I use a lot of Sample and Hold modules. They are an essential component of a Quantussy Cell. It is hard to find simple sample and holds, those without a noise source, a clock, a slew, etc.. The S&H is too often thought of as a random stepped voltage generator. So I keep looking.

I ran across this DIY Kit from Frogleg Synthesis. Actually I found the kit on Synthcube.

The Selective Sample & Hold is a simple dual S&H based on the LF398 S&H chip, but with a twist – there are two possible inputs, and a Select input to determine which one will be sampled between the two. With nothing in the Select input, it acts as a standard S&H on input A, but with a high gate or CV signal patched in (about 4v or above) it will switch to sample from input B. The two output jacks are both the same single sample, but separately buffered.

https://www.modulargrid.net/e/other-unknown-frogleg-synthesis-dual-selective-s-h

Building

This kit requires builders to source their own parts. The BOM has a handy Mouser project link, but several of the parts weren’t available. I was able to substitute, for example a Wima 0.47uf film capacitor for the mylar type. The Wima caps are the big red ones in the photos. These are the holding capacitors.

If you order a kit from Synthcube, don’t forget (like I did) to add 12 Thonkiconn jacks, washers and nuts.

The build document is very good and there is a full schematic, too. He even mentions the point at which you should wash the board if you used water washable solder (which I did). For some reason, he has you install the electrolytic and film caps with no-clean solder. I used washable for those, too. As is my practice with decent PC boards, I soldered the in the chips. I do this for reliability and hardly ever need to replace chip.

Red arrows point to the guard traces around the LF398 pin 6, where the holding capacitor connects. Guard traces on both sides of the board surround this pin where the voltage is being held. The guard traces hook to pin 5, the output of the S&H chip. Seems like good engineering (what do I know), and I’ve not seen it on other DIY sample and holds.

The jack PCB attaches to the main board using right-angle pins. I wasn’t able to source the specified part, and what I got was a little different, but I made it work.

The pins on my right-angle connectors were the wrong way around. I had to put the ends intended to solder into the board as the connection pins and vice-versa. The long, gold pins worked, but I had to carefully align everything. The depth of the pins sets the offset of the main board from the outer edge of the module. It’s a tad behind the front PCB. After soldering, the gold pins are cut off, because they would protrude into any module located to the left in a case.

Though short, the pins made a snug fit. The main board just hangs on them, but the fit is good enough to not worry about it falling off.

It is good to see a keyed power connector. Be sure to align it the right way! If some of the chips look not fully seated, it’s because of the large white box bypass caps I chose. The caps went in first. You might prefer to use smaller 100nf bypass caps.

The scope shows a sine wave (green) being sampled and the resulting output (yellow).

Conclusion

This sample and hold is a relatively easy DIY build. I’m glad to add these to my rack, next to the four Bastl Tromsøs, so that I can easily patch up four Quantussy Cells. The selection feature adds to the crazy possibilities!