Transformers for driving 70V / 100V speakers? Halloween!

Hi!

It's my 'halloween haunted house' time of year again, and I want/need to drive a bunch of 20W 100V speakers from some power amps with 4 ohm outputs.

Does anyone out there have any recommendations or suggestions for suitable transformers? The budget for the overall project is not big, so I don't want to spend a fortune on them. Quality doesn't need to be A+ amazing.
The aim is spooky music and sounds to scare the crap out of people!

I found these at Canford but would be grateful for any budget ideas here.

http://www.canford.co.uk/Browse/21359


Stewart

Just use a power toroid transformer backwards...220/24 or 36V type will do the job.

Regards,
Milan

zebra50 said:

Hi!

It's my 'halloween haunted house' time of year again, and I want/need to drive a bunch of 20W 100V speakers from some power amps with 4 ohm outputs.

Does anyone out there have any recommendations or suggestions for suitable transformers? The budget for the overall project is not big, so I don't want to spend a fortune on them. Quality doesn't need to be A+ amazing.
The aim is spooky music and sounds to scare the crap out of people!

I found these at Canford but would be grateful for any budget ideas here.

http://www.canford.co.uk/Browse/21359


Stewart

Click to expand...

More and more these days, one gets along without an output xfmr. Use a bridged amp. An amp rated at 2x250W into 8 ohms delivers ca 45V rms per channel. Once you bridge it, you get 90Vrms, just 1 dB short of the rated 100. Then you can run about 20-25 of those 20W speakers.
Caveat: the amp must not have significant DC offset at the output. A typical speaker has a DCR of about 80% of the nominal impedance, so some CD current is generally of no consequences. But a transformer equipped speaker has a DCR about only 5% of the rated Z. Any significant DC offset will result in significant idle current in the amp's power stage. Never had any problem doing that on Crown, QSC, Lab Gruppen, MC², Powersoft.

Thanks for the suggestions, guys. They give me at least a couple more options to play with.

We actually have two 'houses' to wire, each with about 12 rooms, and different sound effects in each room.

Scary!

Seems you need about 24. I have 30 off 36V primary 100V secondary 200W toroidal line transformers that I removed from unused Avalon PA system. I'll help you with your budget. Drop me a mail at [email protected]

> 20W 100V speakers

These are 500 ohms.

"50 Watts in 4 Ohms" is 14 volts. 14 Volts in 500 ohms is 0.4 Watts. Depending on racket required, a shy half-watt may be enough for a room. (We used to party with 1-Watt kitchen radios, and sometimes the neighbors complained.) And the 50W amp sure will run cool. Won't get hot until a hundred such loads.

Are you sure you can't break-in and bypass the transformer? They are surely 4 or 8 or 16 ohm drivers. You can drive one or several from your amps, direct or via 10-ohm resistors depending how low the per-amp load gets, how much excess power you have, and the risk of over-tree under-foot wires getting pinched and shorted.

A 1:4 transformer off the 4-ohm out gives over 50V and over 5 Watts. For multiple speakers on a small estate, that's more than enuff. Please put several ohms several watt resistors between the amp and transformer (most modern amps are DC-coupled, and the transformer's DC resistance is much lower than a speaker). The low-V side must be rated over 14V; for pure-clean bass it should be over 40V but I suspect your squawkers' built-in iron doesn't justify such excess. 24V:120V is good. 1A on the 24V side is fine; 0.5A works with tolerable excess loss. 36V:100V "only" gives maybe 5W/speaker from a "50W 4ohm" amp, and you can "only" drive 16 loads before the primary gets lower than 4 ohms. Buying 100W lumps of this spec would be wasteful; if you found a friend with some to spare for a "bad home" then do it.

http://www.radioshack.com/product/index.jsp?productId=2295111

I just wanted to say thanks to everyone & Sahib in particular for their helpful suggestions. It was all a bit frantic in the end, as these things often are, but everything worked, and the halloween event went off pretty much without any gremlins - although plenty of spooks and goblins!

We had over 4000 people attend 'Hallowscream' over four evenings, so pretty much a success!

For anyone who's interested, I put up some of the creepy sounds we used at
http://www.xaudia.com/xaudia/Blog.html

I should now have time to catch up with my microphone meddling and other fun stuff.

Thanks again!

Stewart