The AcmeSwitch
One of the “Holy Grails,” of loudspeaker design, particularly in the bass guitar community, is the loudspeaker that can be switched from 4 ohms to 8 ohms with no negative impact on performance. Unfortunately, this is impossible.
How can I say so categorically that his is impossible? Easy. Allow me to 'splain.
Fig. 1. 8ohm woofers in series result in a 16 ohm load.
The two 8 ohm woofers can also be connected in parallel, as in Fig. 2. This presents a 4 ohm load to the amplifier.
Fig 2. 8 ohm woofers in parallel result in a 4 ohm load.
Those are the only two combinations. Series or parallel. They can be combined to make a 16 ohm impedance, as in Fig. 1, or a 4 ohm impedance, as in Fig.2. Those are the only two combinations. Period.
It is possible to build a system that is switchable from 4 to 16 ohms. It is not possible to build a system switchable from 4 to 8 ohms. Why? Because it is impossible to connect two 8 ohm woofers in such a way as to result in an 8 ohm load. Anybody who says he can is lying to you.
But I thought of a clever way to fool people into thinking I had found a way to do it! Very clever indeed. I call it the AcmeSwitch.
It’s based on a very simple concept: A capacitor has infinite resistance to DC. Any capacitor appears to be an open circuit to a DC ohmmeter. Direct current, or DC, cannot pass through a capacitor.
A capacitor CAN, however, pass AC, or alternating current. This is the key to my deceptively cunning AcmeSwitch. This concept allows me to create a loudspeaker which ACTS as if it has a switchable impedance when measured with a DC ohmmeter, but which reverts to its regular state when being driven by audio signals.
Why? Simple. Because once this capacitor is in series with the second woofer, this woofer is effectively removed from the circuit, when tested with DC.
Fig. 3. Capacitor in series with 2nd woofer, resulting in 4 ohms AC, and 8 ohms DC
Why? Because capacitors come in different sizes, or values. But they ALL exhibit infinite resistance to DC. So what makes my AcmeSwitch work is a matter of selecting the right size capacitor, 1300 microfarads in the case of my DoubleSecret AcmeSwitch. It is possible to select the correct value for the capacitor such that the performance of the cab will be essentially unaffected, while at the same time presenting the ideal switchable resistance to a DC ohmmeter.
It’s brilliant. I must be a genius. It's a method for making a speaker which would SEEM to be switchable from 4 to 8 ohms as a result of an AcmeSwitch that is added only to fool the casual tester, while at the same time havng no effect on performance or on its real impedance one way or the other.
Fig. 4 shows the actual AcmeSwitch as delivered to the unsuspecting customer.
Fig.4. AcmeSwitch circuit
But there’s one problem. It’s a lie. I have decided not to introduce my AcmeSwitch for that reason.
In my 35+ years in and around the speaker business, my AcmeSwitch would undoubtedly qualify as the most dishonest, cynical, and just downright despicable thing I would have ever heard of (if I introduced it...) in the industry. Anybody who would perpetrate such a prevarication for crass monetary gain would deserve to at least be sued for false representation, and at best drummed out of the business for the good of everyone. If someone else did something like this, I would be indignant and outraged. There's no way I could perpetrate such a hoax on the bass playing community, or even on regular people, for that matter.
Not only that, but my lie would be so easy to discover, that I would have to encase the series capacitor in a near-indestructible black box and epoxy it to the inside of the enclosure, to conceal my secret from all but the most determined of investigators. People like me.