Behringer Neutron Fixes and Modifications

Neutron with additional switch for VCO hard-synchronization

The Behringer Neutron offers much for a synthesizer, and although its semi-modular patch-bay does not leave much desired for adding functionality, there are a couple of modifications worthwhile undertaking. Most of these do hardly alter the scope of sounds this little red thing produces but rather adjust and improve some minor aspects addressed in various synthesizer forums.

What has caught my attention there and in the Neutron itself are:

  • the volume of the square wave outputs of both VCOs are rather loud in comparison to the other waveforms and tend to overdrive the filter input stage a bit too much for use in mellower sounds
  • the rather fast ADSR envelopes tend to produce a click when attack and release are at low values, which is very audible on the VCA of my unit when I use a simple triangle wave; especially for ENV 1 (per default routed to the VCA) it makes sense to reduce this click
  • the hold capacitor of the sample & hold circuit loses charge relatively quickly, which results, when S&H is patched to VCO frequency, to pitches falling flat rather than staying on frequency

All three of these things can be adjusted easily.

Further down the line, there are some more laborious changes, such as implementing a different type of VCO hard-synchronization and altering the normalized signal path of the synthesizer (which can all be done nicely with patch cables anyways, so the latter is maybe just a matter of preference). Further things will probably come along.

Part of ADSR 1 of the Neutron, all analogue and built around CMOS gates and switches


Square Level Adjustment

Adjusted level; through-hole resistors also have plenty of space

Because the waveform outputs of the pulse wave on the 3340 is considerably higher than that of the other waves (+12V in contrast to +10V for the ramp, for instance), it is usually attenuated more than the others. Not so in the Neutron, where the ramp (i.e. saw) and square outs are both followed by 18K resistors. While this seems to be a good value for saw, your filter input stage can be a bit overwhelmed by the full oomph of the ramp, especially when both are engaged and/or PWM is active.

With 26K for your new resistors, the balance between saw and square is ace, but 25K or 24k are also fine (with the latter, the square a tad louder than saw). For VCO 1 replace R61, and for VCO2 replace R63. (If you have not lifted SMD components yet, best do a bit of training first; there’s also a little guide in my ressources section).

technical demo with some VU metre aid

Declick the VCA envelope (connection between ENV1 and VCA control)

You can nicely reduce the click audible on the Neutron’s VCA on low attack and release values for ENV 1, which is pre-routed to VCA amplitude modulation. Sice this pre-routing is done by means of a switched socket on the Neutron’s patchbay (when a jack is plugged into the VCA CV socket, the signal from ENV1 is disconnected), and since the Neutron’s patchbay are accessible from the back of the PCB, you don’t need to take off the red faceplate. simply take your Neutron out of the case, flip it over and look for C318, which you enlarge by means of a 1uf capacitor.

ENV1 declick in place

How this works is that the additional capacitor “rounds” (filters) off the ENV spikes a bit while keeping everything snappy about the ENV itself snappy. The capacitor placed there also does not round off any other CV sent into the 1. VCA CV plug or 2. the ENV output at the respective socket on the OUT section of the patchbay. For 1. you would place your cap over C340 and for 2. over C319. As for the value of your additional capacitor, I find 1uf works well. If you use a polarizing cap (the blue ones with the white side), the cathode (white-side leg) goes where the lower leg on the picture is (away from the normal pin of the jack socket).

ENV declick – mod is switched on and off a couple of times

Enlarging the Sample and Hold Capacitor

S&H voltage drop fix off/on

In the audio demo linked above, you can hear the S&H on VCO pitch. In the beginning of the file, no mod is engaged, and you can hear the pitches of the VCO falling flat after a little while. This is on account of the rather limited storage capacity of C42, which holds the sampled voltage until the next sample is taken. If the capacitance of C42 is increased (by just wiring another capacitor in parallel), the sampled value remains stable. This, amongst other things, allows for extremely precise “computer room” sounds 🙂

Location of C42 is on the upper right (new cap not soldered in yet, but basically this works like the ENV click fix)

I’ve not tested the value of C42 nor experimented with different sizes for additional caps but got good results with 1uf immediately, so that’s what I’m going to solder in. but NB I like sloow S&H action – the larger your cap the lower your maximum uptake rate, i.e. if you need fast S&H, you want lower values.


