I got used to having lots of sample & holds in my Eurorack. Whenever I would patch the 5U MOTM, I’d lament having only four sample & holds. It suddenly struck me that I could easily remedy this. And so it came to pass that I designed this Quad S&H module in MOTM format. This is my idea for a basic sample and hold. No internal clock, no noise source, and no slewing!

The Circuit

I went searching for a S&H PC board that used the LF398 sample & hold chip. This is a great chip that’s used in most of the S&H modules I have, in both MOTM and Eurorack. I came across the Barton Musical Circuits 2LFOSH. Not exactly what I needed, but close enough. I needed only the S&H, not the two LFOs. I bought four of the PC boards. Then I set about modifying the S&H circuit to my liking.

My circuit design, above, is based on examining three similar circuits. It’s so simple, I’m going to explain every part. In the most basic of LF398 designs, you typically see the input and output of the module using the input and output pins of the chip directly. That’s probably OK. But I had the four op amp sections of a TL074CN available. So I buffered both the input and the output with simple voltage followers. The input has a 470K terminating resistor for a high-impedance input. And I put a 100 ohm series resistor on the output. The nice thing is that the LF398 is isolated from the input and output jacks. Note that the input is on a 1/4 inch Switchcraft jack with a switched center lug. That lug is grounded to keep any noise at bay when the circuit is idle.

Now consider the trigger conditioning. It’s almost the same as Barton’s, but I made two changes. I have a 1:11 voltage divider setting the threshold at about 1.4V. And I added a 1M hysteresis feedback resistor, a trick I learned decades ago from Electronotes. This insures reliable triggering. The trigger input jack is also normalled to ground, which insures that the initial state of the comparator is low. The state of the comparator is monitored by an LED, again fully buffered so as not to interfere in any way with the trigger. I used the same RC network plus diode as Barton did to generate the trigger pulse.

The holding capacitor is an important choice. I’ve seen different values used, but I went with 10nf, the same as Barton. The Mouser part is 505-MKS20.01/63/5, WIMA MKS2C021001A00JSSD.

I changed the parts used in the power section, too. I had some ferrite beads and substituted those for the 10 ohm resistors. I also used 100nf bypass caps. I don’t know why Barton specified 10nf for bypassing. 100nf (0.1uf) is pretty standard.

This circuit will run on +/-12V too. The trigger threshold would be a little bit lower. The LED and its resistor should be selected in any event.

Building the boards

One of the four boards has both type of power headers. The 4-pin MTA is standard for MOTM and this is the system power inlet. All the boards have Eurorack headers, too, and I utilized those for power distribution (see below). I used the SM (sample), IN, and O pads on the board. The LED has wires running off to where it lives on the panel.

Note that the TL074 is in a socket, but the LF398 is soldered in. I like to solder ICs for reliability, but I had to use a socket for the TL074, because jumper wires had to be soldered to it.

My circuit needed four trace cuts, five jumpers, and one resistor (the 1M) on the bottom. I had to put the 10uf caps on the bottom of this board, because the MOTM power header crowded the two beads on the top, not leaving enough room for the caps and the nylon nut. Again I used nylon hardware to mount the boards. It flexes a bit and insulates as well.

Final assembly

I fabricated a bracket just like I have done in previous projects. It has a right angle bend with holes that match up to the panel jack and LED holes. The jacks hold the bracket to the panel. A single ground wire runs from near the power inlet out to the bus wire grounding on all the jacks. I used jack normal switching to allow a trigger patched to the left side to trigger the right side, both top and bottom. I often like to trigger two S&Hs with the same signal.

Power is distributed from board #1 to the other three by a cable I made from standard Eurorack 10-pin power connectors. This cable will seldom, if ever, need removing.

Testing was simple. A ramp wave goes to the input, a trigger to the trigger input, and we look at the input and output. In the above you can see the blue trace indicating the steps happening on the output, as the trigger rate is about 10X the frequency of the ramp wave.

Installed

You can see how the trigger normalling works from the photo above. A trigger to the left input also triggers the right. One nice thing about the LED monitor is that it shows the pulse width of the input. The pulse width isn’t critical to triggering, though. In fact even ramp, triangle, and sine waves can trigger this circuit reliably.