Bazz339 Posted March 14, 2016 Share Posted March 14, 2016 I recently went to location that is seeking some modifications to an existing system. I am not going to be too specific about makes etc. The system consists of 6 loudspeakers which are not readily accessible. Different models have different power ratings in the same style and size of cabinet. These are fed by a stereo 100V line amp rated at 500W per channel and has 3 of the speakers on each side. The system is used for speech reinforcement in a quiet environment. What power is a 500W amp (presumably with a 0dB input) outputting if the input volume control is at -12dB ? a table I found here would suggest X 0.063 = 32 Watts. Is that correct? Have I used the right table? I did not note the mixer output meters and suspect the level from the desk was well down as well. Measuring the impedance of each speaker cable run (Left and Right with 3 speakers on each) I was surprised how low the impedance was at 27 Ohm on each side so unlikely to be any partial short circuit or similar. To me that means each side is looking at approx 370W of speaker loading, W=V x V/R Again have I got my maths wrong? about 123 Watt per speaker? I am trying to match that to quoted taps on the various likely loudspeakers(3 on each circuit) one model is rated at 125W but quotes taps of 66/35/19/10 (Apparently no 125W tap. (I am querying that with the manufacturer as 3 X 125 = 375W which I would expect to give an impedance reading of around 26.66 Ohm which is close enough to 27 for me)) and another model is rated at 150W and has taps of 150W/75W/38W. How well is quoted power tapping likely to correlate with measured impedance? They want to put a couple of extra speakers in an area and I would normally use something with maybe a 20 watt rating for this purpose, that is often more to do with having a large enough cone to have a bit of warmth and may well be used on a lower tapping. I was contemplating putting the existing loudspeakers on one side of the amp and the new ones on the other as they ideally need a different mix but the impedance reading I got makes this impossible if it is accurate. Because the existing speakers appear to be set to such a (relatively) high wattage, and the volume needed little more than unity gain speech reinforcement the amplifier inputs are (as stated above) @ -12dB so there is not enough level to drive the small additional loudspeakers adequately although the amplifier. I know my options for dealing with this are either reduce the taps on the existing speakers and turn up the amp or to use a new amplifier for the new loudspeakers. It may be seem silly for someone who has been in the sound business as long as I have to be asking these questions but is is not often I have to delve into the maths/theory. I am really just wondering whether my calculations are reasonable or whether I have missed something. Link to comment Share on other sites More sharing options...
DrV Posted March 14, 2016 Share Posted March 14, 2016 What were you using to measure the impedance? If you were just using a d.c. meter the resistance would look very low with all those transformer primaries across the line Link to comment Share on other sites More sharing options...
Bazz339 Posted March 15, 2016 Author Share Posted March 15, 2016 What were you using to measure the impedance? If you were just using a d.c. meter the resistance would look very low with all those transformer primaries across the line I was using an impedance meter to measure the impedance. An Adastra 600 665 now discontinued. Link to comment Share on other sites More sharing options...
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