Arturia Beatstep

Arturia’s Beatstep is both a controller and step sequencer. In sequencer mode it synchronises to MIDI from a computer, but also transmits both MIDI and (more interestingly I think) CV/Gate signals, enabling it to sequence analogue synths.

Step sequencing is inspirational and very productive, so when the Beatstep originally appeared in 2014 I grabbed one as soon as I could.

Here’s one of Arturia’s official videos demonstrating the very basics of how to use it with analogue synths:

 

Trigger Unhappy

There are plenty of things to like about the Beatstep: it’s fun, it does plenty, it feels solid, it’s easy to use, and it’s cheap. However, it does have issues, one of which being that the analogue Gate output does not trigger some devices.

Out of respect for Arturia, who I think have made a fine product, I am not going to cite online complaints regarding this issue — a quick internet search will show how common it is. Not all interconnected equipment is affected, but when it is, the issue becomes a real frustration.

Having experience the problem myself, I initially suspected my own devices (particularly DIY builds), but it became apparent the Beatstep itself was part of the cause. I decided to take a look inside to see what was going on.

Inside the Beatstep

Should you want to do this (and I don’t recommend it unless you’re both competent and nosy) here’s how you get inside the Beatstep:

Pull the knobs off; they are firm but not glued. The metal base plate is held on by several screws; remove it. The single large PCB is held secured to the upper plastic body by several more. The whole thing is quite sturdy enough. On lifting the PCB out of the shell, take care to not catch the rubber pads, as they can be torn with clumsy handling.

Arturia Beatstep: top

Arturia Beatstep: top

Arturia Beatstep: bottom

Arturia Beatstep: bottom

Arturia Beatstep: inner rear

Arturia Beatstep: inner rear

Arturia Beatstep: inner top

Arturia Beatstep: inner top

The bulk of the Beatstep’s PCB is given over to the pads and the encoders. The components gathered towards the upper left are the CPU (an ARM processor) and the various associated control circuitry. The analogue outputs are located at the far left. This is where my interest lies in understanding the triggering issue.

The Gate Output in detail

Here’s a close-up of the Beatstep’s Gate and CV outputs:

Arturia Beatstep: analogue outputs

Arturia Beatstep: analogue outputs

The Gate circuit looks simple enough, and appears to contain just a few discrete components. With some care it is possible to trace the following connections and measure some values:

Beatstep Gate photo with partially traced connections

Beatstep Gate photo with partially traced connections

 

We can turn this into a more intelligible schematic thus:

Arturia Beatstep Gate Output schem

Arturia Beatstep Gate Output partial schematic

 

As we can see, the circuit uses just standard NPN transistors and a few resistors. How does it work?

The gate signal from the processor comes to the base of Q1; during Gate Off, the base of Q1 is held low, sending the base of Q2 high, which turns Q2 on and takes the Gate Output low (to around 0.8V, the sum of the voltage drop of the transistor junction plus 1/101 of the potential between collector and 8V); during Gate On, the control signal takes the base of Q1 high, which turns Q1 on, taking the base of Q2 low. This turns Q2 off, meaning the output is held at 8V during Gate On.

Now we can see why the triggering issue occurs: during Gate On, with nothing attached to the output, the voltage at the output is 8V. However, that 100k resistor means that when you plug something in, that voltage drops. Why?

Imagine you have a sequencer or envelope generator with an effective 100k input impedance. When you plug the Beatstep into that, a potential divider is made with 100k at the top, 100k at the bottom, and the output voltage in the middle. Your 8V Gate becomes a 4V Gate. This may or may not be enough to trigger your external device.

Whether or not the Beatstep’s Gate triggers your gear depends on the kind of input your gear has. If it has a very high input impedance and is designed to work with relatively low trigger levels (eg., some Roland gear needs only 2.5V) then you should be fine. If, on the other hand, your gear has a lower input impedance and is designed to expect higher trigger levels then you might not have such a great time with this. My own Moog Rogue, for example, seems to not reliably trigger from the Beatstep directly, though the Beatstep works well with my Roland CSQ-600 sequencer. The Rogue expects at least a 3V trigger, but its low input impedance means that the Beatstep’s nominal 8V is reduced to around 3.4V (as measured); in reasearching for this blog, I had one session of no triggering, and one session which worked fine.

EDIT: I had included a diagram here showing how this functions, but I later spotted an error. I’ll correct it later, but have removed it in the meantime so as not to consciously midlead. Apologies for any inconvenience.

 

Solutions

I would not recommend modifying the Beatstep. Even if you are handy with SMT soldering, there are probably good reasons why Arturia chose to go with this design, and I’d rather leave it so.

However, it is a simple and cheap task to make a small utility circuit to buffer this Gate signal and overcome any triggering issues you may have that are caused by this issue. Alternatively, if you’re a modular user, you probably already have some buffered multiples or similar that would do the job perfectly.

*NB: I will be blogging a set of basic utility modules for DIY shortly: link to follow*

Advertisements
%d bloggers like this: