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105 dB SPL AT ONE MILE
200,000 SATISFIED ROCK FANS
BY WAYNE YENTIS
Shortly after one o'clock in the morning on April
6, 1974 young people from throughout the West
started snaking through the gates of the Ontario
Motor Speedway in Ontario, California. By two A.M.
over 15,000 people were bedding down on the grassy
infield and at sunrise the crowd was wed over
150,000. At ten A.M. over 200,000 rock fans began
shouting their approval as Rare Earth opened THE
CALIFORNIA JAM, a twelve hour rock concert
produced by the American Broadcasting Company
and Pacific Presentations. In a motel a mile away
seven other groups were standing by to be flown by
helicopter into the backstage compound at the
speedway. They had come from around the world.
Months of careful planning had proceeded the
Jam and the on-site construction and installation was
started weeks in advance Miles of chain link fence
had been installed. Thousands of drinking fountains
and portable toilets were in place. A 45-bed hospital
had been constructed in the infield. A six hundred
foot length of railroad track was laid across the front
of the staging area Dubbed, "The Grand Funk
Railroad," the track supported three moveable stages
constructed on railroad tracks. One stage was to be
permanently set for the closing act, Emerson, Lake
and Palmer. The other groups would alternate
between the remaining two stages. While one act
performed equipment would be set up and checked
out on the remaining stage for the following group.
The plan was to cut the set-up time between acts to
almost nothing.
The concert was to be recorded for commercial
album release and videotaped by ABCTV as material
for future "In Concert" programs. To knit the entire
project together the producers needed a sound
reinforcement company with the know-how and
equipment to saturate the broad expanse of infield,
provide two channel monitor to the stages and feed
signal to the recording vans (Wally Heider
Recording) and the television audio trucks.
They called in TYCOBRAHE SOUND COMPANY
of Hermosa Beach, California. Tycobrahe had
provided both equipment and technicians for most
of the groups on previous tours and had developed
a very sizeable inventory of high power sound reinforcement gear.
In the early stages the Jam producers were estimating an
attendance of approximately 60,000, and the initial objective
was to provide coverage only to a distance of 1,000 feet
from the stage.
One of the first factors considered was the
placement of the staging area within the infield to
take advantage of the prevailing winds. The stage
was set on the western quadrant of the field so the
prevailing westerly winds would carry the sound
eastward across the audience area. "We were lucky,"
said Jim Gamble, Tycobrahe Vice President and
Director of Engineering, "The wind blew just the way
it was supposed to all day long. If it didn't, there
would be nothing we could have done to compensate
. . nothing."
The amount of power required to cover the area
to the 1,000 foot perimeter was swiftly determined.
According to the company's V.P. and Director of
Marketing, Ralph Morris, "We rate our standard
arena system as adequate for 10,000 people in an
outdoor situation. That's a 6,000 watt system. We
use that formula and add in multiples thereof. Of
course, it's only a rule of thumb because the sound
doesn't just go out so far and then stop. But the
formula works."
Some days before the concert it became apparent
that the attendance would almost certainly surpass
the 200,000 mark and appropriately the capability of
the reinforcement system was upgraded. An
additional array of speakers was added on towers at
the 1,000 foot mark These speakers were faced
outward and fed through an 859 millisecond
delay.
All the basic hardware used by Tycobrahe was
designed and built by them and carries their brand
name. The company began manufacturing its own
equipment when it recognized the need for a
specialized sound reinforcing mixer. The
specifications established by their engineering team
called for a mixer that was simple and straightforward,
highly ruggedized and portable.
The Tycobrahe Model MX244 input mixer has
stereo main and monitor outputs, with separate
panpots on each input. It was desiBed solely for
sound reinforcement and can accommodate the
"screaming microphone levels which would clip the
inputs on most studio consoles." Each input has 3
band equalizers with 3 selectable frequencies in each
band. There is no EQ on the outputs so that
inexperienced, or overzealous mixers cannot get into
too much trouble with a single EQ control. Overall EQ
is available for the monitor mix, but those controls are
included in the monitor power amp circuitry. Dual
band limiters are in the outputs of the mixer which
limits about a half a dB from the clipping point of the
power amps. They are similar to Altec dual band
limiters, with a crossover frequency of 250 Hz. 250
was chosen because that frequency is just about the
dividing point between vocals and bass. Dual band
limiting is necessary, according to chief engineer Jim
Gamble, to view from the mixing tower. . .
Ioahing toward the stage prevent pumping the
midrange and highs during those very heavy bass parts.
The amplifiers used at the Jam were also
Tycobrahe products, their 2,000 Watt BFA 2000 bi-
amplfiers. Like the mixers, the amplifiers were
designed specifically for location sound
reinforcement applications. The low frequency
section of the amp delivers 1,500 Watts, and the high
frequency channel delivers 500 Watts. The crossover
frequency is at 800 Hz. The units are packaged in a
rugged portable case, and are mounted in drawers for
easy access. The electronics for both amps are
mounted on a single, fan cooled, heat sink which can
be unplugged and instantly replaced in case of
failure. The raw power supply transformer, rectifier
and filter capacitors are mounted separately in the
drawer, although the voltage regulators are mounted
on the heat sink assembly.
