point for a CV controller. Sad
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Liquids



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PostPosted: Thu May 03, 2012 11:57 am    Post subject: Reply with quote  Mark this post and the followings unread

Does it make any sense to use a larger value (like 1k) for the resistor string to 'reduce' the added resistance yielded by the switches as compared to the overall resistance, at least for now? If so, should I change the values of the 'constant current source' portion of ray's schematic?

Along with that, all the keyboards I'm working with are 61 key...I posted this elsewhere but got no response...do I need to make any tweaks to the values in the current source circuit that drives the keyboard, given that factor?
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PostPosted: Thu May 03, 2012 12:19 pm    Post subject: Reply with quote  Mark this post and the followings unread

Liquids wrote:
Does it make any sense to use a larger value (like 1k) for the resistor string to 'reduce' the added resistance yielded by the switches as compared to the overall resistance, at least for now?


For the usual current driven resistor chain it would not help to make the resistors larger.

What would help is to have the output go to a high impedance point (like an opamp) - which it probably already does - hence the remark, that I read earlier on on this thread, that some resistance in the keys is not really a problem.

The idea is that the resistor chain is relatively low impedance so the taps look like a voltage source, then you buffer the voltage with a high impedance voltage follower, and to that high impedance a couple of hundreds of ohms of contact resistance can be marginal.

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also .. could someone please turn down the thermostat a bit.
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PostPosted: Thu May 03, 2012 12:36 pm    Post subject: Reply with quote  Mark this post and the followings unread

Blue Hell wrote:
Liquids wrote:
Does it make any sense to use a larger value (like 1k) for the resistor string to 'reduce' the added resistance yielded by the switches as compared to the overall resistance, at least for now?


For the usual current driven resistor chain it would not help to make the resistors larger.

What would help is to have the output go to a high impedance point (like an opamp) - which it probably already does - hence the remark, that I read earlier on on this thread, that some resistance in the keys is not really a problem.

The idea is that the resistor chain is relatively low impedance so the taps look like a voltage source, then you buffer the voltage with a high impedance voltage follower, and to that high impedance a couple of hundreds of ohms of contact resistance can be marginal.


I appreciate the comment. This confuses me, though. IF the ideal is to use a string of resistors with as little variance (closest tolerance) as possible, I don't understand how adding switches with resistance of their own, reistance that varys from one switch to another would, as far as I can see, not have an affect on the 'voltage divider' network and hence pitch.

I don't understand current sources....but in my mind, I'd think that if I increase the overall resistance of the string with larger value 1% resistors, it minimizes the significance of the resistance of the siwtches, and reduce the significance of the potentially wide variation of resistance from switch to switch (50R-200R, I'd guess), and then I'd have less issues with consistent pitch (At the keyboard controller level).

If not, than why not just use 5%, 10% tolerance resistors?
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PostPosted: Thu May 03, 2012 12:48 pm    Post subject: Reply with quote  Mark this post and the followings unread

Maybe I understood it wrong - can you refer me to the actual schematic?
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also .. could someone please turn down the thermostat a bit.
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PostPosted: Thu May 03, 2012 1:03 pm    Post subject: Reply with quote  Mark this post and the followings unread

Blue Hell wrote:
Maybe I understood it wrong - can you refer me to the actual schematic?


Absolutely. Very Happy

Article: http://www.musicfromouterspace.com/analogsynth_new/SINGLEBUSSKEYBOARD2007/SINGLEBUSSKEYBOARD2007.php

schematic:

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PostPosted: Thu May 03, 2012 2:01 pm    Post subject: Reply with quote  Mark this post and the followings unread

thanks :-)

Yes, that's the "standard" thing I meant above. The contacts are not in series with the resistor chain, but they provide a tap on it, so their value will not change the chain - and as long as the tap is taken of on a high enough impedance the contact resistance does not matter much.

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PostPosted: Thu May 03, 2012 2:03 pm    Post subject: Reply with quote  Mark this post and the followings unread

And it is loaded here with approximately 10 MOhm ... so the contact resistance needs be small relative to that.
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PostPosted: Fri May 04, 2012 5:08 am    Post subject: Reply with quote  Mark this post and the followings unread

So it is ok to make that circuit with a rubber contact keyboard? I'm not looking for perfection, just close enough that the variance in pitch isn't very noticeable.

