Motorized Pots on an MB-808?

[Roark]

I'm planning out several builds right now, one of which is an MB-808.

I, personally, do a fair amount of automation with VST 808s, so I was wondering how would I go about using motopots for, at least, the BD Tune and Decay, so that I can automate them over MIDI, yet still be able to tweak and mess around.

Is this possible, or just another case of I want way too much out of something?

The hardest part is actually finding motopots. Any ideas? On anything?

Thanks!

[John E. Finster]

Hi,

 

Motorpots are not that easy to find, that is true, but they´re there, they can be found on eBay. The problem is, that they move very slowly compared to motorfaders. The fastest I ever found need 12 seconds (!) for a 300° turn, that´s from one end to the other.

I too considered them once for my midibox, because I wanted to have the "analog" feeling but didn´t want to use motorfaders, because they need a lot of space. But 12 seconds for a full turn seems way too long to be a practical, especially while jamming.

 

my regards

Edited July 24, 2013 by John E. Finster

[Roark]

Hi,

 

Motorpots are not that easy to find, that is true, but they´re there, they can be found on eBay. The problem is, that they move very slowly compared to motorfaders. The fastest I ever found need 12 seconds (!) for a 300° turn, that´s from one end to the other.

I too considered them once for my midibox, because I wanted to have the "analog" feeling but didn´t want to use motorfaders, because they need a lot of space. But 12 seconds for a full turn seem way too long to be practical while jamming.

 

my regards

12 seconds? WOW O_o

Given times like that, I'd much rather go for faders. It would look off, but if it works, it works.

Just got to find some and work out the implementation part. Can the MF_NG board be connected internally, via DOUTs on the Core?

[John E. Finster]

Can the MF_NG board be connected internally

 

no, it´s connected via midi. It works more like a standalone unit, because it has its own PIC core and midi in/out connections. That makes it very flexibel. The LPC17 Core has two native midi in/out connections and 2 more can be added with a small extension board, so 4 midi ins/outs in total.

Atm, you can connect 4 MF modules at 1 LPC core. The income midi messages from the MF modules can be translated into whatever midi event you need.

[Roark]

no, it´s connected via midi. It works more like a standalone unit, because it has its own PIC core and midi in/out connections. That makes it very flexibel. The LPC17 Core has two native midi in/out connections and 2 more can be added with a small extension board, so 4 midi ins/outs in total.

Atm, you can connect 4 MF modules at 1 LPC core. The income midi messages from the MF modules can be translated into whatever midi event you need.

 

Fantastic, thank you!

[Nasrudin]

How about a different approach?

 

going into the circuit of the 808 soundmodules, you could probably replace the pots with different op amp circuits. this way, you can connect the controlvoltage input of your amp circuit to an Aout_NG modul. this solution requires a bit more effort, but the result would be way more satisfying.

 

of course you could do the automation the oldschool way: record the repeating sequence while turning the nobs. get a friend to help if you need more than two hands (actually, this was the common way of mixing an album in big studios before faderautomation was invented).

Edited July 25, 2013 by Nasrudin

[Roark]

How about a different approach?

 

going into the circuit of the 808 soundmodules, you could probably replace the pots with different op amp circuits. this way, you can connect the controlvoltage input of your amp circuit to an Aout_NG modul. this solution requires a bit more effort, but the result would be way more satisfying.

 

of course you could do the automation the oldschool way: record the repeating sequence while turning the nobs. get a friend to help if you need more than two hands (actually, this was the common way of mixing an album in big studios before faderautomation was invented).

 

That....seems like an awesome idea...

I think I'll build up an as-is MB-808, to get a little more familiar with the circuitry, then afterwards, maybe build a rack-mount, 100% MIDI controlled version.

 

I'll have to look into using opamps for this, though. I'm not too familiar with that. The only VCRs I know about are FET and LED-PV styles, which seem a little "cheap" for this.

[freddy]

For another design (MBMixer, though never finished, alas) i went the way of using digital pots fron Analog Devices. They're i2c/spi controlled, might be simpler HW- and implementation wise. Just my two cents...

[Roark]

For another design (MBMixer, though never finished, alas) i went the way of using digital pots fron Analog Devices. They're i2c/spi controlled, might be simpler HW- and implementation wise. Just my two cents...

I've been looking on Mouser and Digikey for some and I think this might be an option, if not THE option.

