In the summer of 1975 Rocking Horse played at the Donnybrooke Speedway, now the Brainerd International Raceway in northern Minnesota.
Rocking Horse was a great band at the time with lead vocalist Tom Baden, David Kurowski guitar & vox, Mark Schramm bass & vox, Ricky Peterson keyboards & vox, and Dale Waracz drums & vox.
We used portions of our PA and rented and borrowed other gear to make a vertical array.
While in high school I had previously built most of the cabinets which had been sold to other bands, the stack was a combination of three PA systems.
Set up had to go quickly, as we were not allowed to place our equipment until after the race ended and the track area was opened to the race fans.
All the cabinets were sealed straight Hyppo and Domino horns (Steve Hall designs), from the bottom there were two Guass 18", an EV SRO 15", a Gauss 12", two JBL 2420 on Recchi fiberglass radial horns, two EV 1823M on EV 8HD horns, and an RCA driver on a University H-600 horn.
The stack was about 10 foot tall.
The "processing" consisted of a Wingo (Mark Winger designed) 1000 Hz 12 dB per octave active crossover, all the cone drivers below 1000 Hz, all the compression drivers above 1000 Hz, and possibly a PAiA kit-built octave equalizer.
A variety of amps were used, the only one I am sure of was the Sunn 200S tube bass amp which I owned was used for a pair of stage monitors. There may have been a Crown DC 150 for highs and a couple Phase Linear 400 for the lows.
Mixers were a pair of Tapco 6000R cascaded for 12 channels: 5 vocals, guitar, bass, keyboard mix, Leslie high and low, kick and snare.
After a day at the races most of the crowd cleared out pretty quickly to head home, I remember the PA sounded good way further back than any of the audience was
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Art