PreenFM2 SMD version

[zelea2]

I'm in the process of redesigning a SMD version of PreenFM2.
The original goal of this design was probably to make it as easy as possible for people to solder their own kit.
Unfortunately  the Cerb40 module cannot be bought anymore so someone else has to solder the LQFP64.

I plan to build 5 pieces of this new design and make it as cheap as possible. The majority of components
will be bought from Aliexpress and the rest from Farnell.
- there will be just one PCB (15x10cm), no sockets.
- the 2 shift registers replaced with MCP23S17 (port expander)
- the 2 DACs replaced with PCM5102A
- OpAmp replaced with OPA2337
- extra MiniUSB socket placed on the main board
- all resistors and caps as SMD 0603
- cheap LCD 40x4 (no OLED)
I believe the total BOM will be well below 100$ (without the case)
The changes to the firmware to accomodate the new DAC and SPI-expander will be trivial and made with #ifdefs

Since I'm not comfortable with this KiCAD tool I have switched to OrCAD+Specctra and I'll make all design files available.
So far I've been able to export the PCB as GenCAD and after a lot of massaging I've now got an OrCAD PCB of the current
design with SMD passives. This way all the sizes and encoders/switches position will be preserved and I'll be able to use
the same cases.

[Xavier]

I'm in the process of redesigning a SMD version of PreenFM2.
The original goal of this design was probably to make it as easy as possible for people to solder their own kit.
Unfortunately  the Cerb40 module cannot be bought anymore so someone else has to solder the LQFP64.

Yes, the goal is to have a fun DIY synth project.
The preenF405 (CERB40 design adapted for the preenfm2) is and will remain available. So nobody has to solder the LQFP64 STM32 if they don't want to.

I don't get what you plan to sell ? the components ? Presoldered SMD preenfm2 ?

The move to OrCAD will move this project away from hobbyist.

Xavier

[solipsvs]

exciting!  can fit easier into euro rack maybe?   

[zelea2]

I don't get what you plan to sell ? the components ? Presoldered SMD preenfm2 ?

The move to OrCAD will move this project away from hobbyist.

I completely agree with you about OrCAD. I'm also in favor of open source and I've been using Linux and free software for the past 20 years.
Unfortunately KiCAD is completely new for me while in OrCAD I've built over 100 boards. Initially I've planed to use KiCAD+Specctra but since
I need to migrate your design to KiCADv4 to maintain the schematic/PCB relation it's almost as much work as doing it from scratch.
Anyways I'll export the final PCB as GenCAD and MIN formats to make it as accessible as possible. Mentor PCAD can also load OrCAD files.

I am an electronics engineer by trade and I design ICs for a living; I also do some board design occasionally. I just want to experiment with
FM synthesis and since I already own too many keyboard synthesizers I was looking for a modular one. I've tried to get a Yamaha DX200 but
they are so rare and expensive. The PreenFM2 seems a very capable choice and I like this idea more than a VolcaFM. Music for me is just a
hobby and I'm in my 4th year of taking piano lessons ;-)

I couldn't get a PreenFM2 kit since they are out of stock so the next best thing is to build my own. Yesterday I've bough all the components
from Aliexpress, including the STM32 (5$) and the LCDs (5$), rotary encoders and switches and I've paid only 150$ for five kits. The prototype
PCBs will be some 40$ for 5pcs 2 layer 15x10cm size. So in the end the whole BOM for this project excluding the case will be less than 50$/unit.
That's how insanely cheap you can spend to build your own PreenFM2. The drawback is that now I have to wait 3-4 weeks for all the
component to arrive from China and probably as much time for the PCBs because I won't send it out before having all the components in
my hand to validate the footprints. For the cases I haven't decided yet - either I'll use a laser cut service and translucent green acrylic or I'll just
buy 1mm anodized aluminum sheets and machine it on my KosyA5 CNC router. I only plan to build these 5 units and maybe sell 3 or keep them all.

Attached are some pictures of my synth's room ...

[solipsvs]

id love one if youre taking pre orders!

[SirPrimalform]

While I have no need of another PreenFM2, I definitely want to see how the build comes out.

[zelea2]

I've got the majority of the components from China so today I've started the new schematic and PCB.
I still need to simulate the analog chain and probably remove/change some of the passive to make it comply with
the DAC's requirements and get a 0dBu (0.775V) signal output.

I'm documenting the progress here https://uglyduck.ath.cx/ep/archive/2017/11/PreenFM2_SMD_Version.html
I've listed there what I think are improvements from the current schematic.

The STM32 still has a lot of free pins. I could even get rid of the MCP23S17 and connect all the switches and encoders directly to the ARM uP.
I've added a second serial interface to connect maybe with another PreenFM2 or sync to other devices.

Before routing the PCB I'd like to ask the owners of this synth what do you miss in your PreenFM2?
What else can be added to make it better?

