OTA Voltage Controls in the Polymoog

Here's a use for OTAs (courtesy of the Polymoog) besides the VCA we already looked at. Technically this could be called a VCA as well. It's taking a voltage from a front panel slider to control the amplitude of filter modulation signals. We will look at the Modulation Amt. control. It affects the amount of LFO going to the filter cutoff. The top half of the schematic shows the front panel slider and other components from the right hand control panel board. The slider itself is a 10k audio taper pot with its wiper terminal in series with a 27k resistor. The 4007 is a CMOS switch. The ZZ control line determines if the output should be in PRE (preset resistors, upper right) or VAR (slider) mode. In VAR mode pin 12 is connected to pin 9 via an n-channel FET. The output of the 4007 is named PVMAN.

The lower half of the schematic is from the top right filter board of the Polymoog. Notice the PVMAN output connects to transistor Q6 which is in turn connected to the control current pin of OTA A9. This is essentially the same pnp transistor configuration we saw with the MG-1 VCA. The input of the OTA is a voltage divider, not a differential input like before. Note that the LFO is ±2.5V. The voltage divider reduces this to 100/(22000+100)*±2.5 = ±11mV. This is within the ±25mV input limit of the 3080. What about the control current I_ABC? With the slider all the way down we can see no current will flow and the OTA will be off. With the slider all the way up to V₊ = 15V the current will be roughly (15 - 0.7)/27k = 0.53 mA. This is also a reasonable result considering I_ABC for the 3080 can be 0.1µA to 1mA.

The output of the OTA goes to pnp emitter follower Q11 which adds several VCF control signals, S+H, LFO, contour, and keyboard amt, which all use 3080s and cutoff (PVCON), which is directly connected from the 4007 without a 3080. The emitter follower in turn drives an npn transistor (not shown) that sinks current dirrectly from the ladder filter. Next time I'm going to take some pictures while my Polymoog is still openned up so I can show what this stuff actually looks like.