October 2018 update
- An exciting development: A very helpful guy called James (see comments below) has given me a few great ideas and some modifications to the “Sensurround card” have been made to his advice. One was a mistake, but that’s OK. Others were worthwhile. However upon listening with headphones and viewing the oscilloscope while “twiddling” the adjustments pots, a limitation of my 15 Volt supply has been identified. The original patent required a 20V supply to the analogue filter section. With only 15 Volts, the output could clip – not just on adjusting the “output” pot (an attenuator), but on adjusting the frequency pot at the digital section. Moreover, the irregular and very high amplitude transient low frequency effects that increase upon downward adjustment of the frequency pot of the digital section could clip the analogue output section unless “rolled back”! Rather than changing the voltage to 20 per the patent (which was too hard), the headroom of the op amp was increased by lifting pin 11 from GND and attaching it to the available -15V output of the PSU! 30V is within limits of the device. More below including a sound file made directly off the card.
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 and suspect that the patent was invalid for want of enabling disclosure. Anyway, 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 than me 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:
A video of the raw waveforms straight off the card (digital and analogue):
And one of the analogue waveform after the 12dB/octave booster:
Here is a spectrum of the boosted signal (note the taper off as frequency increases):
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. 🙂
October 2018 update
This page has been up since 2009, but the circuit diagram was further corrected with the help of James (see comments below). This is the final version:
Note the 2009 wiring changes around Q1 as suggested by Rod were correct, but my lower input voltage (15 compared to 20 from the patent) meant that the 12V Zener supply resistor value should have been dropped from the original 1K to 390Ω to provide the required current to the digital section. Some changed resistor and cap values remain. Each CMOS and the op amp chips also have 100nF ceramic power supply bypass caps and there’s a 100μF electro added across the 12V Zener.
Card with these changes:
Oh – I forgot. It now has a TL054 in place of the LM324. It’s a much better op amp, so I thought why not? Well I wouldn’t have rushed out to buy it, but I had about a dozen just sitting there in the stash.
And dodgey (mostly 1989) soldering, but 5 ceramics are shown:
More importantly, an increase in dynamic range has been achieved by adding a dual rail supply to the output filter section. Pin 11 of the op amp is now connected to a -15V supply rather than GND. A 56Ω resistor bypasses the supply.
The above circuit is correct, it works and is 100% stable but only when an unbuffered CD4011 chip is installed.
Here’s an audio capture that I made straight off the modified card with Audacity. It starts off as a kind of “Galactica fly-by”, but as I turn the frequency pot it transitions into that awesome bone-crunching “Earthquake” with randomly spaced LF transients where it’s best at around the 15 second mark. The trimmer is turned beyond this sweet spot where it sound’s kind of dumb, but I included it for “full disclosure”. 🙂
rumbles 1 ← Download mono WAV. Play this at your own risk! The management assumes no responsibility for the physical or emotional reactions of the individual listener, nor for any damage to anything that it might cause!
Rumbles 2 ← Download stereo FLAC. Ditto! ← This requires a proper browser else it might produce a screen of garbled fonts.
Here’s my own FFT showing that the card has around 105dB of dynamic range:
Original Mod 3 Sensurround control box: