Behringer RD-8 Mods

If you’re interested in one of these, just be in touch.

RD8 instrument schematics can be found here.

RD8 with instrument mods, trigger ins and fx mods

Here’s a new playlist with demos of instrument mods, fx mods and trigger input mods.

Just like the TR808 and the Yocto, Behringer’s take on these iconic circuits offers itself to modifications. The most common amongst those wold be changing the decay of the bass drum, tinkering with the cymbal section, and, in the case of the TR, retrofitting midi trigger inputs.

Since the analogue parts of the RD8 are very faithful to the TR808 schematics, you can easily transfer many of the mods for the latter. Although the RD8 already comes equipped with midi per din/usb, analogue trigger inputs are possible, and since it also sports an FX unit comprising a Wave Designer and a resonant VCF, there is even more to mod.

RD8 wavedesigner/feedback mayhem (only RD8, no post-processing)

My work on the RD8 has been on/off, especially after I shot my first unit… I got a second unit after some interim and fine-tuned the trigger inputs while finding some more neat things to do with your RD8. A Beta Guide for RD-8 modifications contains some info on workable mods, yet eventually this will need some revision.

Rd8 on the mod bench

Different hardware revisions (1st, September 2019, December 2019)

Between my first RD8 from the initial batch and the couple of units I had on my modding bench, I found three different hardware revisions.

  • Units from September 2019: As some early adopters had found issues with noise introduced by touching some of the sound control potentiometers (especially in the Tom/Conga section), Behringer improved the top chassis shielding and implemented some rubber foam pads on some pots (as you can see on the first picture of this page). I never had noise issues with my early unit but more shielding doesn’t hurt!
  • Units from December 2019: In addition to the improved shielding, the rim shot level was greatly reduced in the third hardware revision. In earlier versions, Rim had a very high volume, especially in relation to Clave, which made live switching of both instruments a bit like living on the edge 🙂 Trivia behind this: in the first revision behringer chose 560 ohm for the Rim level attenuation and later on increased the resistor value to 82k – now, in the original Tr808 schematics, the value label of the respective resistor (which has originally 220kohm) is missing, which must have been an editorial error when publishing the manual. A look at the prolific Yoco scene might have helped “guessing” a more suitable value. Be this as it may, early RD8s were soon dubbed as “Rimzilla” (by Allotropes on GS) and are bound to cost 5k second-hand soon (or something)…
meet rimzilla
and learn how to tame it

Summing shenanigans and small oddities

Instrument summing in the RD8 is complex, especially since you have added internal FX, yet some aspects of this setup could have been easier, in my opinion. The 11 instruments of the RD are summed several times. Before leaving the main PCB to the single outputs, they are summed for the headphones outputs – this is why you always hear all dry instruments on phones, whether a single out is plugegd or not. This summing happens two times separately (once for phones left and once for phones right). On the jack board, plugging a single out jack substracts the respective instrument from the main mix. The remaining instruments are summed and this main mix is sent back to the main PCB. In addition, the single instrument signals post-switched jacks are fed back again separately to the main PCB again, where they can be sent to the FX. This FX switching is done by simple “ON” IC switches. As this would normally result in a 50% wet sound, (with an “ON” switch you add FX signal to the dry signal), an additional dry FX Buss signal with a phase shifted by 180 degrees is also added in order to achieve 100% wet FX. In brief: where you send an instrument to FX, you also create a signal that cancells out its main dry counterpart.

Final buss summing and that phase inversion question

The different signal busses

The different busses (dry, wet fx, inverted dry fx, and external audio in AKA “return” signal) are summed at C17 and a final op-amp stage (IC3). After passing C14, the mix then goes through your main level pot and is sent to the jack board output, where it goes through a final opamp and then leaves the RD8. Sadly, the resulting main mix is phase-inverted under certain conditions. There have been quite a couple of (rather adamant) complaints about this, so here is some visual documentation using screenshots from Jscope.

BD through single out
BD through main out (WD on, filter off)
BD through phones out
BD through main out (WD and LP on)
Bd through main out (fx off)
BD not recorded at all…

As you can see from the yellow markings, the signal is phase-correct where the yellow area is on the positive axis. The phase of the main out is not in line with the single outs when FX are off; they are in line when only the wave designer is on; and they are not in line when all FX are on. If this bothers you and your mixing desk/DAW does not offer tools to shift the signal polarity (i.e. invert its phase), you can use the headphones out for recording a main mix using a TS cable (no TRS, please) or keep all instruments on FX send and have the filter off and the wave designer settings at neutral (attack and sustain knobs at 12 o’clock).

