Chip Parade (Part 2)
Inside info — inside gear
We continue our series on popular electronic devices used in music equipment.
Last month we considered the LM13600N and Curtis CEM3320 integrated circuits. Other transconductance operational amplifiers include the CA3080E from RCA. Compared with the CEM3320 and LM13600N this device is a very simple one as it contains just one OTA and does not have a built-in buffer stage.
Figure 2 shows the circuit diagram of a single stage (5dB per octave) low-pass VCF based on the CA3080E. As there is no integral buffer stage, IC1 is used as a voltage follower to give the circuit a low output impedance. In effect IC1 is used as a voltage controlled resistor which forms the filter in conjunction with C4. In this demonstration circuit the control voltage is provided by VR1, and this can be adjusted to set the -6dB point (the frequency at which the gain of the circuit falls to half its normal level) anywhere from less than 50Hz to more than 20kHz, or practically anywhere in the audio range. In real applications the control voltage would, of course, be provided by an envelope shaper or LFO of some kind.
The circuit can be modified to act as a highpass filter by disconnecting the lower end of C4 from the negative supply and coupling the input signal to the free end of this component. R6 and C3 then become unnecessary. The CA3080E can be used in many other types of circuit, such as waveform shapers, VCAs, sample and hold circuits, etc, and it is a very versatile device which is not uncommon in commercially produced electronic music gear as well as home-constructor projects.
Switched-capacitor filters are a relatively new type of circuit, and could well supercede OTAs in the not too distant future. With this type filter the resistive element is replaced by an electronic switch and a small capacitor. The capacitor is first connected to the input where it charges up, and then connected to the filter capacitor where it gives up some of its charge. The switch and capacitor thus transfer power from the input to the filter capacitor at the output, much like the resistor in a conventional filter. With a low switching rate there is little transfer of power and effectively a high filter resistance. As the switching frequency is increased a higher power transfer can be achieved and the effective filter resistance is decreased.
This may seem like doing things the hard way, but it enables complex filters having variable and highly predictable characteristics to be produced. Practical filters of this type usually have a convenient relationship between the clock oscillator and filter frequencies, and with the MF10CN based circuit of Fig. 3 for example, the filter's operating frequency is one hundredth of the clock frequency (one fiftieth if pin 12 is connected to the positive supply rail). The circuit has the clock signal provided by the VCO built around IC2 so that a voltage controlled filter is produced, but this type of filter is obviously ideal for direct control from a microprocessor system or some other form of digital circuit, and in an increasingly digital world these filters seem likely to be used more and more.
The circuit of Fig. 3 is for a 12dB per octave lowpass filter, but by utilising the unused section of the device (the pin numbers for this section are shown in brackets) a 24dB per octave type could be produced. Furthermore, taking the output from pin 2 gives a bandpass action, or notch filtering is available at pin 3. Other operating modes give highpass filtering, and this type of filter can give any of the normal filter type musical effects.
One drawback of this circuit is that the clock signal breaks through at the output, although only at a level of about 10 millivolts RMS. Using a clock frequency of less than about 20kHz would result in audible breakthrough at the output, and in most music application this limits the minimum filter frequency to about 200Hz.
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