The Neutron Slew Limiter changes your CV, which results in bad pitch tracking, when used for pitch CV signals. You can incrae its accuracy by simply bridging  R590 and C588 with some wire.

Slew limiter fix
Slew and vco tracking A/B

VCO hard-synchronization

The Neutron offers oscillator synchronization, yet users soon noticed that VCO sync sounds different here to what one might be used from other CEM3340/V3340-based VCOs (for instance, the delicious electrosmith VCO submodules). Some generous and tech-savvy person opened their Neutron, provided a good analysis, and offered a video tutorial of how to change this. My version of this is fully functional but maybe not as elegant as I remember the other one. However, I am sure you can easily improve it.

What happens in the Neutron sync-wise as is, as far as I understand it, is shown in the CEM/V3340 datasheet in figure 4. This is indeed hard oscillator synchronization, yet only on the rising/falling edge of the waveform (which sounds a bit like soft-sync). What most of us are used to, from synthesizers such as the Prophet 5, is what Curtis termed “conventional hard sync”, as decsribed in figure 5 of the same datasheet (see also Electricdruid’s great page on the 3340). If you want this as a second option, as I did, you need to make a little circuit as shown in figure 5 first and then hook this up to your Neutron.

Let’s do this in reverse, since the connections to the Neutron are the fiddly bit.

Connections to Neutron for conventional hard-syncing of VCO1->VCO2 marked green

These instructions are for syncing VCO2 to VCO1. As you see in fig. 5 pasted into the PCB shot above, you need connections from PIN9 of your second V3340 (that’s the soft-sync input, yes SOFT-sync input) and PIN10 of the same 3340 (that’s the triangle wave output). For PIN9, there is no alternative solder point, yet its corner position makes soldering a tad easier. For PIN10 there must be a good alternative (the terminal of the output resistor just after TRI OUT), yet I have not found it yet. NB soldering in these areas requires a thin tip and thin wires with already some solder applied (suction is your friend here!). Make sure you don’t have a solder bridge between the pins you just wired up.

You then need the ramp (saw) output signal from VCO1, which I took from the top terminal of R62 (the green blob between the two through-hole resistors from the square level fix). If you want to use an external VCO for your sync in as an alternative, you could wire your ramp output to the normal pin of a switching jack socket, so that’d give you even more options.

Finally, you need some ground, and a negative power supply (I took it from the bottom of C157, which buffers the negative supply voltage for the 2164 quad VCA (IC39). (Tested and confirmed by Aart – thanks!).

Then assemble the little circuit as shown in the datasheet, figure 5 and wire up.

Piggy-back PCB with conventional hard-sync circuit (two caps, two resistors, a transistor).
Use a spacer like this to mount your additional PCB.

What you also need is an on/off switch between the negative power supply and the transistor. Important: this on/off switch is not only necessary for deactivating hard sync but also needs to be off when powering up the Neutron since having hard sync on interferes with the VCO tuning routine at startup. This is not harmful, it’s just that the Neutron cannot tune VCO correctly and gets stuck in the routine (if that’s the case, switching sync off and powering up again does the trick).

And that’s how it sounds in comparison:

SYNC types demo: first pass no sync, second as in Neutron, third in the modded “conventional” version.
And another hard-sync demo (Neutron filter bypassed for modded Ladik Synare filter goodness)

Advanced SYNC Implementation with latching switch

This is in from Kees, who worked on an ace way of using a latching switch that is automatically off when the Neutron boots. This way, you bypass the tuning issue/boot hangup described above. This is proper stuff with an indicator LED and Kees generously agreed to have the schematic shared (thanks!).

the circuit
hardsync with latching switch and led indicator courtesy of Kees

More soon…