Technically, I first thought this could be amended easily by adding another inverting op-amp stage, but since the 180 degree phase flip depends on whether the filter is engaged or not, the most sensible act might be to decouple the FX block from the main summing stage and give it an extra output. This way you could still engage FX send per instrument internally but have your main mix neat and a separate FX track at your disposal. Proper modding writeup on this will come soon.

Update 31.08.2020: it seems there’s another trick to have the main out in the same phase as the single outs (7db lower in volume but at least something): https://www.gearslutz.com/board/showpost.php?p=14944398&postcount=9002

The hidden envelope follower in the RD8

Apart from small curiosities in the summing section of the RD8, the biggest surprise in Behringer’s add-on to the 808 circuits comes in form of an unused filter envelope follower.

Unconnected envelope follower for the RD8 FX-section filter

Located around the quad op-amp also used for final summing you can see two unpopulated resistor places (R12 and R42). The pads of these which are conected to the TL074C affect the filter cutoff when fed with audio/CV signals, whith the lower pad of R12 affecting positive amounts and the upper pad of R42 negative amounts. Most likely these were to be used with the FX buss signal (I yet need to see if/what signal is present at the other terminals of the empty resistors). It’s a bit of a shame these never came to live in the RD8 (especially since the hence unused op-amps could have been used for one further main mix inversion so that the Mono out mix is in line with the single outputs, phase-wise). Yet, you can very easily mod your RD8 for envelope follower action. (More on this below.)

Cowbell envelope used to trigger Wave Designer / Filter Envelope Follower

Some Mods for the RD

This section contains some essentials rather than a comprehensive list of what’s possible- for more detailed bending info, check the guide and/or the excellent Yocto/TR modding pages.

Implementing Trigger Inputs for Individual Instruments (revised 4/20)

Firstly, I got all instrument triggers working nicely in parallel with the internal sequencer.

InstrumentMod Point
BDT51 top
SDT52 top
LTT65 top
MTT67 top
HTT69 top
RST24 top (part is upside down)
CPT54 top
CBT56 top
CYT44 top
OHT47 top
CHT46 top
PCB front top view, sequencer step buttons facing south
Trigger input point locations on PCB (CP transistor hidden behind switch)
And most trigger input locations on the solder side of the PCB – note that LT and MT trigs are not present there

When wiring these up, best conect your trigger points via cables to a small circuit board to which you can then connect your trigger input jack sockets. Trigger inputs need to go through standard diodes (cathode in the direction of your RD8 mod points) in order to have external triggering and internal sequencing available at the same time. The clap trigger needs at least a 33k resistor in between in order to work properly, and all other triggers should be protected with small resistor values too (1k).

As per TR808 service manual specifications (p.4), these trigger ins are designed for 1ms triggers between 5-14Volts. (NB that when you have the internal accent runnig, the voltage of that is added to the voltage you feed into the trigs.) An advantage of using external triggers, apart form aspects of timing, which is tight AF by the way, is that you can play each intrument with velocity (if your trigger-producing device is capable of the latter).

Timing stress test for triggers (from Expert Sleepers ES3) – Rim/Hat, MT/HT, Clave/Maracas/Bd,
Rd8 trigger inputs played velocity per instrument (Rd8 sequencer not used)
Trigger rimshot fast enough, you have a synth sound

Hypothetically, you could also easily turn the RD8 into a CV trigger sequencer by tapping the same points listed above and feeding their signal through diodes (anode to PCB point) and 1k resistors out.

Manual Accent Trigger

The accent circuit in the RD8 follows that of the 808, so you can do nice things such as implenting a maual accent trigger button. This gives you a cool performance option by means of which you can fire off accents while twiddling away with accent volume (snare roll anyone?). For manually triggering accent, wire a momentaty on button (normally open, i.e. closed when depressed) between the base of T11 (lower left leg) and ground. T11 is just left of VR19 (BD tune).

Pattern with manual accent play

Bass Drum Mods

Bass drum mod points marked yellow: lower filter, envelope to pitch, longer decay
Mod points for extra decay and pitch env depth on back of PCB.

Bass drum extra decay: The RD8 bass drum can certainly go long enough for decent kicks, yet for them booooomy basses it is too short. You can easily extend the range of its decay by wiring a 1megaohm resistor in parallel to R322. No extra switch is needed here since the range of your decay pot is well wide enough. If this is already to boomy, then use 1.1megaohm. Alternatively, take out R322 and replace it with a 47k trimmer to adjust your desired sweet spot. BTW you can access the terminals of R322 on the back of the PCB.

