Behringer RD-8 Mods

For RD8 MKII MODS, go here.

Apologies for not providing any further updates/ or pending mod descriptions for this project any longer. I describe many mods here (more than the classic mods for the original), including mistakes and spelling glitches, and feel I rather want to spend time on other synth modding/development projects.

October ’21 – time for a bigger update

In the coming weeks I’m adding more modifications. What’s in store? Correction of the phase issues with main out-single outs, some hi hat tweaks, replacing cowbell sound with maracas and some things more.

In the meantime, I’m still chuffed that one of my musical heroes, Jamie Lidell, featured an RD8 modded by yours truly in his HANGING OUT WITH AUDIOPHILES podcast.

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

And RD8 instrument schematics can be found here.

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

Different hardware revisions (1st, 9/2019, 12/2019)

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 ๐Ÿ™‚ Trivia behind this: in the first revision behringer chose 560 ohm for the Rim level attenuation (R513 on the RD8) 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 with oddities

The 11 instruments of the RD are summed several times separately. First, signals are summed for the headphones outputs before they pass to the switches instrument output sockets – this is why you always hear all dry instruments on phones, whether a single out is plugegd or not. On the jack board all instruments are summed again, yet plugging a single out jack substracts the respective instrument from the main mix. The “main mix” of remaining instruments (i.e. not subtracted via single outs) is sent back to the main PCB and mixed, in turn, with the external input and FX output. The signals of “non-single out instruments” are also sent back individually to the main PCB for FX “send” duties. That FX switching is done by simple “ON” IC switches, which would normally result in a 50% wet sound, (with an “ON” switch you add FX signal to the dry signal), so an additional dry FX Buss signal with an invetred polarity is created that cancels the sounds of the respective instruments “sent to FX.” Come again? 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 thing

The different signal busses

Okay, now stuff gets brainy…. The different busses (dry, wet fx, inverted dry fx, and external audio in AKA โ€œreturnโ€ signal) are summed at C6 and an 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. With all that mixing and whatnot there is one draw mentioned oftentimes by users, namely, that the polarity of the main instrument mix as heard at the main output is inverted in relation to the single instrument output jacks. in practical terms, this means that parts of the signal can cancel each other out when using an indidivual instrument out and the main mix together, especially in the hat/cymbal section. If you dislike this and don’t want to solder, you have two options: either use the headphones output as a main out (just use a normal TS cable) or send all instruments to FX and keep the wave designer on but the filter off.

Rewiring FX and Main Mix for Convenience

With these simple modifications you get the correct signal polarity at mix out and repurpose the “RETURN” jack socket to carry a 100% wet FX signal while FX stay out of the main mix. I personally find the latter more useful since you can use this extra FX out in conjunction with a mixing desk or FX pedals or an additional channel on your audio interface for more creative uses than adding just adding the internal FX to your main mix. Send your snare spontaneously to a delay? Can do!

1. rewire summing on main PCB

First, we rewire things on our main PCB. Remove R2, R3, and R4. Pull out the anode of C6 and rewire it (the anode) to the vacant top terminal for R3. With the aforementioned resistors removed, you have teh FX output only available at the vacant PCB terminal of C6, so wire a cable to that for your new FX output (see below).

second strike: rewire post-switched socket instrument mix

Now, we first rewire the mix of all instruments after the switched single outputs and nmake sure it has the signal polarity we need. For that, remove R51 and rewire its vacant top terminal to the top terminal of R67 (keep that resistor in place). Cool, now we have a main mix signal as we want it! [On this picture you see that I also removed the op-amp underneath R51 because it’s not needed any longer- just keeping it in is also fine though!]

thrid strike: rewire FX signal to RETURN socket

Finally, we rewire the RETURN socket to carry our 100% wet FX signal. Pull out C25 and C26 and put the CATHODE of C26 in its former ANODE terminal on the PCB (i.e. white stripe leg goes into the hole *not* marked with white colour). remember the wire you connected to the anode terminal of C6 on the main PCB? This is connected to the freely-dangling anode of your rewired C26. Done – the RETURN socket now carries the 100% wet output of your Wave Designer and Filter FX.