I've just got to get my SMD soldering skills up.

The biggest problem I'm having with all of these options, though, is finding a viable supliment for the 500kΩ BD Decay pot.

Best option I've come up with is two 250kΩ digipots in series, then program them to cover top and bottom ends of the scale.

[Altitude]

Not to pour cold water on you or anything, but what you are proposing involves the complete rework of the MB808.  None of the controls on the MB808 are digital beyond the tempo knob, they are all direct analog controls.  To add digipots, you would need to relay the board and a complete re-write of the firmware and to port it to the new core since you would be looking at hybrid SEQ/MF device (which no one has done and would probably require a TK'ish understanding of MIOS to do successfully).

 

To put things in perspective, the original MB808 took Jeff about 2 years to design, then a team of about 30 ppl about 6 months to work out the bugs in the prototypes and software optimization.  There were 3 revs of boards.  The kicker to all that is that ALL the info and documentation from the 808 forums was deleted so you would be starting from scratch from the orignal 808 schematics.  Also keep in mind that Doug has yet to deliver a single board to anyone despite it has now been almost 4 years since he started taking orders and if you happen to have one of the original 3 rev boards, all the documentation for those is history

If you absolutely must have external control of certain parameters (which I dont really see the usefulness of), I agree with Nasrudin, tap it with a CV

[Roark]

Not to pour cold water on you or anything, but what you are proposing involves the complete rework of the MB808.  None of the controls on the MB808 are digital beyond the tempo knob, they are all direct analog controls.  To add digipots, you would need to relay the board and a complete re-write of the firmware and to port it to the new core since you would be looking at hybrid SEQ/MF device (which no one has done and would probably require a TK'ish understanding of MIOS to do successfully).

 

To put things in perspective, the original MB808 took Jeff about 2 years to design, then a team of about 30 ppl about 6 months to work out the bugs in the prototypes and software optimization.  There were 3 revs of boards.  The kicker to all that is that ALL the info and documentation from the 808 forums was deleted so you would be starting from scratch from the orignal 808 schematics.  Also keep in mind that Doug has yet to deliver a single board to anyone despite it has now been almost 4 years since he started taking orders and if you happen to have one of the original 3 rev boards, all the documentation for those is history

If you absolutely must have external control of certain parameters (which I dont really see the usefulness of), I agree with Nasrudin, tap it with a CV

 

I don't mind a challenge. ;)

I'm going to start out small with just one instrument to get a proof of concept before going full-blown. Bare basics.

I'm pretty decent with programming and tend to learn what to do as I do it (learned to program for the Source engine this way; dive right in).

I'm going to put everything out on protoboard, so I won't need anything from Doug (that I know of).

 

Besides, my work schedule is about to become extremely light, so I need something to do to fill in the gaps. ;)

[freddy]

Thanks for the shower, too ;) but I was thinking basically about a different MB device (assuming the digital controls are more or less already MIDI controllable already). My prototype for MBMixer was a simple Core 8 accepting the CCs plus a bunch of DOUTs to controll the AD digipots. The digipots will simply replace current pots and faders. It kinda makes sense to keep it separate because one part is the SEQ to control the MB808 and another thing (i. e. device) is to control the voice tunes. You can always link it internally using merger and splitter to "look" like a single device. Of course, I didn't consider merging the existing FW to handle both parts which would imply requirement of an additional SCS for the tunes, if needed/wanted.

[Sauraen]

I don't mind a challenge. ;)

I'm going to start out small with just one instrument to get a proof of concept before going full-blown. Bare basics.

I'm pretty decent with programming and tend to learn what to do as I do it (learned to program for the Source engine this way; dive right in).

I'm going to put everything out on protoboard, so I won't need anything from Doug (that I know of).

