Saturday, July 13, 2019

Electronotes Sample-Hold Modded with SMT

Continuing from last time--it's time to keep holding my nose and fab using SMT (surface mount technology).  It's not 1980 any longer!

This time I take a classic Electronotes preferred circuit "SH-2" (EN#61, posted online here) designed by legendary Cornell Professor Bernie Hutchins and add the SMT op amp boards discussed here to make the module a bit more compatible with my setup.


I initially laid out the PCB and built this S/H module in 2017. I believe the design to be 35+ years old--all analog; none of this 10 bit Arduino ADC rubbish!  I am impressed how flexible this S/H is, you can sample damn near anything and run very fast clocks into it and get good sonic results. Go Big Red!

A great Electronotes design! You can get the original schematic here or buy the book; I highly recommend Electronotes Preferred Circuits and the entire Electronotes package to any synth DIYer, you will get a lot of ideas from it.

For me, my somewhat simplified version of the Bernie S/H PCB worked first time, but there were a few problems interfacing it to my modular DIY rig: for external clock, as I initially laid it out: only clock signals going a few volts below ground would tell the circuit to sample. But: many of my clock generators go 0V to 5V or more, not 0V to -5V or whatever.

Lots of ways to change that; for me, it is easily corrected with this familiar op amp inverter:


....you could of course use single stage but I have a buttload of dual op amp boards sitting around, leftover from other projects....

I also wanted to add a bipolar LED to indicate if I was using external or internal clock and (extra credit!) have it pulse for each sample held. Again, time to use the SMT op amp proof of concept board from last time:


For both boards I tweaked the 2 100K trims until the LED colors looked the way I wanted.  For internal: bright green, for external flashing red.  Yeh!


So there it is: the main PCB (Green); the two SMT op amp daughter boards are the red ones on the back.

Only the op amps are SMT, I am still a bit chicken to go all in with SMT! And even for the IC, I could have used PDIP I guess, but this mod would have been harder; the boards need to be small to fit the footprint of a 1U frac.

OK that's it, it was a fun morning modding this board and it made a really useful and reliable S/H even better. Not sure where the SMT thing is going to go right now but I'll think about it.

See ya next time!


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