Hangon, there’s more! The RETURN socket is TRS (and apparently was meant to be used in an FX send/return fashion before the RD8 was thrown into the market prematurely), so let’s use this option too! Pull out C25 on the jack board and wire a 4k7 resistor to the vacant anode terminal (the terminal *not* marked in white). Connect the 4k7 resistor to the anode terminal for C75 on the main PCB. When you use a TRS to two TS splitter cable, you now have an FX input and a dedicated FX output. [NB the 4k7 resistor is chosen for line signals; when using modular level, use at least 47k.]

Increase Main Output Level

The main out level of the Rd8 is on the tamer side, and you can very easily boost it by wiring a 4k7 resistor across R89 on the jack board.

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. It’s a bit of a shame these never came to live in the RD8, 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

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.

Initial โ€œblamโ€ and pitch-sigh: you can increase these by augmenting the value of the 15nf capacitor at the pulse shaper (C260, 15nf) to around 40nf (I have 37nf on mine). Here’s a demo of C260 with an additional 22nf on top:

pulse shape cap up by 22nf

Bass Drum Capacitors: Replace C240 and C241 with film box capacitors (15nf) โ€“ I set the base pitch of the kick on my unit lower by using 18nf caps instead.

tbridge caps replaced with film box and pulse shaper cap augmented

Clearer overall sound on BD: recplacing C72 and C112 with film box capacitors (0.47uf) clears the sound a bit.

Bass drum overdrive (massive): this mod increases the BD level so much that the sound clips into the mixer stage and goes overdrive. Wire a 100ohm resistor and 25k(A/log) pot in series and put these between ground and IC19, Pin13. Vroom!

drive that bass

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.

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

Fuller clap sound: C78 at the end of the clap signal chain acts as a highpass filter. wire a 100nf-200nf capacitor across it to let lower frequencies through.

Reverb level: you can change the level of the reverb component by changing R152 (47k) or replacing it with a 20k resistor and 100k pot. I personally don’t find this yields super intreresting results.

Increase reverb tail: augment C250 by some 20nf-30nf. If you want to go bonkers, use 100nf.

Variable reverb tail length: Augment C250 by 100nf and wire a 1m potentiometer between the top terminal of C250 and ground.

Use Cowbell for Maracas

If you don’t like the cowbell too much but rather have the option to play clap and maraca at the same time, then I have news for you… ๐Ÿ™‚

NB this mod replaces the CB sound entirely, i.e. you use its sequencer track for Maracas trigger and its volume knob for Maracas level. The original CP/MA switch remains unaffected, i.e. when you have MA selected on CP/MA, this also triggers MA (which is cool for sequencer tricks).

you want your new diode in heatshrink tube to play safe and sound

First, let’s make the CB track trigger maracas. Remove D58 and wire a new diode between the top PCB terminal for D58 (anode of your new diode goes here) and the top terminal of D61 โ€“ this way your rim sequencer track triggers maracas.

rewire maracas sound via cowbell volume pot

For re-routing MA sound output to CB, first remove R153 (100k). Then remove C79 and wire the vacant bottom PCB terminal for R153 to the vacant top PCB terminal (anode terminal) for C79. Just using wire is enough here.

Hi Hat Modifications

As for the metal noise and whether it can be tuned – nope. (Not sure if this will be possible on the MKII, yet here the hexinverter is driven from CPU signals rather than a resistor-capacitor-setup.)

More body for the Hats (yep, another one for “guts”): A good deal of more impact can be had by removing two capacitors at the end of the HH signal chain as these remove (not so) low fequency content. For more body, just remove C110 and C129 and bridge their respective PCB terminals with solder or wire.

guts to that haaaats

Metal noise filter tweaks and level adjustments: This on is a matter of taste, really, but I found the HH sounds a bot too distorted and mushy. Trying to obtain a bit of a clearer sound I’ve settled (for the time being) on the follwoing tweaks: wire a 1k resistor between the top of C288 and ground; another 1k resistor goes between bottom of R430 and ground; finally wire a 220ohn resistor between the bottom of CC257 and ground. (First resistor reduces level into pre-hh bandpass, second one sets bandpass cutoff slightly higher; third reduces level of bandpass output).

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)

Further mods for the RD

Well, the old Beta Guide for RD-8 modifications contains outdated info, yet I just keep it linked for archival reasons.

Some further audio impressions: RD8 roadshow