From mr808@teleport.com Wed Jan 25 10:10:36 1995 Date: Tue, 24 Jan 1995 13:32:35 -0800 (PST) From: Mutant Rhythmatist To: Thomas C Butcher Cc: "magnus.acs.ohio-state.edu" Subject: 808 Bass (was "couple things..") On Mon, 23 Jan 1995, Thomas C Butcher wrote: > 4: Anyone know how I can easily increase the decay in the 808 kick drum? You're in luck - the Minister is IN. The following is from the 808 SM: >Sound Generators The bridged T-network filter shown in Fig. 11 is used to generate periodic damping drum sound. This configuration has variations according to application (instrument sound). Values of R and C can be changed. With this circuit, the decay time becomes longer as Q increases. FIG. 11: --------/\/\---------- | R2 | | | |---||----+----||----| | C1 > R1 C2 | | < | |--|\ v GND | | \______________|____ Periodic wave out | / Pulse in ------|/ f = 1/(2pi*SQRT(R1*R2*C1*C2)) Q = SQRT(R2/R1)/(SQRT(C1/C2)+SQRT(C2/C1)) Bass Drum This sound generator is composed of a multi-feedback, bridged T-network including 1/2 IC12 (pins 1-3) as an active element. The decay time of the resonating waveforms can be controlled by adjusting feedback amount by VR6. Immediately after a trigger pulse is fed into the generator, the filter's time constant - when ACCENT is present - is halved and has a resonance on twice its inherent frequency for a half cycle period, then on the fixed frequency with decaying amplitude. This changing frequencies will sound a punchier crisp bass. This trick is performed by the circuit composed of Q41-Q43. > End Quote Analysis by MR-808: Whenever Roland Service Notes speak of a "trick", you know it's going to be cool. In this case, it's what gives the 808 & 909 their characteristic attack (that "crisp" bass sound). Another fun feature of this circuit is that it goes slightly flat (just like a real drum). Listen to the the 808 at maximum decay BOOOOooooommm.... hear it go flat? Looking at the Bass Drum circuit, I realized that the bridged-T description doesn't quite work for the Bass Drum, as the gods added some additional circuitry for reasons for which we can only speculate. There is an additional filter that is in the bridged-T feedback path - maybe they were trying to isolate Q & frequency from Decay (you wouldn't want the tuning decreasing as the decay and Q increases, would you? %-} ). So, lets leave the bridged-T alone & look at the circuit around VR6. My suggestion is to leave VR6 (the decay pot) alone - you might have a hard time finding a pot w/ the exact dimensions you need. My guess is that if you replaced R169 (a 47k resistor) w/ a small 100k pot, you'd be on the road to extended decay & hours of fun. So, how do you find R169? It's on the upper left of the Main Board (the one w/ all the pots for tweaking ACCENT, DECAY, etc. Below VR6 (the DECAY pot, there is a capacitor, to the right of it two resistors. R169 is farthest to the right. It's 47k, so you can double-check by color - let's see, YELLOW-VIOLET-er wot's it-ORANGE (You'd better double check it - I never can remember the 10's multipliers). Replace it w/ a potentiometer & tweak until it sounds right. Be sure to let us know how it goes. Still on my list of things to do is to add some OTAs to the 808 so I can CV the Bass Drum Tuning & Decay. I know you can play melodies w/ a sampled 808 Bass, but sampled 808s just aren't the same. PEACEOUT MR-808