Loudspeakers Ohms Question

Les

Well-Known Member
Hey guys. Got a bit of an out of the ordinary question for ya.

I have a small 20w PA amplifier that I am wanting to connect four loudspeakers to. The speakers are each 8 ohm, unknown wattage. They're not 70 volt, but otherwise they're basically a typical PA paging horn (but a little on the small side with a 6" bell).

The amp has several outputs on the back: COM, 4 ohm, 8 ohm, 16 ohm, 70v. (one speaker lead goes to COM while the other goes to the corresponding impedance). The problem is that all four speakers connected in parallel equal out to be 2 ohms if my calculations are correct. I have heard that improper impedance matching can damage an amp and/or speakers. Think I'll have an issue here? Is there a way I could wire them in series to get them to match up?

The amp is old but usable. It's a Realistic MPA-25 solid state amp. An oldie for sure, but just being used for basic warehouse paging.

proxy.php


proxy.php
 
Last edited:
Wire them in series-parallel as shown in the diagram. Two 8Ω in series = 16Ω. Two 16Ω in parallel= 8Ω. And of course use the 8Ω tap on the amplifier! Even though it's not a bad amp, it would probably self-distruct with a 2Ω load.
 

Attachments

  • seriesparallel.jpg
    seriesparallel.jpg
    4.8 KB · Views: 245
Last edited:
Thanks, you guys are awesome! Been doing some reading and came across this method, but no one else explained it as clearly.

I just realize that I've been assuming, but not 100% sure, that the COM terminal is negative. Am I correct in this?
 
It's audio, so it's AC, which means there isn't really a positive and a negative...
But COM is the common, so you're probably right in connecting between COM and 8, unless we've found a really oddball piece of gear...
 
The amp pictured above uses an output transformer, which you usually don't see in solid state amps (other than ones with a 70 volt output.) Usually, the power output varies depending on your load. When an output transformer is used, 100% of the power is available at any matched output impedance as well as the 70 volt output. One small note: The 70 volt output often confuses people. You will only find 70 volts there when you are driving the amp at 100%. 70 volt speakers then base their wattage on what load they would put on the power amp when the amp is at 100%. Trust me, that math is a lot easier that if you were trying to figure out impedance matching for 20 speakers connected to an output! (in this case, for equal distribution, you would set each one of the 20 speakers to 1 watt.)

In your case, the series-parallel connected to the 8 ohm output is the correct solution and 25% of the output power will be delivered to each speaker.

As for + and - , they are there to keep things in phase as you wouldn't want the speakers canceling each other out if they were mounted in the same cabinet. In your application, I doubt there would be any detectable effect as they are horn speakers located in different locations. From a phase standpoint, positive battery applied to the + on a cone speaker causes the cone to move forward. As noted above, it is an AC signal so there is no real + and - , just the phase relationship.
 
Last edited:
The amp pictured above uses an output transformer, which you usually don't see in solid state amps (other than ones with a 70 volt output.) Usually, the power output varies depending on your load. When an output transformer is used, 100% of the power is available at any matched output impedance as well as the 70 volt output. One small note: The 70 volt output often confuses people. You will only find 70 volts there when you are driving the amp at 100%. 70 volt speakers then base their wattage on what load they would put on the power amp when the amp is at 100%. Trust me, that math is a lot easier that if you were trying to figure out impedance matching for 20 speakers connected to an output! (in this case, for equal distribution, you would set each one of the 20 speakers to 1 watt.)

In your case, the series-parallel connected to the 8 ohm output is the correct solution and 25% of the output power will be delivered to each speaker.

As for + and - , they are there to keep things in phase as you wouldn't want the speakers canceling each other out if they were mounted in the same cabinet. In your application, I doubt there would be any detectable effect as they are horn speakers located in different locations. From a phase standpoint, positive battery applied to the + on a cone speaker causes the cone to move forward. As noted above, it is an AC signal so there is no real + and - , just the phase relationship.

Thanks for the reply! I had a closer look at the horns today. They are marked 5w each. And if I'm reading this clearly, I need not pay much attention to polarity as long as the series-parallel connections follow the diagram? I will probably try to keep it consistent just for simplicity's sake.
 
The polarity is only a concern if more than one speaker is covering a space with another one. If they are in different spaces, don't worry about it.
 
  • Like
Reactions: Les
Indeed the polarity connections only really matters due to phasing and such. It is good practice to follow proper polarity.
Some not nice things happen when speakers out of phase with eachother interact in audio-land. Really for the most part, a speaker does not care, its all the same to it.
 
Thanks for all the help, guys. I got the speakers wired up and everything works as expected!

Now, just to save you all the boring details, I will just say the system got a bit more complex.

Say I wanted to be able to manually switch between a main amp and an auxiliary amp. Would a DPDT switch work? See diagram below:



Basically, something like a Y, but I realize that I can't do that since I'd be feeding one amp's output in to the other amp's output. There is of course also the consideration of powering the amps with no load. That wouldn't be an issue with the Auxiliary amp as it would only be powered when needed, but the Snom amp receives its power over Ethernet, so it would never be off.
 
