Monday, December 31, 2012

Add Sample-and-Hold to Korg Mono/Poly

After my success with fixing the dead keys on my Korg Mono/Poly, I decided to try some of mods that are out there on the Internet.  One that seemed within my grasp was to, once again, follow the Old Crow and add a Sample-and-Hold circuit to the Mono/Poly.  What's the sound of a sample-and-hold circuit?  Well, this video shows how to setup the sample-and-hold on my modded Mono/Poly to get a great bubbly filter effect:



If you're interested in adding this to your own synth, I'd start with Old Crow's description here.  For my build, I used the same schematic as Old Crow, except I chose to hardwire it into MG1, instead of making it switchable between MG1 and MG2.  So, my modified schematic is shown below.


In terms of buying the parts, I went to Digikey, which is the standard place where professionals go to buy their electrical parts.  They've got nearly everything that you could ever want, but unless you know what you want with an engineer's precision, you can get easily flooded by all the choices.  If you've never bought from Digikey, I wrote a post to help guide you through the process.

For this build, let's talk about how to buy a few of the parts in the schematic...
  • The LF398 is the sample-and-hold chip itself.  I don't remember which one I bought, but try this one: Digikey LF398N/NOPB-ND.
  • The ferrite beads are a bit unusual to see in a hobbyist design.  I'm not sure how to pick the right ferrites for the filtering/decoupling task here, but Digikey M8697-ND is probably good enough.
  • You'll need a switch for "SW2".  Any single-pole double-throw (SPDT) switch will work fine.  Try Digikey 360-1801-ND
  • Finally, you'll see a bunch of caps in the schematic labeled "104".  This means that they are 10 * 10^4 pF, which is 100,000 pF, which is 0.1 uF.  These are commonly ceramic caps.  A decent choice might be Digikey BC2665CT-ND.
So, after getting all my parts, I wired it up on a piece of protoboard and then wired it into the synth (using a terminal strip to allow me to easily remove it for debugging).  Unsurprisingly, it didn't work.  This is a common occurrence with my tangled rats-nest electronics.  So, I spent some time poking around and finding lots of errors in my build.  Eventually, it worked!

Below are some pics of it in my Mono/Poly:

Elements of the S-H mod in my Mono/Poly.  Circuit board, Terminal Strip, and Switch.

Close-up View of my Assembled Circuit.  Ugly!

The red wire is how I chose to connect to the KLM-353 board for getting MG1.

Here's how I wired in the shielded cable to VR17 to get the noise source.

I'm not showing how I wired it into the mod wheel.  I've got additional modifications in this part of my synth, so the pictures would have been more confusing than helpful.  Sorry.

Once I got the circuit working, I was able to apply the sample-and-hold effect to anything that the mod wheel can affect.  On the Mono/Poly, that means the filter cutoff, the pitch of all the oscillators, or the pitch of just oscillator 1.  The most useful sounds come when applying the S-H to the filter cutoff.  The video at the top of this post shows that effect.  In my opinion, it's a great drone that can act as a fantastic bed for moody improvisation.  Enjoy!



3 comments:

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  2. My version of the circuit board looks better than yours, mainly because I've done away with the useless ferrite beads and power filtering caps (power coming to the board from the MP4 is already heavily filtered and regulated). Everything works flawlessly. Only the hold capacitor is critical and needs to be polypropylene (or mylar or any low leakage type). I've also modified the clock source and the output destination wires to my personal taste.

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    1. Cool!

      Got any links or youtube videos showing off your modification? I'd love to see how other people implemented this mod and I'd love to see how other people use the sample-and-hold functionality....

      Chip

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