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 two of the SMT op amp boards-- information here--to make the module 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. I am impressed how flexible this S/H is, you can sample pretty much anything in the audio or control voltage realm, run very fast clocks into it and get good sonic results.
Another 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 DIY'er, it can be a source of a lot of ideas.
My somewhat simplified version of the Bernie S/H PCB worked first time, but was a major issue interfacing it to my modular DIY rig: for external clock, as I initially laid the circuit: only clock signals going a few volts below ground would tell the circuit to sample.
But: almost all my clock generators go 0V to 5V or more, not 0V to -5V or below.
Lots of ways to fix this; for me, it was easily corrected with a familiar op amp inverter:
I also wanted to add a bipolar LED to indicate if I was using external or internal clock and have it pulse for each sample held. I used a second SMT op amp proof of concept PCB from last time:
Lots of ways to fix this; for me, it was easily corrected with a familiar op amp inverter:
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| Two stage op amp inverter turns positive going clocks to negative--INV GATE OUT feeds the circuit's clock input, while second stage OUT could be brought to the front panel. |
I also wanted to add a bipolar LED to indicate if I was using external or internal clock and have it pulse for each sample held. I used a second SMT op amp proof of concept PCB from last time:
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| A second 2-stage op amp daughter board provides a bi-color LED indicator for the module. It's in series with the inverter already discussed. |
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!
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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