 

Besides, my work schedule is about to become extremely light, so I need something to do to fill in the gaps. ;)

 

I've wanted to do something like this too, partly to increase my understanding of analog electronics. This is what I would recommend:

• Pick a voice with a distinctive sound (e.g. snare drum), and build just that voice on a breadboard from the schematics. If you have a nice fully-stocked electronics workshop, this probably would only take an hour or two. Hook up the trigger to a microswitch and the audio output to a mixer. See if it works. Twiddle the knobs and enjoy.
• Buy and build an LPC17 core board, and buy a few two-channel-output SPI-input voltage DACs. Hook them up to SPI on the LPC17 and write a small driver program that lets you control their voltage output with CCs or something.
• Once you can reliably control the voltage output of the DACs, find a circuit on the Internet that performs the same function as each portion of the 808 voice that includes a potentiometer, except voltage controlled. For instance, if one of the segments of the snare drum circuit is a low-pass filter with adjustable cutoff, find a simple VCF design online, match the parameters, and hook up the voltage input to one of the outputs of your DAC.
• Replace that segment of the original 808 circuit with the voltage-controlled one.
• Repeat for all parameters of the voice, or even parameters that are fixed in the original design for more control.
• Presto! You just made yourself a digitally-controlled analog drum synth!

[Roark]

I've wanted to do something like this too, partly to increase my understanding of analog electronics. This is what I would recommend:

• Pick a voice with a distinctive sound (e.g. snare drum), and build just that voice on a breadboard from the schematics. If you have a nice fully-stocked electronics workshop, this probably would only take an hour or two. Hook up the trigger to a microswitch and the audio output to a mixer. See if it works. Twiddle the knobs and enjoy.
• Buy and build an LPC17 core board, and buy a few two-channel-output SPI-input voltage DACs. Hook them up to SPI on the LPC17 and write a small driver program that lets you control their voltage output with CCs or something.
• Once you can reliably control the voltage output of the DACs, find a circuit on the Internet that performs the same function as each portion of the 808 voice that includes a potentiometer, except voltage controlled. For instance, if one of the segments of the snare drum circuit is a low-pass filter with adjustable cutoff, find a simple VCF design online, match the parameters, and hook up the voltage input to one of the outputs of your DAC.
• Replace that segment of the original 808 circuit with the voltage-controlled one.
• Repeat for all parameters of the voice, or even parameters that are fixed in the original design for more control.
• Presto! You just made yourself a digitally-controlled analog drum synth!

 

That seems more overly complicated than what I want to do. :/

Why would you design a VCF to work within a VCF and add more boards to the core to control it (AOUTs) when you could use a simple digi-pot circuit and existing DOUTs?

Not to mention that would require a decent amount of hardware tuning, whereas the digi-pots can be adjusted by editing lines in software.

 

I'm not trying to shoot against it completely, it's just something I wouldn't want to attempt. But please, if you can do it, I'd love to see it!

[Roark]

Since a major rewrite (if not from scratch) of the FW will have to be done, considering all the signal routing and core work, would it be better to base it all on an LPC17 core?

I'm doing a more and more research on circuit building for digipots, and I think I'm getting close to ordering parts and trying a few designs out.
I'm only going to do the kick voice for now, to make sure I'm on the right track, then go from there.
Planning on this being a hideous Veroboard build. :D

More info to follow.

Edited August 1, 2013 by Roark

[Altitude]

Yeah, LPC core for sure.  I have used the 7bit microchip digipots and 10 bit analog devices ones for my day job.  The microchip ones have a bit better i2C implementation and are available in a bunch of different pinout configurations (pot to ground, rheosat, etc), the AD ones were simple up/down serial jobs in a painfully tiny micro QFN package.  We use them for setting gain settings and I dont think they are something that would be of use in something like this since the settle time between taps is 1-2 seconds. Further more, there is no way to read the resistance of the part, they way that it is done is that it uC does a reset and then increments the taps to roughly the right spot resistance wise.  I did a bunch of tests to measure the accuracy of this and its in the 5% range.  I think a better approach would be to do something like how old school analog synths saved and recalled patches

[Sauraen]

That seems more overly complicated than what I want to do. :/

Why would you design a VCF to work within a VCF and add more boards to the core to control it (AOUTs) when you could use a simple digi-pot circuit and existing DOUTs?

Not to mention that would require a decent amount of hardware tuning, whereas the digi-pots can be adjusted by editing lines in software.

I didn't know such a thing as a "digipot" existed. Do they work? Are they reliable? If so, then sure; you might even be able to use existing MB808 audio boards, with a veroboard layer either above or below them and pins through to the potentiometer solder connections. Also, video please once you make the kick!

[Roark]

Alright, so I've got something together. I'm not sure if it's right, and this is the first time I've designed something like this, so I'll need some input.

 

I've made this to be a drop in replacement for the physical pot.

