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Peter Wieck[_2_] Peter Wieck[_2_] is offline
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Default Amplifier for passive subwoofers

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