c o n f u s e d m a c h i n e s | v . 0 2


Bass Drum

Decay and Tune
Diode Pitch Envelope
Tone Control

Decay and Tune modification

Here is the fantastic Plutonique9 bassdrum decay and tune mod for the TR-606. It involves reconfiguring one of the opamps in the BD circuit as a feedback stage. It has been tested by myself and works damn well. p9 has asked me to send it along to AH. Unless indicated, all the words are from p9.

Every 606 owner that craves a decay control should have a go. It is not difficult, just fiddly and a little time consuming. I did it in a couple of hours.


A schematic of the BD mod by Frederic Bourgeais. Click to enlarge.

After doing this modification, you are still a little short of having a 808 bass drum in the 606, because the 808 bass drum also has an envelope (which is influenced by the accent trigger) which sweeps the pitch, and the decay time is also influenced by the accent trigger.

This is additional circuitry that i copied out of the 808 schematics and added to a tiny circuit board inside the 606. As you can imagine, this kind of goes beyond the scope of most modifications, cause it involves a little know-how of electronics and interpretation of schematics, which would confuse a lot of people. Still, even without this, this mod still make the kick sound incredible, with smooth controls over both decay and tuning of the bass drum. Just missin' the nice solid punch the envelope circuitry is necessary 4.

1. Remove Resistor R60 and replace with a wire link (or zero ohm resistor). This will boost the volume of the "tight" drum oscillator, and also brings out the "high freq. crispness" of this oscillator, so its not so "muddy" soundin'.

2. Remove Capacitor "C16"

3. Remove Resistor "R56" and replace with a "470k" resistor. Now solder a wire between "R56" (the side the connects with "C16"), going to the point on the circuit board between Capacitor "C23" and "C24" (where they both meet).

4. Cut the trace going to "Pin 3" of IC5, cut the trace as close to where the leg is soldered to the board. Important thing is that "Pin 3" is isolated from any connection.

5. Remove Capacitor "C19" and replace with a "33uf" polarized electro Capacitor. The "+" leg of the capacitor should go into the hole whose trace meets up with "R58" and "C20", and the "-" lead of course should go into the hole whose trace meets up with "R56" and "R59", so make sure you have the cap in the right direction.

6. Remove Capacitor "C20". In place of the capacitor, run two wires from a "2.2meg" potentiometer to the holes left by "C20". This pot is what will control the decay time of the feedback oscillator, you know, boooooooooom!!!!!!!!

7. Remove Resistor "R59", and replace with a "47k" resistor.

8. Remove Resistor "R58", and replace with a "10k" resistor. Now you have to isolate the connection leading to "R58" from the junction at "C19" and "C20". So cut the trace between "R58" and this "Junction". Now solder a wire from "R58" (the side on which you cut the trace) to "Pin 3" of IC5. This is the pin which you cut the trace leading to.

9. Now you need to solder another "10k" resistor between "Pin 3" of IC5 and the positive 15V+ power supply. The easiest place to find the 15v+ is at "Pin 8" of IC5.


**** Step 2 through to step 9

The purpose of all this was to reconfigure one of the two bass drum oscillators in the 606 bass drum to be a feedback stage for the single remaining drum oscillator.

Step 4, the second half of step 8, and step 9 is necessary to remove the original BD trigger signal from going to "Pin 3", and secondly to make the OPamp behave well as a feedback stage, necessary when powering opamps off of single power supply circuits, like the 606.

(10.) Now, this might be tricky depending on how you mount this. You need to add a "100k" trimpot between the points at "Pin 7" and "Pin 2" of IC5. Personally, I "piggybacked" the trimpot on top of IC5 and soldered the legs of the trimpot to the Pins of the IC5 directly. I'm sure theres not enough space on the circuit side of the board, and you need to have access to this trimpot to calibrate the modification.

Also note, that you should probably do this step after you've removed all of the parts around the area (mainly capacitor "C19" and "C20", cause it get a little cramped and awkward to solder to the pins of the IC5 when these parts are in the board.

This trimpot is what will be used to set the Max range of the decay time in the feedback stage, I've set mine so that when the decay pot is turned fully up, you just reach self-oscillation, so between no decay, and self-osc, there is a nice smooth transition. What it really acts as, is just a passive volume control between the output of the Kick drum oscillator, and the feedback stage.
[mc - The 100k timpot is very, very touchy. It's incredibly hard to fine tune with this, though it does work. I found that the key to getting the mod sounding really good is in the fine tuning of this trim pot. I still used a 100k pot, but changed to a pot with better resolution. Earlier, every little thing was affecting it (be it human touch or whatever sending varying amounts of feeback to opamp1. With a bit more care and ensuring isolation, it's all good!]

