goyousalukis

Bi-Polar Question

17 posts in this topic

Hey guys, I am adding CV in and gate in to an old Moog MinitMoog. I have the gate sorted out nicely, and have moved on to the CV part.

I changed one of my AOUT_NG channels to Bi-polar, and I see on the AOUT_NG uCapps page that the range is -2.76 to +2.75. 

The issue I have is that the 37 keybed for the MinitMoog uses a voltage spread of 9V. The lowest note is -4.5v and the highest is 4.5v. So the difference between two notes is approximately 1/4v versus the 1/12v of a 1v/o typical CV.

What strategy would you take to scale the 5.5v  spread to a 9V spread? If I use an opAmp to scale it up, will I get fairly accurate notes, or will it be really out of tune?

I suppose another option would be to use an Arduino Nano to make a note table and convert regular CV to the right voltages.

Just curious if you guys have any ideas.

Thanks! 

Share this post


Link to post
Share on other sites

The range should be over 10V pp. The issue is that the summing circuit in the AOUT_NG is not optimal, thus you don't get the full range out.

See the following for an explanation and workarounds:

 

 

Share this post


Link to post
Share on other sites

Thanks, but I don't think that would fix the issue. Even if I got the 10Vpp, it would still be at 1 v/o right? I actually need 3 v/o, since each semitone is .25v apart.

Share this post


Link to post
Share on other sites

I see, I missed the minit part and assumed minimoog.

First, you need to get the range okay, then the rest is done in software. It is a non-standard tuning so some modification would be needed.

Share this post


Link to post
Share on other sites

I did some work tonight, and was successful with using an opamp to scale the voltage from -4.5 to 4.5. I was able to get an octave in tune using a potentiometer, but of course a scale did not play correctly.

When you say software, do you mean modifying the midibox code? I hadn't thought of that. I don't know if my coding skills are up to that.

Now that I am brainstorming, perhaps it would just be easier to use MIDI instead. There are lots of examples of Midi to CV for arduinos. I would just have to customize the output voltages to match the voltages that I measure at each key.

Time to do some more research. Thanks for your comments!

Edit: This would also solve my problem of what to do if the midibox sent voltages to high. I am worried if I accidentally send more than 4.5v to the Moog. 

 

Edited by goyousalukis

Share this post


Link to post
Share on other sites

Hello

I suspect you "only" have to define your specific table for AOUT modules, so that the full scale DAC output (rescaled from +/-4.5V) give a 3 octave range (if that's what I get for your usecase)

aout_hz_v_table.inc

http://svnmios.midibox.org/listing.php?repname=svn.mios32&path=%2Ftrunk%2Fmodules%2Faout%2F

 

OR you use the actual HW and SW and scale/offset (buffer/opamp gain) to the step you need, which will over scale (at extreme high and low midi note play) your moog CV input.

A schemo will say if it can handle without trouble the extra voltage (maybe just more octave to be played...) or if you need a voltage protection at CV input, you can also just power your AOUT modules with same bipolar voltage as the moog...

Best

Zam

 

Share this post


Link to post
Share on other sites

Thanks Zam, the table would be easy to modify, but then all 8 outputs would be modified. I only need one bipolar for the Moog. The rest of my eurorack is all 1 v/o.

I did try the opamp to increase the voltage by 3, but I couldn't get a scale  in tune.

Last night I hooked up an Arduino with a 12bit DAC, and got pretty good results. Now I need to add the Midi part.

Share this post


Link to post
Share on other sites

Here's a quick video of my first test...

 

Share this post


Link to post
Share on other sites
18 hours ago, goyousalukis said:

I did try the opamp to increase the voltage by 3, but I couldn't get a scale  in tune.

I don't get why it don't work ?

Did you follow the calibration procedure ?

I guess in your use case you need to measure which note is 0V when playing your KB (should be the one in the centre of the key...), then play this note from midi and adjust offset trim to read 0V at DMM, or a tuner at audio out to read the same note as KB play.

Then via midi send the lower note your KB is able to produce, and adjust the gain pot to match (at this point you should read close to -4.5V at DMM). Check with the higher note KB is able to produce. Check again with 0V note and if requested do the process again.

In the mean time I find the schemo, your moog is mainly +/-9V, I don't see why after calibration you can't send lower and higher note/CV to your oscillators (up to +/- 9V), if your AOUT_NG modules is supplied with higher bipolar (+/-12 or 15V), you can zener limit at your gain opamp FB.

By the way, important point, where are you inputting the external CV ?