Last edited:
Yep. You'll need to make sure your DPDT switch is "make-before-break", meaning the first set of contacts to one amp need to break the connection to the center contacts before the contacts to the second amp make the connection to the center contacts. You can check that with a meter. But I think most switches work that way. Otherwise, you would be feeding the output of one amp into the output of the other momentarily when switching.
 

Attachments

  • DPDT Switch.pdf
    14 KB · Views: 202
That's backwards. What you DON'T want is make before break (MBB). The switch you need is a break before make (BBM). No need to sweat this detail, because you'd be hard pressed to find an MBB switch, except in a rotary style.
 
That's backwards. What you DON'T want is make before break (MBB). The switch you need is a break before make (BBM). No need to sweat this detail, because you'd be hard pressed to find an MBB switch, except in a rotary style.
Yea, best bet is a switch that has a "center off" position. Although you will never need that position, it assures that one amp is disconnected before the other amp is connected. The reason that is important is that you never want to connect the output of different amps together (outside of an intended bridge system.) Amplifiers contain an electronic feedback loop that corrects for errors and also helps with the damping factor of the speakers. If two outputs get connected together, these systems will fight each other and things could get ugly, smokey, and expensive ;)
 
That's backwards. What you DON'T want is make before break (MBB). The switch you need is a break before make (BBM). No need to sweat this detail, because you'd be hard pressed to find an MBB switch, except in a rotary style.

This. I described it correctly but reversed the name!
 
Haha - I'm thinking this whole system is going backwards (and sideways).

I might just get a telephone adapter such as this, which can take the output of the tiny Snom IP 4 watt amp and route it through a PA amp to gain more control/power. I'm not sure if I'll stick with the Realistic - it's a little long in the tooth and don't know if it'll like being powered on 24/7.
 
Well I think I figured out how to get the best of both worlds. The warehouse won't get the NOAA Scanner broadcast but...



I mentioned before that the system had changed. We were able to tie in to an existing horn in the warehouse using the phone system but we wanted to also have paging in the office, so I adapted my 4-speaker setup to work in there (basically just horns mounted to the walls above ceiling tiles - janky as heck but it seems to work). I wanted the ability to patch different inputs to at least most of the PA system, hence the reason to at least swap between amps. Now, it looks like I can just use both amps full-time for global paging while patching Aux inputs to the office.
 
Really quick question for you guys.

Well, update first: I got everything hooked up and the system is working great. Yay. Now for the question.

Should I leave this amp powered up 24/7? I mean it's a decent enough amp, but it's kind of old and a Realistic brand, so I don't know what their design intention was as far as duty cycle is concerned. There's no cooling fan in the amp, so nothing to suck in dust. I just don't know if it would be better to leave the amp on all the time or connect it to a timer which would power it down after hours. I know that power cycles are often the most stressful times for equipment.

The amp is located in a well-air conditioned, reasonably-clean Telecom room.
 
Really quick question for you guys.

Well, update first: I got everything hooked up and the system is working great. Yay. Now for the question.

Should I leave this amp powered up 24/7? I mean it's a decent enough amp, but it's kind of old and a Realistic brand, so I don't know what their design intention was as far as duty cycle is concerned. There's no cooling fan in the amp, so nothing to suck in dust. I just don't know if it would be better to leave the amp on all the time or connect it to a timer which would power it down after hours. I know that power cycles are often the most stressful times for equipment.

The amp is located in a well-air conditioned, reasonably-clean Telecom room.

Realistic was a surprisingly good brand, back in the day.
Solid-state electronics (with the exception of electrolytic capacitors, high-intensity LEDs, and moving parts like switches and potentiometers) have lived as long as their invention (since 1950's, for transistors). It wouldn't surprise me if they'll be able to outlive a human.
If that amplifier still works, it could possibly keep working for decades to come, with very little servicing. Keeping it on 24/7 won't hurt it.

Eventually dust will settle in the faders and make them sound "crackly" and "staticky" when you move them... but if you never move them, you'll never notice. (Alternatively, you can move them along their full sweep every once in a while to keep wiping their resistive track clean, but the caveat is this will eventually wear them out. Contaminated faders can be "properly cleaned" and re-lubricated; worn faders need to be replaced. So decide the lesser of the two evils for how you want to use that amplifier. ;-))
 
Last edited:
Realistic was a surprisingly good brand, back in the day.
Solid-state electronics (with the exception of electrolytic capacitors, high-intensity LEDs, and moving parts like switches and potentiometers) have lived as long as their invention (since 1950's, for transistors). It wouldn't surprise me if they'll be able to outlive a human.
If that amplifier still works, it could possibly keep working for decades to come, with very little servicing. Keeping it on 24/7 won't hurt it.

Eventually dust will settle in the faders and make them sound "crackly" and "staticky" when you move them... but if you never move them, you'll never notice. (Alternatively, you can move them along their full sweep every once in a while to keep wiping their resistive track clean, but the caveat is this will eventually wear them out. Contaminated faders can be "properly cleaned" and re-lubricated; worn faders need to be replaced. So decide the lesser of the two evils for how you want to use that amplifier. ;-))

I agree. I would just leave it on 24/7.
 

Users who are viewing this thread

Back