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Stephen Hensley of 2hp Talks Designs, Inspiration, Eurorack, and More

Published on Jun 14, 2020 by Synth Talk

2HP Modular System

I caught up with Stephen Hensley from 2hp to learn a bit more about the background of this innovative Eurorack company. What follows is my conversation with Stephen talking about his designs, moments of inspiration, Euclidean sequencers, synthesis methods and more.

What does 2hp mean?

2hp is the smallest width that a Eurorack module can be.

Are you the founder of 2hp?

Yeah, I’m the founder. It was just me for a while until I had a few people come on to help solder and build. Now it’s mostly just me doing everything, except for distribution.

I share a building with QU-Bit and Electrosmith, they handle most of my distribution and manufacturing. I get to design and create cool things, while they handle getting everything built and shipped out to stores. I help them with engineering stuff and vice versa, so we’re all close friends. It’s a pretty tight-knit group.

How did you get started?

I went to college with Andrew from QU-Bit so we knew each other and have been friends for a long time. He moved to California and moved QU-Bit here four years ago or so. I moved out here from Boston to help them out with soldering and testing.

I was building the QU-Bit Eon modules and had this idea of making tiny modules that do a lot of different things. Andrew agreed and the whole thing spawned 2hp as it is today. We started with six modules for that first phase. Then another eight modules and another eight and however many we’re up to now, around forty-something different modules.

That’s impressive and when did you start?

It was around the fall of 2016.

Did you go to NAMM in 2017 right after starting the company?

Yes. I had one of those tiny little tables in a booth, it may have been in the WMD section.

In the fall of 2017, there were six modules available. In addition to the two analog and four digital modules, I brought eight other new modules. After that, we just kept making new stuff for two years straight. Now we have slowed down and we’re working on getting this lunchbox product out.

How was your first NAMM show?

It was a blast! I’d been to modular on the spot events in LA, the impromptu outdoor modular thing, where I could see a lot of people in the community. There were not a lot of other manufactures at those events. So for me, it was a really cool experience to be able to hang out with other manufacturers and get to know them a little bit more and inspire each other with different ideas throughout the weekend.

You spend all of your time shut in an office designing stuff and you think maybe this is pretty cool or maybe it’s not. It’s awesome to show people and have them react to it and experience that first hand.

Were there any surprises to people’s reaction?

There are several things about 2hp that people immediately point out. How are you supposed to turn the knobs, everything is so close together. There was an overwhelming number of people who didn’t know what they were looking at, it just looked like a pile of wires.


2hp Lunchbox

I’m amazed at how much functionality you can pack in such a small area with 2hp modules, in fact even in a lunchbox

Yeah, we’re excited about the lunchboxes they’re really fun. The first time we got one together, I wondered how much can you do with a lunchbox. Then I went to test it, forty-five minutes went by and I was like oh this is a lot of fun.

It looks like the lunchbox I had in school

Exactly, that was the whole idea. We saw a couple of people doing D.I.Y. boxes with old Star Trek lunchboxes from eBay and cut in rails. We thought man what a cool thing and how fitting for something like 2hp. It is just a perfect perfect match.

Is it coming out soon?

It’s going to be late June or early July at this point just because of Coronavirus delays with everything more or less shut down. We’re doing our best, we have all we have the stuff on hand, we just need the time to get everything installed and start shipping them out.

I see you’re selling it as a lunchbox by itself too

Yes, we’ll be selling it in a few configurations, with rails and power, full of modules, or a traditional lunchbox for your lunch.

2HP Loop, Comp, and Pitch

2hp Loop Comp and Pitch Modules

You have three new modules coming out soon, Loop, Compressor, and Pitch

Yes, they will available later this Summer. We’ll keep everyone updated as much as possible.

How do you decide what to build at this point with the number of modules you have and the endless possibilities of Eurorack?

At first, it looked like we just needed to start running through all of the traditional synthesizer elements with a couple of random ones that were just fun ideas. For this set we kind of went through and said okay what stuff is fairly obvious and simple but we just don’t have yet.

The first two that came to mind were the Compressor and the Loop. If you look at guitar pedals, everybody makes a compressor and a looper.

So immediately those two were the big flags to me. For Pitch, on the other hand, I was just messing around with DSP stuff and thought this would be kind of fun in Eurorack. It quickly became functional after I experimented on it for a weekend.

How did you learn hardware design and DSP?

I went to Berklee College of Music in Boston I studied electronic production and design. It was all about electronic music, synthesizers, and high-level DSP. We worked with block diagrams and understanding the mechanisms of traditional effects, but did not get into the nitty-gritty math of it all.

I got my foundation there and as I was going through that process I fell in love with code and what you could do with it. At the time, a lot of that was learning how to do stuff with graphics, using processing or jitter.

I said okay cool, I can make a plugin. At the same time, I got into Eurorack and then it was just game over for me once I realized I could figure this stuff out.

Learning a modular synthesizer teaches you a shocking amount about the individual intricacies of different like DSP blocks.

For the hardware, I learned a lot of it from doing it myself and I was really lucky to have Andrew from QU-Bit around as a guide.

We would go over the schematics of the QU-Bit modules together so I got some of the foundational circuits from that process. Open-source resources like Mutable Instruments and others also helped me learn how these things work. I gradually got better at it over time. Analog stuff is always harder in that respect. With digital stuff, if you just follow the data-sheet you can get it done. With analog stuff, it’s always prototype after prototype, and tacking resistors all over the place.

Long story short, I’m pretty much self-taught on the hardware side of things. For the code and DSP side, I’m self-taught after learning some foundational stuff.

I worked on an analog compressor design at my first job after college so I’m interested in how that is designed

The 2hp compressor is running on a DSP platform that we designed. It’s all DSP controlled, it would be a lot of larger if we made it with discrete components for an analog compressor would be a little tricky to fit on 2hp.

For an analog design, it would be at least four revisions and all this other stuff to deal with like makeup gain.

The DSP approach is much faster to build in hardware?

Once you have some hardware it is faster. We’ve spent a lot of time dialing in our hardware because it started as a nightmare to fit in the space of 2hp, it’s gradually gotten better and better.

We’re at the point now where it’s almost as good as it can get in terms of assembly. Our DSP platform has changed a lot over the last couple of years. Once you have something that works, you can work on code rather than working on two aspects of the project at once.

I have some fun breakout boards that are larger than 2hp size to explore the DSP platform. The boards have a lot of the possible front panel stuff on them so I can always tinker and adjust what the firmware is going to do and decide which knobs I want or how many jacks I need before I design the hardware.

That’s the other aspect to it, you get to polish a product before you have to invest the time and money into the hardware.

I’m sure you make a lot of difficult choices since you have a limited amount of space to work within 2hp

Yes, especially on something like the Loop, there is such a small amount of space. We switched RGB LEDs to save space, added shifted mode to get a lot of features you get on bigger loopers like different overdubbing modes and reverse.

That’s an art to make those trade-offs and create a product people love, you’re very good at it

Thank you very much.

Marcos Alonso said “When you are designing an instrument, most of the time you are designing the limitations of the instrument. Those limitations are what gives the instrument a special sound or character.”

I can see that. I think a lot of the really fun 2hp modules flourished because of that at least in the design phase. It was easier to finalize something given such strong limitations. It was hard to get stuck in the whole world is my oyster, I can do anything with this piece of hardware problem. If you want to make this thing fit in 2hp, you have one vertical strip of controls. What three things are you going to let the user do?

It could be kind of nice to those limitations sometimes and probably frustrating at other times

It is a comfort for a lot of the modules, that’s for sure.


2hp Verb Bell and Pluck Modules

A lot of people love your reverb module. I’ve had people tell me just get these two or three modules and the 2hp reverb to get started in Eurorack

Yes, I love it too. We’ve heard a lot of good things about it. I’ve got it permanently patched in my set up at home. That was one of our first ten modules.

How did you design the reverb?

It runs on the FV-1 and is loosely based on some of the example reverbs out there. I adjusted some of the feedback networks and stuff like, and changed the sample rate a little bit. The platform has a variable sample rate, so we were able to dial it into spots where we thought that it sounded really good and people tend to agree.

What kind of synthesis does the Bell module use?

That’s all modal synthesis. There are just six or seven modal filters in the guts of the DSP. 2 filters act as a mallet, so whenever you send a trigger there’s a tiny impulse that goes through these 2 modal filters that create a mallet strike. Those outputs end up striking the other four or five modal filters that are configured with a specific ratio to emulate real bodies such as bells and vibraphones. So it’s just a couple of knobs for controlling all of those ratios and how they’re tied to the fundamental pitch. Then you have a decay control to give you some variation on the bandwidth which makes it sound like it’s lasting longer or shorter. Surprisingly it’s only six or seven filters.

We tried a couple of other ideas, the Pluck module is a lot of Karplus-Strong, which is what you think of for plucked strings. I was trying to imagine what you think of when you hear bells. We tried some FM stuff at first but it sounded more like FM bell than real bells. I was messing around adding some stuff to our DSP library and the modal filters were right there and I was like ooh!

