Severl of our dealers are currently working on system designs using the 8500 amplifier and our FreeSpace BMS product. Becase of the complexity of the design and the number of zones, they plan on using individual channels of the 8500 amplifer to power these zones. They realize that a single channel of the amplifier is not going to drive their speaker system to the expected output because of the reduced voltage so they plan on tapping the speaker transformers up one tap setting to allow for the difference in SPL.
My question to the development team is whether you have done any testing with the amplifier using this "non-standard" loading of the amp channels? Would the minimum impedance that they would want to load the amplifiers be four or eight ohms? Do you have any design guidelines that they should be following?
Also, has any testing been done on single channels with high impedance loads, meaning in the neighborhood of 300 ohms? Specifically, does the amp have any issues if you attach something like four loudspeakers tapped at 4W (70.7V)?
Thanks Eric-I was referring to High Impedance transformer loads as well but was on my way out the door to the airport and did not express myself clearly. I have already been taken to task by Mr. Nelson for this transgression
To the development team, we need some direction on this pretty quick. We have several large projects in the works that have this requirement.
I have forwarded the information to our "high power" engineering team. You are asking the right questions. As you probably noted the max voltage from a single channel is about 70 volts, and you could compensate using the tap settings of the loudspeaker.
I expect that we will hear from Bill Becker, lead engineer for PowerMatch, today on this issue.
1. The use of a single amplifier channel to drive higher impedance loads is fine in general. The maximum voltage out on a single channel will be 50 Vrms into 4 Ohms or any higher impedances. (You can drive impedances as low as 2 ohms, but only up to around 31 V, or a total power of 500W per channel. But this may not be relevant to your question).
2. We have tested the amplifier output under a number of load conditions, including open circuit, and it should be perfectly stable and drive it without problem.
3. Your question made us realize that it is possible that the "open circuit" warning may appear in this case. This does not affect the audio output, and is benign. It presently can show if the impedance is over 20 Ohms or so. How it shows depends on your settings:
If the User Alarm setup is per the default, then an open circuit on any channel will NOT light the fault LED on the PM8500, and will not be in the alarm log. You may see it appear in CSD in the monitor screen or the output panel. You can ignore it.
If the user makes 'open circuit' an alarm condition in the CSD alarm setup window, then an open reading will light the fault light on the amplifier and trigger the fault output relay (you can still ignore it)
So I think you should be all set. Let me know how it works out.