Sound Shaping for Percussion
The last article in this series described a simple envelope shaper for percussive sounds based on a VCA using the popular MC3340P integrated circuit. In this article a couple of alternative VCA circuits and a simple voltage controlled filter (VCF) using operational transconductance amplifiers will be discussed.
Operational transconductance amplifiers, like ordinary operational amplifiers such as the 741, have inverting and non-inverting inputs, but they have an additional input called the 'amplifier bias input'. The gain of the device is controlled by the bias current fed to this third input, and with this type of operational amplifier it is the output current rather than the output voltage which is governed by the differential input voltage.
Figure 1 shows the circuit diagram of a VCA which is based on the CA3080E transconductance amplifier. The circuit is powered from a single supply of about 9 to 12 volts, but R2, R3 and C1 are used to effectively give a third supply rail at half the supply potential. The non-inverting input of IC2 is biased direct to this central supply rail, and the inverting input is biased to it via R4. The amplifier is used open-loop, and there is a substantial voltage gain from the inverting input to the output. R6 is therefore connected at the input and in conjunction with R4 it gives about 40dB of attenuation so that the maximum voltage gain of the circuit is reduced to approximately 6dB (two times). R6 also boosts the input impedance of the circuit from only about 100 ohms to a more useful figure of just over 10k. The low value given to R4 is necessary to prevent the circuit from having an excessive noise level. C2 is merely a DC blocking capacitor.
Although it is the output current of IC2 and not the output voltage that is governed by the input signal, by simply adding a load resistor at the output, the output current produces a proportional voltage across this resistor, and the circuit acts as a voltage amplifier. In this circuit R5 is the load resistor. Similarly, although it is the bias current fed to the amplifier bias input that determines the gain of the amplifier, by adding a resistor in series with this input the bias current becomes proportional to the applied voltage, and the circuit effectively becomes voltage rather than current controlled. R7 is the resistor which performs this function in the circuit of Figure 1.
On the face of it this VCA interfaces very well with the control voltage generator circuit described last month since the gain of the circuit is at maximum with a control voltage roughly equal to the positive supply potential, and reaches its minimum figure with a control voltage of just a few hundred millivolts. In fact the VCA will not operate properly if driven direct from the control voltage generator due to the fairly low input impedance at the control input. However, this is overcome by adding a buffer stage ahead of this input, and this is the function of IC1a. R1 is a protection resistor.
The output impedance of IC2 is quite high, and IC1b is used as another buffer amplifier which gives the unit a low output impedance and ensures that there is minimal loading on the output of IC2. Note that the CA3240E integrated circuit has a MOS input stage, and the appropriate handling precautions should be observed when dealing with this device.
The circuit can handle output levels of up to about one volt RMS or so without clipping and producing severe distortion. At full gain the output noise level is around 1.5mV RMS, but the circuit will normally be adequate in this respect, and as the gain of the circuit is reduced the output noise falls by a roughly proportional amount. The circuit draws a supply current of approximately 9mA.
The LM13600N is in many respects similar to the CA3080E, but it incorporates a few useful improvements. The most obvious of these is the inclusion of two transconductance amplifiers in the device, and these have common supply terminals but are in other respects independent of one another. Obviously in many applications the second amplifier will not be required, but the LM13600N is not particularly expensive. Also, it is often possible to use the second sections of the amplifier as a simple voltage amplifier or buffer stage if it is not needed as a VCA.
Other features of the LM13600N are lower noise level, built-in output buffer amplifiers, and linearizing diodes which enable a higher output level to be achieved before the onset of severe distortion.
A simple LM13600N based VCA circuit is shown in Figure 2, this being similar to the circuit of Figure 1 in many respects. The pin numbers in brackets are for the other section of the LM13600N.
Pin 5 is the output of the transconductance amplifier, and pin 7 is the input to the output buffer stage. The latter is actually just a Darlington Pair emitter follower stage which needs external load resistor R9. Tr1 is used as a discrete emitter follower buffer amplifier which gives the necessary boost in input impedance at the control input of the circuit. R7 provides a bias current to the linearizing diodes of the device.
The circuit can handle input levels of up to about 2 volts RMS before serious distortion is produced and the maximum voltage gain is approximately unity. The audio output noise level is only about 200uV, and the signal to noise ratio of the circuit is excellent. Current consumption is approximately 3.5mA.
As the volume of a sound decays it is normally accompanied by a change in the frequency components of the signal. Usually the higher frequency harmonics decay more rapidly than the lower frequency harmonics which in turn decay more rapidly than the fundamental signal. This can be simulated electronically using a voltage controlled low pass filter such as the one shown in the circuit diagram of Figure 3.
Here the CA3080E (IC2) is effectively used as a voltage controlled resistor which forms a simple single stage lowpass filter in conjunction with C2. With the control voltage at or near maximum the cut-off frequency of the filter is just above the upper limit of the audio frequency spectrum, but as the control voltage is reduced the effective resistance of IC2 increases and the cut-off frequency is brought down through the audio frequency range. As the control voltage approaches zero the resistance provided by IC2 becomes so great that there is no significant output from the circuit. Like the other circuits described in this article, the maximum attenuation provided by the circuit is extremely high indeed at about 90dB or more.
Over the past three issues we have considered a number of circuits which can be used in a variety of combinations to produce a number of different effects. Brief details on how to use various arrangements have been given with each circuit, but a recapitulation should prove helpful.
The very simple arrangement shown in the block diagram of Figure 4 gives bursts of high frequency noise which simulate cymbal type sounds. Shorter bursts of noise give a sort of 'clap' sound, and long bursts of pink or red noise provide sea or wave type sounds. An interesting falling pitch noise sound can be produced by using a white noise source in place of the blue noise generator, and using the VCF circuit instead of the VCA.
Simple drum sounds can be produced using the set-up of Figure 4 but using a sinewave oscillator instead of a noise source as the signal source for the VCA. The operating frequency of the oscillator should be fairly low of course, and it is essential to use a sinewave signal or some other waveform which has a reasonably low harmonic content. Using a high frequency oscillator gives a sound which is similar to that produced by a triangle.
By using ring modulation it is possible to produce a range of more complex sounds, and the arrangement shown in the block diagram of Figure 5 permits metallic sounds to be generated. The basic arrangement shown in Figure 5 only gives the sum and difference frequencies of the two tone generators at the output of the ring modulator, and this gives a bell-like output if the two tone generators are set at almost the same frequency so that the difference frequency is no more than a few Hertz. Bear in mind that the main output will be the sum frequency which will be at double the pitch of the tone generators, which in consequence should be set at half the required pitch. Good metallic sounds can also be produced if the tone generators are set some musical interval apart, but they should again be set slightly off-tune in order to obtain the best effect, and the effective pitch of the output signal will be at the sum of the two input tones.
A useful refinement to this arrangement is to have a mixer between the ring modulator and the VCA so that the output of one tone generator can be added to the new frequencies generated by the ring modulator, or unbalancing the ring modulator is a crude way of achieving much the same thing. When used in either of these ways it is probably best to have the tone generator which is fed through to the output as the main signal which determines the pitch of the output, and use the ring modulation to add a suitable amount of non-concordant signals to give the desired effect.
This is really only a guide to some of the effects that can be produced, and with a little experimentation it is possible to produce other percussive sounds such as wood-block sounds and some which have no mechanically generated counterparts.
Feature by Robert Penfold
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