Minimoog on a 951 keyboard...

[noddyspuncture]

Hi again,

I have what might seem a strange query... because I have installed one of my Mini's into my Modular system.

Wanting to use the system live I thought it might make sense to have the Mini in there as backup if ever the oscillators misbehave or even if there is say, a fault... Modular power supply goes etc...!?

The Mini is independent and has it's own power supply... but I wanted to use the keyboard circuit - just the contacts... not the 951 electronics... and wire it directly via a multi-way change-over switch into the Mini.

Looking at the schematics of both the Mini and Modular - the keyboard contact circuits are virtually identical, so it should work..!?

So I made this modification and indeed, it works great... with just one small little annoying symptom... which is....

If I hold a note and then press another note ABOVE the held note... there is a slight pitch change (the held note goes slightly sharp) and this is more and more pronounced the further up the keyboard you go. Hold the lowest key and press any others and the symptom doesn't happen. You need to go to around half way up the keyboard before it becomes *noticable*.

The very worst case is if I hold the highest B and then press the very top C just above it. The pitch of the held B raises by about a quarter note. You can almost "trill" the held note by hitting a HIGHER note, which shouldn't happen, as the circuit is low not priority.

I hope I explained all this OK...?!

When the Mini is connected to it's OWN keyboard then this symptom doesn't happen... anyone have any ideas to the cause and as to how it could be solved please?

Cheers,
TOM

[CZ Rider]

I would recommend using a seperate power supply for the 951. Keith Emerson did this same mod to his 951. You could still use the multi connector with the +12/-6 disconnected, to bring in the pitch CV and S-trigger in to to lower panel. That would eliminate any problems with the 910 power supply and the Minimoog input.
I tried to put a new 951 keyboard with resistor string into the Mini keyboard input and it did not work very well. First, the Mini uses the lowest note as reference for 0 volts input as an F. While the 951 lowest key is a C. And the Mini keyboard circuit did not have enough current drive to play the notes at the top correctly. Last few notes went flat on mine, so it would need some modification to the Mini circuit. Plus switching in/out the two different power supplies between modular and Mini might not be the best solution. Safer to run the 951 seperate from both.
The easiest way was to use the Mini keyboard input as a 1/4" pitch input, that kept both the glide circuit intact and filter tracking. That way any CV input and trigger would drive the Mini. I even put in a three-way rotary so I could quickly change from a keyboard (CV or MIDI with converter), or sequencer, or Moog accessory. And it also needed the lefthand controller section wired to a panel. Used the pitchbend as the coarse tune so the 0 volts equaling F did not matter, as this had more than an octave range.

Worked for me!

[noddyspuncture]

Thanks for the info Terry.

So you tried hard wiring a 951 contact board onto a Mini too...?

Strange that it didn't work... as my attempt works really well and is in tune all the way up.... but for this slight problem with pitch variations with two notes pressed it would be perfect...!

By the way I have built the 951 it's own power supply, and when controlling the Mini via CV from that supply everything is fine... but I'd like the option of also keeping the Mini completely separate if at all possible.

There must be an answer... these are identical circuits - the 951 just has more notes...!

Cheers,
TOM

[drogoff]

Tom,

The keyboard voltage on the usual mono-synth, including the Mini, is directly affects by the power supply voltage so pretty much anything will throw it off. From this schematichttp://www.fantasyjackpalance.com/fjp/s ... -schem.gif, it looks like the 8.3mA just comes from the +10v supply. Anything yanking on that will affect pitch. Also, the bottom end of the keyboard resistor chain is ground. But ground isn't always ground - especially when you're starting to wire the Mini and the Modular together!

Should definitely isolate the power supplies and be really careful about where grounds come together for both audio ground noise and CV ground issues.

Isn't analog fun???

[noddyspuncture]

Thanks David,

And I know exactly what you're saying... but when I have this symptom the Modular doesn't come into the equation... basically I am just replacing the Mini's keyboard contacts with the Modular's - diverted, wire for wire.... an identical circuit apart from it being 5 octaves...!

When the two ARE wired together (via CV & trigger) there is no problem at all...!

Cheers,
TOM

[drogoff]

OK. But 5 octaves means 5 volts across the keyboard when it was designed for 3.5v. I don't have time to trace it all out but maybe something in the Mini doesn't like the higher voltage. You can emulate this but just using the Mini with its keyboard but inserting 170 ohms (the 17 key difference times 10 ohms - can just tweak/measure a potentiometer lying around) between the bottom end of the Mini's resistor chain and ground. That should make it act like the top 44 keys of the modular's keyboard. I'd guess you get the same problem.

[noddyspuncture]

Good idea David... I might try that... but the "higher voltage" you mention doesn't come into it either... as I am using the Mini's keyboard circuit... which is still the 10volts it supplies to it's own contacts but now it's supplying it to the contacts of the 951 keyboard ....!

[drogoff]

Good idea David... I might try that... but the "higher voltage" you mention doesn't come into it either... as I am using the Mini's keyboard circuit... which is still the 10volts it supplies to it's own contacts but now it's supplying it to the contacts of the 951 keyboard ....!

Yes, but the voltage at the top end of the keyboard is 1/12 volt * number of key = 3.666v for the normal Mini but 5v with the extra keys/resistors from the mod keyboard.

[Kevin Lightner]

I haven't spent a lot of time thinking about this, but here's where I'm at:

1. Perhaps the Mini doesn't have the current necessary for the extra keys or something's been overlooked (key resistor value, wire resistance, etc.)
Negative voltages necessary or present?

2. Why not make your own current source and driver in the 951 itself? It can put out its own calibrated CV and gate voltage.

[noddyspuncture]

David it isn't an actual "tuning issue" I have... the tuning is perfect, for the full 5 octaves, as long as each key is pressed individually, but when playing you usually hit/clip other notes (which usually shouldn't affect anything)... my problem is that hitting a higher note affects the pitch of a (lower) held note. There is an unwanted "interaction" coming from somewhere...

[drogoff]

That makes sense. The Mini and Modular keyboards are the same constant-current resistor chain as duo-phonic keyboards like the Arp Odyssey. If you tapped the voltage at the top of the resistor chain and subtracted it from it's normal voltage (3.6 mini, 5 mod), you'd have the 2nd CV. And, whenever it's not zero volts is the 2nd gate.

So, let's say you press the 2nd lowest C. That should be 1 volt. And, with no other keys pressed, you'll have the 8.3ma times 12 * 10 ohms -> 1 volt. Now, while holding that, you then press the highest C. Because the resistor chain is in the transistor's loop, it maintains (or tries to) the constant 8.3mA, so the CV is still 1 volt. However, you've now shorted out 47 resistors so that whole chain looks like a total of 120 ohms to ground instead of 430 or 600 ohms. So now the current source transistor (Q9 in the Mini schematic) has to output 1 volt instead of 3.6 volts. I'm still not sure exactly what the deal is, but something about the output of Q9 having to sit at 5 volts instead of 3.6 volts is causing problems when you press high and low keys at once and force it to a low voltage.

Again, I'd try the 170 ohm experiment to verify it's not something in the modular's physical keys. And then, if it acts the same, try sticking a separate 10-volt regulator to drive the keyboard circuit. I see on this schematic that the +10volt has voltage sensing which can always cause headaches since you're sensing at one place but using it in lots of other places so when the regulator works to keep it exactly 10 volts on the board where the sense line is it could be going up and down on other boards.
http://www.fantasyjackpalance.com/fjp/s ... -schem.gif

Kevin is the expert on this so I'll let him suggest a place to tap a higher voltage for a local regulator.

You can thank my eye-doctor for being late and having WiFi in the waiting room for this post

[EricK]

Tom,
I saw your video about a week ago where you demonstrated your Moog, please post some pictures of it. I think it was really unique to include a mini in a modular cabinet like that and it certainly looks nice also.

Eric


Here it is by the way:
http://www.youtube.com/watch?v=oJh7zK8nbzQ

[Kevin Lightner]

I'm not sure it's because of anything to do with the action and 2nd voice intentions or designs.
Some contacts rest on a common buss bar when not in use (like the Mini). Others may not.
I don't know. I'm flying blind here like most of you. I've never hooked a 951 action to a Mini either.

What I do know is that a Mini expects its lowest note to be zero volts and an F.
The lowest note on a 951 is a C.
So in order to get the voltage up to the highest C on a 951 might take some modding.
The Minimoog wasn't designed to go that high and voltages won't go negative once it encounters that lower F on the 951.
So I still think it's a current source thing, but might also be just how the 951 action is wired up.

[noddyspuncture]

Thanks for all the responses guys... much appreciated...

By the way Kevin - the "low note being an F" issue is really the least of my problems. I found a real easy get around... I just added a small trimmer in series with the orange wire to the wiper of the pitch bend pot. That way I can tune that pesky F to a C very easily.

Also, the only difference (after much original research into if this could work) in the two keyboard units is that the 951 doesn't have the trigger's "a rest bus-bar" - so consequently it has only 5 wires running from the assembly. The Mini has 6, because it does have the trigger's "at rest" bus-bar... which on the schematic is marked as "unused" anyway, and even though it does have a wire running from it, on the circuit board it is directly connected to all the trigger springs (trigger bus).

But as this extra bus-bar is for the trigger side of things I can't see it being relevant to the slight pitch fluctuation issue...!?

Cheers,
TOM

[noddyspuncture]

Having played with this problem for most of the day I have come to terms with the probability that it's a symptom I am stuck with... not the end of the world, as I was only going to use the Mini wired directly to the 951 keyboard contacts as an emergency measure - like a panic button. So what I think might be an idea is to incorporate into my changeover switch, an extra function which grounds the resistor chain at the 'F' 3 1/2 octaves down from the top of the keyboard. In effect giving me the same keyboard length as the Mini has and then the bottom octave and a half of the 951 will become redundant in this mode. In CV/Trigger mode I have the pleasure of both Mini and Modular perfectly in syncronization...

I am going to try that next.... if it works I shall be happy....;c)

Cheers,
TOM