Benjolin loop mod

Hi guys,

I’ve built a benjolin from Robs original PCB layout over on Electro-music.

I want to implement the Loop mod from the old Casper Electronics site. Does anyone here know or have built this?

Is the mod an addon or are the compenents in the below schematic part of the og circuit?

I am not familiar with the Benjolin (I am sure others here can help), but I noticed one 100nF (.1uF) bypass capacitor for the TL072. The good practice is to have two of them, one from 4 and one from 8, to ground.


I will look back at my notes , I built 2 differnt one nd alot of circuit on. off made mods


I had a look at the schematics at, looks like the components in the schematic you posted are to be added to the original circuit.


yeah, all of these get added into the original benjolin circuit.

Here is mine that I finished up last year!


That looks really impressive. What value pot did you use for the Loop offset? Is with woth adding in? Also are the attenuators worth having?

I’m trying to keep my build pretty simple with only adding LEDS and the Loop mod from Caspers old site.

Using thonkiconn connectors instead of banana jacks as well.

Did you add everything onto one board or is it a mash up inside?

I’m designing up a daughter board for the mods, inputs and outputs. Just learning Eagle so it will be slow going :smile:


(yeah I know, at least 20 chars…)

yeah everything is on board one for these -
It was a collaboration project between me and a friend on another forum.
I can’t remember what values were used - it was whatever casper or jolinlabs called for -
OH thats another good place to check - Jolin labs did the Ben2 module in euro and pretty much added every mod to it as well - check their schems on github so you can see how everything was implemented.

The loop mod works okay - the switch for bit length is 100% a welcome addition - also i highly suggest adding a switch for internal or external clock - you can get some real nice drones with the clock disabled and letting it just scream on its own.


People still use Eagle? Okay. Go for it!


I couldn’t find Jolin Labs on GitHub. They do have schematics on their own site:

There’s also a BOM and Gerbers for their module — it’s 0402 SMD though! :dizzy_face:

1 Like

I actually have 2 of these pcb and panel sets. I’m going to build one and compare it to the OG!

I have a stereo scope setup which helps sooooo much with these tiny builds.

I got into Eagle from the Mutable boards and building guitar pedals from the site

The guy who runs that site put up a library and comprehensive tutorial around converting schematics to boards using Eagle. I have also mucked around in Diptrace and KiKad.

What is your PCB designer of choice?

I use KiCad. Seems like most people here are using that or EasyEDA.

Note that (standalone) Eagle will be discontinued in two years in favor of Fusion360 Electronics. I tried running Fusion360 once and it was awful — just ridiculously, painfully slow. Granted it was on an old Macintosh and I never did a direct apples to apples comparison to KiCad. The only use I made of it was to convert old Eagle files to its newer format which can be imported to KiCad.

Part of it is my preference for open source software, and I use a Linux computer these days so Autodesk products are out.

My one attempt at 0402 was under a microscope too, but that didn’t help with the fact parts just seemed to vanish into thin air when I tried to pick up and position them. Maybe I inhaled them. It was an SMD practice board so I didn’t have much incentive to pursue it further. For me 0603 was manageable, just.

1 Like

I’ve been looking at the Benjolin schematics. One bothersome thing is the -9 V regulator. Certainly it’s possible to buy these but they’re much scarcer than positive regulators. DigiKey has only one in THT and it’s minimum quantity 100. Mouser has one in THT. There aren’t many more options in SMD.

You could use an adjustable negative regulator but I note the TI LM337 in TO-92 packaging is end of life.

And of course you can buy TO-92 LM79L09, or at least something claimed to be that, on AliExpress or eBay but I don’t regard them as reliable vendors.

Anyway, there are options available. But I’m wondering why it’s needed in the first place.

Looking at the schematic, most of it seems it’d work with ±12 V, maybe with some adjustments to resistor values. The one thing that is problematic is the CD4021 is being powered with ±9 V, rather than the more usual positive and ground. Since the maximum supply voltage difference is 20 V, you could not use ±12 V.

Presumably the point is that the desired Rungler CV output range is ±9 V. But if that’s the case I’d think one could simply use an op amp to scale a 0 to +12 V CV to ±9 V. Then the CD4021 could be powered with +12 V and ground.

Not sure if it’s worth pursuing but it’s what I’m thinking about this morning.

1 Like

I can confirm that -9V regulators are hard to find. For some reason, it’s easier to find -8V regulators.

They are, though not by a lot. The Kassutronics Wavefolder schematic specifies ±9 V regulators but it can be built with ±8 V ones, which is what I did. Here it seems to me if you did that it might work but would reduce the range of RUNCV.

These look like they will do the job

Another thing I’d recommend is to add a 1k current limiting resistor between the input jacks and diodes. There probably is a resistor on anything you’d plug into the input but no point in risking it.

Also, if anyone is looking at the CV/Audio input jacks mod… it strikes me as very peculiar. The way it’s implemented the input resistance varies from 10k to 0 (!!) rather than the usual and better 100k. I’d do it like this:


(The 100k pot with 51k input resistor combination makes the pot response somewhat nonlinear, but not grotesquely so. A 10k pot would behave more linearly, but that’s a lower input resistance than is best practice.)

Yes, that’s the one I was referring to. Like I said, they are available — this month! — but not too common, and in a year or so, who knows? As mentioned, the TO-92 LM337 is going away.

I found this version of the benjolin which has some mods not found in other versions (i think). The clock input looks especially useful.