Yeah, I’ve noticed a lot of people in Eurorack like it too. It’s one of those things you can do with a computer without really thinking about it. You just get a pattern together and loop it without even knowing it’s a Euclidean pattern. I think in Eurorack it is so hard to dial in unique rhythmic patterns without explicitly sequencing each note. Euclidean generators and similar techniques have become so popular because it is something you can set, turn some knobs, and get good results without having to explicitly program each step. I think a lot of people get into Eurorack to avoid programming each step.

Euclidean based patterns can yield results that are almost generative or a bit of controlled chaos. You see the approach in iOS apps too, but maybe modular is where it took off

Maybe, I could see that because I’ve never seen a computer plugin that does Euclidean pattern sequencing or anything like that. Most software doesn’t have that many compositional tools, whereas iOS apps and Eurorack modules do. I think those generative tools, whether on an iOS device in your hand or in Eurorack where you’re experimenting and not using traditional methods of composition, people find some comfort with Euclidean rhythms. They are usable and rhythmic, but they can be generated from just a pair of numbers or a control voltage.

Modular people are focused on a different part of the process

A lot of the people I know focus on the tonal qualities of what they’re doing versus the melody or composition.

That’s why I up until fairly recently before the Eloquencer and a couple of these other big sequencers came out it was rare to see something more than an eight-step sequencer that wasn’t massive. For the most, part everybody was fine with looping an eight-bar sequence and making it sound cool or have sixteen different things tied to the one sequence. If you want to compose a long piece of music, there are still a million ways to plug your computer into a modular system.

How do you see people using the Logic module?

I love that thing but we haven’t seen that many people using it yet. I understand this module can be confusing or at least finding a place where you would want to use it is not obvious most of the time.

You can use the OR section as a gate summer. For example, if you have a snare drum pattern triggering on two and four, and patch a Pressure Points into the second channel, either of those will cause it to trigger so you can create fills with the Pressure Points. At home I keep a QU-Bit Tri-ger hooked up to the OR module along with a snare drum so I can add extra hits on the fly.

For the AND, you can do cool trap-style hi-hats. An example would be using a 4ms Quad Clock Distributor (QCD) driven by a bunch of CV inputs so the rhythm is always changing and gate that with a slower control like an LFO. This gives you rhythmic pulses of very dynamic sounds. As long as one input is high, the other input will pass through. I’ll use a QCD or the 2hp Div and have one channel of quarter notes and another channel of sixteenth notes or triplets to get these cool rhythmic stutters with just a couple of cables.

It has a lot of possibilities, but none of them are obvious.

I’m sure you see your modules in a lot of different systems. Are there any systems that have stood out as really interesting or unexpected?

It’s always cool to see our modules in Richard Devine’s cases. It is always hard to see stuff in those cases anyway and then when you can spot a little 2hp in there it’s like oh yeah! Other than that, we’ve seen some people do some very cool tiny cases with an Orthogonal Devices ER-301 and a 2hp Trim and a Verb along with a Pam’s Workout. It’s always cool to see people using 2hp modules for what they are, little tiny gap fillers.

Freeze Module; That was one of the most fun moments of inspiration that’s turned into a module very quickly.

Are there any modules you want to talk about?

I always love talking about the freeze because it was shockingly popular for awhile. It happened over a weekend because of being bored. We had the DSP platform working and the verb and delay were more or less ready to be released. I had some extra time and thought what happens if I just use this delay line like this, get it all weird and lock it. Then I added the sample rate control on top of that and it just started doing all these crazy things. All of the controls started slowing down and being weird because I was clocking the chip at four kilohertz or something ridiculously slow. For something that was only supposed to have less than a second of delay time, it ended up being almost a minute of delay time due to the low sample rate. It was just disgusting sounding for a whole minute, but I had a blast with it. I told my friend to check this thing out he’s like holy $#!*. Let’s make a front board and call this thing a module.

I was just messing around for a little bit and it turned into this super fun thing. I’ve seen people use it in crazy ways I use it and almost every patch. That was one of the most fun moments of inspiration that’s turned into a module very quickly.

The stuff that generates sound or manipulates sound tends to be the most popular. The utilities like the Mixer and VCA are popular too. You can have a million of them and still not have enough of them. Why get an 8hp 4 channel mixer when you can get a 2hp 4 channel mixer?

What modules have been the most fun to make?

A lot of them are fun for different reasons. Analog stuff like the VCO were fun because I’m poking around on chips that have been around forever. I’m trying to get them to work correctly but also maybe sound a bit better or different than they’ve always sounded, plus attempting to squeeze it onto a tiny little board.