Some silly ideas:
- RGB LEDs or NeoPixels
- MIDI Through
- I2C OLED displays
- capacitive switches or sliders
- a stereo amplifier and socket for headphones

[lokki]

a stereo out jack and a dedicated headphone out would sure be great. other than that i am pretty happy. i would wish there were 4 outputs instead of only stereo so that you could route each instrument to another channel. but that is not gonna happen without adjusting quite a lot of the firmware and i am also not sure if the stm32 would support another stereo dac for the additional outs.

[zelea2]

i would wish there were 4 outputs instead of only stereo so that you could route each instrument to another channel. but that is not gonna happen without adjusting quite a lot of the firmware and i am also not sure if the stm32 would support another stereo dac for the additional outs.

The STM32F4 has two I2S interfaces and the PCM5102 are cheap and in small packages. One problem is the size of the 6.3mm sockets; there is not enough room for 4 of them so I'll have to switch to two 6.3mm stereo sockets instead.  But as you said the main problem is the firmware support.

Xavier, do you think the design I'm doing will be of any interest? I will publish a diff file with the changes of the git firmware (mainly using #ifdef's for the differences). Should I bother to ground one currently  unused STM32 pin so the future firmwares could distinguish between my hardware design and yours?
This will be just to report on the LCD that you are not using the correct firmware for the board and stop the code from running.

[lokki]

two stereo jacks would be awesome...

[Xavier]

Xavier, do you think the design I'm doing will be of any interest?

This is a great project. But i'm not sure who the target is...

[zelea2]

This is a great project. But i'm not sure who the target is...

The target are hobbyists who want a PreenFM2. After I proof the design someone else or a small company can manufacture the PCB with all the SMD components soldered and sell that as a kit. The end user will only have to solder the encoders, LCD, switches and sockets. With a stencil and a soldering oven and even a manual component place these PCBs can be manufactured very fast and cheap. Small companies who have automated pick and place machines can churn these boards in tens per hour.

The strength of this project is the firmware and the hundreds of man-hours you guys put in to develop it. So any derivative design should be able to reuse this firmware without too many modifications.

Keep in mind that a new Korg Volca FM can be bought for 150$. It can also load DX7 patches, has a case, MIDI and a sequencer. It doesn't have the same flexibility as the PreenFM2 though. The out of stock PreenFM2 kit and a metal case is 212E + 70E (approximately 330$) and this is one reason the Volca FM is selling so well.

I will build these SMD versions PreenFM2 mostly for myself and I'll solder them by hand. I do have a powerful stereo microscope and a steady hand so the only drawback is that it will probably take me an hour per board.

So I think you can use my build to proof a cheaper design, add a few new developments if you want and make the PreenFM2 more accessible and popular.

[Xavier]

The target are hobbyists who want a PreenFM2. After I proof the design someone else or a small company can manufacture the PCB with all the SMD components soldered and sell that as a kit. The end user will only have to solder the encoders, LCD, switches and sockets. With a stencil and a soldering oven and even a manual component place these PCBs can be manufactured very fast and cheap. Small companies who have automated pick and place machines can churn these boards in tens per hour.

The strength of this project is the firmware and the hundreds of man-hours you guys put in to develop it. So any derivative design should be able to reuse this firmware without too many modifications.

Keep in mind that a new Korg Volca FM can be bought for 150$. It can also load DX7 patches, has a case, MIDI and a sequencer. It doesn't have the same flexibility as the PreenFM2 though. The out of stock PreenFM2 kit and a metal case is 212E + 70E (approximately 330$) and this is one reason the Volca FM is selling so well.

I will build these SMD versions PreenFM2 mostly for myself and I'll solder them by hand. I do have a powerful stereo microscope and a steady hand so the only drawback is that it will probably take me an hour per board.

So I think you can use my build to proof a cheaper design, add a few new developments if you want and make the PreenFM2 more accessible and popular.

I was hopping you were not going to write that.
I really like the preenfm2 like it is today.  I would like it to remain a DIY project.

If like you say a small company manufacture the PCB, i would really feel robed.

[zelea2]

I really like the preenfm2 like it is today.  I would like it to remain a DIY project.
If like you say a small company manufacture the PCB, i would really feel robed.

From my point of view it is a completely DIY project - I'm even gone route my own PCB.
I have no intention getting into the commercial side of this, I'm earning enough money with my daytime job.
I will just proof my SMD version and make all the design files available.

But surely making it cheaper and available to more people can't be a bad thing for this project.

[zelea2]

a stereo out jack and a dedicated headphone out would sure be great. other than that i am pretty happy. i would wish there were 4 outputs instead of only stereo so that you could route each instrument to another channel. but that is not gonna happen without adjusting quite a lot of the firmware and i am also not sure if the stm32 would support another stereo dac for the additional outs.

I kind of have to decide between 4 outputs and headphones amp because the amp will be only stereo. Also if I will add a headphones amp it will be a Class D amplfier like the stereo 3W PAM8403.

If I go for 4 audio outputs then I'll have to replace the dual volume pot with digital control pots (let's say MCP42010) and replace the actual pot with another encoder. This extra encoder can control independently the volume on all the 4 output channels (so volume + pan times 2).

Anyone else thinks that having more audio outputs is a good idea? What other changes would you like to see in PreenFM2?