1221 Oscillator Expander

The page for the 1221 oscillator expander module is live:


yes thanks and we are sowaiitng for the price in store section,want 2 at least

2 Osc Expander would be apply to a likely 3 Performance VCO mixed / 3 Kosmo synth voice mixed scheme.

-Fumu / Esopus

Here they come


Hopefully the VCOs stay in stock!
Need to figure out how many things i need to order this time around…

1 Like

a whole synth voice im trying to get finished and out before chrisrtmas, it also works pretty well with this too!


  • almost done
  • I got no action on the lfo… debugging is fun :slight_smile:
  • next stop VCOs. I currently ony have two working ones, their friends are stuck in my backlog…

AND there is an update to the BOM:
13 100k resistors are needed, my version of the BOM said 12


A synth DIYer who doesn’t have a couple hundred spare 100k resistors lying around!? :laughing:


just to prevent confusion…

I started with the 100Ks because they are the (low profile) component with the highest count. when inserting the 100Rs I stumbled upon one unpopulated instance of 100K. I started doublechecking and counting… I found that I made no mistake and added one more 100K. If i had found the unpopulated 100K instance later with more resistors already soldered in the hunt for the error would have been considerably longer. especialy for those of is who can’t read resistor-stripe-codes…

(edit: a few less typos)

1 Like

I finally got around building this, seems to work, but LFO led looks weird. I have bi-color led there, it seems like the both colors are on all the time. Or at least it stays on the same brightness constantly. LFO itself seems to work, so I need to check what’s up with the led.
And I forgot how hot and one-sided the output from the 1222’s is, putting them directly to the funky filter overdrives it too much, and using my own VCA distorts it quite easily too. I need to tame the signal levels a bit, and maybe add something to sum constant voltage there to counter the dc-bias.

Well, seems that the lfo isn’t really low frequency oscillating, but problem is that the frequency is very high and that’s causing the problems. All components seem to be right ones, so this debugging isn’t progressing…
What should actually be the frequency range of the lfo?

RC = 56k x 100 nF = 5.6 ms implying ~ 28 Hz. Simulation gives about 1 Hz at one end of the pot, ~23 Hz in the middle, ~100 Hz at the other end.

Ahh, now it works. I had 100k instead of 100 ohm for R7.


RV5 in the schematic is a 20k trim potmeter. I only have 10k and 100k trim potmeters. For those who already built this module, how much K-Ohm do you think was needed to make it tuned to 1V/Oct? Do you think a 10k or a 100k trim potmeter could be used?

(Ofcourse I could add one 20k trim potmeter to my next parts order, but that will take some time, and I would love to finish this module during this weekend… :smiley: )


I think (=uneducated guess) the resolution (number of thurns per ohm) on a 100k pot is not fine enough. i replaced a 10k pot with a 20k (on a 1145 LFO) and it got realy fiddly calibrating it. with 20k vs. 100k the difference is even bigger…

If the pot has about 25 turns for 0 to 100k then 1 turn roughly translates to ab out 4 ohm. compared to 0.8 per turn on a 20k

i might be wrong… in that case correct me!

Thanks! Your reasoning matches mine, so I will give the 10k trimpot a try and see whether that will suffice. If not, I can always replace it with a 20k trimpot in the future.