User manual

16
Slide Slew Times
The Slide Time pot gives you longer slide times than normal and can give you shorter
times when running from CV In.
The slide time – how long it takes for the internal CV to the Oscillator (and Filter
Tracking Pot and CV Out) to slew from one level (and therefore oscillator pitch) to
another – is proportional to the sum of three separate resistances. Resistance is an
electrical term similar to impedance. A firehose has a low resistance to water flow while
a drinking straw has a high resistance. The slew time is theoretically infinite. The times
mentioned above, in the New and Changed Controls section, of 360 milliseconds etc.
refer to the approximate audible time it takes to reach the final frequency. But in any
resistor capacitor arrangement, the final voltage is never precisely reached – it is only
approached closer and closer as time progresses.
When the Slide is activated, the internal DAC or the CV In drives a capacitor – which is
like a large tank. If the source of Control Voltage (Voltage is like the pressure or level of
water) has a high resistance, then it will take longer to bring the capacitor (tank) to the
final level, so the slide time will be longer. The three resistances in the slide circuit are:
1 The resistance of the DAC (Digital to Analogue Converter) in the TB-303’s
internal sequencer. This is 100K Ohms (100,000 Ohms). Alternatively, if you
are using an external source of CV, the resistance of the source of CV which
is likely to be very low – 100 Ohms or less (equivalent to a firehose).
2 A 3.3K Ohm (3,300 Ohm) resistor in the Devil Fish CV voltage protection
circuit. When thinking about slide, this is a very low value – close enough to
zero.
3 The resistance of the Slide pot: between 0 Ohms (anti-clockwise) and 500K
Ohms (500,000 Ohms = very high resistance equivalent to a long, narrow
drinking straw) when turned fully clockwise.
The usual rather fast slide of a TB-303 is caused by the 100K resistance of the DAC
feeding a 0.22uF capacitor. If you are using the internal sequencer and have the Slide pot
turned fully anti-clockwise, you will get almost the same slide speed, due to the 100K
DAC resistance and the 3.3K protection circuit voltage giving a total of 103.3K Ohms.
By turning the Slide pot you can add to this resistance to give a slower slide time – up to
a total of 603.3K Ohms, or a time 6 times longer than normal.
If you are running from an external CV, and the Slide pot is fully anti-clockwise, then the
resistance will only be 3.3K Ohms (assuming that the source of CV has a very low
resistance). This low value will give you a very short slide time: so short that you will not
hear it. If you turn the Slide pot a little towards the clockwise position, you will be able to
find a position which gives you the usual slide time, where the Slide pot is at about 100K
Ohms. By turning it fully clockwise, you will get a total of 503.3K Ohms – about 5 times
the usual slide time.
If you want extremely long slide times, feed the CV into the Devil Fish through a resistor,
where the value is 470K Ohms or above. Values above several Meg Ohms may result in a
loss of pitch accuracy. There is a trimpot inside the Devil Fish to adjust for the bias
current of the slide buffer op-amp, to minimise the mistuning which would otherwise