70 volt pa amp problem for fire station

[tcash158]

I got a pyle 120 watt peak 70 volt amp and ran my cdm1250 speaker level line into the amp and it is running 5 8 inch speakers with a 70 volt transformer on each speaker and 2 horns in the pay. If we turn the pa input to about 1/4 way up and the amp output to max or anywhere else all of the speakers sound distorted. If you turn the input down some to where the speakers sound good even with the volume turned up none of the speakers are very loud or I should say loud enough at all and the radio guy that is a good friend of mine says we need a bigger amp. Any ideas will help thanks.

[Tom in D.C.]

You didn't offer any data on how much power those speakers are pulling, or where the transformer taps are set, so no one knows how much power you're trying to pull through the system. Figure up the power draw and then make your decision as to whether or not the amplifier has enough power to drive everything you've got connected. Often times horn speakers are set for power like 15 watts each or even higher. Also, depending on how you set up the input circuit to the amp, the radio's output may be overdriving the amplifier which might result in the same problem.

[MP 6.7]

If you're using the speaker output from the radio, you're most likely overloading the input of the amplifier. Speaker level signals are far hotter than line level signals which typically average about one volt. Speaker level can be anywhere from a few volts up to over a hundred for a very large amplifier. Even though the audio PA in the radio isn't that big, it's still probably overloading the input of the amplifier which is expecting only a couple volts. You may have better luck using pin 11 (Flat/Filtered RX Audio) on the accessory connector which will give you line level receive audio. Ground will go to pin 7. I'm not sure how to switch between flat and filtered audio on the CDM's; it may be in CPS, but I can't say for sure as I don't own one.

On the audio system side, everything looks like it's in order as long as you aren't overloading the amp. With seven speakers and that 120w amp it doesn't look like you are overloading it. As a general rule for 70v sound systems, the sum of all the used wattage taps on all the transformers in the system should not exceed 80-85% of the amplifier's rated power at 70v. The extra 15-20% is there to account for line and transformer inefficiency which is not included in the wattage taps.

[Jim202]

I got a pyle 120 watt peak 70 volt amp and ran my cdm1250 speaker level line into the amp and it is running 5 8 inch speakers with a 70 volt transformer on each speaker and 2 horns in the pay. If we turn the pa input to about 1/4 way up and the amp output to max or anywhere else all of the speakers sound distorted. If you turn the input down some to where the speakers sound good even with the volume turned up none of the speakers are very loud or I should say loud enough at all and the radio guy that is a good friend of mine says we need a bigger amp. Any ideas will help thanks.

My first question to you is how are you connecting the CDM radio audio to the PA amp? Hope your
not grounding one side of the speaker output from the CDM to ground. You can't do that. You will
blow the audio output in the radio. You would be better off in obtaining a transformer to try and
match the impedance of both the radio and the amp. Second, make sure you add a coupling
capacitor between one leg of the transformer and the CDM radio. There is a DC bias that is on
the speaker output and you don't want to short this with the winding of the coupling transformer.

I suspect that between killing the DC bias on the radio speaker wires and trying to couple the
radio directly to your 70 volt PA amp, you have caused a couple of issues. Pull the wires off
the radio speaker before you blow the radio audio amp. Obtain the needed parts and go back
and do it the right way. A short time fix would be to simply add a capacitor between the PA
amp your coupling to on the ground side input. This at least will stop you from grounding
that side of the radio speaker output.

Another solution would be to use the accessory connector fixed audio output on the CDM radio.
This way, no matter how much someone plays with the radio volume control, the level going to
the PA amp will always be the same. I forget which pin on the accessory connector is the fixed
(filtered audio) audio output. It is selectable in the radio programming software between the
discriminator output (not squelched) and the fixed filtered audio (squelched). Want5 to say it
is pin 11, but don't hold me to it. If you use this pin, you should not need all the fancy extra
material. However, it won't hurt. if you use a coupling transformer, this fixed audio output
also has a DC bias, so use a coupling capacitor of say 0.1 uf to keep from shorting out the bias.

[Bill_G]

Excellent point Jim. Without having a schematic of the amp (or reverse engineering the input), the decoupling cap probably isn't necessary since most amps have a cap in line already on the aux input to prevent DC offsets from causing popping. However, the aux input is single ended, and definitely tied to chassis ground which would be a big problem for the CDM spkr output probably sooner than later. The CDM det audio output is the best choice. However, now we run into the possibility of a ground loop between the two devices bringing us full circle to either caps or transformers. Hello Radio Shack and their little 1:1 transformers. Those things have saved my butt a number of times.

[Will]

Use the filtered audio output from the CDM it IS a line level output. This goes to a LINE input on the PA amplifier. The radio's volume control does not affect the filtered audio output level/volume.

I STILL recommend a coupling capacitor to block grounding the output of the op amp in the radio. 1 to 4.7 uf non-polarized capacitor.

[jackhackett]

Use the filtered audio output from the CDM ....
I STILL recommend a coupling capacitor to block grounding the output of the op amp in the radio. 1 to 4.7 uf non-polarized capacitor.

According to the manual the filtered audio output has a cap inside the radio already. You can use an external one if you want, but you shouldn't need it.

[Will]

According to the manual the filtered audio output has a cap inside the (CDM) radio already. You can use an external one if you want, but you shouldn't need it.

Just EXTRA insurance for the radio's protection.

Note: very few Motorola radios have the internal capacitor.

[jackhackett]

Note: very few Motorola radios have the internal capacitor.

As far as I can tell just about any of the radios with the "16 pin" accessory connector (CDM, GM300/Radius/Maxtrac, M1225, SM series, CM series) have the RX audio out capacitor coupled.

Spectras don't appear to, and I'm not sure about MCS2000 series, the schematics for those are so $*@# poorly done as to be almost completely useless.

But like you said, adding another one externally can't hurt.