The analog stuff is always fun for completely different reasons than the digital modules. I enjoy making them all, it’s all a blast for me.

You’ve found your place in the world

It’s amazing. I never thought I would get to do exactly what I want to do all day for a living. It didn’t start that way, for the entire first round of mixes I built the entire surface mount boards by hand. That was eight hours of doing a jigsaw puzzle every time. It’s nice to be able to focus on designing things and all the fun stuff now. I also have a group of people here that are so awesome, professional, and great at keeping stuff going out the doors.

You don’t have to worry about being a pure anything. You can make it different and people will like it for being different.

What are particular challenges for Eurorack? It feels like the wild west, if somebody has a crazy idea, they will find an audience

On one side, it is easy because weird is cool in the Eurorack community. You don’t have to worry about being a pure anything. You can make it different and people will like it for being different. Or it doesn’t have to sound like a lexicon 3800, it can be something unique and that’s what people like about it.

I think that’s what drew a lot of us to it in the first place. Nobody’s done stuff like this and I get to make something that’s completely my own with all of these little art projects that everybody’s made to fit together into their systems. That aspect of it is great.

The other side of it is let’s ensure that everything works with everybody else’s stuff. It’s always a surprise, we keep older modules around to test with things. If you plug in the gate from here does it trigger correctly or does something weird happen? The spec is not very well established and there’s no enforced specification for Eurorack. Everything is plus or minus 5 volts or plus or minus 10 volts. That’s both the pro and con. Again the limitations are not there in the sense of what we were talking about earlier and there is this freedom to do things. You see new ideas all of the time. Seeing people release things like CV recorders, what an idea that could only exist in Eurorack.

Are there a limited range of analog chips that everybody’s using to make modules?

I think the popularity of synthesizers, in general, is forming this resurgence of integrated circuit development. Some classic chips are becoming popular again but I think this popularity is going to cause new chips to come out that haven’t been available before.

We use the 3340 chip in our VCO. Those are very popular and I think a lot of Eurorack companies have released oscillators based on that chip. The 3340 goes all the way back to the Curtis days and it’s seen a resurgence. It was recently re-issued and other companies are working on different versions. There are a couple of other chips like that and I’ve heard of a few other modern IC manufacturers working on new oscillators. A few companies have created completely discrete oscillators based on discreet Buchla modules. There’s no way I’d ever fit that on a 2hp module without going crazy.

What about tools used for design?

For the hardware stuff, we use Eagle CAD by Autodesk. For firmware, I use Linux command-line tools, Vim, compilers, and Make to build stuff. I’ve been on windows for a while for a lot of reasons, some good and some not so good. Recently I’ve started using Visual Studio which also has its ups and downs for completely different reasons. There’s an integration that works well with our DSP platform. Those are the two primary tools. Of course, I use all of the Adobe Photoshop and illustrator and stuff that for doing a lot of the mockups and the asset design.

How do you come up with your ideas?

It’s fifty-fifty. One aspect is either an idea I’ve had for a while or random inspiration like the Burst or Freeze. Any modules that aren’t standard old things generally fall into that category of fun projects.

Then the other aspect is asking what things are we missing. What are standard function things that we just don’t have yet like Loop, Comp, and EQ? We have a spreadsheet here with sixty things we don’t have yet. The list is endless, it’s always a balance between choosing a standard function we haven’t made yet or something more unique and fun.

Any future things you want to talk about?

I’m excited about the lunchboxes. I mean they are lunchboxes and they are cute. They’re small enough that you can play with them on the couch while you’re stuck in quarantine for maybe the next six months.

When I get to engage with people and see that the things I create are actually making people happy. That’s definitely been one, if not the most, rewarding aspects of the whole thing.

What part of the whole experience has been the most rewarding?

When I get to engage with people and see that the things I create are actually making people happy. That’s definitely been one, if not the most, rewarding aspects of the whole thing. I enjoy events like the NAMM show where I get to see our customers and how excited they are about 2hp. For instance, a family from Japan with their two kids stopped by our table and they just had a ball for thirty minutes playing with these lunchboxes. It was so cool!


It’s great to hear Stephen talk about his experiences building the company, products and the Eurorack community overall.

There will be more articles to come highlighting the person and the process behind the instruments we love. Check back here or subscribe to the newsletter if you would like to hear more Synth Talk.

If you have suggestions or topics you want covered please contact me. 🙂

The 2hp Site

Future Music Magazine: Exploring the 2hp Range (2018)

DivKidVideo on the 2hp Verb (2018)

If you have suggestions or topics you want covered please contact me. 🙂

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