a-912mk2 70v constant voltage amp wiring???

[georage]

Hi. Is anyone here familiar with wiring constant voltage public address amplifiers? Something bad happened to the TOA 912mk2 at the warehouse. It looks like it had a little campfire on the circuit board. I'm sourcing a new board or a new amp before attempting a repair, but I'm not sure how to tell if the speakers are safe to hook up to the new amp. There are about 5 speaker cables ready to hook up.., all in parallel, they read 2.6 ohms and the system is supposed to be a 70volts constant voltage. I've been advised that impedance doesn't matter with the constant voltage type amps. Does that sound right?
Thanks.

[Jon Snell]

No, that is not right. I presume you have a TOA amplifier?
The amplifier is a standard amplifier with a coupling transformer on the output. The coupling transformer is set for either 70volt line, (USA) or 100volt line (UK and EU).
You really need an impedance meter but as there is only 5 speakers, I would check they are all wire with the transformer tappings set to the output you require for that speaker.
The DC resistance should be higher that 2.6 Ohms. The cable supplying the speakers can be as high as that!
The DC resistance should be around 30 - 50 Ohms.
Check the transformers in the speakers and disconnect the inputs to check the input DC resistance is no lower than 50 ~ Ohms for a 10Watt speaker.

[Helmholtz]

A 70V public address distribution is a "high" impedance system.
As the amp can deliver 120W, the minimum load impedance is found from Vē/120 = 41 Ohm.
Each speaker should have its own step-down transformer, which makes sure it gets the correct power while the primary impedance is high.
No use measuring load DCR as this is not impedance but rather the DCR of all speaker transformer primaries in parallel.
As the amp has an output transformer, the load DCR doesn't matter.
Total load impedance can be much higher than DCR.
Do all speakers produce sound?
If one of them has a transformer with shorted turns, this would significantly lower total load impedance and would overload the amp.

[georage]

alright! Thanks Guys! So there are actually 52 speakers. most of them are way up in the ceiling and cant be reached. We took one down in the entryway and it was tapped to the 1 watt setting, but thats just one out of 52. I read the label "High impedance system" suspected something was off. OK so 41 ohms and up! I did take DCR readings of each pair of wires, which was 64, 64, 32, 128, cant remember the last one, maybe it was open. So i put every wire under the same 70v and com lugs. Thats how you do it right? See, someone took all the wires off before I got there and wired in a 4 ohm amplifier which worked for a few minutes and then "blew up".

Anyway. I suspect that the 64 ohm loads are correctly wired speakers. Would I hook those is parallel??? that would lower the DCR to 32 ohm, below 41ohms. Maybe I need to go back and re-evaluate the system.

[Helmholtz]

The amp doesn't see DCR but impedance.
If there's no short, impedance might be 10 times larger than DCR. In fact there's fixed relation betwee DCR and impedance.
Yes, the speakers are typically wired in parallel.

To find the impedance you will need to measure voltage and current with an AC signal (say at 400Hz).

[Tom Phillips]

...There are about 5 speaker cables ready to hook up.., all in parallel...

I suggest that you consider hooking up each of the five lines individually when you first test the new amp. That will allow you to determine if each sting is working properly and identify any string that may have a bad speaker or bad step down transformer. At least then, if you need to access installed speakers you will have far fewer suspects.

[Jazz P Bass]

Here is good write up on CV systems from Crown.

[georage]

thanks jazz p bass. I will give that a read in due time. Thanks for all of your support! i aquired a spare cv amp and a practice speaker so i can do some experimenting. i wanted to compare my dcr reading with a single good speaker. on the new speaker i measured 254ohms dcr on the 1.2w tap and 54ohms dcr on the 10watt tap. i know that dcr and impedance are different measurements. while im waiting for TOAelectronics to get back to me about a replacement board, id like to learn as much as i can about cv amps and speakers.

iIs there a simple test to measure impedance? im used to using a resistor in series to check voltage drop and calculate current. is it that easy? or is there really an impedance probe tool? It would be great to test each speaker circuit for problems. after all, something caused the old amp to over heat and discolor the circuit board of the original amp.

[Helmholtz]

A DMM can't measure impedance.
You'll need something like this: https://www.nti-audio.com/en/applica...mpedance-meter
or this: https://www.ebay.com/itm/40231912627...0AAOSwcKJfAEvd