KOSMO Audiohacker - looking for mistakes/feedback

[sebastian]

Hi, I am trying to build a KOSMO variation of the nootropic audiohacker board. Now I would like some “additional eyes” from you to spot stupid mistakes before I design and order a pcb for it
My plan is to use it as a bitcrusher/sample rate reducer and also a freeze module, similar to the 2hp freeze Sam uses.
If you have other cool ideas what could be done with a 12bit ADC -> DAC Arduino, I would love to hear!

I added a TL072 to convert from ±5V to ±2.5V and then exchanged the original opamp for an MCP6004, where I add 1.25V to have a range of 0V to 5V for the ADC. For the output I convert 0V to 5V from DAC to 0V to 10V and then AC couple it to the output. The part for CV and pots is from Mutable Instruments’ grids.

It should be a 5cm KOSMO module in the end, it will be quite crowded! The jacksockets and the circle for the big button are just a reference and will be cut out later (will be connected via molex connectors).
This is just a first idea of how it could look:

Feedback welcome!!

[SYNTHGUY57]

wider and notso crowded but love to see it in all phases of construction, great idea there

[CTorp]

Great module to kosmotize!

[analogoutput]

I haven’t gone over the schematic carefully, but for the layout — Yeah, if it were me, I’d make this 75 mm wide to reduce panel crowding. (I also would put the jacks on the bottom, or on the left side if necessary, but not on the right, but that’s me and my dominant right hand’s personal preference.)

Also “length” is misspelled.

Praising with faint damns here, looks pretty nice!

[antoine.pasde2]

U5B in channel 2 looks like it’s got its + and - inputs swapped.

Might also want to add negative voltage protection on your inputs. (Q1 and A1 wouldn’t enjoy negative voltages).

[sebastian]

Thanks for the feedback!

@analogoutput

Yeah, if it were me, I’d make this 75 mm wide to reduce panel crowding. (I also would put the jacks on the bottom, or on the left side if necessary, but not on the right, but that’s me and my dominant right hand’s personal preference.)

Hm… 75mm could be a good compromise, I did not want to make it 100mm, but maybe 50mm is too dense… I wanted it to be small, because I would like to be just a small effect that I can put in where I still have some space in the case I will make a file for the panel and see how it looks without all the “invisible” components.
For the left-right stuff: I am left-handed, so I prefer it this way, but maybe I can work out a symmetrical version.

[sebastian]

@antoine.pasde2

U5B in channel 2 looks like it’s got its + and - inputs swapped.

great catch! thanks!

For reversal protection, would you replace the 10 ohm resistors with diodes (which?) or would you add diodes to ground?

[ChristianBloch]

1N5817. I always put those in in stead of the resistors when I can.

[fredrik]

@sebastian

For reversal protection, would you replace the 10 ohm resistors with diodes (which?) or would you add diodes to ground?

There’s quite a few options, some better than others:

Typical module power circuit

Different schools: 10 ohm and capacitor near power header (this basically results in an RC low pass filter, but with a voltage drop over the resistor depending on the current). Usually somewhat pointless (the resistors, not necessarily the capacitors). 10 ohm without capacitors (either using them as fuses, expecting them to die if something’s shorted, or possibly just cargo culting, trying to do the above but forgetting how things work, see “power sins” here for more). Ferrite beads (if done …

[analogoutput]

You’re probably aware that isn’t what @antoine.pasde2 was referring to in his comment about negative voltages, but just in case, it isn’t. It was about protecting components against negative input voltages.

[sebastian]

Oh, no I misunderstood that! Thanks for pointing it out!
For the Arduino I think it is pretty safe, because there is the MCP6004 before the inputs, which limit the voltage due to the rails. But for the trigger input I don’t know what happens if I put in negative trigger voltage… will the transistor blow up? The part is from some other schematic (probably also Mutable Instruments) and I have not yet seen any protection for triggers. Or are you talking about something else still?

[analogoutput]

I think you’re correct that the audio input on the Arduino is safe, but the CV inputs are not.

I’m not sure about the transistor, I did basically what you did on dac/ino (taking it from the Synapse project it was based on). But I guess the emitter-base breakdown voltage is 6.0 V and that can be exceeded if the input goes much below -6 V?

[sebastian]

But CV inputs go through the MCP6004 as well, this is the CV circuit from MI grids, where the CV is offset by the potentiometer. The output of the MCP6004 can never go beyond its rails (0V and 5V).
At least this is how I understand it, maybe I have mixed up my labels?

[analogoutput]

Sorry, didn’t notice that op amp was not on ±12 V rails!

[antoine.pasde2]

@analogoutput

Sorry, didn’t notice that op amp was not on ±12 V rails!

Same here, I hadn’t noticed that, sorry. Those inputs should be OK.

As for the trigger input, to quote the excellent web site https://northcoastsynthesis.com/news/design-mistakes-in-synth-schematics/

“The problem is, what happens on a negative input voltage? Then the transistor’s base-emitter junction is reverse-biased. For small negative voltages, no problem, it just won’t conduct. But the absolute maximum reverse voltage rating for a 2N3904 transistor is 6.0V (even less for some other common transistors). Apply more than that much negative input and it will go into Zener or avalanche breakdown, passing current in the reverse direction. The current will be kept small by the 100k series resistance, but the transistor isn’t designed for it and may be permanently damaged. This input cannot safely accept a voltage below -6V. A quick fix would be to add a basic switching diode, like a 1N4148, from ground to the transistor base, to pass any reverse current and prevent the transistor’s base-emitter junction from seeing a larger negative voltage.”

[sebastian]

That website really has good resources on how to improve your design!

Would it be bad to put components on top of each other on different sides of the board? Like this:

I can save some space like this, the Arduino will be socketed on the backside, so there is space in the z-axis. But maybe bad practice?

[telec16]

Not really a bad practice, it’s just harder to debug. But I to that quite often !
The only thing to take care of is to separate well enough the digital and analog circuits (try not putting an opamp under the Arduino).

[sebastian]

Good to hear! These are memory chips, so no problem there, but I have some of the audio traces crossing paths with die digital (SPI) traces… I should probably improve that, right?

Upper left IC is ADC (MCP3201), upper right is opamp (MCP6004), Bottom left is opamp (tl072) and bottom right is DAC (MCP4921).
This stuff makes routing pretty hard! xD

[eric]

If they are crossing at a right angle there shouldn’t be any problem.
Problems arise when analog and digital run parallel close together.

[telec16]

It depends on a lot of factors, but to avoid crosstalk you can:

• Use only high level/low impedance audio signal when near digital one (especially clocks)
• Separate traces with ground
• Only cross traces perpendicularly, do not route traces parallel together, even is they are not on the same side of your PCB