BD decay pot turned form zero to full (with mod implemented)

Envelope to pitch: the amount by means of which the short pitch envelope at the sound onset affects the bass drum is regulated via R372 (6.8k, R166 in the 808). You can lower the value of this resistor using a switch and a fixed resistor (2.2k) and pot (5k) in series or go an easier route and wire a 100k potentiometer, followed by a resistor (I prefer 4.7k) in parallel to R372. When the pot is on zero, you don’t have any noticeable effect and when it’s full, there is a more pronounced “smack” in the kick, which also responds very nicely to accent. Also here, you can access the terminals of your resistor on the back of the PCB

Lower tone filter cutoff: if you find the bass drum with the lowest “tone” setting still too clicky, you can enlarge the value of C177 (0.1uf) by wiring another capacitor in parallel – I find another 100nf cap hits the sweet spot. Easiest way to wire your new cap in is between the middle pin of the tone control pot and ground.

Some Snare Mods

Higher Level: The RD8 snare sound is very likeable, yet in places maybe a tad to low in volume. You can increase its level by lowering the values of R198 and R199. Since the snare sound consists of two components – a body that’s made out of a T-Bridge networks and a noise component, you can also change the volume ratio of these. If you want more snappy noise, just reduce R199, and if you want a bit more body impact (a version I prefer), lower R189, for instance by wiring a 100K resistor across the terminals of R189 (see pic above).

Tuning the Snare oscillators: the main body of sound in the snare is generated by two T-bridge oscillators that can be mixed by the “tone” control pot. The frequency of both is fixed: T-bridge1 is a bit lower, while T-bridge2 is tuned higher (also, T-bridge1 is less filtered, while 2 gives a duller thud). A classic way to implement adjustable tuning is to replace fixed resistors in the T-bridge path (a 820k resistor for T-bridge1 and a 1megaohm resistor for T-bridge2). The way these circuits are set up, however, also means that the lower you tune your oscillator the more it loses level, so the useable limit is within bounds. A working alternative to replacing the rather large resistors with (rather large) potentiometers, is to replace those smaller resistors (R414 and R268) feeding the T-bridge circuit to ground.

Snare mod points marked yellow: snappy noise replacement, oscillator tuning, component levels

What works well for me is to replace R414 (680ohms) with a 330ohm resistor and 1k pot in series for oscillator 1 and to replace R268 (2.2k) with a 2.2k resistor and 1k pot. This extends the tuning range of oscillator 1 in both directions and extends that of oscillator 2, which is already quite high downwards. To save some space, I have both mods on a dual gang potentiometer. Generally, I find this a more convenient way in comparison to replacing the larger value resistors (marked yellow but unlabelled in the picture above) because you can wire everything up from the back of the PCB (R414 and R268 connect directly to right-hand legs of the nearest yellow film-box capacitors). That’s how it sounds:

Snare tuning demo (with some noise level mod in between)

Alternative snappy noise input: C295 (0.022uf capacitor just left of the LT/LC switch) is where white noise is fed into the “snappy” component of the snare sound. If you want to play with sound source variations, lift that cap up (upper side) and use a switched socket or switch for alternative noise sources (such as the metal noise used to the hats or any other source – try a saw wave, for instance). You need some resistance in between with the value of your resistor matched to desired effects.

Snare with saw wave input instead of white noise for snappy component
Behringer model D played into snare snappy components and tom reverb component

Rim/Clave mods

Apart from adjusting the level of rim (in case of Rimzilla), I find variable rim/clave pitch the coolest of the bunch of options (you can also adjust the decay and how much rim is distorted, but these mods are not igh on my own list).

Rim/Clave pitch: replace R380 (1k) with a 680ohm resistor and 2k potentiometer in series, this way both sounds go a bit higher and a ways lower. With a yet smaller resistor and larger potentiometer (up to 5k) you can massively increase the span and turn clave into some screechy techno sound (with a lot of sonic energy around 1khz, so prepare for ringing ears too). Best point for soldering your new resistor+pot is on the back of the PCB (see clap mod pic below) – the other terminal goes to ground.

Clave/rim tuning with extreme values for pot (5k) – later sent through fx

Clap Mods

In technical terms, the 808 clap is a composite of bandpass-filtered noise (the reverb component) and a complex envelope network opening and closing a VCA (the “rattle” bit at the begining of the sound, called burst by Roland). Behringer have already implemented some “mod” by locating the internal trimmer for the off set of the burst in the 808 to the chassis outside in form of a knob.

CP mods
CP mods and RS pitch mod on back of PCB.

