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.
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.
Version 1 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 with 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 with a sampled 808 Bass, but sampled 808s just aren’t the same.
Version 2 by Robin whittle
Solder a 390k resistor in parallel with R164 (47k). This boosts the gain of the Twin-T feedback path. In the machines I have worked on, this provides enough extra gain for the circuit to self oscillate when the decay pot is nearly at its clockwise limit.
If this does not get you oscillating, then try a 330k or less.
The decay depends on how close to a gain of 1.0 the circuit is. Since there are a number of +/- 5% resistors in the circuit, some TR-808s have markedly longer or shorter decays than average.
This mod is very simple once you find the resistor location – just solder it on the back of the PCB. There is no need to take the PCB off the front panel.
This involves lifting the PCB from the front panel and removing or cutting a track to R165 (also a 47k). Replace it with a 100 k pot.
I use a 100k log pot, and this gives fine control over the higher pitches at the anti-clockwise end of the range.
Thanks to Robin Whittle.