klem kedidelhopper wrote:

I need to install a 60 Watt paging amplifier on a job. The maximum
load at this time is 42 Watts, however we need to have a slight amount
of room to move in the future. So I felt that 60W unit would be a good
cushion. I currently have in the shop a nice Technics 100 Watt mono
amp that I rebuilt and have no plans for. I would really like to sell
this to the customer to use for this application. The problem is
though it does not have the 70V output that I need. So I had two
thoughts. The first is can I buy a transformer that will have a
primary of 4, 8, or 16 ohms and a secondary which will provide the 70V
output that I need, and handle at least 60 Watts RMS? The second
thought is this: I have a number of 60Watt amplifiers in the scrap
pile with good transformers I'm sure. The schematics show these
transformers hung on the outputs as though they were a speaker load. I
have no idea what the primary impedance could be though on any of
these transformers, but I was wondering if it is anything close to
typical speaker impedance? If it was then perhaps I could pull one of
these from a junker, hang the primary across the Technics amplifier
output, (which I think is 8.0 ohms} and then have my 70V output? Has
anyone ever done anything like this? Of course I would do a load test
on the amp/transformer combination before selling it. Thanks for your
thoughts on this. Lenny


Lenny, the biggest difference in the amplifers is that a PA amp is
designed to be used 24/7/365.25. The transformer adds isolation from
the amplifer, as well. I see no problem, as long as you're willing to
stand behind what you sell and tell them that the transformer was added
to match the speaker wiring. MCM sells transformers for this
application. We did it back in the '70s to clean up installations done
by knowitalls, and their victims didn't want to pay for another
amplifier.

As far as matching constant voltage speaker system impedance, keep
these numbers in mind:

70V = 5000/W which means your 100 W amp would be 50.00 Ohms.

25V = 625/W which means your 100 W amp would be 6.25 Ohms.

100W is in the ballpark of what I would specify for a 60W load, if
you beleive that there will be more speakers installed at later date.
It could drive up to 120W load, in most installations with no problems.




--
You can't have a sense of humor, if you have no sense.