According to Gamble, "Each of the amplifiers has
its own voltage regulator, although they are fed from
a common raw supply. This is necessary because of
the very heavy power demands, especially from the
bass amplifiers. Often the demands are so heavy that
the AC supply for the entire concert site fluctuates
up and down with the music, dropping from 120
volts to as low as 90 volts. We feed the voltage
regulators from a + 7 5 VDC raw supply and regulate
down to +55 volts. This allows for nearly a 30%
reduction of input voltage before the amplifiers will
fall out of regulation.
The monitor amps are packaged similarly; the
electronics are mounted on a replaceable fan cooled
heat sink, installed in a separate drawer in the same
cabinet with the bi-amp unit. A six band graphic
equalizer is included for the monitor amps to counter
feedback problems that arise on stage due to the
proximity of the monitor speakers.
The Tycobrahe loudspeaker units are the result
of several years of research. In explaining the reasons
behind selecting the elements of the Tycobrahe
system, Gamble says, "bass horns are the most
efficient, but there is a tendency to produce peaks
and nodes as you walk across in front of the
speakers, differences m levels of as much as 10 dB.
The infinite baffle, is of course, the flattest, but it is
really inefficient. The bass reflex has the next flattest
response, and is more efficient. If you stack a lot of
them together in the riBt configuration to get good
bass coupling, you get a very flat response without
those peaks and nodes, as you move through the
audience. That is why we use, mainly, the bass reflex
enclosure. We have tried all sizes from 7 to 30 cubic feet
and there was a point where more cabinet volume didn't make
any difference. That was around 10 cubic feet. Our
enclosures are 10~ cubic feet and have 2 JBL 2220A's,
a 2482 driver with midrange horn and 2 075's tweeters
in them."
To support the speaker arrays and amplifiers
two, fifty-four foot, six level towers were installed,
one on each side of the stage. The first level was a
utilitarian platform left empty. On the second level of
each tower were two eight foot bass horns driven by
18-inch woofers. Seven smaller rear loaded horns
were mounted on the third level. The upper three
sections of each tower supported fifty Tycobrahe
bass reflex cabinets. Care was taken to construct the
towers and mount the loudspeakers so the speaker
elements were directly over each other in a vertical
line to keep the system in phase.
Following a standard company procedure,
speaker lines are run individually to each woofer in
the bass reflex enclosures because, as Jim Gamble
pointed out, "If you put the two woofers in parallel
across one line, there's a chance that they won't react
exactly the same and you get a reflected impedence
back across the speaker lines. Also, two speakers in
paraUel present twice as much of a load in series with
the speaker cables. Any resistance in the cables will
then become twice as significant and more power will
be dissipated in the cables. It's a matter of
maintaining as much efficiency in power transmission
as possible.
Gamble went on to talk to R-e/p about the sound
dispersion: "A lot of people told us our bass was
going to roll off. They said we were just going to
have a hell of a time getting bass way out there. I
said that's just not true. The air disperses high end,
not low end. And I was right. We found we had to
really crank up the high end to get the tweeters out
over that broad an area. We had about 225 tweeters
in the system but what was really predominant was
the bass. It was solid and it sounded good.
"If you put enough speakers together you get a
big, wide plane to project off of and that acts as a
huge coupling board and that's what we were
depending on that coupling. With that much
coupling you can really project it out there. A lot of
people say if you don't have a horn you can't get the
bass out there. Well, that's not true. We even found
that the horns we did use contributed very little to
the bass level."
"The system developed 54,000 watts RMS and
we measured 105 dB SPL at a point onemile distant
from the stage. At the mixing tower we measured 120 - 126 dB,
and that was about 160 feet from the stage. We were originally
told to put the mixing towers up 50 feet from the stage, but no way
were we going to be that close."
The amplifiers were installed on the towers with
standby units in place and a compliment of spare
components handy. Each bank of amplifiers was
manned by a technician who monitored the meters
and was ready to exchange any amps that failed. A
complete lab and repair station in a Tycobrahe van
was located in the backstage compound.
Two Tycobrahe mixers fed the main system.
They were installed on a tower in the audience area
160 feet from the stage. The cabling between the
mixing tower and the ampUfiers was redundant. A
spare line was available to each tower and the tower
technicians had the ability to switch lines instantly
in the event of a failure by using specially
constructed switchboxes.
One main system mixer was assigned to each of
the two stages for which Tycobrahe was
responsible. (The Emerson, Lake and Palmer set-up
on the third stage was mixed by their own
personnel.)
Two additional MX24-4 mixers were used to feed
the on-stage monitor systems. One was installed on
each stage behind the acts so the monitor mix was
completely independent of the main system feed.
