The whole secret of the synthesizer is its ability to produce a given result for a given set of voltage inputs. This ability is called voltage control. The question is how do we go about producing voltage control?

There must be as many different ideas on voltage control as there are synthesizers. Some units use Field Effect Transistors (FET) as voltage controlled resistors. The technique is rather simple, but lends itself well to non-critical applications such as phase shifters (see POLYPHONY, Vol. 3, #4) and non-tracking filters. The technique is so simple it bears inclusion here, although this article isn't really intended to discuss FETs.

Note that the control voltage must be negative. This will require a simple inverter on the input. Certainly this is not complete, but will have to do for the time being.

Another approach is the differential amplifier route. We've all seen this in VCAs from various manufacturers:

You'll find that you can't totally fault this approach, either. The differential amplifier is the basis for most of the op-amps available. Grab a Linear Data Book and see! In addition, it has served Bob Moog for many years, being the very heart and soul of his equipment.

The list grows. If you have a PAIA 4710, you know how it may be used as a VCA. You may already be getting ideas about how it may be used in other areas as well.

One company has even made a success of the high-speed, pulse width modulated switch.

This uses a high-frequency oscillator (50K Hz to 100K Hz) and a pulse width modulator. If you think of the switch as always open, the resistance is 100K ohm. If you think the switch is always closed, the resistance is 0 ohm (shorted out). If the switch is on 50% of the time (50% duty cycle) the resistance is 50K ohm, etc., etc., ...

There is one IC available that really suits our purposes to a "T". It is the CA3080 Operational Transconductance Amplifier (OTA for short, thank heavens). Those of you with 4730 filters should recognize it immediately. What you may not recognize is its extreme versatility.

Before we get too heavy into building something, we should know something about how the 3080 behaves. When properly used it can be invaluable. When improperly used it can be a source of terrible distortion.

The following is a pinout diagram of the 3080. Hopefully this will allow you to at least get the pins hooked up right!

At first look it seems to resemble a common LM301 or LM748. Indeed, the pinouts are the same.

Well, almost. Notice pin 5 the "IABC" terminal. This is the secret. "I" is the most universal symbol for current. ABC is Amplifier Bias Current. The relationship between IABC and output is simple: With NO current there is no output. As the current increases, so does the output. Very simply, what we have is a resistor that responds to changes in current. Voltage to current is a simple trick, so with some minor changes, we can easily come up with a VOLTAGE CONTROLLED RESISTOR. HOORAY!!!

But not so fast. The 3080 is easily misused. We can not simply plug in an input, stick on a control voltage, and expect it to work. There are certain rules to follow.

1) The input voltage on pin 2 or 3 cannot exceed 100 millivolts. There seems to be some confusion on this by the way. Some sources recommend no more than 10 mv, others up to 200 mv. My experience has been that 100 mv won't hurt.

2) The IABC must be limited in some way. This means a resistor in series with pin 5.

3) The output is a high impedance current, quite a change from the low impedance regular op-amp. Care must be taken not to load down the output. Incidentally, the spec sheets claim that the output can sustain an indefinite short to either supply voltage or ground. This is fortunate for all of us butter fingers!

Keeping these in mind, we're going to try to use this thing! Let's try to get something useful before we go.

This is a SIMPLE VCA. Nothing fancy, just something to get you going with the CA3080. The 10K and 1K resistors on pin 3 divide to signal to the required 100 mv. The resistor on pin 2 is to balance the resistor on pin 3 (logical, huh?). The 10K resistor on the output is a load resistor since the output is a current, not a voltage. The 741 is a follower circuit to give the desired low output impedance. It may be a compensated 748 or 301 as well.

Remember, this is only a SIMPLE circuit. It may or may not have a unity gain output (1 volt in equals 1 volt out), and it will probably thump. No problem. Play with this for mow, and we'll get to an advanced version in a future column.