100V line amplifier help

[cedd]

Paging all 100V line experts!

I'm working a night tonight to try and locate some 100V line circuits that have been disconnected from a now removed amp rack. I have a marshalling box with several circuits arriving at it, but no idea which speakers each circuit drives. The aim is map them out and eventually to recommission some of them.

Last week I ordered a cheap 100v line amp from CPC, with the aim of simply connecting it to the circuits in turn and playing some tone through them. The amp hasn't turned up (UPS tracking suggests it was attempted delivery on Friday after we'd closed, and has been stuck on "next day redelivery" since). 

I don't have another 100V amp available. I do have tonnes of regular ones though sat at home.

Is there anything I can do to get sone audio through these speakers? I'd toyed with if using the transformer from a speaker backwards might work, as I'm only driving them at low level. Not sure if I'd be letting magic smoke come out though. 

I don't need much sound, just enough to walk around and hear which speaker is which. 

Any suggestions?? Or anybody in west Leeds with a spare 100v amp that I can easily get a signal in to? 

Thank you!!

[Lamplighter]

Hi Chris

You don't need to worry unduly about matching to the 100 v line to identify your circuits. A 100 to 200 w amplifier at 8 ohms will easily give you enough level without a matching transformer. If it is easier then a 10 w amp with a speaker transformer in reverse will suffice. I would tend to use pink noise as it is easier to locate a speaker where there might be more than one in earshot. Music works well but you always get a quiet bit at the wrong time.

Brian

[DrV]

1 hour ago, Lamplighter said:

Hi Chris

You don't need to worry unduly about matching to the 100 v line to identify your circuits. A 100 to 200 w amplifier at 8 ohms will easily give you enough level without a matching transformer. If it is easier then a 10 w amp with a speaker transformer in reverse will suffice. I would tend to use pink noise as it is easier to locate a speaker where there might be more than one in earshot. Music works well but you always get a quiet bit at the wrong time.

Brian

That's exactly right. You only need 100V to get the full nominal output. Any amp which is capable of driving 20 or 30 volts will give you enough level to identify speakers. In fact using the minimum required to be just audible will probably help overcome the problem of distinguishing multiple speakers in earshot that Brian mentioned.

[pstewart]

The signal generator from a tone probe set will make the speakers sound.  That wasn't what I was expecting to happen when I tried to trace some speaker cabling in our library....

 

Philip

[sunray]

As has been said any amplifier will do. I did exaclty this exercise in a shopping centre fitted with 32 zones of voice EVAC. Initially we tried locating failed speakers using the main system (450W per zone) but lack of volume control capabilities made it difficult. we used a small amp of around 30W at 8ohms to provide a low level signal on a zone by zone basis.

The thing about such a system is if properly designed there are no apparent gaps in the sound but running at a much lower level allows the gaps to form.

 

As Philip has mentioned the tone set may be enough for the job, the same is true for a loudspeaker impedance meter.

As an aside to this I'll shortly be sorting out my 'store' and disposing of my PA hire stock including a number of 120W mixer amps, typically TOA A512 sort of thing if it helps.

 

EDIT: Another aside: I've been using Eagle horn loudspeaker for 50 years 100V tapped at 15W and regularly run a row of them on a 120W 4ohm amplifier without a step-up transformer.

Edited March 13 by sunray

[sandall]

3 hours ago, pstewart said:

The signal generator from a tone probe set will make the speakers sound. 

If all else fails, dabbing an AA battery works just fine.

[cedd]

Thank you everybody. One of my trusty old crown amps worked a treat and we were able to locate all of the zones. Can confirm that my impedance meter also drove the circuits a-ok. 

A good result! CPC amp still hasn't turned up!!

[sunray]

11 hours ago, cedd said:

Thank you everybody. One of my trusty old crown amps worked a treat and we were able to locate all of the zones. Can confirm that my impedance meter also drove the circuits a-ok. 

A good result! CPC amp still hasn't turned up!!

I was surprised the first time I used an impedance meter just how efective it is for the purpose, did you think the same?

PM sent

Edited March 14 by sunray