Oct 03 2009
This wonderful box of tricks was put together a few years ago to control the input signal to my home cinema subwoofer amplifiers. It is mono and houses two ESP power supplies, a 12db/octave booster, an 8-band subwoofer equaliser and an optical output power limiter/compressor (all ESP projects).
The subwoofer output from an Integra DHC 9.9 decoder is connected to the input socket.
A balanced line driver and XLR output socket were added since the photos were taken.
The best thing about the project is the pseudo-random earthquake noise generator circuit used as a “test rumble” feature to impress my friends.
It is activated at the flick of the left rotary switch which powers the circuit board and triggers an “input selector” relay.
I built the circuit back in 1989 from the diagram provided at Fig. 10 of US Patent 3,973,839. This was the first Universal Studios “Sensurround” patent and the circuit is supposed to be what created the earthquake rumble effect in each cinema in which Earthquake was shown in Sensurround in 1974/5. Control tones on the film switched the circuit’s (continuous) output into the subwoofer amplifier bank (BGW or Cerwin-Vega power amps) during the movie and varied the gain of the preamp as required.
The circuit did not work because Fig. 10 was full of errors – whether deliberate or not I do not know, but I have my suspicions. It sat in a box for 17 years and I dug it out for this project. I did not have the expertise to identify the errors, but Rod Elliott who is much smarter in the area than I was able to model it with software and identify the problems. That got the thing working, but it was very unstable and would run for only a few seconds before stopping. Applying power again usually got a few more seconds out of it and adjusting the input voltage and changing a few resistors extended the time to about a minute before stalling, but it was pretty hopeless.
With a little lateral thinking I finally figured out what the stability problem was and substituted an unbuffered NAND gate chip onto the card (modern buffered types were not around in the 70s!). It then worked continuously without falter and cranks out an awesome rumble with great transients extending just into the infrasonic range. Rod didn’t believe that my substitution could make any difference, but confirmed that it did and here is a frequency response graph of the electrical signal that he took straight off the card:
Here is a spectrum of the boosted signal (note the taper off as frequency increases):
If anyone wants to know what it sounds like, a wav file can be downloaded here. Please heed the warnings!
It works so well with the big subwoofers that it causes the windows in a friend’s house in the next street to rattle all at once! When calibrating it, my immediate neighbour thought there was tunneling work under his house! It is therefore only activated in the middle of a week day when people are supposed to be at work or playing golf or something.
Note track corrections around Q1, lower input voltage, some changed resistor and cap values and (optional) conversion of high-pass to band-pass output filters.
Original Mod 3 Sensurround control box: