[~misfit~[_3_]]

Once upon a time on usenet Peter Wieck wrote:
On Tuesday, August 14, 2018 at 11:15:25 AM UTC-4, ~misfit~ wrote:
Once upon a time on usenet Peter Wieck wrote:

Hi Peter, a quick question if I may? I bought four second-hand
ex-shopping centre rack mount 100w PA amps cheaply a while back,
mainly for the J49/K135 output devices. They use output transformers
of a similar size to the main PS transformers. (Turns out I actually
like the way the whole amplifier sans output transformer sounds so
am using those in a project.) I tried to find out a bit about how
this sort of system works but had no luck. Can you offer any help?

I was wondering if there was any way I could re-use or re-purpose
the output transformers. They're a big hunk of silicone steel and
copper, seems a waste to just throw them in landfill.

Cheers,

Sean:

Likely those are more properly a line-matching transformer designed
to operate (typically) at a constant voltage (70.7 - 100 VAC),
variable current system. The typical (tube) OPT operates at full B+
voltage (350 VAC+ in many systems, as compared to the 70.7 V (or so)
in a solid-state PA system. The amp feeds the primary side (4-8 ohms)
and the secondary is a constant-voltage (70-100V).

As above, what you have are, again most likely, the step-up
transformers used at the amplifier to raise the voltage into the
hundreds-of-feet of distribution wiring to the speakers. There is a
similar but much smaller transformer at each speaker to
drop-and-match the output to the particular speaker (4-8 ohms). Look
at it as similar to what happens when AC power is sent up to a
transmission line, then dropped for the individual consumer via a
series of transformers.

However, as they are typically fully isolated, they will provide the
same level of DC protection as a conventional OPT.

https://www.electronics-tutorials.ws...er/trans70.gif

Shows you the sequence-of-events.

Now, as to re-purposing: There is very, very little current on the
primary side of the typical output transformer. If you have a Variac
and a VOM, you could try bringing the *SECONDARY* side (70-100V side)
up to the limits of the Variac (140 VAC or so) and see what you
measure on the primary side. That will give you the (approximate)
turns-ratio.

Now, of course, you could also put your amps 100 yards from the
speakers, and feed them from the amp by putting the appropriate
transformer at either end.

https://adn.harmanpro.com/site_eleme...s_original.pdf

Here is a white-paper on how it all works.

Peter Wieck
Melrose Park, PA

Thank you kindly for that pdf - it explains everything I was curious about.
Unfortunately I don't have a Variac - I keep looking for one at the right
price but no luck so far. So the 4 x output transformers are useless to me
and will be thrown in the rubbish if I can't find a recycler who'll take
them.

Cheers,
--
Shaun.

"Humans will have advanced a long, long way when religious belief has a cozy
little classification in the DSM*."
David Melville (in r.a.s.f1)
(*Diagnostic and Statistical Manual of Mental Disorders)