The delay in the system that fed the auxiliary
speakers at the 1,000 foot distance was accomplished
by using tape delay recorders augmented by an
Eventide Clockwork digital delay unit. It was found,
however, that the delay system and auxiliary
speakers were not necessary. The primary on-stage
amplifierspeaker array was sufficient to cover the
entire area. Although the delay arrangement was
stipulated in the contractual arrangements if the
attendance estimates approached the 200,000 mark,
the Tycobrahe people were confident that the on-
stage system was adequate and the company is
philosophically opposed to delay.
The effectiveness of the reinforcement system
was checked prior to the Jam by simply driving to all
points in the infield while recorded music was fed
through the system.
A three-man Tycobrahe crew was assigned to
each of the two stages for set-up and miking. The
microphone selection was very straightforward;
Sony ECM22p's were used for the drum overheads
and Shure SM-57's were used on each instrument.
The SM-57's were
chosen because, according to Jim Chase,
Tycobrahe's director of operations, "You have to
use a real close-pattern cardioid dynamic
microphone on any instrument that is going to be
put through the monitor system. Other people use a
lot of wide-pattern stuff, even good cardioid
patterns, but not good enough. There's only one
mike that works for us and that's a Shure SM-57."
Everything on stage was miked except the bass,
which was also taken direct, so as to deliver through
the system the distortion and other effects created in
the guitar amplifiers. As Chase points out, "We don't
much get into studio techniques in terms of miking
or direct instrument feeds. The groups are looking
for a sound and they want to be in control of that
sound. They don't want us to color it or change it.
We'd make it clean if we had our choice. If we were
to take the instruments direct we wouldn't get the
musician's tube amplifier mushing and his power
supply saturating and the speakers moving to maximum
excursion, and all the other things that create
the particular noises that you can get out of a
guitar amplifier. The talent wants that and
they want us to simply reinforce it."
At the Jam the signal from each mike on stage
had to feed four separate and distinctly different
entities; the main Tycobrahe sound system,
the Tycobrahe monitor system, the
Wally Heider's 24-track recording vans and the
ABC-TV videotape trucks. Two vans were used
by Heider, one for each stage, and it was de-
termined that the Heider mixers would feed
the signal to the ABC-TV mix. From the early
planning stages the interfacing of the four
systems was considered one of the primary
potential trouble areas. As Ralph Morris stated
it, "The equipment was all different and the
concepts were all different. And, of course,
each group of engineers had different points of
view. We anticipated ground problems as with
any interconnected system so we left plenty of
time to work them out. Sure enough, when we
plugged it all in, it hummed!"
In considering how to isolate the various
signal feeds from each microphone any resis-
tive method was quickly discarded because of
the substantial gain loss that would occur.
Instead, each microphone fed a separate four-
winding transformer. The transformers were
special-ordered from Sescom and mounted in
boxes with ground-release switches.
The installation of equipment began on the
l Tuesday before the Jam with Wednesday and
Thursday devoted to the set-up of speakers and
electronic hardware and the check-out of each
individual system. On Friday morning the
main system, monitor system, recording and
TV systems were interconnected for the first
time. It was then a question of methodically
going through the systems and eliminating the
ground loops Ralph Morris told the story of
one potential trouble area:
"At one point we found we were still picking up
an additional ground on some channels which was
then not an overall problem. So we started going
through them and found one connector with the
shell wired to groumd. Some manufacturers make
them that way and they have to be disconnected. At
that time our chief engineer said,'Oh, my God, we're
going to have to go through 180 connectors and clip
the grounds on all the shell connections!' But most
professional audio engineers will remove the case
ground if they use that brand of connector so as it
turned out, that was the only connector in the
system that was causing the problem.
"Our MX24-4 mixers are wired with all internal
grounds returned directly to the center tap of the
power supply secondary. If a mike cable with the
connector shell wired to the shield is plugged in, this
creates an exaggerated ground loop and a lot of hum,
even if the input is turned down. We did this on
purpose so we could instantly recognize an
improperly wired cable."
Another interesting point was the special
attention paid to the delivery of power to the site.
Separate mains were run to accommodate the
Tycobrahe equipment and isolate it from the power
source feeding the on-stage equipment of the
groups. Otherwise, as Jim Gamble pointed out, "We'd
have had the whole band playing and sucking up the
A.C., especially on the bass notes. The power gets
eaten up by the low end instruments. Consequently
the AC starts dropping on every low note. It can
drop from 120 volts down to as low as 90 volts.
Instead of more power when you need it most, we'd
get less."
All-in-all the problems involved with bringing 12
hours of an ultra high level of audio entertainment to
a potentially volitile audience of over 200,000
supercharged rock fans seemed to be very few. ..
Very few, indeed, as typified by the comparative
brevity of what the producers and their sound
contractor had to say about problems.
Perhaps a statement from one of the producers
summed it all up: "We were delighted to do without
any lurid post concert headlines."