Loudspeakers Weird speakers...~90 ohms ?

[leistico]

Hi there...

As I'm normally used to 'plug and play' gear (clearly labeled, simple math, 'newer', etc) this is one I feel I should pitch to the forum for consultation.

I need to set up some on-stage monitors for use in a musical. My previous M.O. in this space is to run a single speaker off-SR as a fill monitor, considering our orchestra is SL upstage of the pro. The orchestra is miked and mixed and pumped out the mains as well as onstage for the performers.

Anywho, my situation is this: I'd found eight speakers stashed and collecting dust, along with an ancient (I think it has tubes--when tubes were new!) amp (no jacks, just screw terminal strips on back for output AND input) The speakers boxes are about one foot cubes, homemade, with terminal strips on the back. As they were unlabeled old gear, and as I was thinking of chaining several across the front and sides of the stage for better monitor coverage, I grabbed my ohmmeter just out of curiosity. I ohmed one out at about 89 or 90 ohms. They all were around that, between 85 and 90 roughly.

I chained four of them together in parallel and got the resistance down to about 22 ohms, hooked them to my amp (500w at 8ohm) and gradually fed signal to them. They seemed to work fine, and the amp didn't show signs of complaining.

Just how Bad an idea was this? I could probably chain six for onstage, and wire up two as ballast as need be to get the impedance down to 8ohms, but would that overtax the amp? I don't need it cooking on me mid-show (we're a poor community theatre, and the speakers/old amp were found relics from when we moved in to our current space a few years ago)

Thoughts, cautions, better ideas?

sean

 

[avkid]

I have no idea what to suggest.
But pictures would be awesome......

 

[leistico]

Okay, upon further examination, the 'tubes'comment may have been hyperbole, but it's pretty old and crusty. Dunno if it works, not sure how to wire it if it does. I'm guessing the one amp drove the eight speakers (mounted in coves 10' above the floor -- this used to be a Soroptimist's Hall and then a community center, dates back to late 40's early 50's)

Please forgive my crappy RAZR phone camera, but here are pics of the things... two pics of the speakers, a front and back on the amp and a closeup of the wiring section of the amp. Any thoughts?

 

[leistico]

Okay...feeling idiotic...

After a bit more detective work, I'm thinking that the reason they're that high impedance is because of a *transformer*... I'm thinking they're 70V speakers *headdesk-headdesk-headdesk* The 70V screw and the com screw are loosened on the unit, none of the others are.

So, I just parallel-connected 4 70v speakers to a 250w Inter-M 8ohm amp, driven by a digital stage piano. Apparently none of the magic blue smoke escaped from anything. I think I need to read up on 70v systems--this is new to me.

 

[FMEng]

Yes, they are 70V distributed line speakers. It's a method to drive many speakers from one amp channel, in parallel. When you see lots of ceiling speakers in a restaurant, that's how they are wired.

Inside the box is a transformer that raises the impedance, so each speaker draws less current, and requires more voltage for a given amount of power. The distributed line amp also uses a transformer to match the transistor or tube stage to the combined impedance of the speakers. In addition to allowing many speakers together, it also allows the use of smaller wire over longer distances without much loss.

Typically, each speaker transformer has several input taps. Each tap is likely marked in the power delivered to that speaker when the amp is driven to its maximum. (A few transformer taps were marked in ohms, but that is rare.) By choosing the taps, one speaker can be louder than another to zone them. The total of the tap powers added together should not be more than ~80% of the amplifier's output rating.

It does not matter if the taps add up to far less than the amp's rating. All you need to do is ensure that the load impedance is equal to, or greater than the amp's output impedance. Adding up tap powers is just a convenient way to handle the math to do that.

If the Inter-M amp drives them loud enough without matching the line, it certainly won't hurt to run them that way. Depending on the speaker coil impedance, you could run two or four of them with the transformers removed. (My guess is they'll be 16 ohm or 8 ohm speakers.) Just keep in mind that these speakers are probably made for moderate levels of voice amplification. Expecting them to handle music at high levels may kill them. They will handle higher levels if you cut off the amount of low frequency energy you give them. In that day, 100 Hz was deep bass in a large space.

I'd probably use the old amp and speakers for low level backstage monitors. They would handle that just fine.

By the way, you can't accurately determine the impedance of a speaker or transformer with an ohmmeter. The ohmmeter uses DC, which just measures resistance. Impedance is an AC value. All you can do with an ohmmeter is measure whether the coil is open, or not. DC resistance of a coil will always measure lower than the AC impedance.

 

[leistico]

Thanks for the info on this. I think those speakers and that amp will become our new (!) house monitor system, feeding a house mic into the kitchen..er, scene shop, SM booth and dressing room.

One question comes up though. As the back of the unit is screw-terminal connections, I'm guessing I'd just use a servicable cheap dynamic mic wired to the terminals (through much zip cord or other small-gauge extension)? I've got a cheezy little Realistic mic with a 1/4" TS connector--can I just slice that off, strip the wires and run it to the terminals on the back? I'm guessing that's the way to do it, but if you know differently I'd appreciate any advice I can get.

Thanks!

 

[FMEng]

Looking at your photo, the amp is built with a balanced mic input, and your mic is un-balanced. Amps of that day usually used transformers on the mic inputs. Therefore, connect the mic's signal conductor to one mic terminal, and the mic's shield to the "SH" terminal. To make the transformer work un-balanced, connect the remaining mic terminal to the SH terminal.

Unlike active balanced inputs, transformers have to have both sides terminated.

 

[jeffmoss26]

Yep, a classic Dukane paging amp...those things are workhorses.
Some older amps have an octal socket that looks like it is for a tube, but is for an input or output transformer.

 

[FMEng]

Yep, a classic Dukane paging amp...those things are workhorses.
Some older amps have an octal socket that looks like it is for a tube, but is for an input or output transformer.

The octal sockets would be for input transformers. Output transformers are too large to plant on the end of an octal base.

Empty octal sockets on an older amp may mean the that the mic inputs are for un-balanced, high impedance mics unless the transformers are added.

A few old amps also used octal sockets for speaker output connections, with various taps for 8 ohms, 16 ohms, 25 Volt line, 70 Volt line, etc.

This thread has turned into a bit of an audio equipment history lesson. I hope that doesn't mean I'm an old fart.