pa_system PUBLIC ADDRESS SYSTEM (by David Lawyer, 1995) THROOP HALL HARD of HEARING SANCTUARY THROOP HALL: Throop Hall had two curved sound columns (speakers) and has two XLR (balanced) microphone jacks. The actual microphones are kept locked up somewhere. The cables from both the speakers and the microphone jacks go to the projection booth high on the S. side where amplifiers, etc. are located. The signals from the mikes are amplified in the projection booth and the output is sent back to the stage over a 70v. line system which feeds thru knife switches near the E. door to the stage. These switches permit the 70v feed from the projection booth to be switched to portable speakers which one may place on the stage. One could also connect directly to the speakers via the knife switches by setting up an amplifier (along with other audio gear such as a CD player) placed on the stage but there are two requirements: 1. the amplifier must have enough output to feed a 70v. line (most do, even though they are not rated at 70v. and can't put out 70v. -- but they can put out enough so that the sound may be heard loud and clear in Throop Hall) and 2. It is necessary to use jumper cables with alligator clips to connect to the knife switch as it was not designed to be used in this way. The bass response of the sound columns is not too good, perhaps due to the presence of 70v. line transformers. A plug in jack is needed so that people may plug in their own on-stage amplifiers to the hall speakers. But what type of jack to use? Professional PA equipment often uses standard 1/4" phone jacks. Home audio often uses just ordinary wires which connect to a push-button terminal. Some use phono jacks (RCA jacks) or miniature phone plugs (1/8"). It would be nice to have two monaural jacks of each type. One could also have just one type and use plug adapters (likely to get lost). If the amplifiers in the projection booth have been previously set up, then one may turn on power to them by just flipping the switch in the hall just outside the projection booth. This turns on the electric power to the 120 volt AC wall jack where the amplifiers are plugged in. PROJECTION BOOTH At one time in the past the amplifiers in the projection booth were used for amplifying the sound from the microphones in the pulpit. The problems with this is that anyone adjusting the sound in the projection booth can't hear or see what is going on in the sanctuary. See the document "av_wiring" for what cabling was used. The following description is as of Jan. 1994 and it is hoped that it soon will be obsolete as new audio gear needs to be added/substituted. The input mixer MX-300 (Numark) takes its input from the two microphone jacks on the stage front. Additional input comes from line-input jacks which are fed by audio players in the projection booth. One may also connect a local mike to the above mixer for use as a projection booth mike. The output of this mixer is fed to a pair of Realistic 20 W amplifiers (loose on the right) which in turn drive the loudspeakers in the hall. Note that the output from a modern phono cartridge is not enough to drive the line input jacks of the mixer and a preamp is needed. A Realistic mixer is currently being used as such a preamp for a turntable that plays too slow. The Wintec Stereo Amp 6005 mounted in the console is not hooked up and probably doesn't work. It should either be fixed or given away. It is high fidelity (see response curve in center drawer) and may be worth saving. FUTURE PROJECTS: Should the projection booth be continued to be used for amplifying sound, or should this be done on the stage (or both)? I think the answer should be "both". The advantage of using the projection booth is that the audio gear is relatively secure from theft. A disadvantage is that if recordings are played in the booth someone may need to be there and miss out on the dancing (or whatever) in the hall. Although there are programmed players, many people are still using tapes which require manual changing (and even CD changers cost more than single play ones). Also, one needs a same-room player if spontaneous requests from participants are to be played. It also would require connecting up a remote control line if one wanted to stop the music for announcements, dance lesson instructions, etc. Other proposals: Remove line transformers from speakers to improve bass and make it easier to drive them from home audio equipment that has no 70v line output. Reinstall line transformers in basement between projection booth and knife switches or eliminate transformers by use of heavier cable in run from projection booth. Use of existing heavy cable would put the speaker circuits inside the same multiconductor cable as the microphone circuits, possibly resulting in feedback (ringing). Obtain CD player(s). Other audio gear could be locked up on the stage (build a cabinet for it?). Provide jacks to connect existing sound columns to audio equipment set up on the stage. SANCTUARY (part of article written for Tidings but never published) The main PA system consists of two loudspeakers installed in the mid 1990s. They are mounted on each side of the chancel. The cable for the left one goes into the wall and then comes out several feet to the W. The remote audio control box is located in the S. rear of the sanctuary. A conduit was installed in the 1990s to run audio wiring in. It goes from the closet behind the chancel (to the N.) to the remote audio box. The pulpit also has a conduit connection. Many years ago, the PA system in the sanctuary was stolen and was replaced by an old system of 4 speakers mounted on columns. For only $10 one may purchase loudspeakers which sound significantly better that the existing ones. Why haven't they been replaced? It turns out that just replacing the speakers (and the amplifiers etc.) will not solve much of the problem. They create a detrimental interference problem if monaural sound is used (as it is from a microphone). This results in cancellation of certain frequencies when ones ear is at a distance from two speakers such that the difference in distance is a half wavelength. The effect is like a "comb filter". This makes it difficult to hear clearly. An ideal solution would be to just have one loudspeaker (or array of speakers in one enclosure). Even in this case there is still some adverse cancellation due to sound being reflected (echoed) from the walls. Luckily, we have an "acoustic" ceiling designed in the 1920's with padding behind the visible burlap. New speakers should be highly directional so that the sound hits mainly the audience and not the walls which are highly reflective and cause echoes. Such speakers are both quite rare and expensive. HARD of HEARING There are various ways to transmit loud sound to the hearing impaired. Infrared and FM require special receivers and headsets. One would leave a driver's license as a security deposit for checking out a receiver and headset. Another system is a magnetic field created by a wire loop (a health hazard ?). Some existing hearing aids can pick up such magnetic fields: About 10% of hearing aids which fit inside the ear and about 50% of ones which don't, can "hear" such fields. In order to use headsets (earphones) one often must remove their hearing aid or else the sound will be too loud (or the hearing aid overdriven). Some people want to conceal their hearing aids and are reluctant to remove them or to be seen using headsets. We used to have some earphones and jacks in the sanctuary for the hearing impaired. Some of the wiring is still there. If our sound were better, perhaps there would be less need for special arrangements as the use of personal hearing aids alone would be satisfactory. SUPPLIERS: Speaker City, 115 S. Victory (S. of Olive), Burbank, 846-9921 JD Audio Visual Co., 1713 E. Walnut, Pasadena, 792-6682