I have a Yamaha YK-10 that has been sitting in the corner of my studio waiting until I get time to do something with it. If it can't work as a CV controller, then maybe I will be making another top octave synth.

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PostPosted: Fri May 04, 2012 5:17 am    Post subject: Reply with quote  Mark this post and the followings unread

Cynosure wrote:
So it is ok to make that circuit with a rubber contact keyboard?


It depends on the actual contact resistance and the input resistance of the buffer amp used - for the schematic above it must be small compared to 10 MOhm.

10 kOhm would give an error of 1 in 1000 or 0.1% and I'd expect the contact resistance to be way lower than that, but to be sure : measure it Wink

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PostPosted: Fri May 04, 2012 8:29 am    Post subject: Reply with quote  Mark this post and the followings unread

Blue Hell wrote:
thanks Smile

Yes, that's the "standard" thing I meant above. The contacts are not in series with the resistor chain, but they provide a tap on it, so their value will not change the chain - and as long as the tap is taken of on a high enough impedance the contact resistance does not matter much.


Thanks. I have been thinking/picturing it as that the switch resistance would become part of the voltage divider...but really, it just adds resistance between the voltage divider in series with the bus. yes!!

Blue Hell wrote:

Cynosure wrote:

So it is ok to make that circuit with a rubber contact keyboard?


It depends on the actual contact resistance and the input resistance of the buffer amp used - for the schematic above it must be small compared to 10 MOhm.

10 kOhm would give an error of 1 in 1000 or 0.1% and I'd expect the contact resistance to be way lower than that, but to be sure : measure it


If it helps - I agree that one should measure it for each particular case. However, on the 2 keyboards I've been screwing with that use the rubber/carbon contact switches, I get ~50-220 ohms per switch on both keyboards generally speaking.

If I recall correctly, the resistance dips upon press/measurement - not sure if that is the DMM's lag time, or if the actual switch resistance drops over time when held.

I need to figure it out by holding one of the bubble keys down for a while, and only then connecting the DMM, to see if it does the same kind of apparent resistance drop at the same speed upon connection, or if it instead 'reads' the lower value of resistance across the switch more immediately by comparison. Anyhow, the switch resistances should be smallish in most cases.


I have a few more questions.

Ray says this design has 'low note priority.' But for those who have used this, if you press two notes, do you get the 'real' low note,' or detuning of the note when pressing 2 or more keys?

Also, I posted this elsewhere but got no reply...

On the webpage, Ray first mentions a 37 key keyboard, and then at the end mentions calibration:

"Calibration is easy. This procedure assumes that you have the circuit connected to the keyboard resistor chain, one end going to circuit board point KBTP (keyboard top) and the other end going to ground. We want to have .8333 mA flowing through the keyboard resistor chain which means that .8333 mA will be flowing through R5 (475 ohm 1% metal film resistor). When .8333 mA flows through a 475 ohm resistor is causes a (.0008333 Amp x 475 ohms) 0.3958175 volt drop across R5. So with the highest impedance most accurate DVM you have adjust R3 until you have 0.3958 volts across R5 and voila you will have .8333 mA flowing through the resistor chain which in turn will give you 1 volt per octave from your keyboard controller."

I'm using 61 key controllers. On one hand, I assume that if the amount of octaves mattered or would require more than the 1k trimpot's range of adjustment, it'd be designed this way....However, I want to make absolutely sure.

Q: outside of the +/-15v or +/-12v variation and appropriate changes noted on the page and schematic given that variable, will all values and calibration function and procedure be the same as specified, utilizing a 5 octave (61 note) keyboard controller? Will the value of R2 R3 or R5 definitely need changing?

I should mention I am simming this in LTspice at this point for clearer info, measurements, and answers, as I haven't finished the PCB or keyboard at all yet.
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PostPosted: Fri May 04, 2012 8:57 am    Post subject: Reply with quote  Mark this post and the followings unread

Liquids wrote:
Ray says this design has 'low note priority.' But for those who have used this, if you press two notes, do you get the 'real' low note,' or detuning of the note when pressing 2 or more keys?


