You can never have too many Sample & Holds! I’ve been on the lookout for a 4hp triple S&H module forever, it seems. Never found one to my liking until now. Biopower Audio’s DM (for Dopamine) hit the mark. Not just a triple S&H, but also a gate to trigger converter.
I decorated the panel with Bananuts, visually better to distinguish the Gate in (silver), Trigger out (red), Signal in (gold), and Signal out (black).
The Kit
DM is available assembled or as a full kit with SMD all soldered, one of the easiest forms of assembly. I ordered two kits.
The main PCB faces to the inside of the two PCB sandwich.
Modification
The trigger output pulse width by default is a fixed 1 millisecond. During sampling, the LF398 is put into follow mode for 1ms. This means that the signal being sampled is passed through to the output for 1 ms, while the voltage is acquired. I found that length quite long. Most LF398 circuits need only 100 microseconds or less for acquisition, depending of course on the size of the holding capacitor. I emailed Biopower Audio, asking which capacitors set this timing and for a schematic, and received a wonderful reply from Eric Schewe of Biopower Audio. He included a schematic of the circuit and pointed out the capacitors in question, C1, C2, and C3, all 10 nf. I spent some time reviewing four or five LF398 circuit designs, before settling on a value of 330 picofarads. These narrowed the following window from 1ms to 50us, about 1/20 of the original time. Acquisition still works fine.
In the photo above, the blue line is the trigger output from the module. Note that the follow mode begins shortly after the pulse starts to rise, quite a bit below its final 10V peak. (The long fall time of the pulse is due to it not being loaded by anything but the scope probe.) Holding begins as the pulse falls below about +8V. Notice the noise coming through for 50us. Before the modification, this was 1ms, long enough for the noise to be audible briefly in the output.
Application
Here’s the Modular Grid link for this rack. There were already nine sample and holds in this rack and now there are seventeen, counting two in the SSF URR and one in Bagai.
