The Bass Drum circuit is built around two Bridged-T 'oscilliators', IC5A and IC5B. These are not really oscillators, but 2-pole bandpass filters. After a transfer function is derived (I can give the circuit analysis and maths for anyone interested, but I am sure you're not so...) for these circuits, the impulse response will be found to be in of the form:

        h(t) = cos(at)e^bt

which is very useful. It basically states that we can get a exponentially decaying sinusoid out of this circuit, if we subject it to an impulse. The impulse function, delta(t) is defined as having zero width, infinite height and an area of 1. Obviously, this can't be achieved, so we will approximate it with a narrow pulse.

By manipulating the values of R1, R2 and C (both capicators are the same) we can change the amount of resonance and the natural frequency of this system.

In this case IC5A is the low end of the drum, and has a frequency of 60Hz. IC5B is the 'click', and has a frequency of 130Hz.

Now for a circuit description. The BD drum trigger (a short 12V pulse) appears at the port marked 'BD'. This goes to a pulse amplifier and level converter (Q14 and Q15). The pulse is greatly reduced in amplitude (to about a volt) by the voltage divider R63 & R64. R47 and R51/R52 form a voltage divider that biases this pulse at about 7.5Vs. This is done because the TR606 has no negative rail, so all op-amps have their non-inverting input biased to 7.5V (ie. 7.5V = ground).

This pulse then is fed into the two tone generators IC5A and IC5B, and their outputs are joined and mixed. C21 is provides a little lowpass filter to smooth off the nasty pulse from IC5A and IC5B.

VR3 controls the level of the Bass Drum in the mix. R57 is the resistor for the bass drum output when it goes to the final mixer. By decreasing this you can increase the overall volume of the bass drum in the mix.

The AC port driving the pulse amplifier (Q14 & Q15) sets whether the bass drum will be accented, by changing the amplitude of the pulse, and therefore the amplitude of IC5A and B. This port is connected to all pulse amplifiers on all drum circuits, because in the TR606 only whole steps can be accented. This is the same as the 808, but the 909 can have individual accented instruments.