response 1 of 33:

Nov 1 21:08 UTC 1999

A few quick browses didn't find any items with source, or whatever, at least
for what I'm trying to find.

I'm trying to find a public address 70 volt speaker line transformer.  I'm
building a PA amp to connect to speakers in a building, and I need a 50+ watt
transformer that goes from 4 or 8 ohms to a 70.7 volt line (all the speakers
then have little transformers to go the other way, standard sort of setup).

Any ideas or sources?  Even Purchase Radio didn't know, although they could
find a 35W transformer attached to a wall plate and volume knob.  Maybe a
couple of those linked together...

response 3 of 33:

Nov 1 22:59 UTC 1999

Why not use one of those x-formers backwards?

response 5 of 33:

Nov 2 00:03 UTC 1999

re: #3:  60watts > 10 watts

response 7 of 33:

Nov 2 02:35 UTC 1999

These are a common PA system item.  I think Radio Shack used to sell them.

response 9 of 33:

Nov 2 12:18 UTC 1999

Well, the trouble is that "everybody" knows what "70.7 volt line" is, so
nobody bothers to describe it.  It basically takes the usual 8 ohm speaker
input/output and converts it to one with higher voltage and lower current in
order to reduce line loss in building wiring.  It requires a transformer at
each speaker, and usually at the amp (although I now suspect that amp
designers these days just found a way to drive the line directly rather than
pay for a heavy transformer).  Maybe 70.7 volts for 1 watt at 8 ohms?

The difficulty is that with reduced use at the amp end, the higher power
transformers are getting hard to find.  I was able to find a 100 watt
transformer 15 years ago when I did something similar at Pioneer High School,
but so far I'm only getting blank looks.  10 watt transformers are easy to
get, but the big one for the amp is another story altogether so far.

response 11 of 33:

Nov 2 16:20 UTC 1999

Right, my Mouser catalog has the same part (imagine that!).  W == watts.  I'm
curious about what "70 volt line" really means, too.  One interesting thing
is that the speaker side transformers often have multiple taps on the line
side, to allow different amounts of power to be tapped off (i.e. the speaker
in the warehouse gets 10 watts, in the Inner Sanctum the speaker runs off a
measly 1 watt).

response 13 of 33:

Nov 2 20:59 UTC 1999

You could run some AC into the input of one of the 10w speaker-side
transformers and see what voltage you get on the output; this will tell
you the turns ratio.  You could either be brave and run 110vac direct
into the input (if it's designed for 70vac audio, it probably has some
margin of safety), or you could run the transformer in series with a
resistor -- a 100w incadescent lightbulb would make a fine resistance,
and measure the voltage on both sides of the transformer.

Once you know the turns ratio, you can experiment with experiment with
ordinary 60hz "power" transformers, or you could wind your own
transformer.  Do audio transformers use the same magnetic material as
power transformers (silicon iron?)

response 15 of 33:

Nov 2 21:58 UTC 1999

If this is a PA, hi frequency fidelity may not be important.  But if it
is, then you probably want to get something better.  Other things to try
would be a ham swap meet, or an electronics distributor.

response 17 of 33:

Nov 3 02:54 UTC 1999

Try it and see.  I believe telephones cut out around 2kHz.  Music is
more likely to have obvious problems than speech.

response 19 of 33:

Nov 4 00:39 UTC 1999

And even with a 5-6 kHz cutoff, speech intelligibility is adversely
affected.  The reason you have such a hard time spelling things to people
over the phone is that some of the frequencies in the sibilant consonents
get cut off, making them sound more alike.

At any rate, even if frequency response weren't a problem, I doubt a power
transformer would have the proper impedance to match the amp output.

response 21 of 33:

Nov 5 21:01 UTC 1999

actually phones clip anything not ni the 300-3400Hz range, but the highest
a human voice can go is around 5KHz, though that is only when singing. normal
voice is well within that range. 

response 23 of 33:

Nov 6 01:00 UTC 1999

Bit of a breakthru:  Out of the 20+ electronics books I checked in the local
library, only 2 had any significant mention of these transformers.  One of
the books promised more detail in volume 2 (which of course was not
available), while a crusty old speaker design book mentioned the formula for
picking the speaker's transformer:  70^2 / (desired watts).  So a 10 watt
speaker tap would have a primary of 590 ohms, and a secondary of 8 ohms.  So,
if I want a 60 watt transformer for the amp, and the amp can drive an 8 ohm
load, then I need a transformer that has about 10 to 1 turns ratio.  Hmmm,
I wonder how a 120vac to 12vac transformer would do here?

This must have been more of a hassle in the old tube days, when you would have
to add up all your speakers by power tap and make the amp match.  Solid state
amps only need to have a maximum load, while tubes also needed a minimum load.