While the Systron-Donner Series 80 is the oldest and rarest analog computer, this is the most modern and most available. The Comdyna GP-6, shown in Figure 1, is a relatively small and inexpensive computer that can usually be found on ebay. It was manufactured from 1968 to 2004. The Comdyna GP-10 is a very similar device, but optimized for analog signal processing applications. Figures 2-4 show our four GP-10s in a special case where they share power supply and control. This type of system is often used in synthesized music applications.
These computers are fairly limited in capacity, but are perfect for classrom and breadboarding applications. I have my GP-6 set up to solve the differential equation
x'' = -x
The solution, of course, is a sine wave. (Since sin(x)' = cos(x), and cos(x)' = -sin(x)).
Unsuspecting engineers passing the museum are often dragged in for a demonstration of the principles of analog computing using this example on our working GP-6.