Creating a Speaker Cabinet having either a single 8 ohm input or two 4 ohm inputs.

I suppose the first question to answer is why would one want to do this? For me the first reason was to have single cabinet to use with my modded Ashdown Superfly amp as well as for general use. In the case of the Superfly, having two 4 ohm inputs is useful because this amp has two separate power amps internally, which are each rated 250 watts into 4 ohms.

But this could also be useful for anyone who wants to have a cabinet (maybe a 4x10, or 4x12 cabinet) that can be used as an extension cab for a normal configuration, or with a stereo power amp (such as the QSC I use), driving each channel from a single input.

The cabinet I applied this to is a BagEnd D-10. This is a 2x10 cabinet originally with a 4 ohm load. I got two of these to provide an optimal load to the Superfly in a 4x10 configuration. Eventually I decided I'd like to have a 2x10 cabinet for the Superfly, together with the possibility of using it as an extension cabinet with either my GK or Markbass amp.

The first thing to consider is that the cabinet was loaded with two 8 ohm drivers wired in parallel to obtain the 4 ohm load. Clearly one cannot achieve 2 4 ohm loads with the original drivers, so I searched for some 4 ohm replacement drivers. Using each separately I get the two 4 ohm loads, and in series, the 8 ohm load.

After quite a bit of searching and cabinet response modeling I decided on the Eminence Legend BP102-4. These are rated 150W, which is a good match for the 250 Watt rating of each of the Superfly's two power amps.

Cabinet modeling suggested these would work well in the the ported BagEnd cabinet and also if I decided to seal the cabinet. I prefer the sound of sealed cabinets because there's less distortion in the region where the port is reinforcing the low frequencies, so it seemed like it could be interesting to have the opportunity to try it. Sealing the cabinet did leave open the possibility of too much response with very low frequency signals, so a high pass filter at about 40 Hz might be necessary.

The cabinet has both speakon and phone plug connections (which I don't favor and would not be useful for the dual impedance configuration). I decided to use speakon pins 1+ and 1- for a 4 ohm input, 2+ and 2- for the other 4 ohm input, and pins 1+ and 1- for the 8 ohm input. Also the ¼ inch phone plug was to be removed and replaced with a toggle switch to select which impedance to use.

Now the interesting part – how to make it impedance selectable? Scribbling on some paper resulted in the following schematic for switch wiring which would make it all happen. A 4 pole double throw (4PDT) switch was found at a local electronics supply store, though only 3 poles were needed. If you can find a 3PDT switch, you might save some money. The one I got had contacts rated at 10 Amps, which is more than sufficient.

So now the cabinet can be switched from two 4 ohm loads in one switch position, to a single 8 ohm load. For use with the Superfly, a custom cable was built with a speakon for each Superfly output connector and one for the cabinet, and wired as shown. I was able to find some 4 conductor 12 gauge wire at the same electronics supply place where the switch was found.

Btw – I tried the cabinet both ported and sealed, and ended up making it sealed despite the low frequency excursion issue.

Here's the cabinet with the switch installed: