Batboard

I will be using a maxtrac and some external speakers for a simple alerting system at our base. I plan to wire up about five 8 ohm speakers. Because I did not spend long enough in electrical engineering, how many speakers will safely work, and is there anything I should do to help make it work?

Becareful in connecting additional speakers to the Motorola
radios. The audio amplifier is not designed to drive
anything beyond the original speaker load. Chances of
damaging the audio output amp are very good.

If you need to have a number greater than the original
speaker, it would be wise to run the audio through an
external audio PA type amplifier to drive the needed
quantity of speakers. You might even want to consider
going to a 70 volt distribution system for all the speakers.

Jim

rrfd43 wrote:I will be using a maxtrac and some external speakers for a simple alerting system at our base. I plan to wire up about five 8 ohm speakers. Because I did not spend long enough in electrical engineering, how many speakers will safely work, and is there anything I should do to help make it work?

you can use 4 8 ohm speakers in a series paralles setup and be fine, that will keep the impedence at 8 ohm.

You could wire up 4 speakers in series/parallel, but you might not be pleased with the results.. remember the volume gets divided among the speakers, to be loud enough you'll probably have to run the radio at full volume, and that might not even be enough.

Another thing you'll want to think about is individual volume adjustments, you don't want to turn it up to be able to hear it in the garage bays and find that it's blowing your ears out in the break room.

A 70Volt system would be the way to go.
A Bogen C-20 20Watt amp and 5 4Watt speakers with volume controls will probably run you in the $300 range, that would probably be the minimum you'd want.

(2) 8 Ohm speakers total. Period. In a station, you'll be running them at higher levels and will overheat the audio amp.

As stated, use an external amplifer with 70V feeds. With this, you can install speakers that have their own volume pot.

We build remotes with a 2 ohm speaker,a microphone and a 5 watt 20 ohmvolume pot.We put as many as 6 on boats,all in paralell,never had any blown audio amps,oh and we ground the speaker low for a 4 wire hookup.
This is widly done here in the oilfield,by many radio shops,it works.
mod

all 6 in paralles is 1/2 of a 1/2 ohm and that's not good on any amp, that's way too low and it'll run way too hot. even 6 8 ohm speakers in parallel is down to 1.7 ohm and too low.

I knew someone would do the math and say it wasn't good! I agree with the math,but.......There are 1000's of maxtracks on boats,production platforms all over the Gulf of Mexico for as long as maxtracs have been around.
In actual use, it works! with no blown amps,and secondly,you are not supposed to ground audio low,and thats a myth! ohm out any maxtrac/radius
speaker low to chassis or power ground,what do you get? continuity!
Keep in mind not, all remotes are turned up to max all the time,so the impedence changes with the pot settings.
CDM 750's are in the gulf working also,but with 2, 70 volt transformers used in a 1 to 1 configuration so that the radio doesn't see the grounded speaker low,or the impedence mismatch.
If you still don't want to believe me about the maxtrac's,you could use the transformer thing to set your mind at ease.
mod

Thanks for the input everyone....I came up with an audio amp and hooked it up through there.....Works ok, just wish I had one of those bottomless budgets.....

n5tbu wrote:you are not supposed to ground audio low,and thats a myth! ohm out any maxtrac/radius
speaker low to chassis or power ground,what do you get? continuity!

You're half right. Maxtracs have a complimentary symmetry audio PA using discrete transistors, the output goes through a large electrolytic cap to speaker high, and speaker low is connected to ground.

The Radius models on the other hand tend to use an IC audio PA with balanced outputs. The outputs each have a DC bias of half the supply voltage on them (around 6 volts) and grounding either speaker lead will generally result in some nice special effects (mainly smoke), and a trip to the repair shop.

I don't recall if there were some Radius models that didn't use an IC amp, I don't recall coming across any.. the GM300s, M120s, and CDMs certainly use them.

One danger of connecting a Maxtrac speaker to vehicle ground instead of speaker low to save a wire is that at some point in the future they might just swap the Maxtrac out with something else, say a GM300 or CDM. Someone who doesn't know better will look at the connectors, decide they all look the same as on the old radio, and plug them in, then wonder what that acrid smell is... and yes, I have seen this happen.

You guys are making a lot of general comments about a specific radio.

The "proper" external speaker for the Maxtrac (both 5 pin and 16 pin) was a 2 ohm speaker. Part number for the actual speaker is a 5084561B03. On MOL shows as SPKR 5" PM 2 OHM

The radio has one side of the speaker referenced to ground. It was the GM300/M120 series that went to an isolated speaker.

You could run up to 4 seperate 8 ohm speakers in parallel to achieve a 2 ohm load (add in some speaker wire and you will still be above 2 ohms).

Ok, now that I've got access to the manuals to supplement my failing memory... looks like some of the Radius models used a transistor audio PA, such as M100, M206, M214. The IC amp is used in M10, M120, GM300, M1225, SM series, CDM, probably some other ones I've forgotten.