Speaker basics

[Shopkeeper]

I am attempting to revitalize a built-in Nutone Intercom and speaker system. One of the 8" speakers has blown and it is a dated 25 ohm 2 watt paper cone. I have picked up a conventional 8" - 8 ohm speaker but need to match the impedance. What is the best method to do this? And if it is just adding simple resistor does this go on on one leg of the speaker terminals?

Shopkeeper

[kg]

the best method is to use a speaker line transformer, also known as a constant voltage speaker transformer.

[52 Bill]

Adding a resistor in series with the 8 ohm speaker will "work", but it will not perform like the original one did. What is the specific application of this speaker, background music?, paging?, door intercom?

[Shopkeeper]

speaker basics

The application is with a Nutone intercom, door chime & music system. The amp puts out 25 watts and needs be equipped with 25 ohm speakers. One of the 8" ceiling mounted units is shot & 25 ohm speakers are not readily available. I picked up a conventional 8" (8 ohm) 20 watt speaker to put in the system -- assuming I can match the impedance w/o a bunch of trouble. The suggestion of putting a transformer in the mix seems solid but adds to the cost??

Shopkeeper

[52 Bill]

There is no easy way to fix your problem other than getting another 25 ohm speaker. The 25 volt transformer system is usually for use in larger systems, and I don't believe that the Nutone is designed to work with one.

The problem you face is that adding resistance to the new speaker will present the correct load to the intercom system, but the signal going to and from the speaker will be greatly reduced by the added resistance. So if this is a door speaker, the person will not hear your voice very well, and you will not hear him very well either. If this is a ceiling speaker, it will play music at a much reduced volume compared to other speakers connected to the system.

[kg]

if you run the speaker as-is, it will draw 3x as much power from the amp as the others will.

depending on its efficiency, this may or may not cause it to be louder than those others.

regardless, the amp will be working harder.

now, if you insert 16r or so of resistance in series with the replacement speaker, the amp will now be working the same as before.

however, that resistance will get hot. that energy USED to be going to the coil, and is now being lost as heat.

again, depending on the speaker efficiency, it may end up softer, louder, or just the same as the original.

the big advantage of the matching transformer is that it allows you to change power to the speaker load WITHOUT that penalty/overhead of lost thermal energy.

[Shopkeeper]

52 Bill,

Per usual there is never just a simple fix!! I will see what the resisted speaker will produce and if it is too low for hearing, I will look at the WEB for 25 ohm replacements.

Thanks,

Shopkeeper

[Shopkeeper]

kg,

Found some 8 ohm speakers with a 70V transformer wired to them. What are your thoughts on this?

Shopkeeper

[kg]

shoppy,

let me ask you a few questions... you said the amp is a 25w amp. how many speakers were attached to it? were all the speakers wired in parallel, or series, or some combination? the topology is important. are there separate zones? there may very well be a way to hook it up without any extra parts or fuss...

[Enzo]

25 ohms is an odd value. 32 ohms is not common, but has precedent. For example the drivers in the AMpeg 8x10. See if you can find a cheap 32 ohm speaker somewhere. Parts Express has a TON of speakers to chose from.

[anthillrich]

I am not an expert at all, but aren't most intercoms, muzac systems, etc, run from 70v systems, hence requiring a transformer?

and I'm sure you can find an intercom speaker without much trouble at all.

ADI is sure to stock them. They are a security system/data networking/everything supplier.
google them.

I know there are other companies along the same lines, thats just the one I happen to know off hand.

[Shopkeeper]

All,

There are a total of 9 zones in the system. Most are intercom/door speaker systems w/ 2 of the 8" as additional music on the lanai. There were no transformers on the external speakers.

Shopkeeper

[Enzo]

By any chance, did you CONTACT Nutone and ask if they could provide the speaker?

[Shopkeeper]

Enzo,

That was my first search, No longer in stock at Nutone.

[J M Fahey]

Higher-than-"normal" impedance speakers are practically the norm in intercom speakers.
25 ohm ones do not sound unusual for me (in that area), I've even seen some old 45 ohm Philips ones.
As with everything non-standard (or popular), there must be *some* shop specializing in that, or, worst case, maybe some manufacturer, different brand, still has some equivalent speaker.
Sorry can't be more specific but that's a clue.
Compare this to finding Dr Thomastik gut strings for cellos:not the average staple at Guitar Center, but I know a couple shops in Buenos Aires, where I live, that sell nothing else (besides violin resin, odd reeds for clarinets and fagots, etc.).
In a similar way there must exist a couple shops specialized in intercom parts.
Good luck.

EDIT: Look what the cat brought:
NuTone: Speakers: <br>Music, Door, Intercom
There it says:

Intercom Speakers
Use a separate intercom speaker and intercom remote control to gain the advantage of a smaller intercom control at eye level. Place the speaker high on the wall or on the ceiling. All speakers are designed to work with any NuToneŽ remote control (see NuTone models starting ICA). Can be used with NuTone 'Master' station models IM-5000, IMA-4406, IMA-3303 and IM-3204. Speakers rated at 25 ohms impedance.

Maybe they do not have your *exact* speaker model any more(which, as you say, is dated), but they sure are hawking "25 ohm louspeakers", as of Today.

EDIT2:
forget everything I posted above: here you have them from $24 up:
http://www.google.com/products/catal...45363854763635