From mr808@teleport.comWed May 17 13:16:40 1995
Date: Tue, 24 Jan 1995 13:32:35 -0800 (PST)
From: Mutant Rhythmatist <mr808@teleport.com>
To: Thomas C Butcher <tb303+@CMU.EDU>
Cc: "magnus.acs.ohio-state.edu" <analogue@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