Best

Zam

 

 

Share this post


Link to post
Share on other sites

Hi Zam, 

I have gone through the calibration procedure to some extent. The center key is at 0V. I'm sure someone more competent than me could get it done with opamps. I tried an opamp with a 1K and 2K resistor, which if I got the math correct will amplify the voltage by a factor of 3. So If I send a C# at .083v (1/12) from the midibox, it would amplify it to .25v, which is what the moog expects. When I did this, I did not get an in-tune scale. I then swapped the 2K resistor with a 10K pot, and trimmed it until I was getting perfect octaves from the Midibox. But then the scale between the octaves was not correct. 

Here's the schematic from the satellite, which is much clearer than the MinitMoog schematic that is available. They use the same main board. I am applying the CV in to pin 4 on the accessory jack. This feeds into the DC Summer via a 150k resistor like the keyboard circuit does.

If I get a chance, I will redo my opamp circuit and re-measure the voltages.

Thanks for your replies

Justin

schem2.png

Share this post


Link to post
Share on other sites

OK...

I think that you maybe do this the wrong way. (I just find the whole service manual for the satelite)

Let try this:

First, measure Q2 emitter voltage (junction Q2E/R48/R51) for lower key press, and higher key press, you'll have the KB voltage range that feed the summing amp...

I suspect you won't have a +/-4.5V range here... this will define the requested range at R13/A22 junction for the same behaviour at summing amp as summing resistors have same value (R13 and R48)

Only when we know that we can go further... (and maybe just change R13...)

 

I guess you won't have any correct result whatever CV system you use if we don't clear this.

 

Best
Zam

 

side note 1, as this "accessory socket" is offered with +/-9V(J1-1 and J1-3) for the external pot biasing or whatever you use to initially modulate at A22, I guess that's the limit you can feed to the summing amp (whatever the scale VS octave is)

side note 2, I don't find J2-3 (coming from J1-5) which might be for trig ???

 

 

Edited by Zam

Share this post


Link to post
Share on other sites

Update.

My math confirm your 3V/oct at J1-4

If you want to use 1V/oct CV you can change R13 from 150k to 50k, maybe it's the simplest option...

Best

Zam

Share this post


Link to post
Share on other sites

Thanks - I posted on MuffWiggler, and the fine gentleman suggested calculating the appropriate resistor value, but I didn't know how to do it. I will try your suggestions today, as soon as I get home!

I also searched for what the last two accessory jack points connect to, but I also couldn't find it. I will try to track them down. Then I could just make an outboard adapter without modifying anything on the original synth.

Thanks

Share this post


Link to post
Share on other sites

Find the connector end, redundant to the external filter jack !!!

For the math, I see that we have a 1 octave sw that send 9V to sum amp via 450K (442+20K trim, I suspect 450K nominal calibration setting) I deduce that 1 octave is 20uA for the summing amp... after that it's all ohm law again...

Best

Zam

Share this post


Link to post
Share on other sites

Side note: this entry point CV at summing amp is a OSC modulator, you won't trig note here ! nor modulating the filter via key

Share this post


Link to post
Share on other sites

Ah, yes, I see that J2 is the External Filter jack. So if nothing is connected to that jack, the accessory jack can be used to control the filter. I'll have to see if pin 6 is connected to anything. To me, it seems strange for Moog to provide a CV input with out some sort of gate. On my synth, the jack is actually covered by a plate with rivets, so I guess they decided to not pursue any accessories.

Thanks for listing your math and deductions. I will try to understand it. 

In your last comment, which CV entry point are you referring to? The J1 pin 4? I am not triggering there.

I am sending an 8V trigger through a diode and 2.2k resister to the junction between R195 and C41.

I first tried the 5v gate from the midibox. It triggered a note, but not at full volume. Once I scaled it to 8V, it works perfectly. That does give me the idea of adding velocity, or accents...

 

Share this post


Link to post
Share on other sites
55 minutes ago, goyousalukis said:

To me, it seems strange for Moog to provide a CV input with out some sort of gate

as I say, it's a OSC modulator, not a note CV, you have to think it as a "pitchbend"

maybe that's why you encounter strange behaviour, if KB trig and/or sample & hold circuit keep some voltage at IC9/Q2 output, then your ext CV at R13 is just adding/subtracting to sum amp what's coming from R48...

I suspect you should "reset" KB trig in the mean time you gate from external if you want to use J1-4 to "play" a note

Best

Zam

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!


Register a new account

Sign in

Already have an account? Sign in here.


Sign In Now