Reverb level: you can change the level of the reverb component by changing R153 (100k) or replacing it with a 20k resistor and 100k pot. I personally don’t find this yields super intreresting results.

Clap density: you can change the density of the initial burst by altering the value of R147 (82k). Just remove the resistor and replace it with a 47k resistor and a 100k pot in series. This way you can nicely (although not too drastically) spread or squeeze the burst in addition to the offset pot. You can acces the terminal of R127 from the back of the PCB (the other termonal is ground).

Clap filter frequency: you can easily make the cutoff of the bandpass filtered noise component of the clap variable by replacing R253 (10k) with a 4.7k resistor and 25k pot in series. Here too, you can access the necessary terminal from the back of the PCB (the otehr terminal is ground).

Clap densityand filter frequency mods

FX Section Mods

I also had a closer look at the FX section again, initially trying to find ways to bypass the silly phase inversion thing in the instrument summing sections, and found a couple of hidden gems. There is a much louder pre-volume dial output of the main mix, and FX parameters of the wave designer and the resonant filter can be modulated by external CV.

Modding the “Return” socket for feeding external audio through FX

With the solution for FX insert switching chosen by Behringer, adding an external sound input that is to go through the FX buss, needs to be fed to several places simultaneously. his is because feeeding an external audio signal into the FX Buss also automatically results in an inverted dry version of the same signal (since this is how internal instruments are phase-cancelled when sent to FX). You can do this as follows: remove R2 and connect the top solder pad of R3 on the PCB through a 15 resistor to the anode of C75 and also through a 20k resistor to the anode of C58. (I tried other values with 5k difference but these resulted in bleed.) Alternatively, you can use this on a switch and toggle between dry return (switch to bottom pad of R2) and wet.

Behringer model D oscillators into the RD8 filter with CV on cutoff
Different loops sent though RD8 FX and spiced up with internal instruments
External input signal that properly passes FX unit

Direct FX outputs – LP out and HP out

You can easily tap two points on the solder side of the PCB to have direct outputs from the lowpass filter and the highpass filter. These signals are available simultaneously and regardless of your filter buttons’ status. The signal, of course, is post-wave designer.

Direct filter outputs
LP/HP outputs hard panned with filter sweeps at full resonance

FX send output

At the top of R4 you can tap the dry fx buss (the sum of all instruments sent to FX used for phase cancellation, read above). You can use this very nicely to send your instruments to an external FX processor, like to a delay in the example below. If you want the instruments sent to fx to be silent in the RD8 audio path, engage lp filter and put cutoff to zero, so that all instruments sent to fx “vanish” from the RD mix while being sent to your external FX.

Individual RD8 instruments sent to external delay through output from fx buss

Filter envelope follower usage

While IC3-D (top left part of the quad op-amp) is used for final buss summing, the remaining three op-amps are used for filter cutoff CV purposes. IC3-A is an unconnected cutoff CV connection with negative polarity (a decay envelope fed into it makes cutoff sweep upwards). Best feed CV into the bottom pad of R24 for using this. The output signal of this op-amp is mixed with the current from the cutoff potentiometer at IC3-B. Here (at the bottom of R23) you can feed in cutoff CV for positive polarity. At IC3-C, you have an unused cutoff CV input with a larger capacitor that gives your incoming CV or audio signal (yes, this takes audio too!) some slew, i.e. envelope follower action. Best use bottom pad of R12 (unpopulated) for this. Any incoming control signal should be buffered with a 1k resistor (at least) and if you want to retain the cutoff potentiometer action, you need to go through a diode when using IC3-B or IC3-C. With a switch you could toggle between positive FM, negative FM and envelope follower. While positive and negative cutoff CV can be applied to points on the back of the PCB, the envelope follower input at bottom of R12 cannot be accessed from the back.

Access points at PCB back for Cutoff CV (positive/negative polarity), Resonance CV, Wave Designer detection input (D21) and direct LP and direct HP outputs.

Modulating the Wave Designer

You can feed CV or audio signal to mess with the wave designer in various places. The easiest way is to feed your control signal to top of D21, which is the entry point for the amplitide detection circuit. Go though a small value resistor and a diode when using this. If you want to go wilder, fire some control signal to the bottom of R308.

External input modded to play through FX unit and be used as mod source for FM and AM
Fx buss modulates and Wave Designer at bottom of R308 and filter cutoff (negative FM)

50% wet FX rather than insert

If you remove R4, the FX busss woks in parallel rather than as an insert (this is because phase cancellation is deactivated).

more soon

Some further mod impressions: RD8 roadshow