U P D A T E as we have obtained the manual for this unit, the technical/electronics information will be updated soon!(july 2015)

On the EG&G-PARC Selective Amplifier model 189
Stand-alone Analog Computation equipment used for Electronic Music

Some equipment models by the EG&G-Parc company will be introduced, and a more detailed description of the Selective Amplifier model 189 will follow. The analog computing functionallity of this instrument will be dealt with, including application examples related to electronic music techniques.
The ARP model 1047 Multimode Filter/Resonator will be refered to.

These North America based companies EG&G (Edgerton, Germeshausen & Grier) and PARC (Princeton Applied Research Corporation) of New Jersey (Princeton roughly between New York and Philadelphia), with many domestic and international sales offices, have produced a range of specialized electronic instrumentation for nuclear research facilities, for example modules for NIM/CAMAC applications, and other scientific equipment in electro-chemical research and industries, with US defence, government and universities as main contractors. As seen before, these kind of companies did grow rapidly after WWII and during the Cold War era. The PARC company was acquired by EG&G (historically famous for high-speed photography of nuclear blasts) around the mid 1970's.
Currently PARC has become Ametek Inc. and EG&G has become PerkinElmer.

The models we'll get into here have been developed during 1966-1975, so at about the same time the ARP 2500 analog modular electronic music synthesizer was developed. We therefore find similarities in component use and general pcb and front panel layout/construction. ARP (after founder Allen R. Pearlman, formarly TONUS Inc) was also based in the North-East of the USA.

PARC is known for its development of the Lock-in Amplifier concept.
The Telesoniek Atelier (TA) features the EG&G PARC model 121, very useful for particular sound modifications. In short, a known reference signal is mixed/multiplied by the unknown input signal, the result is amplified and low pass filtered. Its main application is the detection and measurement of signals buried in noise (signal recovery). In the TA studio both simple and complex sounds can be heavily distorted by this instrument, and be transformed into crunchy sounds most effectively by the use of the lock-in amp's variable filter.
This unit can also process dc voltages.
Please find more details in the referenced literature down below.

Another interesting instrument is the model 193 Multiplier/Divider, offering a variety of direct analog signal processing on two input signals supplied to the instrument. This is clearly a stand-alone analog computation unit, with specific functions that originally could be found as separate function modules in the larger scale analog computers
of the 1950-1990 era.
By what we know from those kind of analog computation electronics we'll expect many biased diode arrays in that unit (no technical documents available here right now, any documentation is highly welcome).

The EG&G Parc model 175 Universal Programmer generates functions we can also trace back to analog computation techniques. The unit contains logic controlled integrators as ramp voltage generators, and uses comparators to set the ramp limits and logic control signals for switching and proceeding.

Find the Model 175 operation and service manual here:

This instrument (of which two are available in the TA studio) immediately shows on its frontpanel four to us interesting outputs:
band pass (0 degrees), notch, lo pass (-90 degrees), hi pass (+90 degrees).
Thus we can conclude quite certainly that we deal with a state variable (or multi-mode) filter here, and a large dial promises us a frequency range of 0.1 Hz – 110 KHz.  Added are controls for Q factor (resonance), AC or DC input coupling, and a pre-amp.
At high Q values the signal amplitude of the model 189 drops down quite dramatically, but this can be compensated by switching in the pre-amp boost.
As we up to this moment don't have any technical documentation on this model 189, we will draw out of our experience and extract its general functions and design.

We can recognize the function of this model 189 as a result of working with analog computers. In programming a state variable filter/resonator on an analog computer, we'll need two integrators and one inverter connected into a loop, and complemented with several potentiometers. Feedback around the first integrator will control the Q-factor or resonance (which is infact a control for 'damping', as the program initially will generate an 'undamped' oscillation). An additional summing amplifier is used to combine high pass and low pass filter outputs to perform the notch function. 
The model 189 made some arrangement to accelerate the set-up time, so instead of only the usual ten-turn potentiometers (and changing the capacitor values of the integrators) we find additional frequency range switching, and Q switching. Added also is a flexible pre-amp for signal conditioning before entering its main function: the variable filter/resonator. All filter outputs are directly available at the same time, which ofcourse is the main feature of this kind of circuit.

As shown earlier (Kulk 1994) ARP technician Dennis Colin (Colin 1971) has based his model 1047 filter design on excisting analog computation practice and technique. That state variable filter/resonator has proved to be very useful in sound synthesis and spectral modification. A similar filter type was part of the excellent Oberheim SEM (Synthesizer Expander Module) series of synthesizers during the mid/late 1970's.

