Whole Building audio system

[FMROB]

Hello boys,

Have a need to run three separate speakers with three volume controls in three different rooms about 300 feet from a CDM1250 so the fire crew can listen to the radio.

My question, I am thinking about using 6.5" ceiling mounted speakers, decors-style mono volume controls, and a 30 watt amp.

Can I connect the audio line output of the CDM1250 into the amplifier? Any suggestions on this, hints??

Thanks, Rob

[chpalmer]

Depends on what your using as an input.

You might try using the discriminator audio pin 11 with "filtered audio" turned on in rss if it provides enough signal which it should. That way the volume pot of the radio wont have any affect on those speakers and can be used to control the local radio speaker if desired.

But yes it could be used.

[Tom in D.C.]

On the input side I would first try a direct connection into the
amplifier from the CDM. We'll assume that the only speaker outputs
will be the three remote units, so once you set the correct level
between the CDM and the amp all further volume adjustments will
be done at the individual speakers.

On the output side, you will go nuts trying to make a 4, 8, or 16 ohm
speaker circuit work direct over a distance among three speakers. What
the pros do (I used to be one of them) is to convert the low impedance
speaker output to 70 volts using an inexpensive 8-ohm-to-70-volt
transformer. You'll need four transformers, one at the CDM and one
at each speaker to bring the 70 volts back down to 8 ohms. The speaker
volume controls are actually stepped transformers rather than volume
pots, and they work in "clicks" rather than continuously. You can mount
them in the speaker box enclosure (if the speakers have enclosures) or
in a single-gang box near the speaker. On speaker size, all you need is
the cheapest 8-ohm speaker you can find, but it should be eight inches
rather than six inches in diameter.

The last job I did like this had a number of fix-tuned VHF, UHF and lowband
receivers all fed into a distribution network at 70 volts. At each speaker
location there was a volume control unit as well as a receiver selector switch.
That was at least ten years ago and the system is still running fine today.
If you do it right the first time you're good for many years of reliable operation.

All the transformers, boxes, speakers, etc. can be bought at Radio Shack and
similar low-priced outlets. If I were doing a job today on my own I'd start out
by shopping for the stuff online to save time.

Good luck.

Regards,

[Will]

Depends on what your using as an input.

You might try using the discriminator audio pin 11 with "filtered audio" turned on in rss if it provides enough signal which it should. That way the volume pot of the radio wont have any affect on those speakers and can be used to control the local radio speaker if desired.

Be SURE to use a capacitor in series with the CDM Filtered Audio output on pin 11, + side to the CDM radio. That can be fed into the line input on an inexpensive PA amp that has 70 volt speaker out put. I use a 2.2uF cap.

Volume control on the radio does not affect the volume on the PA speakers.

[FMROB]

Why use a capacitor? Is it bad not to use it? Thanks, Rob

[Terriers618]

We have 17+ speakers wired up in our station using a CDM1250 as the audio source. Here is how it works - speaker output on the CDM accy port to the amplifier. The audio is set to low-medium on the CDM and then controled further from the amplifier. The amplifier runs the audio out to all the speakers over CAT5 cable (dont ask who did the setup because I dont know....) anyways each speaker has its own built in volume knob. The system works well. Best advice is to keep the volume on the low side of the CDM and turn up on the amp side otherwise you will get the obvious distortions....

Is there a way to set the speaker out on the CDM accessory to be a constant level independent of the volume knob?

[Will]

Why use a capacitor? Is it bad not to use it? Thanks, Rob

Rob,

The Filtered Audio output of the CDM is an opamp and it has a DC level on it, shorting or loading down the output will burn out the opamp. The capacitor blocks the DC voltage from the Public Address amplifier input.

[Will]

Is there a way to set the speaker out on the CDM accessory to be a constant level independent of the volume knob?

Yes, use the Filtered Audio output on pin 11 to feed the 'house' amplifier. This output is NOT affected by the radio's volume control.
Use a blocking capacitor to prevent damage to the opamp IC in the CDM radio, 2.2 Uf + to the CDM side works well.

[FMROB]

Will,

Thank you, I will be setting the system up this weekend, I will give it a shot.

One more question, would you use a low-z or hi-z input on the amplifier. I guess it would be either the phono plug or xlr input?

Also, would you have to use a matching transformer 1:1 or just the capacitor will do?

Thanks, Rob

[Tom in D.C.]

Low impedance, using the XLR connector with the XLR
converted to two pin rather than three pin use. Tie the
ground in the XLR to the low side pin, leaving the hot pin
as the positive.

A capacitor blocks DC and passes AC. The audio signal is
an AC signal, so what you're doing is isolating the receiver
from the amplifier for DC.

[Will]

One more question, would you use a low-z or hi-z input on the amplifier. I guess it would be either the phono plug or xlr input?

Also, would you have to use a matching transformer 1:1 or just the capacitor will do?

Thanks, Rob

No transformer needed. Use the hi-Z input, AKA line input.

[FMROB]

Ok, Lost here.

Line input or hi-z is the 1/4 phono input jack?

low-z is the xlr input?

On the phono tip is usually hot and ring is ground, is there a standard diagram on XLR?

Which one should I use?

Thanks for all the help, this board has always rocked. Rob

[GMC]

Like it was said above you are looking for an line input usually called an AUX input on the amp. The type of plug will depend on the brand but it is usually a phono type plug. Some amps will have a terminal strip and some do use a xlr type and have a switch to select (bogen comes to mind). As far as the xlr connection it is standard..... Pin 1 is ground, Pin 2 is +, and Pin 3 is -.

[FMROB]

Thanks guys. I set it all up on the bench and it works well. It has little his in the audio when there is no talking, but being ceiling mounted no one will notice.

Thanks again, Rob

[FMROB]

Here is another stupid question.

Low Band Maxtrac pin 11 for audio. I am getting squelch noise without the radio RX anything. When a signal comes in the squelch goes away and the audio comes thorugh. I assume that this is non descriminated audio?

I need a pin for RX audio that will not do that. It needs to provide just RX audio without squelch.

Thanks, Rob

[FMROB]

OK I think I found the stupid answer. There is a jumper inside of the maxtrac that selects between filtered and desc. audio, moved said jumper and it works. Thanks, Rob

[Will]

OK I think I found the stupid answer. There is a jumper inside of the maxtrac that selects between filtered and desc. audio, moved said jumper and it works. Thanks, Rob

That is correct, the jumper needs to be on the FILTERED position.