 

I've only done the BD Decay pot for now, just to make sure I'm headed in the right direction. The two pots are 250k, so they're wired in series.
I'm still not sure how to figure out the feedback resistance, so that's not filled it yet.
I chose a quad-opamp for the BD voice, since it has 4 controls (minimizing parts).

I'm pretty new to opamps, so, as I understand it, since the d-pot voltage is 5V and the opamp's voltage is 15V, is the input voltage multiplied by a factor of 3 to be relative to the voice circuit?

 

The DPot calls for an SPI interface, but can this be simply done with DIN/DOUTs and programming, or would I have to use an SPI bridge?
If it can be done with DIN/DOUTs, I was planning on using a dedicated external crystal for the clock (all d-pots).

Please, don't sugar-coat it. I learn better that way. ;)

 

 

I didn't know such a thing as a "digipot" existed. Do they work? Are they reliable? If so, then sure; you might even be able to use existing MB808 audio boards, with a veroboard layer either above or below them and pins through to the potentiometer solder connections. Also, video please once you make the kick!

 

 

 

I'm learning how they work and how to use them as I go, so it's trial and error, but definitely, the first bit of sound I get out of this thing, I'm going to document the hell out of it!

 

The veroboard route is exactly what I want to do, for prototyping all this, anyways. That way, if this is a lost-cause, I can just keep building an regular ol' MB-808.

Edited August 2, 2013 by Roark

[Sauraen]

Problem: For this to work, you have to just replace the potentiometer from the original circuit with the appropriate pins of your digipot (let's forget about the 250K / 500K issue for now and assume you wanted a 250K pot to start with). However, if I'm reading the datasheet for AD5235 correctly, you can only apply a voltage to its pot pins between that chip's Vss and Vcc, which can't be farther apart than 0 and 5 volts. Therefore, as far as I can tell, you can't use this chip with the +15 / -15 volt MB808 board. It will not work to "amplify" the resistor with a 3x op-amp circuit (or 6x with a constant subtracted to get the full range); an op-amp circuit does not have the same electrical characteristics at all as a resistor. For starters, a resistor conducts signals (not to mention current) in both directions, while an op-amp only conducts signals in one direction and is designed to conduct almost no current.

 

As far as I can tell, you have two options: first, find a digipot chip that can itself be supplied from +/- 15 v (this may not exist, I've looked for similar things for voltage DACs to no avail). Second, since that little segment of the original MB808 circuit that includes the pot, capacitor, resistor, and single op-amp is just an amplifier with a capacitor across the feedback with a variable effect, you could replace that chunk with a VCA circuit, add the capacitor in an appropriate spot (you'd have to experiment), and control that with your digipot.

 

Edit: You may have another option. Do a search for "voltage controlled resistor". Looks like you can use a JFET as a resistor; one article recommends PN4119A; buy a few, plunk them in and see what happens. Worst case scenario you fry some transistors; I don't think you'd even fry the op-amp. Connect the two ends of your digipot to +5 and 0, and the wiper to the gate of the JFET; then connect the source and drain of the JFET in place of the resistor, trying to match the polarity of that capacitor next to it. One problem is that the JFET has a resistance 1/100th what you need, but you shouldn't have to chain 100 of them, there should be a way to modify the values of the capacitor and resistor in the MB808 circuit to get the range of decay times about the same.

Edited August 2, 2013 by Sauraen

[Sauraen]

As far as the SPI connection, it's easy, connect it to J19 on the LPC17 core the same way that the DIN/DOUT modules are connected to J8/9.

 

J19:SC - AD5235:CLK

J19:SO - AD5235:SDI

J19:RC1 - AD5235:#CS

And if you want the LPC17 to be able to read data from the digipot:

J19:SI - AD5235:SDO

 

If you have a bunch of dpots connected, you have two options:

• If you want one frame of your synth's operation to refresh the values in all of them (with different values of course): chain their SDI and SDOs (i.e. make the SDO of the first go into the SDI of the second, etc., and the SDO of the last into J19:SI). Connect all their #CSes and CLKs together.
• If you want to only refresh one at a time, connect their CLKs, SDIs, and SDOs together, and connect each of their #CSes to a different IO pin on the LPC17. Examples include: J19:RC2, all of J10, all of J5A and J5B if you're not using them, all of J28...

You will of course have to write the module driver for them one way or the other, but that's not very difficult; when you get there you can use the same code I used to control the OPL3 from LPC17.