AS “АLFA RPAR”

Joint Stock Company ALFA

Riga, Latvia www.alfarzpp.lv; alfa@alfarzpp.lv

6

2019 v.1

AS3394E

AS3394E

As an example, assume the highest cut-off frequency is 24KHz. C1-C3 becomes 33nF and C4 is

33nF/57 = 580pF. The frequency at zero control volts is approximately 1300Hz; 24KHz will be reached in

4,2 octaves, or at -1,6V. The filter can be opened up to >40KHz using these values, but CV feedthrough

becomes excessive.

The resonance VCA feedback circuit has been designed so that as the resonance is increased the apparent

loudness remains constant, providing a much richer resonant sound. Note, however, that the peak-to-peak

level of a pule/square wave actually doubles as resonance is increased, due to the ringing on its fast edge

transitions.

VCF MODULATION

The modulation VCA allows the VCO triangle wave to modulate the reference current of the VCF, and hence

the cut-off frequency. The Modulation Amount control voltage (Pin 6) controls the 'depth': at maximum setting

the VCF is swept from a very low value to twice the unmodulated frequency. Since the modulation is linear,

the apparent filter frequency does not shift as modulation is increased.

One application of this control is to set the filter into oscillation for obtaining linear audio FM of one VCO by

another (using the VCF as a VCO). However, an equally interesting application is to audio FM the filter while it

is filtering normally. The result is strong timbral effects, especially with some resonance added.

FINAL VCA

The final VCA is a low noise, low control feedthrough design which substantially reduces fast envelope click

and pop noises without the need for a trimmer. The output of the filter is essentially AC coupled to the VCA

input by means of the bypass capacitor connected to Pin 21. Thus, its value along with the internal 11K

resistor sets the low corner fre

quency; a value of 4,7µF results in a -3dB point of 3Hz. This pin may also be

used to extract the filter output signal before it is passed through the VCA.

The final VCA control scale is approximately audio taper. The first 20dB of attenuation from a control voltage

of +4V to +2,5V is linear. The next 60 to 80dB of attenuation is from +2,5V to 0V is exponential. This allows

the natural sound of exponential decays to be produced with simple linear envelope control, which is much

simpler to generate in software.

The signal output at Pin 23 is a current with a voltage compliance of Vee+1 to Vcc-1. This allows the outputs

of multiple 3394s to be mixed together by connecting all Pin 23s together. The summed current can then be

converted to a voltage simply by a resistor to ground, or an op-amp with feedback resistor. The major portion

of the 40%

tolerance on the maximum output swing (nominally ±210µA) is due to the monolithic diffused

resistors setting the currents. Although the output variations part-to-part are typically much less than this

(about 5-10%, ed.), output levels as well as the filter initial frequency may be more closely matched by

matching the resistance between the Vcc and Vee pins.

INPUT CONTROL VOLTAGE BUFFERS

With the exception of Waveform Select whose control voltage is not critical, the input bias current to all control

inputs is typically less than 1 nA, and have maximum values of 3 and 5 nA. Hold capacitor values can be in

the range of 2 to 20 nF before causing multiplexing problems and DAC output instability.

The amount of

droop is given by:

Vdroop = (Ibias

– Tupdate) / Chold

With a hold capacitor of 10nF and update time of 10 ms, maximum droop would be 5 mV or between 0,06 and

0,12% of full scale. Typically, the droop is a factor of 10 less than these worst case values.