[chart2006]

I have an old PAStrong Electronics mixer with integrated amp. The model number is PA4120S and unfortunately I can't find any information on the web about it. Well my problem is I'm not familier with the setup. It has 100 volt, 70 volt and 8 ohm isolated transformer outputs of which I'm not for sure what they are used for. I know that obviously that the com/8 ohm output is for the speaker connections but what of the others? Plus there is a 24VDC connection as well. I can't see these being connections for any additional speakers. Thanks

[JohnWill]

The other two are also for speakers, either 70V or 100V audio transformers are required.

Here's some additional information.

A 70 Volt Sound Distribution System is actually quite simple once you understand it. Here is the concept behind it.

Most audio amplifiers can handle an output impedance down to 4 ohms (sometimes 2). Most speakers have an impedance of 8 ohms (sometimes 4). Once you connect two 8 ohm speakers in parallel you have reached the 4 ohm limit of the amplifier. If you connect them in series, you get 16 ohms which is safe for the amplifier, but inefficient in terms of power transfer. You could connect any number of speakers in combinations of series and parallel to achieve various impedances, but the volume of sound from each speaker will vary significantly unless you have the design perfectly balanced. No matter what you do however, you cannot control speaker volumes independently.

In a 70 Volt distribution system, a transformer is added to each speaker which increases the speaker's impedance significantly. This allows you to connect a large number of speakers in parallel to the amplifier. The amplifier sends a higher voltage than a regular 4 or 8 ohm amplifier (up to 70 volts) in order to compensate for the high speaker impedance. By connecting the speaker transformers at different wattage levels, or by adding 70 volt attenuators, each speaker can be set to the desired volume independent of the others.

In a 70 volt system, you can add as many speakers as you desire as long as the total power requirement of all the transformers added together does not exceed the power output capability of the distribution amplifier.

[Soundy]

John nailed it - to use the 70V or 100V outputs, you need the appropriate speakers, or the appropriate transformers to add to your desired speakers. Otherwise, just use the 8-ohm outputs.

[chart2006]

I now understand the idea of the 70/100V outputs and it seems this is not something I'm wanting to do unless I'm running speakers to various locations which is understandable for the distance. What are the additional hookups for then? I noticed one is not labeled which is next to the 'com' and a 24VDC output. Judging by the outputs this is not a stereo amp just a mono setup? The inputs offer both left and right. I'm debating whether or not just to purchase a whole new setup or not but if this works for small events despite it weighing about 50lbs I'll use it. I'm needing a PA system for small event my club is putting together. Also I'm wondering if it is stable at 4 ohms. I have two 8 ohm speakers I'd like to connect, in parallel, over the independent channel but with my unfamiliarity of this system I'm not sure. Most amps are but this one doesn't specify.

[guy2]

I own one of these things ,unfortunately when i was busted i had to use it as a every day amp.

There not good for clean music playing but they do serve their purpose ,120watt rms is plenty loud seeing how there is no need for full range sound since it probably cant push it anyways.

Mine 100 or so rms and it's plenty loud with an 8ohm 15'' paper loud speaker.

[Soundy]

"COM" is the common for the speaker outputs - in your case, you'd connect your speakers to the COM and 8OHM terminals.

The 24VDC, I would suspect, is for either supplying power to a powered mic or other such accessory, or for powering the unit from something other than 120 or 240VAC line power (although this seems unlikely to me - it would make more sense if it took 12VDC input).

[JohnWill]

I'd guess the 24V is an output, and a voltmeter will confirm this.