The only significant difference between the ARP filter and the regular analog computer filter/resonator program is the possibility of (logarithmic or linear) voltage control of the filter frequency (across the whole range), and of the resonance amount. However, analog computer programmers can accomplish logarithmic transfer functions with the help of a good bunch of multipliers and additional amplifiers, or by using variable diode function generators with which in fact any arbitrary function can be approximated.

The Q factor switch has nine positions in its range, including full resonance, to enable the transition from a (damped) resonator to an (undamped) oscillator, and therefore will produce a sine wave nicely.
At the filter outputs we now have sine waves at different phase angles (90 degrees apart) available, and with an additional inverting amplifier a socalled 'quadrature oscillator' is completed (usefull in 4-channel spatial sound projection if combined with VCA' s).
By taking the bandpass output as X-input, and low pass output for Y-input to an oscilloscope, it will display the phase-plane representation of oscillations generated by the filter/resonator. Here enters a quite powerful analog computation technique: investigating the response of a (simulated) system to a single pulse signal.
The controls of the Selective Amplifier set the parameters of that system.

So we now actually have a simulator, an electronic analog of a mass-spring-damper system with which we can freely experiment in real-time and look for responses to all kinds of stimulus/excitations/forcing functions: sine waves, unit pulses/step voltages, square waves, and random noise. On the scope display we can register the results of various system parameter values, which in this case are the frequency range, the damping factor (Q), and the input value.

The unit can function in a very low frequency range, and process DC signals. The following instrumentation set-up was made in the TA studio for the purpose of illustration:

- the selective amplifier was set for it's lowest frequency range, and as input signal we apply a very slowly varying random analog voltage generated by a Hewlett Packard 3722A noise generator. All four selective amplifier outputs are used at the same time, and connected to a Hitachi 4-channel oscilloscope for display.
Top trace is the band pass output, followed by the low pass, high pass, and notch outputs.
On the pictures you'll see the different responses to the analog random voltage, the phase differences can be clearly noriced.
As we change the random voltage output waveform from 'analog' to 'binary', we can notice the typical waveforms that occur when step-type signals are processed by such an analog computer circuit/program.
By increasing the Q factor, the circuit become more resonant and starts to oscillate ('ring').

To summerize, one can find many interesting application for this EG&G model 189.
In audio range, these filter/resonator/oscillator system outputs are interesting to listen to, to investigate, and to apply to sound synthesis. The phase phenomena inherent in analog filter design can be studied, e.g. by connecting the bandpass output to the left channel, and lowpass (or highpass) to the right channel of your stereo mixer (channels panned hard left and right) , and then sweep through the frequency range, using noise or pulse waves as system inputs.

In sub-audio/low-frequency range, one can generate control voltages for oscillators, filters, amplifiers etc. (anything that is voltage controlable). Remember that, by means of parallel processing, four versions of the input signal are available.
It all may sound simple, but it's nice and just cool to realize that you're working with an analog computer.
If your approach is from that angle, new insights will present itself; study of the analog computation literature will open up more techniques for analog sound synthesis.

INSIDE the EG&G PARC model 189 selective amplifier
As technical documents are not available yet, please refer to some circuit board pictures above and below. Most circuitry is discrete, we can see some use of Op Amps, and a component side with a wide spaced component layout. All multi-wafer switches are connected directly to the foil side. The yellow colored type of capacitors can also be found in ARP 2500 series modules.
If anyone has schematics and/or service documentation on these units: please do contact me!


This model 189 is a stand alone hardware unit representing an analog computer function with many applications, both in analog computing itself and in electronic music sound synthesis.
To my opinion it is, alongside the multiplier, the most useful analog computer function
to be incorporated into the electronic music equipment repertoire.

The other EG&G Parc units that have been described briefly present additional analog processing capabilities wourth looking after.


- on EG&G co-founder Elgerton:  
- A lock-in amplifier primer: Princeton Applied Research,
- Lock-in amplifiers: principles and applications (e-edition) by Mike Meade
- all about analog computers, techniques, further reading, compiled and written by Bernd Ulmann:  >>http://www.analogmuseum.org/english/

- Dennis P. Colin, Electrical design and musical applications of an unconditionally stable combination voltage controlled filter/resonator, JAES Dec 1971, Vol 19, No.11
- 1047 filter data sheet, Arp 2500 Manual, Newton Mass. 1971
- PARC, Instruction manual Lock-in Amplifier/phase detector model 121, NJ. 1968
- Korn and Korn, Electronic analog and hybrid computers 2nd edition, McGraw-Hill, 1972
- Thomas H. Wells, The technique of electronic music, Schirmer books, 1981
- Hans J. Kulk, Reader: The electronic analog/hybrid computer, Faculty of Art, Media and Technology Hilversum, Utrecht High school of the Arts, 1994


To Bernd Ulmann in thanks for his excellent work on keeping the art of analog computation.
Hans Kulk, February 2014/May 2014.