(11.) Remove Resistor "R66" and replace with a 50k or 100k potentiometer (50k should do). This of course will act as the "Tuning" control of the bass drum oscillator. The 808 bass drum did not originally have this control, but I've modded mine so it does, and it sounds good.
[mc - The tune resistor values needs to have a definite high and low value set (so use a pot + low valued resistor in series). Turning the knob too far (I cant remember if this was too high or low) resulted in the 606 squeeling very loudly at about 4000Hz. Ouch on the ears!!]

(12.) Solder a Silicon diode across resistor "R65". Diodes are polarized. The Anode side of the diode should be connected to the "ground" side of "R65", and the Cathode should go to the side which connects with "Pin 5" of IC5. I'm not 100 percent sure the full purpose of this, but the 808 has this at the trigger input of the BD oscillator, I guess it clips the trigger signal for consistency or something. If your foggy about what side is what of the diode, check this;
[mc - Having the diode around the right way here is important. You wont get sound if it's in backwards]

(13.) Remove Resistor "R57" and replace with a "100k" resistor. This is necessary to correct the impedance of the Bass Drum audio out, as the apparent volume of the bass drum is a lot louder than the original levels.

(14.) The last step is to calibrate the bass drum decay with the trimpot which was mounted between "Pin 2" and "Pin 7" of IC5. You should turn the bass drum decay pot up full (so the resistance of the pot is 2.2meg). Program a simple bass drum pattern and Adjust the trimpot for maximum decay time, to your taste, this may be just at the point of self oscillation (for long deep booooooooms) or just a tad more so that it does self oscillate when cranked. Anyways, wrote that up in about an hour, I'm pretty sure there are no mistakes, [mc - there aren't!] but wording can get confusing some time.

-- Plutonique9, Plutonique9@symbiocom.com

[mc - extra comments]

- The BD isn't as punchy as the 808 due to the lack of the envelope. Can't quite get that BOOOOOOOOOOOOOOoooooooooooom. It's more like a BOOOMmmmmmmmmmmmmmmmmmmmmmmmm. :-) It's a fairly loose sound, so to restore/add punch to the 'BD sound' the envelope is actually required. I'd suggest adding the envelope circuit below. If you so desire, you can get very close to the original 606 BD, but with more punch and a *hell* of a lot more.

- Having the BD self osc. is very cool when used with the tuning. :)

- There's a very fine line between self osc. and about a 2 sec or so decay. This is why the decay range trimpot must be have a high resolution. A multi turn pot is ideal.

- Be careful when cutting the PCB traces, keep it neat!


Diode Pitch Envelope

This is a great mod (yet again from Plutonique9), that allows the *extreme* range of sounds available with my modded BD. Basically, you're making a simple voltage envelope that tracks the pitch of the BD, following the BD decay.

Anyhow, just solder two *Germanium* (important) diodes together in parallel, but switch one around so it's reversed. I hope that makes sense. Now solder a 50k or 100k pot (experiment) to one end of the diodes. Now the free end of the diodes needs to be soldered to the junction of C23 and C24, and the pot goes to ground. That's it. You can also experiment with the diode configuration, I also got different but cool results with the diodes in the same direction. I'm going to add a switch later to select the envelope type (and also no envelope at all.) Just remember to use Germanium diodes, or it wont work.

p9 says "Now the Bass drum tune in your 606 will always set the base pitch. Even with the diode env. implemented, the pitch will sweep from its highest point and sweep down to the pitch set by your Tune Pot. Two elements are going to effect the Pitch sweep depth.
1. The amplitude of the drum itself. An Accent will increase the depth because it increase the amp of the drum osc.
2. The resistence between "Side B" and ground."
So make sure you jam in some accents with this to get the best effect.

Tone control

This mod is quite nice in the way that it allows you to control the tone, or more specifically the initial 'crack' of the BD. This is most effective when used with accent or with the diode pitch envelope above at high settings. It's not the most important mod here, so if you're short of room, you can leave it out, but it's nice to have another knob for your BD! Anyway, in the p9 BD mod above, replace the wire link that replaced R60 with a pot. It's that simple. You can do this mod without the p9 BD decay mod also.