Mac Kerr wrote on Mon, 17 September 2007 06:09 |
Speaker transformers don't "expect" anything. The main thing you lose by using an amp that can't put out 70V into the system impedance you have is the ability to drive the system to its maximum volume. When you are playing low level background music that 70V amp isn't putting out 70V either. When it does put out 70V, each of the speakers will be receiving about the power it is tapped at. |
Ken Freeman wrote on Mon, 17 September 2007 15:45 |
Hey Jeff, This is not rocket science, but does require some math as Dave noted above. I direct drive 70 Volt all the time. I often explain this little slice of audio as a lawn sprinkler system, which works just fine as long as you keep the pressure (voltage) up and don't draw too much water (watts) from the system. Our rule of thumb is to use an amp that can generate more than 700 watts into an 8 ohm load. For large scale systems, you do need to figure in the wire as part of the load that the amplifier is seeing. If you do not, your calculations for actual delivered SPL may be off. Ken |
Ken Freeman wrote on Mon, 17 September 2007 19:13 |
Yes, that would be watering the garden with a Fire Hydrant. Ken |
Jerry Turnbow wrote on Tue, 18 September 2007 12:04 |
The "easy" math for 70.7 volt systems is to remember that power = voltage squared/impedance, and conveniently, 70.7 squared is 5000. |
Bob Lee (QSC) wrote on Tue, 18 September 2007 22:44 |
Perhaps even more important than load reactance is that if the speaker transformers saturate, they may put a voltage spike on the line when the cores un-saturate. This phenomenon tends to happen mostly on lightly loaded lines, and it used to frequently take out some older direct-drive 70-volt solid-state amps that either didn't have flyback diodes to absorb the spike or had inadequate ones. |
Bob Lee (QSC) wrote on Tue, 18 September 2007 14:44 |
Perhaps even more important than load reactance is that if the speaker transformers saturate, they may put a voltage spike on the line when the cores un-saturate. This phenomenon tends to happen mostly on lightly loaded lines, and it used to frequently take out some older direct-drive 70-volt solid-state amps that either didn't have flyback diodes to absorb the spike or had inadequate ones. |
Ken Freeman wrote on Sun, 23 September 2007 19:38 |
Yes, as Master Bob has noted, it can get a little squirrelly if you don't give the amplifier something to drive. I have always found that putting a 1000 feet of 14 gauge cable (about a 4 ohm load) seems to settle things down. Due to the odd venues that I end up in, I often build these systems while they are hot so we can check them as we go...Then just remove the dummy load and continue to the next system or branch. |
Tim McCulloch wrote on Sun, 23 September 2007 20:00 |
Lock down the DriveRack. I had a club install where they kept blowing up 15s. The rig was for syndicated foreground music. They kept it pretty loud, but not pushing it hard enough to clip, or even make it sweat hard... it turned out the cleaning crew would crank the system far louder than the bar crowd would tolerate. All the way up... They shouldn't have been anywhere the controls were located, but they did anyway. We put in a locking rack and disabled the remotes behind the bar... the maintainence issues went away. Someone thinks those little speakers can do car stereo bass. Lock down the DR or call a clergyman for an exorcism if the lockout doesn't work. Have fun, good luck. Tim Mc |