So close and yet so far. Today’s Tech Tip will be an exercise in connecting the dots.
“I have two JBL 4-ohm speakers and when I put them together on an amp, it adds up to a 2-ohm load. The 2-ohm load freaks out any of my power amps including my new Mackie powered mixer (808m). It does this clipping kind of noise at high levels. When I use one, there is no problem. JBL pretty much told me “too bad” and to use multiple power amps. They also said the cabs can’t be rewired successfully. I once used 2 Marshall guitar cabinets and to keep the ohms lower (or higher) my roadie made me a special “series” cable. Do you have any idea what it could be or other suggestions?”
First, we need to all make sure we understand how parallel impedances “add” up. That information was handily covered in yesterday’s inSync. You might want to stop here and review before reading more.
So two 4-ohm speakers in parallel make a 2-ohm load. It is true that some power amps do not like to drive into impedances much below 4-ohms so it’s not surprising you’re having problems. If you must drive two 4-ohm speakers (or a net 2-ohm load) from one amp that can’t handle that low an impedance then the only workaround is to wire the speakers in series. Unlike parallel impedances (which divide), series impedances add up. So two 4-ohm speakers in series present an 8-ohm load to the amp.
You do have to make special cables to wire most speakers in series. The positive speaker lead from the amp must be connected to the negative lead of the first speaker, then the positive lead from that speaker must be connected to the negative lead of the second speaker, and then its positive lead connects back to the negative lead on the amp. This makes a big loop that current must travel through, which is what constitutes a series connection. The specific steps of how to build such a cable are beyond the scope of inSync. If you know how to solder wires to a standard 1/4 connector then you can figure this out. If not then your best bet is to enlist the help of a technician who can.
Keep in mind that the speaker cable you create will be a nightmare to use because it will be one big loop that connects to both speakers and the amp all at once. It is possible to design modularity into such a cable so it can be broken up into pieces. There are many techniques for doing this, including modifications to the cabinets themselves, but again the details are just beyond what we can tackle in inSync.
Also keep in mind that the typical amp will produce far less power into an 8-ohm load than it would a 4-ohm load. In this series configuration you will now be dividing less power among your two speakers than if you just hooked only one of them up. That’s why it’s usually best to just use two amps (or both sides of a stereo amp) to operate the two speakers that can’t be driven from one.