The idea is that a constant current flows through the resistor ladder - when you press multiple keys you'll still have that constant current flowing through the lowest note's resistors, and so you will get the voltage of the lowest note pressed. Now in practice a constant current source is nver completely load independent (and you do change the load by pressing multiple keys, shorting out some resistors), so in practice you will get a tad of a change. How well this is handled for the actual circuit is hard to tell for me, but it does have some stabilizing feedback so I expect it to be no problem.


Quote:
I'm using 61 key controllers. On one hand, I assume that if the amount of octaves mattered or would require more than the 1k trimpot's range of adjustment, it'd be designed this way....However, I want to make absolutely sure.


Calibration would not change for that in principle, however there is one issue you can run into. As the current through the resistor chain is assumed to be constant the voltage over it wil go up with 1/12 volt for each key added. At some stage you will run out of the dynamic range of the circuit - where exactly that will be is another thing thats hard to tell.

A rough estimate : R2 + r3 is about 5k worst case, times .8333 mA is about 4 Volt. The transistor will need one or two volts to operate properly, so assuming a 12 Volt supply there is like 6 Volt left for the resistor chain. That in turn means there can be a total resistance there of about 7.2 kOhm, subtracting the fixed one of about 500 Ohm that leaves 6.7 kOhm, which divided by 100 yields 67 for the maximum number of keys. This is a bit rough, so 61 keys may be a bit much .. with 15 V supply you'd have more headroom.

Quote:
Q: outside of the +/-15v or +/-12v variation and appropriate changes noted on the page and schematic given that variable, will all values and calibration function and procedure be the same as specified, utilizing a 5 octave (61 note) keyboard controller? Will the value of R2 R3 or R5 definitely need changing?


Hope my previous section shed some light on that too.

Quote:
I should mention I am simming this in LTspice at this point for clearer info, measurements, and answers, as I haven't finished the PCB or keyboard at all yet.


Good idea Smile

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also .. could someone please turn down the thermostat a bit.
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PostPosted: Fri May 04, 2012 11:42 am    Post subject: Reply with quote  Mark this post and the followings unread

Is the value of R5 critical?

It'd be a lot easier to calibrate by measuring across R5 if it were 1k or 100R, etc.

With a 6k string (61 keys) and 1k for R5, I'd still be under 7k total....

I'd think that going down to 100R for R5 would affect the low notes as the constant current source might start to waver under that load...?

As I simulate in LTspice and change what would be trimmer R3 to get the correct voltage drop across R5, It seems like my 'high note' voltage is below 5V. Is that a problem? I can tweak it to be above 5v especially if I use +/-15v. Or is it just the scale (difference in voltage from note to note) which matters more than the effective 'highest' voltage the buss sees?

If not, is it wise to make R4 or R6 variable via resistor+trimmer in order to control the voltage at U1 non-inverting input more precisely so as to make the 'high' note as close to 5v as possible?
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PostPosted: Mon May 07, 2012 10:08 am    Post subject: Reply with quote  Mark this post and the followings unread

Liquids wrote:
Is the value of R5 critical?

It'd be a lot easier to calibrate by measuring across R5 if it were 1k or 100R, etc.


Not too critical, but why not just measure over 12 100 Ohm resistors to get 1.0000 V ... I mean that's what you want to happen in the end, everything else is indirect.

Or alternatively what richardc64 said in the other thread (press a key and then one an octave apart and see that you get a 1 Volt difference on the output).

And the absolute voltage on the output is not really very intersting.

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PostPosted: Thu May 10, 2012 11:01 pm    Post subject: Reply with quote  Mark this post and the followings unread

Haven't been back in a while...
For those wondering, yes I used a rubber pad keyboard. It is a Casio CT-360.
Also, it works great. I get very consistent voltage drops across 4 octaves. It's not absolutely perfect--between the lowest and highest octaves there's a tiny shift, but it's not enough so that the synth sounds out of tune at all. I'm not exactly a perfectionist though, so others may have different opinions. Over more octaves you might have more trouble.
I'm actually not even really sure what the issue is that folks have been talking about. I'm relatively new to electronics and just came up with this on my own. I do fail to see how the rubber pads would add more resistance than the banana plug probe that the mini-controller uses though.
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